F18ht mast on I-20

I am considering trying to use a HT mast on my I-20. I read that the HT mast would need to be beefed up to live on the 20. What are the more experienced thoughts about this? I would love to take an opportunity to put a lighter mast on the boat, but I would rather not have something that would be on the verge of breaking with the extra heft of the 20.

Im no expert but I've spent a lot of time on both boats. The HT weighs maybe 2/3rds(or less) of the 20, I think the rig is taller too. My guess is that the ht mast would snap like a twig. It is wayyyy lighter than the 20 mast. Is your 20 mast one of the old carbon ones or a new aluminum one?

The I20 mast was very old carbon technology and far heavier than it was robust. Over what it was tasked with originally, you're really only asking the HT mast to carry a jib and cart around another 125 lbs of boat. It would probably be more sensitive to mistakes but I bet it would survive.

You must have very skinny crew!

F18HT was designed for 2 up. I know us N20 crews tend to be larger but still...if an A-Cat mast can handle my fat bottom surely a F18HT mast can?

Does a mast care how much weight is attachted to the bottom of it?

What if you stepped it to a dock and sheeted it in? Would it snap instantly? The dock is never going to move...

I think it cares how much sail you put up on it, so you may want to also use a F18HT main, no jib and an F18HT spin instead of an I20 sailplan.

Masts most definately care the weight of the boat they are attached to.

When designing a mast the most important input is the righting moment and yes if you step your mast on a dock and sheet it on in conditions that would normally make you fly a hull when on trapeze you will almost certainly break it. The only time this is not the case is where the builder as run into a minimum practical thickness limit and as just has had to build a stronger than desired mast.

Where would the "Load" be focused, and where would it snap, if you stepped it on a dock and sheeted it in? At the hounds? Or at the spreader attach point? Or would it crumple? How about if you put zero downhaul on it, would it still break, and where?

Most of the masts I've seen break were rolled up in the surf or over-downauled and then side loaded by some other force being added, ie. crew falling on it in a capsize, or the mast hits something on the way over, ie. a mark, or the water, hard. I'm sure you can 'overload' any mast, buy hanging say, five 200lb. guys on trap wires off a mast built for only two trappers, but that would be caused by the downforce of the 1000lbs. hanging down from the trap wire attach point, right? What force does the base add, and why does it care?

Where it breaks would depend on the mast and how it is engineered and built, but realistically it will either deform due to increased compression or at the hounds.

The masts you have seen break are on the boats they were designed for. Would you put an A class mast on an F18?

Back to the original question:
Can you put a 18HT mast on an I20? Calculate the total righting moment ( boat weight x half beam + crew weight x ( beam + 3')) if they're within a reasonable tollerance ( say 15%) you're probably good to go but you've eaten into your safety factor.

I think if you put and A cat mast on an F18, and only put an A cat sail on it, no jib, no spin, and only one trap wire with only one sailor on it, it would be fine. You may not go very fast, as the F18 is a 'Dock' compared to a light A cat hull, and the A cat sailplan doesn't have the horse power of the F18 sailplan, but I don't see where the mast is going to care what it's attached to, at the bottom. If you keep the same amount of stay tension on the mast when you step it on the F18, as it had on the A cat, how would the mast even know what it was attached to?

A lighter platform will lift and pivot as the rig loads up (or pitchpole)...that reduces the wind angle and the loading on the mast. The compression increases on the mast as the boat and sailors get heavier.

As I asked earlier; if you keep all the loads the same, same stay tension, same sails, same crew wt. etc. how does the mast know what it's attached to?

I submit that it doesn't.

The amount of lift generated by an A cat rig on an I20 platform may not fly the hull, but it won't just snap because it's on an I 20.

If it breaks on one boat, vs. another, you must have added more stress to it. So, how did you add that?

If you add a jib and a spinnaker to any A cat, the mast is not going to like it. If you do the same with a F18HT mast, it's not going to like it.

But those loads you added were up the mast, not at the base.

Wooter....! Splain it to them!

The act of flying a hull would relieve some stress, but if you put five 200lb. guys sitting on an A cat, not trapping, just sitting on the upwind hull, would the rig snap? No, you must overload it first by adding more...Wind. If the hull can't fly to relieve the added lift from more wind, then you have a problem.

You could argue that if you take an I20 with an A cat rig out in 30knots of wind, the mast will snap, but that's because the (heavier) I20 platform didn't fly a hull to relieve stress, as soon as the A cat would have, and that has allowed you to add more stress to the rig.

