# Trimaran Crossbeam Static Load Test

Discussion in 'Multihulls' started by cuica, Sep 3, 2014.

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### cuicaJunior Member

So if I have designed and built my first crossbeam (I haven't, yet), I assume I should perform a static load test.

Does this involve the displacement of the ama?

Should a single crossbeam be able to support the full displacement of the ama or some multiple?

Are there any other tests that are generally recommended?

Is it usual to test the first crossbeam that I build to destruction, and if so, what force should it be able to withstand without breaking?

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### Tom.151Best boat so far? Crowther Twiggy (32')

I'm not saying you should do this - I'm just saying that IF I was designing my own crossbeams (not using factory built alloy or carbon tubes) - I would definitely test them because... you need to test three things: the design, the materials, the building workmanship - and whether the three work together as expected.

I would...
Calculate the fully loaded (with all stores & equipment) weight of the boat

Calculate the maximum righting moment for the boat (the same number used to design the beams) and use that to determine the load to be applied to the end of the beam. (This should be the righting moment that will overturn the boat.)

If using mostly wood for the beams, I would weigh the components individually before building the beam to insure that the wood matched the expected characteristics.
I would weigh the beam(s) to verify that it meets the weight of the design for the materials used.

Set up the cantilever bending test so that i can measure the deflection of the beam during the test (at least at the loaded end and at the midpoint)

Plan to 1st test up to the 100% of max righting moment and measure the deflection every 10% of the way up to the 100% load.

Compare the actual deflection during the test to the calculations of what the deflection was expected to be.

Now, I'd sit down with a beer and see what I got and how it matched the expected deflection.

Of course - if the the test shows MUCH more deflection that expected the beam is weak. But I don't really know why do I? Could be that (a) one of the components is understrength, (b) the assembly is faulty (bad epoxy batch, fillets too small, or improper or missing reinforcement, etc), or even (c) that the calculations were in error.

So, how do I find out? I test the obviously weak beam to destruction - which will hopefully shed light on where the problem might be.

Of course if the test shows actual deflection that just matches the expected deflection then the beam might be perfect . Yes, I'd be rather happy but I think I'd continue the test to 200% of the maximum righting moment - as that's the minimum safety factor I would use for the static load case.

Good luck, but instead of trusting luck continue bringing your assumptions, design, and calculations to forums like this - lots of good help to be found, you just have to sort though it all.

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### cuicaJunior Member

Tom.151, when you say you would calculate the righting moment for the boat, presumably the maximum righting moment that can be applied to the end of the beam is given by the displacement of the ama, or am I missing something...?

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### Richard WoodsWoods Designs

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### Tom.151Best boat so far? Crowther Twiggy (32')

Hi cuica,

The maximum RM (righting moment) is determined by the weight of the boat multiplied by one half of the beam of the boat (assuming a normally symmetric trimaran). This does assume that the ama does not submerge when the maximum wind condition occurs - which would be the conservative approach for calculating the maximum value.

If the ama does not submerge, then the maximum RM is the moment that will capsize the boat - once you've got that much you can't possibly get any more load on the boat (for the static load case) because the boat goes over.

And just to be conservative with the load for the main beam I would consider that the main beam must be able to support the full load - which it very nearly would need to do in worst conditions.

So for an example, my trimaran in (cruising trim) weighed 3250 pounds and was 30 feet wide. So, the maximum RM for that boat was 3250 lbs X 15 feet = 48,750 foot-pounds of righting moment.

Hope that helps,

PS - pay close attention to what Richard Woods says.

PS2 - tell us about the general dimensions, sail area, and weights of the boat you are planning - it makes it easier to consider the approach to some of design issues.

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### MarmosetSenior Member

Newbie so be gentle with me, but! Anybody ever do testing via g-load like airplane wing? Ala suspended at tips cranked in the middle with cable and scale? New to boats but have done aero work, a small slide on winch base allows off angle testing under tension as well. Of Yes one still has to do calcs to see what it is we're siting or expect to see.

