# calculating torsional stiffness hull

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

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1. Joined: Aug 2002
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### gonzoSenior Member

I see now what you can't mean

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### PammieSenior Member

OK, next time I will draw on paper.

No, it has always been a catamaran. Maybe the confusion is in talking about a middle hull (but not a hull into the water). A drawing would have been more clear... Sorry for the confusion. Does it change your view? I think not.

I'll do my sums! Prove to myself that it'll work? Sure, but I allready made a mistake (besides the ones that I am not yet aware of) so I hope you or anyone will comment on it.

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Errr..yes, since you drew this:

which shows 3 hulls!!

chinaseapirate likes this.
4. ### chinaseapiratePrevious Member

to be honest I dint even realize what the mast was in you're first figure even though I think ive seen the diagram before in multihull article. I thought it was a big vicegrip simulating a torsion test. Sorry for the jibing about the "extra hull", I didn't know you were more informed on the subject. I thought just NA student., female at that (truth is honest- PC is lying, "thou shalt not lie"). I'm fine with "spare hull" but still catamaran nomenclature. I'm just hanging around to pickup anything about torsion or composites. I "calculate structures" by estimating my boats are 75% the strength by of a Kurt Hughes boat and 120% the strength of a James Wharram boat. Both cases by displacement/weight AT fixed length. If I'm "too" light I just add laminates to the shear clamp and keel once. Twice I've attempted building a "big cat" over 48. Once abandoned and once it turned into trimaran. I will "watch" how to make beams. I looked at Bredt's formula..I am ok with that, Lol.

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### gonzoSenior Member

You still fail to explain how you calculate the strength of a structure. To make something 75% or 120% of a value, that value has to be determined first.
Further, your derogatory comment about female students is completely out of line.

6. ### chinaseapiratePrevious Member

Well sure you can. wanna see? KH=120 JW=75 CSP=.75KH=90. thats it- just "declare" them. Kurt Hughes declared hi cylinder moulded hull 5 X as strong as double diag hull...And the consensus was Wharram's boats had a untarnished failure record... So I "safety factored" the cylinder moulded hull by 4 and gave the Tikis 100% times their declared payload/weight X weight/payload of similar length Hughes boat. I used Moondance and a Tiki 46, I could go on about how moondance had 120 sheets of 1/8 ply per hull and he he nearly raved about how light and "clean" and easy his bulhkheads and stringer were at only 15% of the hull weight- I hope it's understood that I'm guessing at these numbers now at this point but that that is how I calculated the amount of wood I needed to build the 69 ft tri out of one cat hull multiplied by 64/27 because of the length difference. The 3/4 ply that was actually available was under dimension so i just made more stringers, I had (4) 1 1/2" x 5" stingers in one side of 72' by 54" bow to 42" stern hull panel. Frames ere 2 1/2" x 12"outside over a
core of end grain coconut lumber 1 1/2 x 9 with grain running to beamwise not vertical and 1/2 ply both sides, when it was done we jacked the bow up with bottle jack had people on top of the hull and measured zero(sic) deflection.
As for what you may have been asking, the big longitudinal load (which I calculated as good enough on the 70 ft real bangka by test described above (similar in fact to the torsional calculation method described by Richard Woods where you jack the cat up on 4 corners and let one off (much easier(of this I'm not certain because I can't do one of the comparisons ( can you (Gonzo)?)) than Brendt's equation by the way))) was a structure from a different thread and I did complete it above (you just have to plug in the actual moment) which I did. I posted the result with using estimation of 2000 - 5000PSI allowable load. so I'm done with structuralizing- I lost the scratch paper... but you can always just declare it to be variable x=7560^3 x 144/50 IF the macro composite's allowable stress is > 2500PSI+ - 50.

Last edited: Apr 11, 2018
7. Joined: Oct 2008
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Gonzo

I would give up...
China....e, is just sprouting verbal diarrhea and not actually saying anything sensible. Wasting bandwidth basically..

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### gonzoSenior Member

However, since he defines calculating a structure as "declaring it to be a variable" the rest of his posts have a context.

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### PammieSenior Member

Well Ad Hoc, here's my sketch to clear things up Now you know why I rather use a computer...

10. Joined: Oct 2008
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Hi Pammie...at last simple sketches....thanks

Hmmm..im getting confused again though.

Ok...are you using designing a catamaran...or a trimaran?

Since each post flips from one to the other...makes it hard to advise. As both require different approaches.

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### gonzoSenior Member

I think that is a low hanging pod. It will add stresses from pounding, but they are usually of soft mounts, so they are not part of the beam structure.

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### PammieSenior Member

It is indeed a pod as can be seen by distance to the waterline. Minimum clearance is 670 mm so I have some spare (800 mm). Not for the pod itself therefore it is rounded (and small). I calculated slamming loads according GL HSLC: max 10 kPa for the pod part, which is lower as slamming loads for hulls: 34 kPa

13. Joined: Oct 2008
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Hi Pammie

Sorry..been snowed under with work.

The basic design of the 2 beams, is pretty straight forward. As it is a simple catamaran.
What makes it less easy is the fact that you have a central accommodation (weight) on them and with a compression load from a mast on one of them too.
From your point of view, less easy to calculate, owing to your original enquiry into this to begin with.

Have you got an all up weight, displacement, of the whole vessel when finished?
Have you got an all up weight of each hull and also the central accmd section too?
Have you got a handle on the mast loads, in particular the compression from the mast - assuming it is a stayed mast?

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### rxcompositeSenior Member

What is this Bredt's formula? Any link or explanation?

15. ### chinaseapiratePrevious Member

Well there is wiki description, complicated math that i'm not understanding, and then the link to calculating panel stiffness with holes cut in them (for house construction). I'm waiting to see, if it is decided eventually, that a chosen ideal max deflection of the rig loaded front beam (without use of dolphin striker) will determine the beam section used, alone without consideration of added hull moments and torsional resistance. But i believe they went to email communication...

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