I doubt it would be a problem if you put the F18HT mast on the I20, as long as you also use the F18HT sail plan, ie. no jib, smaller spinnaker. I also doubt the I20 will be as fast as it would be, using it's own sail plan.

I'd love to hear from someone who actually knows what they are talking about, like Ben Hall and/or Pete Melvin, regarding mast design for hull wt. etc. I think it comes down to compression loading, as to how much weight you can hang on the trapeze, how much downhaul, and how much main sheet and forestay tension you put on the mast.

Don't forget rig geometry. The angles of the shrouds to the mast affect compression loads. The height of the attach point on the mast and distance from the base of the mast to the attach points on the hulls fore/aft and port/starboard should be the same.

The stiffness of the mast will have an impact. As the mast "bows" out, the loads are no longer pure compression.

I don't know much about the designers of the 18HT. They probably designed it to some load case with a 50-100% margin and then tested it to the design case. The problem is I don't know what that case is. Some boats are designed to stouter than others. I remember a picture of a TheMightyHobie18 from testing. There were reaching with ~6 people on the boat with wind blowing solid whitecaps, to see if anything would break.

You just explained it yourself as well as I could. Added to which, the I20 carries more sail area.

For the same reason you put your rig at risk by putting five large guys on an a-cat while it's blowing 20, an a-cat mast trying to support an I20 sail area and typical crewing weight would be asked to do more than it was designed for...and it would likely break.

I'd like to hear from Ben or some other mast builder as to the designed strength of masts, if there is some 'magic formula' as to how much sail area and or downforce (cunningham, trapping wt. etc.) they use to decide how strong to build the masts.

I doubt if any beach cat mast would break based soley on the lift being generated by the sail itself, no matter what the wind strength.

I think what causes masts to break (other than flipping the boat onto them) is the compression force of trapping, added to the backward bend caused by mainsheet tension, and foreward pull of the forestay and/or spinnker.

Still, I haven't seen too many masts break under sail forces alone, only when the crew lands on them or they roll in the surf.

If the mast was to reinforced, how and where would you do it?

As Scarecrow says, it depends on righting moment. The mast has to support not only the two guys on the wire but also the flying hull (which for sure is heavier on the I20) via the shrould. On top of it you have to add the main sheet load (~50kg x 7 or 8) which pulls the mast down and aft and the resulting downward force of the forestays, which keeps the mast in position (a bit more than the load from the main sheet). The downhaul will add additional downforce and bending, but this is fairly comparable between the two boats.
You can pull the main sheet harder on the boat with the high righting moment (as the light boat will fly one hull earlier), hence the higher loads.

A mast fails due to compression and bending, a special buckling case. Constant material and thickness sections will fail around the spreaders. A carbon mast has most likely more material at the spreaders, hence it might fail somewhere else.

The formula given above should fair enough to estimate how much higher the loads could be. If the mast can handle this, is an other question.

Cheers,

Klaus

Add another set or two of spreaders?

Timbo, you usually come across as a pretty sensible and switched on guy, but you really need to step away from this thread. Every one of the loads you have defined above is related to the righting moment.

"compression force of trapping" clearly part of righting moment (don't forget same force from side stays).

"mainsheet tension" you apply mainsheet tension to hold the leach of your sail up. Your leach opens up because of the force of the sails, the luff is held closed by the mast (which is held up by the righting moment) and as a result you tension the main to get a similar amount of support. The as such the mainsheet tension is proposional to the available righting moment.

"foreward pull of the forestay" see above plus add in tension as applied to offset mainsheet tension.

"foreward ..... spinnker" drive from kite is clearly proporsional to righting moment.

etc etc etc.

There are many of examples of keel boats adding ballast to their keels in some cases making the boat lighter by taking it out of the bilge making no changes to the rig or sails then breaking the mast in their first serious race after making the change. A great example of this is the 98' yacht Wild Thing, which lengthened its keel and reduced the bulb size shortly before the 2009 Hobart. Having made no rig changes from the way the boat had been rigged for previous years (in fact using old delivery sails) they proceeded to break their mast on the delivery to Sydney for the race.

As stated above the first step in designing a mast is to calculate righting moment as this effects two things the compression on the mast from stays and trap wires and the side force the mast will see from sails as the total side force will only every be equal to the righting moment that stops the boat from heeling over.

See if you can find a copy of "Principles of Yacht Design", Larssone & Eliasson their is a good section on rig design.

As I said above there are practical limits on how thin you can build a mast but as this is a carbon mast the practical limit is about 1/100 of an inch and that definately won't do the job so won't have been an issue in the 18HT rig.

