Boom design advice?

Flight Risk needs a new booom, and I'm contemplating a T
shaped design similar to ones seen on ORMA 60s (see attached sketch). The original boom (see next pic) had to be lengthened when it's gooseneck was moved down 22" from its original position on the wingmast, to the main crossbeam, just below the maststep. This changed the angle of the underside mainsheet traveller (ala 18 meter square) that serves as vang.
The original boom was also strengthened (w/ a carbon tube bogged and slipped inside) at the same time it was lengthened, and still there was noticible flexing. So the compression forces seem big. (The main is an EP squaretop with a 34' luff and 11' foot). So now the boom's too heavy as well.
So the proposed plan is to vacuum bag carbon uni to the flat surfaces of a couple of Sitka spruce planks, cut to profiles similar to the sketch, screw 'em together in the T cross section, hi density fillet either side of the joint, layer of 2" tape followed by a 4" tape, tape all three edges.
So it's advice on the details I'm asking for.

Thickness of the boards (.75", 1", 1.25")?
Width of boards @ widest point (8", 10", 12")?
Weight of carbon uni (6 oz)?
Does carbon come in a woven tape (what weights)? S glass?
Are the tape widths proposed for joint reinforcements appropriate/sufficient?

I think the design offers a lot of strength and pretty light weight, and I think there's less than $500 in materials. But I'm open to other affordable ideas. I can get a 2.25" x 10' or 12' carbon spin pole blank for around $450 retail, but I still need to modify it for the underside traveller. And is a pole like that stiff enough for this application?
Other souces for carbon poles? Other concepts entirely? Baseless crticism and ridicule?
Thanks!

Dave

Here's a pic of the current boom.

Well, I do not have any advice on the boom but, wow, that boat looks nice. First of all what type of boat is that and is that a custom paint job?

Sorry to get off topic I was just impressed!

Wouldn't a triangle tube be stronger and stiffer?

Don't get hit in the head by that thing <img src="http://www.catsailor.com/forums/images/graemlins/frown.gif" alt="" />

If you attach the mainsheet to the sail via a loop similar to the 'A' class setup, there's no need to make a truss of the boom member.

Just an observation Dave, the tack of your sail (and therefore the set of your sail) would surely be happier if it was held in line with the rest of the luff, closer to the mast, by a slide in a track?

I'd go round carbon - www.forterts.com. Tony can engineer you the right size tube, and its going to be a lot less work than building a T boom.

If you need a custom attachment at the travelor end of the boom, I'd modify the Round Forte section by adding carbon and epoxy to suit your needs.

Or you can use a LOOP around the tube, and shackle it to the sail.

It seems to me the reasons the ORMA booms are shaped the way they are don't mean anything to the boat you're applying the design to.

If I understand it correctly, the flat section of the boom on such such boats serves two functions -

One is that it forms an end plate of sorts for the main. Unless the foot of the main is close to the flat section, there will be no end plate effect. The picture of your boat shows the foot of the main to be well separated from the boom. Thus no end plate effect, and your class rules may specifically not allow end plates or may calculate your sail area differently if they are used.

Two - the flat area makes a nice platform for controlling the bottom part of the main when reefing. Again, I don't think this applies to your boat.

I'd go for a carbon tube with the appropriate main traveler built onto it. As there are no benefits of the t-section for your boat, I'd think it would only get in the way, add windage, and hurt like hell when you hit your head on it.

But I could be wrong...

Can you tell us in what direction the boom flexes ?

Of what material is the current boom ?

My current analysis is two fold.

First where does the flexing come from ? With the boom fitted to the mainbeam there is absolutely no reason to have the boom under compression. On beach cats the boom is under compression to actively rotate the mast but clearly this is not the case with your setup. In effect you can just release the car on the underside of the boom as fixing it is absolutely pointless. The car on top holding the sail still needs to be fixed to set the draft in the bottom of the sail, however this loading should be nowhere near the level that is required to bend the boom under compression. And if it is then your boom has a serious design flaw; one that is not to difficult to solve. But more about this if this is truly the case, which I really don't believe.

I think the boom is flexing downwards because your forces don't line up well. If your leech car is much to the rear of your mainsheet car (on boom) then these loads create a bending moment that acts all along your boom. A first quick fix is to fit a slide to your tack corner of your sail and hold that corner close to the mast. It appears a couple of centrimeters can be won that way on the total misalignment of maybe 15 centrimeters. This is already a 15 % improvement.

Is it possible that you load up the boom and sail and give us a picture of the flexing boom. For I suspect the solution is really simple and easily homemade for 100 bucks.

If the misalignment of the boom cars are the reason for the flexing then what you want in the way of weight is a non-prismatic boom. Meaning one that is not constant in its crosssection. This means additional weight can be won with respect to a straight carbon section. Not too mention alot of money.

Pretty much the advantage of carbon is that it is twice as stiff for a given weight as aluminium. Aluminium and other metals are pretty much equal in their stiffness/weight ratio's. Forget about glass laminate as the has a worse ratio between stiffness and/weight then aluminium. This carbon advantage is ONLY attractive when a designer is seriously limited in the volume AND weight he can accept for a given element. If either one of these is NOT limited then their is no reason to spend more money on carbon as a simple adjustment of the design will result in exactly the same performance.

Homebuilding a boom for your boat can be as simple as getting cutting out some ply sides (non-prismatic) and just gluing two strips of aluminium to the top and bottom, closing of the box. For weight reduction you could cut large holes in the sides as was done at the rear of your current boom. At the point of the cars you'll just glue in a piece of solid wood (between the ply sides).