Barry

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### DCockeySenior Member

You are confusing stiffness and strength. Excess deflection for a given load means the beam is not as stiff as predicted. It may or may not be as strong as predicted.

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### catsketcherSenior Member

Great unknown

Gday Marmoset

I think the problem with this question is that a lot of designers are a little unsure of what they do themselves. I have asked about beam strength around Australian designers and builders for years and no one has ever said
"You need to get the load to be such and such". I have even found this with retired designers who owe me a favour and I am sure are being candid - maybe most don't really know from a purely theoretical stance but understand well from a boatbuilding perspective.

Indeed one of the most spectacular structural failures in Australian multis was designed by engineers who were proud of their achievement and then threatened legal action if anyone asked them about the problem in a talkfest.

I think this is the case because in Australia most designers are builders first. Even some of our most successful cruising cats are expansions of previous designs and so not designed from scratch. Your Pescotts, Chamberlins, Orams, Hitchs, Schionnings, Arbers, Perrys, are all designed by boatbuilders, people who really know how to build a boat and can work out how to do it better than the plans they get given. So they do.

I asked one of the list above how to design trimaran beams. Eventually after much evasion (I was working for the man for a few months) I got told to reverse engineer a successful boat which is if course what these guys do when they design a new boat - they use the structure from a successful one.

As to plus 5 G - 3 G loading like in aircraft I don't think there is the public theoretical knowledge of what goes on behind these boats to do this. Ad Hoc says you have to design for the load condition but we have problems even speculating about this. If a tri beam has to act as a cantilever (flying the main hull) what about when it is old and heavy, or the beam has undergone years of fatigue, or the effect of falling from a wave? (when it will weigh maybe twice as much when it hits the bottom)

S0 pick a good tri of a similar size and length. Bucc 24, Marples CC designs, a Piver Nimble would be especially good as they are old and very tested (I would neglect the deck and underwing inm calcs) and work out their own safety factors and use them. You can't pick a load case if you don't know what the boats out there are using themselves. When you have done that you can go back and safely engineer your own basing it around the beams you have already checked. Be careful making up your own load cases - we have a few stories (20 year old) of new designers getting excited about new beam designs and then the tris losing a float - once on the maiden sail.

I would like to look at the reverse engineering if you do it.

cheers

Phil

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Then i suggest you're talking to the wrong designers!

As DCockey pointed out, they are most likely confusing the two issues and/or do not know how to calculate the loads either.

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### frenetteJunior Member

Now add the max crew weight on the windward rail. So 4 beer drinkers on the windward rail is about 800lb (being kind maybe closer to 1,000) * 29' is close to 24000 ft/lb.

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### MarmosetSenior Member

We'll if you say I gotta drink beer for testing sign me up! Remember now when the wife pops her head out in the shop just put your hand up and claim "empirical testing!"

But yeah there's the first tweak over just a wing, now your looking at whole boat plus buddies minus one float. And something like a Buccaneer would be an easy baseline, aluminum tubes well known, the bulkheads and mounts would be the ? Part. What was mentioned about floats I could see that all day long, someone just wiggles the poles hanging out and calls it good, ignoring ama mount or integrity of mount to ama.

Barry

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### cuicaJunior Member

Posted a reply here then went back and read some more of the very interesting comments above and am rethinking...

Many thanks for all your input so far!

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### cuicaJunior Member

The boat I'm considering is a 10 meter proa with displacement of about 1000kg and a beam (distance between centers) of about 4.5 m.

I'm presuming the calculations should be similar to those for a trimaran?

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### Tom.151Best boat so far? Crowther Twiggy (32')

I found your proa help thread -- decide which you're going to maintain.

Calcs may be "similar"... maybe -- but the design case will be rather different, which means the beams will be very different for a proa compared to a trimaran -- you mislead the reader when you want one thing but ask a different question because you "presume" things and don't inform the reader (asking for trimaran beam help when you're planning a proa)-- not a good way to get good results.

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