Sail7seas, yes another set of spreaders will help as you are reducing the unsupported lengths and therefor making the mast less likely to buckle, but who wants multiple spreaders on a beach cat?

Timbo, if you're still not convinced rig your boat on the beach, strap it down tight pull on the sails and come back after a week to see what went first.

Spreaders.. Who wants spreaders at all

Look at this nice mast. Originally the moulds were designed for the Formula28-XOZ but now re-used to build a spreaderless wingmast for a Farrier 22.

http://1.bp.blogspot.com/-a8bdJHlpC...h6yj_2cvhE/s1600/2012-05-25+22.50.28.jpg

http://3.bp.blogspot.com/-cY7CZvzvF...WHC9uIvIp8/s1600/2012-05-26+02.10.14.jpg

Scarecrow, I'm not a designer, or even an engineer, but I did take some engineering classes....many, many, many beers ago.

Please draw me a force/vector type diagram and show me where the additional forces are applied when you step a mast on:

1. a dock
2. a heavy boat
3. a light boat

As I said earlier, if you put the exact same amount of rig tension on it, it has zero idea of what it's stepped to.

What DOES change, is the amount of WIND (generating lift force, sideways to the mast) it takes to flip a light boat, vs. a heavy boat, vs. a dock. In our little beach cat world, we usually flip over sideways long before the mast breaking point is reached.

Those clowns on Wild Thing broke their stick because they went out in too much wind. The added lead just let them keep the boat upright while it broke.

I think we are both saying the same thing, you are calling that force: "increased righting moment", I am calling it too much lift.

The end result is the same. But to answer the original question, will the HT mast work on an Inter 20?

Probably, as long as you don't put your wife and 4 kids and a barbeque on board and go out in 30knots.

;^)

The better question is WHY put the HT mast on the Inter/Nacra 20? Did the stock telephone pole break? Are you thinking the more flexible rig will let you select better sails? Maybe you should order a new spinnaker and see what that does to your boat speed before getting the upwind sails/rig sorted?

I think he said he wanted to save some weight up top.

I once owned a Javelin F18HT and an Inter 20, the Javelin mast was MUCH lighter and easier to step!

It broke. Sad story.

Rig tension increases beyond your initial settings with wind power. As the mast/sail catches wind and creates lift/force, it pulls on the cables holding it upright and to the platform. These cables have a high degree of down angle to them...so...adding tension to the cables results in considerable downward force (compression) on the mast (pulling it harder toward the mast ball). The more the boat/platform/dock/concrete foundation resists the motion of the sail/mast/cables, more compression force is carried by the mast....until it goes pop.

I'm not drawing a diagram.

I dont understand why this is so complicated. An HT hull weighs like 75 pds? An I20 hull weighs like 125pds or thereabouts. When you fly a hull on the HT the shroud (which is attached to the mast) PICKS UP 75 pds out of the water and into the air. When you fly a hull on the I20 the shroud (which is attached to the mast) PICKS UP 125 pds of hull into the air (where it is not bouyant).

The difference between flying a hull and strapped to a dock is not a useful analogy unless you managed to fly a hull on the dock, which breaks either mast. Just dont fly a hull on the I20 and you'll be fine, but what fun would that be...

Jake, in your example...what actually caused the tension to increase, for the mast to break?

More wind.

So we could say you exceded the mast's 'wind limit', what ever that is. Maybe they should come with a stamp on them that says, "This mast was designed to support xxx sq. feet of sail area, at XX knots of wind, on a 500lb. (boat + crew) 8'6" wide catamaran platform" or something like that?

It's not complicated, just don't put two big fat guys on the F18HT and go out in 30 knots!

here is one that will not explain much, but still is the best mathematical diagram i ever saw on this site...

Nice ballast!

It's an idea that was sparked when I remembered the ht/a-cat thread. But there is no close HT that I know of for sale. Just exploring options. Unfortunately I have a brand new set of sails that were just built for te 20 mast so it would probably mast more sense to stick with that.

that should answer all your questions

The I20 mast is thick, especially above the hound and the I20 kite is huge compared to the HT kite. My concern would be what would happen if Bach is out having fun with 3 on board (which he does), flying the kite in sporty winds, stuffs a wave, and the mast is not in the best rotation??

Typically the I20 mast survives this. Not sure the HT mast would. Will he have to re-position the spin hound and mast hound on the HT mast, and what affect will that have?

Great discussion, but looks like he will need to find another carbon I20 stick, to complement his investment in new sails.

Wait...what was the question again?

