# calculating torsional stiffness hull

Discussion in 'Boat Design' started by Pammie, Mar 17, 2018.

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1. ### chinaseapiratePrevious Member

Glad I don't use computers to tell me how strong a boat is. I saw a catamaran stuck in the trees after a hurricane in multihulls magazine 25 years ago. The limb it broke punched a hole into it and it stuck there. Back in the water a week later...

Here is a picture of the west coast's greatest designer of his time's pioneering attempt to move away from his solid "single torsion beam" design. The two hulls weighed LESS than 400 lbs each. It was 40 ft. The pod was if I remember correctly 600 lbs. They needed that pod. The boat raced the Transpac, California to Hawaii. They had a choice, 6 crew sleep in the hulls, the crossbeams, or design it as a trimaran? (Rudy is rolling in his grave). It was rebuilt after the race with twin aluminum tubes. Took off the beams and cabin, added the aluminum and dropped 2,000 lbs! so they could keep beating a new 60' foot cat named Double Bullet(some of the time).

Last edited: Apr 7, 2018
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### gonzoSenior Member

chinaseapirate: A single beam connected at the center of mass is the worst possible location. It is likely that it will self destruct when the waves are at the resonant frequency of the boat. I am not sure what makes you glad not to use computers. Perhaps you enjoy the tedious math and like doing it by hand. How do you calculate structures?

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### TANSLSenior Member

Do you really think that the frequency of the waves can come into resonance with the frequency of vibration of the boat structure?. I think that would deserve a more detailed explanation.

4. ### chinaseapiratePrevious Member

No, a single beam further away from the center of mass (for example either end) of the two parallel members its connecting will be subjected to more stress(in layman's term) than if it were at the center. I did not that suggest it be used either. I stated that a single beam with a cross section dimension the same as the either hull it connects would endure the least stress of any other placement of that beams material. As I excluded using its placement there (keel to deck) for other obvious reasons I moved ahead in thought process. Beams rarely break anywhere but at the hull connections by a combination of stresses. A single hollow beam will have twice the hull attachment contact than two hollow beams of equivalent weight placed anywhere else.

Last edited: Apr 7, 2018
5. ### chinaseapiratePrevious Member

I test my boats during construction. I jack the hull up, usually while upside down, to see if they deflect. I do not use strongbacks. I scarf together pre lofted, pre cut, pre marked for bulkheads/frames epoxy coated 3/8 marine plywood panels together, bond the outside component of the eventual sheerclamp to the top and a stringerish looking piece of wood to the inside bottom. I use these 2 long panels (1 day to fabricate) as the structural foundation together with the frames/bulkheads (I'm using my 32 ft one off as an example). Next day start SCREWING together the prefab frames BONDING only #6,starting in middle with #6 then #7 then #5 then #8...until #1 and #11. zero and #12 are bow and stern - leave out, deal with later on. Frames were simple mini box beams, (2)1" x 3/2" bonded between 1/4" ply 4 1/2" wide slab sided to the bottom(top-upside down) 1/2" x 2 stringer notch. Below the notch the frames were elliptical, deepening from semi circle at #6 to bow and flattening to 13 1/2 W x3" D at stern. When #6 is dry enough and its dark enough, we started adjusting the semi wiggly "structure" to line up with the laser sights (we only had little toy lasers back then but they would range more than 50 ft with new batteries). The frames have "legs" extended above the sheerclamp and settled in the sand. Tedious but doable. 3/32 tolerance. Very tough to explain in a foreign languge, i remember arguing "no, the holes are not wrong/hindi mali ang mga butas butas" couldn't tell anyone why, just that we couldn't just move the holes. Really simple once the laser went through three or four in a row finally everybody understood. And much easier to get the straight because the legs were less liable to readjust themselves once they were further from the action and were easy fix with rocks around the legs. Once everything remains straight after all "shoring up" with rocks is finished. Next remove the screws one frame at a time, carefully remove the frame, and bond it with epoxy. The rest of the boat SHOULD take only take a week with 3 workers the designer/builder included if they had done the work before. less than 100ft^2 of easy strip planking, 120 ft^2 of glass 6oz ,(no price discount and ***** to find. Strips 3/4 x 11/16 from less than a dozen 2x4 x20s Lauan - big discount cheaper than home depot hem/fir). Finish sheer clamps, 2 big stringers (3/2 x 7/2) purchasing preformed bamboo akas for 3\$ each less for the 6"+ bamboo amas (I put 2 on each side but really only need one like my friends suggested for my size boat), bamboo removable floors done custom (3 hrs 1 expert 60ft2-10cent/ft2 +a meal and 2 beers) It didn't actually get done in a week but realistically could have. It took about a month and a half, although the actual work hours the same as a weeks worth. I let the locals handle the complicated installation of the 65 lbs 10hsp briggs stratton. Contact price \$12.
So how did I do the calculations on structural integrity? I had read multhihulls magazine since 1975, even had all the available early editions back when all but 1 or 2 were available still. I built described above in 1990 maybe 89. That was the largest up till that time but it wasn't my first from 87 - 90 I built 5 that I can remember now 4 were less than 16 ft and three of those were "compounded plywood" what you call stitch and glue now. I saw the interior of "Crusader"- Norm Cross design quite extensively many times. Everyone I ever talked with about boat construction said he was a genius and Crusader was the strongest toughest heavy weather windward trimaran ever built even my neighbor Bob Hanel aka Double Bullet who was a snob, never let me on his boat to race, and he had a key on his all four deck hatch covers, parked on adjacent slip to my father's boat Imua. Basically I had no calculations. I just did it.