The advantage of this setup is that flat alu strips can easily be bend to follow the non-prismatic shapes you have cut in the sides. The ply sides are only there to stabilize the alu strips that take all of the load. Of course the alu strips can be replaced by carbon if you really want but alu strips is a simple way of quickly getting results by an amateur. The advantage of ply is that it is MUCH lighter then carbon laminate. Think 2.3 times lighter and don't underestimate the load baring properties of ply. In effect ply is nothing more then a naturally produced carbon based fibre matrix. Glues or screws are easily available to fit the box together. This much lighter ply can possibly allow you to homebuild an equally performing boom for less weight then a constant section carbon boom and for less cost. Professionally made custom non-prismatic booms are far too expensive to even consider.

Like I always say. Carbon is for lazy engineers.

Wouter

I like your composite boom design. can you develope it a little?
Carbon is for lazy engineers. Think I like that one even more...

I can but I'm doing this between two "really-must-do" design projects and another "really-want-to-do" design project. So I'm a little pressed for time. Meaning I will only persue it further if it actually is seriously considered to be build.

Ake, if you are going to use it on your mini650 then we surely can continue, just let me know.

I may have something you may like. You are going to use a heavy boom vang on the mini boat right ?

What do you think about a robust but lightweight hollow timber tube of varying crossection reinforced with two alu strips on top and bottom. It is made from curved timber planks. And then rounded on the sides by routing and a little bit of sanding. This leaves a flat surface (but curved lengthwise) to secure the alu strips to, you can also replace the alu with carbon cloth if that is what you want.

This trick (but excluding the alu or carbon strips) is used on traditional Dutch sailboats to get lightweight tapered timber masts without upsetting the traditional feel of the boats.

I love what you are doing with the mini, I regulary check your webpage.

Wouter

No hurry the winter is long, but I am quite serious about it. Are considering a wooden mast and boom for my PE 21.
I kind of like those little odd ideas.
Then I have also the plans for a Blade F16 coming from Down Under.
By the way, coming down to Belgium for Christmas.

Will the mast be rotating ?

Profiled or round.

I will type something up over the coming time and send it too you.

Belgium ? Well everything I got in the way of boats is tucked away in some commercial winterstorage. I live in the city. So I can't show you any stuff, but we can can drink a beer over discussing boat design.

Wouter

If you go carbon pole, I would use a 4" dia.

The compression strength is increased by the cube
of the increased area. 2.5 to 4 is huge.

I don't believe Wouters analysis of being able
to rid yourself of compression loading is correct.
Unless you can get the mainsheet to pull on the boom
at 90 degrees and you can get the clew to align
at that point (you can't) you are doomed to having boom
compression. No big deal. If your existing deflection
is minor, consider living with it.

Nice boat.

"Carbon is for lazy engineers"? Wouter, I would like to introduce you to Mr. Tensile Strength and Mr. Density .

It will be a rotating Wing profile. Like the idea of a beer.

Wow! That's a wonderful response! Thanks guys!
Here's more in hopes of answering questions and or clarifying things.
The current booom started life as a 2" hand laid fiberglass tube. Malcom immediately (after sailing it the 1st time), wrapped most of it with layer of Divinycell and another layer(s) of glass. The day the wingmast snapped (another story) I noticed the boom flexing to leward enough to cause concern, it was a 15 kt day, max. Part of trying to analyze why the mast broke where it did, Malcom and I figured it couldn't hurt to take the compression force off the mast and transfer them to the main crossbeam, which I'm pleased we did.
By dropping the boom down to the crossbeam, it needed 14" of additional length to have the outhaul work properly. Bogged up a very stiff, thick walled, 8' carbon pole and slid it inside. This did actually stop the flexing really, but at the end of the season I could see stress cracks in the paint in the area just beyond where the 8' tube stopped. So some signs of weakness, the wrong angle for the mainsheet traveller, and excess weight brought me to thoughts of a new boom.
Seems not everyone is catching the concept of the underside traveller for the mainsheet, which I like the theory of, and will try to relate as clearly as I can manage. Here's how Malcom, the boat's designer and builder, described what he was after.
His main goal was to be able to use the main traveller to spill power in the gusts, which allows the main to retain it's shape, w/tight leach, not twist off and spill air. He claims that with skilled crew running the traveller (4 to 1 purchase, continuous sheet), he could keep the boat on it's feet, accelerating in the puffs, not losing energy by heeling excessively or allowing the power to spill out of the main with too much leech twist (associated with the rise of the boom) when you dump the mainsheet. By dumping the main traveller instead of the mainsheet, with the underside traveller on the boom, the aft end of the boom swings to leward without rising (leech twist, spilling). The pull downward on the boom (and clew) is always perfectly vertical and constant regardless of how far sheeted out, neither adding nor subtracting from the boom compression.
So the theory sounds nice, but I can't say that I've been able to test it much, as I mostly sail the boat solo or with unskilled crew. And I don't yet trust the traveller as much as I do the mainsheet in a panic dump. It's still a new and somewhat intimidating boat when fully powered up, and I'm trying real hard to keep it upright , at least until I have a more comprehensive plan to right it than I do at this time.
John and others, good eye! Sorry for the sloppy rigging! I just threw the main up the day I pulled the boat, so I could take the picture to try to have a chance at determining the angle between the underside traveller and the top edge of the boom. The lowest sail slide isn't even in the track! I normally lash the tack directly to the mast when sailing.
Thanks for suggestions on pole diameters and sources, I will pursue those options too, though cost is, unfortunately, and issue.
Anyone else have thoughts on triangular sections?
Wouter, you once again are very generous with your thoughts, though I don't have a real clear picture of the design you're offering. Simple sketch maybe? Seems like the T concept I offered shares the intent of maximizing material where it's most needed.
Keith, your right in that I don't need any of the properties you mentioned, the endplate effect, sail storage platform, and ? . But it does seem to offer substantial resistance to flexing out of column, in both directions, with a minimum of weight, and within my modest skills. I don't like the windage either though. The design
does allow easy accomodation of the underside traveller.
Please keep the comments coming, I'm far from decided on the best course of action here.

