I want to purchase the Beams for my Blade,
From the plans the beams are 80mm dia alu with 2mm wall,

Wouter wrote:


I understand that If I buy a larger dia with less wall thickness I can reduce waight,

Wouter can you help me figure out what would be the wall thickness for a 100mm dia?

also what alu tube (dia/wall) do you recomend for the boom and spi pole

from the plans the spi pole is 3m, shuld i get longer? 3.5m?

Thanks,

I am no engineer, by far, but increasing the diameter will also increase surface area. As you increase surface area, weight will rise proportionally unless you reduce wall diameter. But how thin walls can you use in real life, before buckling become a problem?

Erez,

Pretty much the minimal wallthickness you want on your beams is 1.6 mm. But this is very hard to get for 100 mm beams. At these large diameters 2 mm is almost an absolute min wallthickness that is carried by commerical suppliers. By far the easiest section to get commercially is 80x2 mm and hence its use in the Blade F16 homebuild plans. And added advantage is the fact that an alu 80x2 beam is pretty lightweight, they are 3.5 kg per beam. The only downside to this section is that its bending stiffness is sufficient but not excellent. Still, the end stiffness is similar to production Tiger F18's and Nacra F18's. The newer beams are stiffer though, but at a weight penalty.

A 90x1.6 mm round alu beam will be 12 % stiffer and 15 % lighter. This was considered too little to justify the extra effort in sourcing these sections and the larger frontal area. The last point is not unimportant. The frontal area needs to be smoothed into the deck or else it will not shed water properly. This can really be a drawback during a dive, it will impact negatively on dive recovery.

My advise therefor is to stick with the 80x2 mm beams.


>>also what alu tube (dia/wall) do you recomend for the boom and spi pole

Boom 50x1.6 mm ; this is really the BEST section there is for the F16's

Spi pole 40x1.6 or 40x2 mm ; I personally have the 40x2 mm and that has withstood all the abuse I put it through. It feels like 40x1.6 mm would also do the job but at less weight.


>>from the plans the spi pole is 3m, shuld i get longer? 3.5m?

I have a 3.5 mtr spi pole with my spi tack block fitted at 3.25 mtr. The 025 mtr sticking out I planned to use for holding a weathervane. This is actually a good place to have a weathervane, however I never did install it. I sail mostly be feel now. I think all the newer F16 spis are cut for 3.5 mtr poles now. For some reason the balance of the boat is not much affected by the longer poles. Personally I advice to start at 3.5 mtr and then find the best point for the spi tack. You can always make a pole shorter but never longer.


Wouter

Don't forget what the type of alu in very important (6060 - 6082 - 7070 etc.)and the mechanical data really change!!
I have a schedule containig this data, i can post them.
Emanuele

The company that I use offers the large diameter and small walls. i.e. 101.6x1.6 or 90x1.6, but I agree with Wouter when he reccomends sticking with the 80x2 reccomended.

See

http://www.worcester-aluminium.com/html/rt7.htm

Also the material to ask for is 6082-T6, you may find that the larger diameter/thinner wall is not available in that spec as it is often more decorative than structural.

All the best

Gareth

A Good rule of thumb is that a tube will buckle when the stress reaches 0.25*E*t/r

Where
E = Elastic Modulus (70,000 for Al)
t = thickness
r = radius

So for a 80x2 it will fail at 860MPa and 100x1.6 will fail at 550MPa. The Yield strength of a good Al like 6082-T6 is 250MPa so both values are above this which is good. I personally then like to add a generous safety factor to these which is why I prefer 80x2. I have experience of 76.2x1.6 beam tube buckling on one of my boats.


Gareth

Actually, aluminium alloys don't change much at all in the way of density and stiffness from one alloy to the next. That is when looking at alloys with relatively good corrosion resistance and good extrusion behaviour. Most significant differences from alloy to alloy are to be found in the ultimate (break) stresses. However this is not really an important factors in beams, boom or masts. In these components one is far more interested in the amount of stiffness one can get for a given weight. Yield stresses don't seem to dependent on the type of alloy, they are however very dependent on the applied temper (T-codes). This is an after (heat) treatment that improves certain specs.

so

6061-T6
6005-T5
and
6082-T6

are what you are looking for.


Note that the T codes ARE important as these have a VERY big influence on yield stress. This is the level at which a tube will not break but maintain a permanent bend after being overloaded. In boat design we want all components to straighten out after we remove the (peak) loadings. So a minimumal level of yield stress should be maintained.

Having said all this I have not encountered any supplier that delivered 80x2 mm tubes of any other specification then the ones above. 80x2 tubes are clearly structural components and they use structural alloys when producing them. But guys like Hans have bought and sold much more alu tubes then I have and so they should be able to much better inform you here.