:^)

what is the best rotation?
is it with the rotator (arm) facing straight back (so the spin pulls on the front of the mast...

or rotated 90% (so the pull is on the side of the mast and supported by the diamonds?)

with the kite up, you want the rotator off (running free)

spin pulls sideways...so mast rotated at 90 degrees gets the thickest part of the mast to support the spin loads (particularly since the diamond wires don't go that high).

thanks guys, that's what i thought and how i fly, just making sure.

The Bimare Jav2 circa 2002-2005 mast is tougher than a old school Nokia phone. I have punished my 18HT relentlessly over the past 5 or so years and can say confidently that the mast will survive an I20 application. By the way. That mast is totally class legal in the open 20 class.
I believe that version was made by RIBA composites. Do some homework and you'll be impressed with their product.

Of course the mast doesn't know what it's attached to; it's just a stupid mast. But the boat beneth the mast matters a lot whether or not it will break. Stepping it to a stationary object is not a good comparison. Also wind speed would be a constant in comparing the two boats. Wind doesn't increase when you put a mast up on a N20. Best case in point is downwind sailing, spin up. As you sail down wind the apparent wind decreases the load on the rig. The highest loads will be during acceleration, and when the boat suddenly slows stuffing into waves. Compound that with waves causing the top of the mast to whip. There are much higher stress loads than mounted on a stationary object. Up wind the apparent wind is increasing the wind velocity, add waves again and you'll have much higher stresses than stationary.

Mainly the this will cause over-stress on the mast: Downwind boatspeed is key, if you can maitain boatspeed close to the true wind speed the rig is not under stress even though the boat is moving fast. As waves slow the boat down, pressure on the rig increases. This will happen on a heavy boat to a greater extent than a light boat.

You guys have missed the point of the platform swap. The harm to the mast would be done by the increased weight of the platform creating more righting moment on the mast.Has nothing to do with the wires or any of that stuff. That being said I think the HT mast could handle the task fine with proper boat handling.

If you are concerned about Buckling?

http://en.wikipedia.org/wiki/Buckling
>3.The boundary conditions have a considerable effect on the critical load of slender columns. The boundary conditions determine the mode of bending and the distance between inflection points on the deflected column. The closer together the inflection points are, the higher the resulting capacity of the column.<
add SPREADERS if you are really concerned.

jeeze, just let the boy put it on there and have him take pictures or describe the results (snap or no snap).

I have one more option. I could get a blank tornado section from Forte for just under 3,000 clams shipped. Anyone have experience with Forte?

I see in your original post you have new sails for an I-20. I am not sure if i missed this: is the goal solely to have a lighter mast or did you snap your current stick?

The i20 mast is snapped. Getting my sails re-cut for a softer mast isn't a big deal according to my sail maker and will cost about $300 to have it done.

I know a guy that has a carbon I20 stick he wants to get rid of if you're intersted.

I would definitely go for an i20 mast if you can find one. I know someone with an extra I20 stick too. The HT is 3 ft taller and it uses a different mast ball size. I'm sure you can do it but it will be a serious pain to accommodate the change. Good luck and let us know how it goes. Cheers.

The mast ball and related problems are not a huge deal to me, but I would love to hear about the i20 mast.

Check your PM

So this project will go forward, a very nice HT mast followed me home today!

On a side by side comparison, the masts are very similar. I was surprised that the spin bail on the HT was much lower.

Did that ring any alarm bells?

Make sure you get cash pledges from all those who advised you to buy this mast before you go ahead and rig it.

If it breaks it breaks, I don't have a ton of money in the thing. It it doesn't break, it will be worthwhile. If it does break, o well I have a spare i20 stick anyways.

what was the cause of the original mast fail?

It blew over in a storm.

gotcha... sucks

Any suggestions on preventing this in the future? Its the second such incident I've heard of in the past month. Currently my boat is on the beach, rig up, with the dolphin striker tied to a screw-type stake (used for beach umbrellas). So far so good in winds up to 37 kts.

Sand or hard packed soil? For sand, bury 2 2x4s a few feet deep and run lines from the 2x4s to the shrouds or trapezes. If hard pack, get more of the screws to add multiple tie down points.

Go to the sides/ traps with 2 tie-downs ( stakes, boards trees, signs,etc.) and not your dolphin striker they can bend out. Don't ask me how I know. Plus you get less leverage to the middle, engineer boy. tshan is right about burying boards, I just use 2X 10s or 12s 2 or 3 feet long.