The boat was built 5 or 6 years ago by Malcom Davy, Downunder Boatworks, Kelseyville, CA, a boat restorer who built it for himself, as a challenge and a project I think. It's 24' x 14', maybe 750 to 800 lbs now, between Malcoms early modifications and additions, an my repairs and reinforcements (ongoing this winter), cylinder moulded, canted hulls, assymetrical daggerboards (very effective!), sweet rudder system, 36' Barlowe design, Gougeon Bros technique, wood/epoxy (and now carbon uni) wingmast, and waycool central pod system for storage.
I drug the boat home to WA a year ago Oct, spent the winter sanding it, and had my car guy paint it. Corvette Yellow, Viper Red, $900 for the paint alone! Have proceeded to trash the paint job in one short season, shoulda found all the week spots and practiced docking FIRST, then painted it!
Overall, despite a lot of early breakage, I'm absolutely enamored with it! It has a powerful, silky feel to it in light air, strong creation of apparent wind, and produces plently of adrenaline starting around 14 kts windspeed. It's the challenge I was looking for , I expect to be thoroughly entertained for years to come.

.Dave 509 276 6355

Dave
You are my hero for singlehanding that beast. Wow.

I think most of us get the constant leech pressure idea. My Acat has a curved travelor that accomplishes the same thing, but with much more simplicity. The clew of the sail is attached to the mainsheet via a loop and none of the leech tension is transfered to the boom. The outhaul is attached to the clew, so the compression forces on the boom are minimalized.

N6.0 does this will a free floating car under the sail on a loose footed sail.

I think the issue you need to solve first is lining up your clew with the travelor so that the force is 100% downward. This will relieve pressure on your boom. From the pics you posted it doesnt look like the clew and the travelor are lined up. Recutting the sail might work. Or moving the travlor, or both.

Bill

Wouter

Thats the stupidest thing I've ever heard from you.

Bill

Dave, just a quick question. Your boat have assymetric boards, have you got much experience with other cats? If so, what would you say the performance and handling difference between assymetric boards and symmetric boards are?


If a boom can be buildt as stiff and light with a ply/alu composite, why use expensive carbon?
You guys should begin to get used to Wouters pointed and exaggerated style so you dont take stuff like this too seriously <img src="http://www.catsailor.com/forums/images/graemlins/laugh.gif" alt="" />

I got your attention now ehh guys !

"Carbon is for lazy engineers"

as in what a pocket calculator was for a student today who checked my claim that 35 times 37 wasn't 1645 (the answer he wrote down in his paper).

Lets, see if you guys are smart enough to figure that one out under 1 sec ?


Also I haven't seen too many carbon fibre dolphinstriker straps, surely this doesn't have anything do with the "superior" stiffness or tensile strength of carbon laminate would it ? That is if it has such superior qualities.

Ordinary stainless steel will kick carbon's butt in these area's, my friends. Just ask Goran Marstom who is now replacing all the carbon stays and trap lines on the M20 for plain stainless steel.

Also simple oak wood will kick carbon's butt in the low density department, being twice as low. There is a 600 years old oak beam holding up the ceiling at the old Hijink family farm. I want to see the glue in the carbon-epoxy matrix see do that.

Now lets combine these two materials in a composite boom. Composite in this phrase used in it original meaning = "made up of different materials" and not in its current slang meaning = "GRP" or "CRP"


It is just like I said boys :

Carbon is ONLY attractive when a designer is seriously limited in the volume AND weight he can accept for a given element. If either one of these is NOT limited then their is no reason to spend more money on carbon as a simple adjustment of the design will result in exactly the same performance.

When a given modern engineer doesn't know what to do, he then appeals to the devine status that carbon seems to enjoy in todays society to get himself of the hook.

Anybody else noticed some people exhibit a Pawlov reaction when some poster discusses the issues he has with a given setup or component ?



You've bend an alu boom ? Buy a 350 dollar carbon boom.

Your boom flexes under load ? Well just order a 600 bucks custom job at your local carbon shrine.

Can't right your Nacra 5.5 singlehandedly ? Well, the solution to your problem is a 4000 US$ Hall spars carbon mast.

Want a good singlehander ? Buy the latest 20.000 US$ A-cat and then learn to bitch about its PN handicap and the fact that everybody is buying 14.000 US$ spi equipped FAD boats.

I remember last time, when "my boom flexes" Dave Farmer broke his mast, that the first advice was again that he should not attempt repairing it but just order a new carbon mast.


Lets face it guys, for some of you carbon has become a religion.

As in the answer to all things, being beyond question and scientific analysis.


Me personally, I've developped an allergy to carbo, I've seen it abused too often. The only density reduction the use of carbon achieved effectively in my situation was that of my wallet. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Wouter

You are right here.

The main line of though I'm following now is that I don't feel that there are big loads on the in te sideways direction. So no material is really needed here. Neither stiff nor much of it. But their should be enough to prevent the boom from buckling or breaking in case it hits the stays or something else.

If there are any real loadings on the boom I suspect it is almost exclusively in the vertical place and mostly in the rear part of the boom. To check this I want to know how the current boom flexes.

Lets stick to your T-shaped boom for now, to keep things simple.





In the diagram above I've draw the forces that I suspect are dominant, that cause the flexing.