I have sound reasoning. And reason to take offense with just about anyone who would say I put peoples lives or welfare in jeopardy. I know the actual weight of a boat I build. Epoxy is easy -10lbs/gal, douglass fir .32 X63 sea or 60 fresh 3/8 ply = exactly 1ft^3... I know the designed displacement. example above- since I know the water line beam's outline with respect to to centerline is closely approximating a section of a circle with a radius of C because I lofted the frames off of a simple x2+y2=C I forgot off hand what C was 28 years but its not to hard to find. I wanted the transom to rest 3" below the waterline. I chose the bow to be 4 out of the water for simplicity and no overriding negetives in doing so. The beam was 27"at waterline. The side panels were the key to the whole concept of the design and build simplicity and they were was "placed" 2 1/2 inches below the waterline because now when spread apart 25 1/4 inches apart measured from the bottom of the stringerishy reinforcement and the ends brought together at the bow and 13 1/2" apart at the stern each panel now follows a cylinder of same radius C. no need to care about that functions intersect with the water line because it affects little

Last edited: Apr 6, 2018
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### PammieSenior Member

@ chinaseapirate
A bridgedeckbeam is certainly an option. Possible as big as hull, but not longer (x) than 2,50 m. I'll have to calculate weight. Both beams should be less than 110 kg. Measuring deflection after you build it can be done, but the methods to correct a problem will be limited. (Well... unless you start to build it all over ). So if there is a way to calculate/estimate it beforehand that would be better I think.
The middle 'hull' I'm talking about is to directly take forestay/ mainsheet/ mast-loads, and unloading the hulls. Is sometimes also called backbone/ spine. Later on I decided to use it for a more comfortable sitting position. A pod is used to be able to steer dry.

@TANSL
wasn't that the problem of team philips?

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### TANSLSenior Member

I can not answer you, I do not know what was their problem.

8. ### chinaseapiratePrevious Member

team phillips lost use of one of their "floats" about 1/5 of way between ireland i dont remeber where they were going. probably new york or up st lawrence river somewhere. im outta here unles less gonzo wants anymore background info on how I (maybe me only left but i doubt it) calculate structure...

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Well, it all depends upon the configuration and, if, and i do stress if, there is sufficient shear flow area in the members.

Yes it can be calculated by hand and by FEM.

If you are able to give a sketch, of the whole arrangement, of what you are proposing it removes the ambiguity and guesses from my side to provide further guidance.

10. ### chinaseapiratePrevious Member

anyone one who wants to know the displacement of a 30 year old open deck homebuilt can request that also. single beam is going to trump gonzo's knowledge base of "multi species" wood composite monocoque bridgebeam because he can't prove his beam is better without describing it. I can design around anything his computer tries., because with a single beam i got a double stock pile of wood in any specific place, incase i miss any "new inovations" double beam is obviously the clearly logical starting place but as i explained last night ths what i would use to for practical reasons. hull resonations..team phillips...well somethin i dont need to know. has nothing to do beams anyways. my memory cant be that bad . it what hull failure im sure. i was picking them to win too.

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### gonzoSenior Member

Yes, I would like to know how you calculate a structure, what you mean by "stress", and how you determine that the hulls won't be excited at their resonant frequency by positioning a single beam at the center of mass.

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I'd love to know...

Ok...so lot of words, yet still no method for your calculations given....

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### PammieSenior Member

Not much time to react: just putting my sketches online. I have 2 options differing in frontbeam position. The one with the foremost position has some mast placement issue's. The spine section just after the frontbeam is empty. Is going to be cockpit, winchdek, etc. Loads are diverted left and right of here to bulkheads in the frontbeam (where the kink is).
Hulls are not that boxy . Also picture of the biggest bulkhead. Hulls are long and pointy; in fact a forward extended 8,5 m hull.

14. ### chinaseapiratePrevious Member

did i get stress and strain wrong? I'm not going to go proofread to see if I told you I that I "calculate structures", if I did I misspoke, I do that often. If stress is not the psi measurement often represented by a greek symbol not available on my keyboard that results from solving the equation moment force / section modulus * the distance from the nuetrual axis of the beam in question to the edge of a symmetrical (about the neutral axis) beam's cross section, or the translated edge of a non symmetrical one's cross section , perpendicular to the moment force, then that would be strain, and stress is young's modulus over strain. Do you really believe i'm incapable of reading an appendix table correctly in a reference such as Gougeon Bros on wooden boatbuilding. I can do moment calculations on simple beams like a 740ft length 105 width 24 ft THICK ship to derive the max allowable stress to come up with a fairly accurate safety factor whether I have a appendixtable/cell phone/internet in front of me or not, some even WITHOUT KNOWING the the psi rating of the solid material in question. I thought you knew this already by the response you gave when I first made this claim.
, this does not affect the design as to its seaworthiness.

So you think my single beam looks like a tuning fork. Great observation, I would never have thought of it. I thought you were referring to an amplified pitch or heave response by getting in sync with a wave length, most studied land-lubbers know that but unless you have been at sea you won't understand what part of marine engineering does not apply to multihulls unless you prefer it to apply. You really wan't more info? All right then, sign a disclaimer. Gonzo will stop assuming CSP 's reading level is as poor as his writing skills and will provide more substance.

Last edited: Apr 9, 2018

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Sorry, perhaps I should have clarified..sketches...i.e drawn on paper and scanned in...much easier and quicker to grasp what you're doing.

So you're now going to a Catamaran rather than a Trimaran?

The location of the beams are not ideal, but it can be done. The aft beam will take a higher load since it is further away from the centre of rotation. Thus so long as you have done your sums...and proved to yourself it'll work..great

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