I will add that if using the screw-in type anchors with an eyelet, make sure the eyelet is at or below surface level. the boat may slide a little and the eyelet can damage/puncture your hull. You may also want to secure your wing mast from rotating freely.

use the redneck insurance policy:

many of us in my area use these tree augers shown above with good results (up to about 45-50mph) on our beach. above that wind they pull out and old tiedown lines snap.

In this picture from 2 years ago i have 3 tie downs in play. 1 on each side with rope going across the hulls, and one out of shot aft of the boat that has a line wrapped around my beams.

these days i would move my tie-downs in a bit and also work my trap handles into my tie down rigging

A good outdoorsman would put those anchors in at an angle and get enough hold down power to hold that boat down in a Hurricane. I anchored a 6.0 in my yard with 2 of those ran in at an angle and it held through Hurricane Ike.

A good sailor would take the mast down in a Hurricane

Wasnt my boat but it was in my yard. The anchors were already in, so I just tied it to them. Had bigger fish to fry getting my boats to safety. Allways fun to experiment with someone elses stuff

as i said "these days i would move my tie-downs in a bit and also work my trap handles into my tie down rigging"

however, there is no way in the world i would leave my cat on the beach/or with the mast up in any hurricane. That would be grossly irresponsible, negligent and liable if/when your gear hurt someone/something

Like I said, it wasn't my boat. However, by looking at those photos, it's clear to see why those anchors pull out when you have them straight in the ground. Not rocket science.

yep, and using your traplines puts all of the resistant leverage at the top of your mast where it can do the most good against the wind blowing on your .... wait for it, MAST.
Not that I'm one to be giving advice on tying your boat down this year. I know how to do it, sometimes I just neglect to do it.

Ok, so the mast I have has no base. I plan to cut a new one from several layers of stainless. Right now I am considering changing the mast step system to one similar to the AHPC version, and no longer using the ball. Is there any other systems I should look at and take note from?

They replenish our beach (causeway) with very soft and fine sand (think kids sandbox). I wouldn't trust anything to hold (besides buried boards) in +50mph wind when it gets water logged (regardless of angle). Even if i had a great, trusting system ... the cat parked down the beach often doesn't...

I even carry spare augers to tie down other peoples boats in summer if i am concerned they can flip on me

It is simple, winds predicted above 25-30, mast comes down. boat goes home.

The Riba section is shorter and heavier than Marstrom's M20 mast, which is longer and lighter than the N20c mast. That mast holds ~380 lbs of crew weight with a 24sqm main and 24sqm spinnaker double trapped reaching.... granted, I've burned holes in it with my eyes where I think its going to let go, but they never have. Every one of those masts that have come to the US have broken, some to design/construction failure, others to flips or mismanagement on the water or beach. That mast has to be handled carefully!

There is one crazy project boat that uses the Riba section with a LOT of sail area and huge beam dimensions. The mast was easily the best part of the Jav HT.

That's not much of an endorsement....

I'm interested to hear how this works out. He got the HT mast from me, I was a little surprised he chose to go this route, I expected him to get the i20 mast instead.

I have a repaired carbon i20 mast if anyone wants it.

Jake,
The Riba HT mast is a quality mast. I only know of one that broke and that was from getting tossed a couple hundred yards in a tornado (it was mine, so I know the laminate, fixed it and sold it.) Its got a better profile thant the i20 mast.

Riba makes chassis parts for Ferrari...

Someone asked about the Tornado section that Forte makes. Its way heavy, I bought one and returned it. Its an older style profile. Lars Guck ground the mast track off a aluminum T mast and gave it to Forte, so you are getting an oversized section based on an outdated shape. I wasnt impressed.

So on a rain day I got to doing a little more work on this project.

It seems that the easiest, simplest mast base could be made to use a tapered cone and seat for the mast step. Pretty much the same that I have observed on some A cats. Is there any considerations I should make before adopting this system?

I measured the mast section at ~2.3125 and ~5.625, that was not using calipers but tape measure. I believe this means the HT mast is very similar to an A mast section? Obviously its much heavier.

I need to decide whether to make new rigging, or move the hound on the HT mast. New rigging will be made in either case, but it makes sense to me to try to use something off the shelf. Any thoughts on using a "T" hound?

The tapered cone and seat should be fine, that's quite similar to the orginal design. How will you attach the new/moved hounds? Just rivets into the tube makes me nervous, although that may be how the factory issue is attached. I sure wouldn't lower the hounds and have more unsupported mast above. And if you were to raise them, wouldn't you want to reconfigure the diamonds too? And then you've probably changed the mast bend characteristics, not a problem if you can have a main built to fit.