The black line gives the level of bending stresses inside of the beam. Clearly around the mainsheet point the internal bending moments are the largest. These reduce down to zero towards the ends. The internal shear forces are not drawn, but these are rather small (and constant) in the front part of the boom. In the small rear part these are significantly higher but still constant. I suspect that a decently thick planck will be able to transmit these shear forces, but reall calculations need to be performed here.

Clear the bending loads are not constant along the boom and so it pays to give the boom different crossections at different points along the boom. The wood is most there to act as spacers between the strip of alu or stainless steel on the top and bottom. These two strips take by far the most of the bending loads. The wood middle part handles the shear stresses.

The drawing on the bottom is the top view. Clearly I made the boom thicker in the middle and narrower at the ends. Clearly a cosntant crossection boom will be weakest in the middle so I've compensated for this by moving some material from the end to the middle. The wood will be strong enough to take any loads in this place, because they won't be very high.

You can just cut the top (horizontal) and bottom (vertical) planck by the outline and then cut out smaller sections to save weight (circles). You can attach the top strip with the top planck to the bottom plank by just screwing through all these sections with woodscrews. Some timber glue may well help but is may not be necessary. Now you only need to fold the bottom strip along the bottom contour and secure that with screws as well and the boom is ready to take the fittings.

By the way, I forgot to draw the rear section taking the traveller rail. Just adjust the outline of the bottom planck to get the right angle for the rail. Here I would put a U-shape metal strip over the bottom and secure that side-ways to the plank by schrews. Fit the rail to this additional U-shaped section. This should transmit the loads better to the plank.

The cut-outs need to be well places to save weight and not degrade stiffness and strength to much. You can buy saw-bits that can be mounted to your drill or simply saw out hexagonal shapes with a suitable blade. Both will work. This way of building the boom will be both easy and fast. Most likely it will be cheap as well. You can test the setup using some old planks you got in your shed and a section of alu strips (hardware stores). You can then test this and determine how it performance and adjust the setup.

Remember, wood like Douglas Fir have only 28 % of the density of carbon laminate. Meaning you can use 3.6 times as much (in volume) as you can in carbon for the same overall weight. This makes the boom alot more stable under load as here volume is just as important as material stiffness. Yield stress of these wood types are still at 55 % of that of aluminium. But aluminium is again 5 times more dense meaning that the wood per USED VOLUME can carry more load then aluminium ! This makes wood a very attractive material to make the main body of the boom from. Here you need volume for stability BUT you DON'T to want to accept much weight here.

A strip of stainless steel is 10 times as stiff per volume as the same volume Aluminium and about 7 times as stiff as a normal carbon laminate of the same volume. By the way stainless only weights 3 times as much as alu and only 4 times as much as carbon laminate. So stainless steel is what you want for the strips (like a on a dolphin striker strap), but it is expensive. Alu is dirt cheap. Carbon is hardly more interesting then alu (in the way of stiffness per volume (only 30 % more) and very much more expensive. So it will be either stainless or alu. Added advantage is that both metals are easier to work with. Just bend them around the slightly curved contour and screw them to the planck. You can finish the boom and sail away minutes later. No sticky fingers, curing time or chemical lungs.

Both the bottom contour and side contours are slight curved.

Now I want to see any constant carbon crossection even beat this simple wood/metal setup both in stiffness, strength AND weight. More is possible if we spend some more time optimizing the design.


Wouter

Rolf:

Having sailed a Hobie 16 with asymmetrical hulls and now a Hobie 17 with symmetrical hulls. I say the symmetrical sail a LOT smoother. It seems to have a better feel to me, especially going downwind. But the centerboards may have something to do with it too.

Dou Snell

Rolf,
I've had an TheMightyHobie18 Magnum, and still have a SC20 with symmetrical boards, and Flight Risk just points substantially higher. If I pick out a point on shore as I'm heading in, and attempt to maintain a straight course, I can see the boat working to windward. Have done windward leeward with J boats and 35' racer/cruisers and I can either point as high as they at their upwind speed, or bear off 10 degrees and beat them handily to the windward mark, in less that 10 kts of wind. My other boats (admittedly in my hands) haven't performed that well. The difference seems very noticable to me, though I doubt I'm as sensitive as you experienced racers. So I'm puzzled that they aren't tried more commonly. It may be that the required switching of the boards with each tack/jibe is too much hassle or time, for racing. Pretty fabulous for long tacks/reaches though. Having the "wrong" board down works well pulling the boat to leeward on downwind runs too.

Dave

Bill,
Yeah, I like the curved traveller a lot, though it would be tough/expensive to retrofit this boat with one. The current system provides the same constant downforce on the clew, even when the clew is not directly over the mainsheet. And I can see that not having those 2 line up will stress the boom more than a direct connection between mainsheet and clew. But I think I'm committed to this system, and I don't think it'll be difficult to build a boom that can easily handle the forces generated by the misalignment of clew and mainsheet.
I think the major compression forces are produced by the outhaul resisting the desire of the clew to move forward(towards the mast) when pressure is applied to the body of the sail. Without the outhaul pressure the sail would want to bag out to leeward, which would entail the clew moving forward. This was very noticable on the original main of the Reynolds 21, which was boomless. When it blew over 15 kts, it bagged seriously, even with the mainsheet attached the the furthest forward hole in the clew plate. How does the N 6.0 get around that?
Thanks for the input, I'm still trying to come up with clear pictures of the forces involved here, and it reall y helps to get a bunch of different perspectives!

Dave
And all of this is theoretical, it's clear that there are signifcant compression forces here, evidenced by the flexing experienced.

That is not the only possible conclusion that can be drawn.