Otherwise follow these links for more info :

http://en.wikipedia.org/wiki/Aluminum_alloy

http://en.wikipedia.org/wiki/Tempering

http://www.aalco.co.uk/technical/datasheets/Aalco_Datasheet_Al_Specifications.pdf


More general sites with specs :

http://www.aalco.co.uk/technical/aluminium.html

http://www.azom.com/materials.asp

can someone post these comments on our open source F16 design wiki ?

Good luck,

Wouter

This is a good conversation for me, because I'm also thinking about this now.

I was thinking about getting both boom and spi pole from same section, because I was hoping to get it anodized (I need them black <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />) and here in Finland it seems that I must have at least 6 meters section to be anodized. This would propably mean compromising on the boom (i.e. 40x2 mm or something).

I got the idea of black sections one evening and now I'm pretty much obsessed about it... Let's face it, carbon is fun only because it's black <img src="http://www.catsailor.com/forums/images/graemlins/wink.gif" alt="" />

--
Valtteri

My personal perference for a min wall thickness of 2 mm is that the wall will be alot more dependable under the loads of the bolts and fittings. As Grob, I too like a bit of meat (safety factor) to give security under load and abuse.

However my mainbeam is only 1.6 mm thick as all Taipan beams are and it is holding up well. There are some reports that after several years of usage small cracks may develop in these beams. 2 mm wallthickness will take care of preventing that to a large extend. So in my personal experience 1.6 mm as a min is acceptable but 2 mm is just a little more dependable.

Wouter

This isn't quite true as I have often been offered 6063-T6 as an alternative to 6082-T6 and this has a much lower yeild strength.

I think that the alloys that Wouter mentions are all good, what you will find is that 6061-T6 is what is offered in the US, whereas 6082-T6 is what is offered in the UK. Not sure about 6005-T5. I was told by my supplier that 6082-T6 is the reccomended alloy for a marine enviroment, this is certainly something to do with its corrosion resistance and could be to do with this alloys propensity for anodising.

Gareth

valtteri,

Both my spi poles were never anodised. The first was on my P18 and the second is on my F16. Eventually a non-anodised alu tube will colour a little darker grey with a matt feel, but apart from that it holds up just as well as an anodised tube. Anodised tubes will be more silver in colour and shine more.

If anodising is a problem for you then just forget about it. The only alu sections I would recommend as anodised are the mast and beams.

If shine is important for you then contact www.catamaranparts.nl he will send you the anodised stuff. Black anodised if you so desire.

Another trick if you really have to have black tubing on your boat is to powder coat them. Sometimes these companies are more relaxed in what they handle.

Wouter

I admit that 6082 is not an alu alloy that I'm very familiar with. Most of my dealings were with 6061 and 6005. So please everybody double check my comments.

What I do know now, after checking the specs, is that 6082 has a much higher ultimate and yield stress levels then the other commonly encountered alloy types. While maintaining the same desnity and stiffness. So thanks for the heads up Grob.

However with beach cats only a certain minimal value of yield stress level is required for beams. In effect, any additional yield stress beyond a certain level does not improve the boat design in any way because other factors like minimal wallthickness and extrusion considerations prevent optimal use of the higher level.

In my dealings so far, all alu alloys with tempering above 200 MPa yield stresses were well suited to cat beams, booms and spi poles. But if I'm mistaken in this then I welcome anybody saying so. I'm always open to learn new stuff.

I just read that :




So the guy offering you the 6063-T6 as an alternative to 6082-T6 may still have given you good advise as for example a 80x2 mm beam of either material will behave exactly the same in 90 % of the possible applications. The only exception being the really highly stresses situations where the amount of associated flexing is not a limiting factor. In catamaran beams however it is the flexing that is one of the main considerations, as a result the cat beams tend to be overdimensioned and under stressed. If the latter is the case then the 6063 material could be cheaper and have a better surface finish for the same performance.




6005-T5 is interesting for the following reasons :

source ; http://www.aalco.co.uk/technical/datasheets/Aalco_Datasheet_Al_Alloy_6005_6005a.pdf



It was also advised that 6005-T5 suffers less from shape distortion during the heat treatment then 6061-T6. Note that the code T5 also indicates a slightly different heat treatment proces. Therefor 6005-T5 has certain production advantages over 6061-T6 and I'm guessing over 6082-T6 as well. One of these is that thinner walls can be had with less rejects per production batch.

But the upshot of all this nerding is that all quoted alloy's will be fine if they come in a 80x2 tube. The differences are too small to matter.