The boom could well be under hardly any compressional loads at all and be flexed by bending moments. This situation is actually far more likely.

Wouter

See the attached screen dump of an excel sheet. It shows the automated calculations that produce an aluminium round tube with the exact same (bending) stiffness AND weight as a given round tube made out of carbon laminate.



All the numbers given in blue can be modified at random and an equivalent tube will be produced. This mathematic problem is fully determined, meaning that a solution (of equivalence) can ALWAYS be found. Although it must be said that not all produced wall thicknesses and outer diameters may be realistic because of other engineer considerations. Example ; a wallthickness of 0.1 mm is impossible to achieve by extrusion, a common production method for alu tubes.

The excel sheet allows all kinds of materials to be entered.

In the screen dump I've entered the typical material property values for aluminium and carbon laminate. A little further down I defined what could be typical dimensions of a carbon A-cat boom (0.04 x 0.002 mtr.). Clearly a fully equivalent boom can be produced in aluminium, both in weight and stiffness, when the extrusion has the dimensions 0.059 x 0.001 mtr

How stupid is this ?


For obvious reasons I left out cost-equivalence.

Obviously some mathematical model needed to be developped that could easily produce these results. Good engineers will do that, lazy engineers will just just think "whatever" and try to solve the design problem by throwing lots of money at it, covering their lack of skill by using exotic materials.


For reader who are interested in learning something more. The closer two materials are in their stiffness and density ratio the easier it is to produce an equivalent beam in both stiffness and weight. Pretty much any a-symmetry ratio (R_E/R_Ro) that is close to 100 % will make the use of one material over another unimportant. That is unless other considerations like cost, ease of production and stability of thin walls are not factored in. This a-symmetry ratio between aluminium and carbon is not particulary far from 100 %. This leads to the situation where it is relatively easy to find good alu alternatives to carbon laminates. A similar analysis can be performed for glass and kevlar fibres with respect to carbon.


A counter example : Lets compare dyneema lines to stainless steel cables. The asymmetry ratio is now about 700 %. Makes the carbon/alu ratio look like a minor league comparison.

Wouter

Call me "a lazy engineer", if nothing else, because I'm not reading through all that text (I prefer "efficient"...but whatever). You claim that you can create a tube of the same weight out of aluminum with the same bending characteristics as one from carbon. Well no $hit Sherlock! However, the diameters are not going to be the same.

Jake,

You don't hear me say this often, but :

If you are too $%#%@ lazy to even read the post then your comments are completely void of any meaning. They are are truly baseless.

And cut the BS about "efficiency". It took me 30 minutes to develop and implement the mathematical model producing the equivalency results. Even at a going rate of 100 buck per hour, this excersize would have been well worth the time of any professional engineer as the price difference between alu and carbon tubes is much bigger then that. THAT, my friend, is true "efficiency". And also what engineers are supposed to do, thats what they are trained for. To find solutions to design problems that satisfy the criteria and be as inexpensive as possible.

I don't know exactly how to intepretate this quote :



If you agree that this can be done, then you have just confirmed all my points.

If you disagree then, well, you have just proved to be one of those engineers that prefers gut feelings over true science. A dressed up monkey pretending to be an educated man. We already have far too many of those in our societies today. Please assure me that you are not one of those, I would consider that a loss.

I like you personally Jake, so take this post as a single issue disagreement, even though it is a very fundamental one.

I simply can't stand dumb people thinking that they have anything meaningful to say without having put in the effort to understand what is being discussed.

Wouter

Don't give Jake such a hard time Wouter, the problem with your spreadsheet is that you could calculate that a chocolate boom can be made as good as a carbon one, Yes it would have a large diameter and small wall, just like your aluminium one would.

When you go on to say "a wallthickness of 0.1 mm is impossible to achieve" I think that also makes your post pretty meaningless. (although I think you meant 1.0 mm)

In general terms most engineering materials except carbon have the same stiffness/weight ratio.

Carbon is the worst material for lazy engineers, because it is not homogeneous and can have different properties in different directions. It is a very hard material to design with. But can be amazing when used correctly.

As far as the efficiency of materials goes, you make a good point about efficiency of materials that are volume constrained, but most of the engineering on a cat is weight constrained. The only volume constrained case on a cat I can think of is where you are deigning for least drag, and then it is only really applicable with situations where the material is purely in tension. Like Dolphin striker support and stays.

Gareth

I have been following this thing for some time and watch how people think about things. I see for some reason that Wouter is anti carbon and he is an alu fetish, but i have to say that the boom he did designed will be working but you will never been hit by this shape because the damage would be severe.

Quote:
Carbon is the worst material for lazy engineers, because it is not homogeneous and can have different properties in different directions. It is a very hard material to design with. But can be amazing when used correctly.

I agree with this because it is the biggest problem with carbon, you don't have 1 type of carbon but several types and than not only the type of cloth or UD but also in flexibelity and than you have a lot of different types of resin where you can work with and i can a sure you that you don't have the time to follow the whole development in carbon and resins because it is going to fast.
The thing Wouter is right, is the money and carbon will be more expensive at this time than aluminium but i am also following the prices for the metal market (stainless,Alu)and i have to tell you that the prices for metals is going up more rapid than carbon.
The trick on this is that you try to get a design that is within the budget you have, or in alu,wood or carbon.

Regards,
Hans

What I meant to imply was that it didn't take me reading your entire post to realize that it's crap. You list materials that have a better tensile strength or lower density but not BOTH in the same material. You list some sort of table that's silly over complicated in that it ignores the diameter of the aluminum required to achieve the same bending properties as if it is not important. You're only presenting part of the picture because if you present the entire one, it weakens your argument. Again though, it's like arguing with a pig.

I think it's probably a decent idea for this boat to have a beam that starts with wood to be cost effective. But I was responding to your statement that carbon is for lazy engineers. Just go look at an F1 racer and tell my how "lazy" those engineers are.

Exactly !

So you need to look at each individual case in detail to determine what material to use. That is my main point. It is 180 % opposite of making everything from carbon laminate because it is so "good".

Also my alu boom doesn't have a LARGE diameter, it just has a LARGER diameter. There is a difference here. A chocolate boom would have a diameter nearing 1 mtr, that IS an impractical large diameter.

Clearly the larger diameter of alu is still so small that it is not an argument for not using it as a boom.





An alu wall thickness of 0.1 mm (0.0001 mtr) may be impossible to extrude but a wall of 1 mm (0.001 mtr) isn't. Therefor my alu boom example still stands as that one uses a 0.001 mtr = 1 mm wall.

This is high school stuff guys, please don't waste our time by making such rookie mistakes.






There are other materials like wood, plastics and even metals like copper that have noticeably different ratio's.




It is really not hard at all to design with carbon. Because it is so light, you can just pile up more and more matts on eachother (and under different directions) till the bloody things holds up under the loads. Without getting hit back by heavyness or a large volume. That is why it is so favoured by homebuilders. They hardly do any real designing.

It may be a hard material to design WELL with but apparently even aluminium is that for most engineers. This has nothing to do with the material but rather with "laziness"




That is not the full picture. If I'm volume constrained then I can still make an alu stiffness equivalent to carbon, I just can't make it the same overall weight. So we are back again at my initial statement. Carbon is preferable over alu when an engineer is limited BOTH in volume AND weight. If either one is not limited then it is not immediately clear which material is to be prefered. It will then dependent on other considerations like cost, availability, and stability of the component (wall thickness)





Correct. When it comes down to alu or carbon laminate the difference is volume are often so small that the increase in drag is negligiable. How much more drag will a 59 mm boom have over a 40 mm boom ?




Wrong again.

Situations with ONLY pure tension favour carbon a little more as here you can't play with factors like diameters to make both components the same in stiffness AND weight. This because both stiffness and weight are influenced to the same degree by a changing length measure. In bending, torsion or buckling this is not the case and here stiffness changes faster with varying length measures then weight. This disportionality is the reason why it is possible to design a tube with equal stiffness and weight when using different materials. Think about this.

The reason why carbon is not used in stays and dolphinstrikers is because it requires too much volume to get sufficient stiffness and strength and carbon is more sensitive to abuse and degradation and often this is compounded by being more expensive as well.

Stainless steel dolphinstrikers can be 3 mm thin (while being 25 to 40 mm wide). An equivalent carbon strap needs to be at least about 2.5 times as thick or 7.5 mm thick. It is alot easier to smash a 3 mm strap through a wave top then a 7.5 mm thick plate. Additionally when a stainless strap is hit very hard it will bend but maintain the beam in shape. Carbon will probably splinter or fracture and fail, bringing down the mast.

Stays is somewhat different. Stainless steel 1x19 cables have the same stiffness per area as carbon laminate. Dyform stainless steel cables have a better stiffness per area then carbon laminate. Again stainless will survive much more abuse then carbon which is important in stays. But most important of all, 1x19 steel cable costs 10 times as little as an equivalent carbon cable. Really the only downside to stainless steel cables are that they weight a little more. But if that is a problem then make your stays out of dyneema or similar fibres. Such fibres will shame both stainless and carbon but with the drawback of being much less abuse resistant then stainless. This last property is really holding back the use of fibres for stays.

Mind you the use of fibres of trap wires is an old trick by now. For nearly 10 years now a portion of the Dutch cat sailors are hanging of 3 mm dyneema or 2 mm D12 lines, with great succes. Much lighter then either stainless 2 mm 1x19 or carbon and significantly cheaper then both as well !

Currently there seems to be a drive to build cat hulls out of kevlar. It is lighter then carbon and it is also cheaper. The superior stiffness of carbon fibres is not really used in the hulls because the crossectional area of the hulls is already so large (think again about our alu/carbon example) that the minimal wallthickness needed allows other material to relatively easily arrive at sufficient overall stiffness. In the case of hulls this favours kevlar and seriously limits the drawbacks of using S-glass.

I guess my mainpoint I'm driving at is that cat sailors would be wise to understand these principles. It will allow them to not get scammed into paying alot of money for boats with fashionable materials that aren't really any better in what they are supposed to do.

Example : If you have the option to have your hulls build in S-glass, kevlar or carbon with the additional cost going up from glass to carbon then don't consider carbon, get Kevlar. You'll get the same quality in the hulls for less cost. If kevlar is not an option then seriously think about how much a little extra stiffness is worth in relation to paying significantly more.

Thanks for reading through my lengthy reply.

Wouter

It's getting way too heavy in here.
MMMMM, chocolate booms...delicious. They would be great for those long days of distance racing. Just take a bite off of the back end...

Jake,

I'm truly sorry to say this, but you are a dumb [censored].

All the things you state as counter arguments were adressed in my initial post.

But I guess you missed them because you were too lazy to actually read what I wrote.



Let me put it in a kindergarten format for you :

a 59x1 mm alu tube will have the same (bending, torsional, buckling) stiffness and weight as a 40x2 mm carbon tube.

Wouter

Hello Hans,




My boat has :

ply hulls
epoxy resin and glass tape as joints, with some carbon and kevlar local reinforcements
aluminium mast and beams
carbon laminate rudder boards and stocks
Glass vinylester daggerboards
stainless steel dolphinstriker, stays and mast/beam fittings
dyneema trapeze wires.