Wouter

Thank you all for your time and effort.
I am grateful for the knowledge I receive here.

generally speaking I do not tend to go against the plans instructions, How ever building your boat givs you some space to customize the boat to your own preferences and that is of value.

some times there are limitation <img src="http://www.catsailor.com/forums/images/graemlins/crazy.gif" alt="" />

I can not find marine grade ply wood in Israel
and importing is very expensive
how critical is that issue?
What can i expect if I use regular ply? if it is an option at all?

should I open a new thread for this?

Thanks,

There are many types of ply and many methods for making plywood. Marine grade ply is a high quality ply without voids inside the layers of wood, and without repairs to the wood veneers inside the ply or the surface.

When you begin to stress the ply, any voids in the ply will cause problems (ply will probably break). You can repair the voids by filling them with epoxy, but it is very time consuming and generally not worth it. If you dont fill the voids, and the ply survive the stressing/steaming, you can get problems later with strength, moisture and probably delamination as well.
Marine plywood is usually made from mahogany, which is a rot-resistant high density and strong wood. Gaboon okume is the most usual one.
Regular plywood are often made from woods with properties less good than Gaboon okume (strenght/weight). Often there is just one good surface, and a core made from softer low-quality wood. Lately, there has come some mahogany skinned plywood with poplar as core on the marked for use in furniture. Avoid it for use in boats!

Plantation grown okume grows too fast, so it is not as good as the plywood made earlier from forests. But we got to think about the environment..

If you think about it, the plywood is a relatively minor investment compared to what other funds you will put into the boat when you buy sails or deck gear.

In short. You can use other ply, but there are severe risks.
Perhaps Phill will have some good ideas for either sourcing some marine ply or other alternatives.

A last tought. If you could get okume veneers, it would be relatively easy and straight forward to make your own high quality plywood..

The plywood is not expensive , the shipping is




Rolf, I think you just found me a good solution <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />



I was told that a good way to know if plywood is suitable for boat building is to take a piece of the ply and put it in boiling water for an hour, then dry it in an oven with high temperature for 2 hours, then do this procedure 2 more times.
if the plywood layers are not separated then it is suitable for boat building.
I ran this test on several kinds of ply and only one passed,
it was birch ply, the problem with birch is it's heavy weight.
The question is, can I use 3mm birch ply instead of 4mm okume? will it be as strong?

Marine ply is the best quality ply you can buy with proper glue and the best surface finish, but the next best is exterior ply. Not sure about void or surface finish problems, but the glue used is suitable for marine environments.

Erez,

I would not use the 3mm ply as we have built F16 hulls from the Blade drawings & the folded up curve of the hulls turned out a little different than the frame drawings that were supplied. The 3mm did not stress the glass layers on the keel line enough to fold the hull into the desired shape. Stick with 4mm.

One other point - we used Gaboon 3 & 4mm grade marine ply & the quality was a little suspect. During the construction period, we encountered many problems with bits of core missing & the ply failed when stressing ( like Rolfe mentioned).

Ironically the ply came from Israel.??!!
<img src="http://www.catsailor.com/forums/images/graemlins/blush.gif" alt="" />
Regards

Indeed, One factory in Israel Used to be a major supplier for marine grade plywood worldwide, unfortunately the factory was closed many years ago, It was cheaper to buy made in china.

Thats a shame. I think years ago someone told me that the Israeli Gaboon (okoume) was the stuff that you should try to a hold of.

I bought some sheets 2500mm x 1200mm (a little longer than standard) interstate, and the quality in some respects was questionable, but I didn't have too many problems. I'm pretty sure it was from China, can't quite remember, but China rings a bell.

I don't think you can build these types of boats, with the stresses of building and sailing, with anything but a good Marine plywood, I think it will end in tears.

Regards

you should contact someone in the unicorn class in uk , unicorns are built from tortured ply , Gary Piper is current proffesional Unicorn builder and should be able to give advice , Bob Dorks too has built many . I think they use exterior ply as it bends easier than Marine when taking up shapes for a cat . above folk can be found through unicorn website http://www.unicorn-cat.co.uk .

I also had an A class cat built for me from a kit supplied by Roger Dewen a noted Unicorn and A class sailor, I think Roger got the ply he used manufactured to his own spec , the fella that put the hulls together for me - Bob Higgins claimed it was the easiest boat to pull together he had ever built , even though the A Class had more rocker (harder to form ) than the unicorns he had built , ply was 3mm thick , when wested both sides (no cloth) was strong enough , though I was always carefull when capsized to stand on bulkheads or deck edges .

Roger can be found on uk A class website in for sale section , he's selling a boat at the moment

http://www.aclasscatamaran.co.uk