I can be accused of many things, but not of anti-carbon or alu-fetish tendencies.

It is as I always said. Look at the individual design problems in detail and choose the best material for that application.

Sometimes this is carbon, other times this is aluminium at other times it is something else.




Agreed, this was a quick knock-up of a design. I have an idea of how to make one that is less hurtful when hit. It is a little more complicated and I didn't have time to work that one out fully, including drawings etc.

I'm probably going to propose that design to Gato for his mini650. Roughly speaking it is a round section made from straight planks that has been routed and sanded to have round sides. the flat top and bottom planes will take the carbon cloth or aluminium strips to get it up to sufficient strength and stiffness. This building method, but without the carbon or alu, is indeed used to produce lightweight timber round spars for masts and booms on traditional Dutch sailing boats.

Wait a minute; I have a document somewhere I written in 1999 or so ...

Wouter

Lightweight timber spars as used today for traditional Dutch sailing boats.

Gato are you reading this ? This is a good option for your mini650.

This building method allows you to build hollow tubes of plain timber plancks. It even allows you to make tapered hollow sections. Some builder decide to coat the outside with a layer of glass to increase the stiffness even more.

I think the drawings are selfexplanatory.











My idea is to keep the top and bottom parts flat (only route the little points off) and use the flat surfaces to glue or screw carbon cloth or alu strips too.

The timber properties should be be enough to withstand the loads in the sideways direction.

If you don't to spend much time rounding the sides then you can just route the sharp points of and keep a multi chined pentagonal outer shape. That won't hurt to much at all when hit by your head.

Also if you don't really want a perfectly round section then a good simplifications would be to not to use 8 elements but only 6. Two larger ones on top and 4 on the sides. You would then only have to round the sides a little and leave the rest as it is.

I suspect that this 6 element hollow spar (possibly tapered) could well be just as strong, stiff and light as the earlier (simple) design. It can be homemade realitively easily and cheaply. Ofc ours different cross section profiles can be made by varying the angle under which the elements are glued together.

As I said earlier this setup is used as masts and booms on (small to large) traditional Dutch sailing boats. These spars can take a good loading before failing. And they are surprisingly light.

Wouter

Wow ... this is impressive. I'm a dumb [censored] and need stuff in kindergarten terms for me. Can I note for the record that the diameter of the aluminum tube is NOT the same diameter as the carbon tube? Why exactly have you resorted to calling me names and insulting me?

As is so often the case you should take Wouters engineering based arguments with a degree of scepticism.

Gareth

Wow way out of hand. Wouter there was no need for name calling. You are reminding me of a certain individual called Sam, remember him?

Hey! Gimme my thread back! I'm still looking for concrete designs, with dimensions.
Wouter, I actually like your design, what thickness for the timbers (planks), how about maximim width? Aluminum strips on the 3 edges? Thickness? The same width as the thickness of the timbers?
I'll consider an aluminum pole, What dia, and wall thickness?
A 4" dia carbon tube has been suggested, anyone agree with that? Wall thickness? Most affordable source of such a pole?
Any other thoughts or considerations on my original T proposal?
Other designs?

Dave

Carbon, Aluminium and now CONCRETE <img src="http://www.catsailor.com/forums/images/graemlins/confused.gif" alt="" /> <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Quote:Gato are you reading this?

Would not like to miss it for all the carbon in the word. I will try Wouters solution for the boom. There is for les than 50€ in timber for a 4 meter spar and I have time enough to give it a try. At least I will not be doing something stupid in stead.

Jake,

I like you as a person, so it is not about that.

But I do think a great definition of "unwiseness" is to first state that you didn't read all of my inititial posting and then follow-up with claims about what you feel I failed to mention.

You could at least do me the honour of actually reading the article you are about to critize ? Fact checking and that sort of stuff.


Lets look at what we wrote okay.

You wrote :




When I had already written in the post you replied to :




To copy the expression you used first : "no $hit Sherlock".

I had already stated in my first posting that the diameters wouldn't be the same. Add to this that such an observation is trivial (obvious).

A component having the same weight, same stiffness AND same dimensions as another component would practically have to be made of the exact same material, wouldn't it ?

Pardon my emotional outburst. I got a little annoyed chasing wraiths created by a person who clearly didn't read the whole posting. And the "INCOMING" picture and "No $hit Sherlock", "Mr tensile strength" and "Mr density" quote's aren't much justification to calling you dumb, or are they ?

I hope you can now understand my use of descriptions in line of the word "stupid".

If you just had read all my posting and took a little time to reflect on what was written then you would have recognized the "uselessness" of your comments before you wrote them down.


Wouter

Dave,

I can't give you dimensions unless I know what loads are acting on your boom. Magnitude and direction. Contrary to what some people believe, I don't do guesswork.

>>Wouter, I actually like your design

Which one of the two ?

After some though I would personally do it even simpler in your case. Just two vertical planks and alu strips as top and bottom. And have the the outward facing corners of the timber plancks rounded at a woodshop. That should be sufficient to score "good" on the hurting your head aspect.

The sides of the boom can then still be given a non-constant contour optimizing for load baring and weight.


>>Aluminum strips on the 3 edges?

I really doubt wether you'll need that. Just alu on top and bottom (can also be carbon of course)


>>Thickness? The same width as the thickness of the timbers?

Probably not. Alu (carbon) is much stiffer then timber and also weights significantly more per volume. So you are looking at relative thin alu strips compared to the timber. And that is all you'll need anyway.


>>I'll consider an aluminum pole, What dia, and wall thickness?

Also for this I need to know the loads and such. However 80x2 round sections are pretty much commonly available and should hold. Such a section should only weight 3.40 kg (7.5 lbs) per 2.5 mtr (8 feet) length. This is probably bigger then needs be but it will give you a feeling of what we are talking about. If such a section is made out of carbon then it would still weight about 2.30 kg (= 5 lbs). So how much are you willing to spend on 2.5 lbs weight savings ?


How long is the foot of your mainsail ? And what kind of purchase are you using on your mainsheet ?



>>A 4" dia carbon tube has been suggested, anyone agree with that?

I find that a larger diameter. I think you can do with less. Note that 8 foot 80x2 mm alu tubes (about 3 inches outer diameter) are also as rearbeams on homebuild Blade F16's. There it handles sheet loads of about 300 kg at midbeam. Surely it will hold up as a boom.

But lets give you a proper design, so allow some time to get the laod data of your boat and run them through some calculations.


>>Wall thickness?

If going for a pure aluminium tube then you never need more then 2 mm walls. If you can get 1.6 mm then that would be perfect as that would be 25 % weight savings.


>>Most affordable source of such a pole?

Without a doubt your local alu supplier.


>>Any other thoughts or considerations on my original T proposal?

It can be done better.


Wouter

Thanks Gato,

Can you send me the pics of the building and its completion ?

Or just signal me that they are on your blog.

I would love to have those pics.

Wouter

Ok, it will probably take some time but I will not forget to send you the pics.

>>Wouter, I actually like your design

Which one of the two ?

I like them all, am most attracted to the 1st, which is closest to my T shape, but honestly, I think it's the appearance that appeals to me, and that's probably not the best reason to choose it.

After some thought I would personally do it even simpler in your case. Just two vertical planks and alu strips as top and bottom. And have the the outward facing corners of the timber plancks rounded at a woodshop. That should be sufficient to score "good" on the hurting your head aspect.

So, the vertical planks would have an elliptical shape (narrower near the ends), and the alum strips would be constant width and thickness? Maybe 2" x .25" alum strips? A max width of the side planks of 6" to 8"?

>>Aluminum strips on the 3 edges?

This question refered to the original T shape.

>>I'll consider an aluminum pole, What dia, and wall thickness?

Also for this I need to know the loads and such. However 80x2 round sections are pretty much commonly available and should hold. Such a section should only weight 3.40 kg (7.5 lbs) per 2.5 mtr (8 feet) length. This is probably bigger then needs be but it will give you a feeling of what we are talking about. If such a section is made out of carbon then it would still weight about 2.30 kg (= 5 lbs). So how much are you willing to spend on 2.5 lbs weight savings ?

Type of aluminum alloy?

How long is the foot of your mainsail ? And what kind of purchase are you using on your mainsheet ?

The main has a luff of 34', foot of 11', square top. Currently running 10 to 1 mainsheet, I think about 12 to 1.


Thanks Wouter, and others, I'm enjoying this education.

Dave

I suggested 4" tube, but if you look around you may
find the remains of broken carbon spinnaker poles of
the standard 3.5" size for cheap. They also sometimes
come with the fitting, making gooseneck fab a bit easier.

Another common source for carbon spars is broken Melges
24 masts. The original masts were weak at the spreaders
and many, many M24 owners have the pieces laying around
in their back yards. Great for makeing strong booms out
of.

Wouters design is pretty cool. One other thing lazy
designers do (besides use carbon fiber) is sometimes
they design elegant creations without regard to the
labor involved. They sometimes add 20 hours of labor
to save $300.00 in materials, those pesky critters do.
His design looks like fun to build for the hobbiest,
though, and I bet it works.

Thanks Mark, those are good ideas. Is there a Melges site? And nice to hear someone support Wouter on occaision. Anyone have thoughts on how to connect with those with broken spin poles?
Dave

>>So, the vertical planks would have an elliptical shape (narrower near the ends),


No, just straight but with rounded corners.


>>and the alum strips would be constant width and thickness? Maybe 2" x .25" alum strips? A max width of the side planks of 6" to 8"?

Are you still serious with this approach ? If so then I will run the math on it and try to get you the best balance between weight and stiffness.


>>Aluminum strips on the 3 edges?

>This question refered to the original T shape.

I don't think the side edges need any alyu. Just top and bottom will work. However if you want more side-to side-stiffness then the much efficient weight setup would indeed be to replace the alu strip on the top by two on each side (with same combined crosssection area)


>>Type of aluminum alloy?

Anyalu you can buy at your local hardware store will be fine. Anodising is not really necessary. Most commonly available alu types are of 6061, 6005 and comparable alloys anyway. There is not much difference between these alloys at all.


>>The main has a luff of 34', foot of 11', square top. Currently running 10 to 1 mainsheet, I think about 12 to 1.

All right, I don't oceans of time so which setup is preferred by you. Round alu tube or T-composite setup. If you want I will run the numbers on one of these setups this weekend.



>>Thanks Wouter, and others, I'm enjoying this education.


Well that is the whole intention !

Wouter

I think this debate about Alu or Carbon is all about perception ? Aren't all boats female and the really sexy ones seem to have more sexy Carbon than plain Jane Aluminium. To prove my theory have a look at the attachment.

Now, Answer this question: What colour shoes was she wearing.

If you don't know then I reckon you're into Carbon. If you said red then stick with Aluminium (and get a life)

My immediate thought was about hooter size and pole length. <img src="http://www.catsailor.com/forums/images/graemlins/wink.gif" alt="" />

That was too easy! Hahahahaha.

tack

Dave, Dave, Dave, you know that cats and concrete don't play well! Why would you consider concrete designs with all of the exotic material available these days?