# The Elements of Boat Strength for a Catamaran

Discussion in 'Fiberglass and Composite Boat Building' started by BlockHead, Jul 3, 2019.

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1. Joined: May 2013
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I'd appreciate any thoughts on the following questions.

1) Can the The Elements of Boat Strength be used to inexpensively estimate the scantlings of a catamaran hull (only, not bridgedeck nor beams)?

2) If so, is the Scantling number (Sn) calculated with the BOA or with the Hull Beam?

3) If it's calculated with Hull Beam then is the volume doubled (for two hulls) before being divided by 1000?

Thank you for your time and consideration.

Last edited: Jul 4, 2019
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### gonzoSenior Member

I don't think you can calculate the scantlings of a catamaran hull without taking into consideration the bridgedeck or beams. It would be like ignoring the rig loads on a sailboat. Also, note that Gerr is very careful about pointing out the limitations of the method. It gives scantlings almost identical to the Herrshoff method for traditional construction. However, very narrow hulls fall outside of the method's scope.

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### TANSLSenior Member

With all my respect for a book, which is of enormous value, I would not use "The Elements of Boat Strength" even to calculate a FRP monohull (narrow or wide hull, that does not matter at all). At present it is considered mandatory to carry out a series of analysis of the layers of the laminates that this book does not even mention.
That book, based on a number that, as you know, is difficult to apply correctly, determines a minimum thickness for each area of the ship. But the minimum thickness is only one of the elements, and is not the most important, which must be studied when dealing with non-homogeneous materials, such as FRP.
In case you had to expose your calculations to a regulatory body, it is very possible that they did not accept those made through this book. Even regulations such as ABS, which for years had formulas to calculate thickness only, have abandoned this method or have completed it with additional calculations.
A catamaran is a complicated ship with which there are no shortcuts. It is impossible, in several areas, to extrapolate the calculations of a monohull to a catamaran because they do not have absolutely nothing to do with each other.

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I apologize for not being clear that I am looking for inexpensive estimates only.
I promise not do anything substantive nor put my nor anyone else's life at risk based on these estimates

You bring up good points about the bridgedeck or beams.
That's why I specifically stated, "hull (only, not bridgedeck)".
I have however edited my question to also specify the exclusion of beams.

You are correct in that not only does Mr. Gerr specifically addresses "vessels with D:L ratios under 100. He also states that vessels <10' and >120' and those with a maximum speed of 45 knots fall outside the method's scope.
Additionally, Mr. Gerr also specifically addresses internal structures around shroud chainplates, masts, keels, deck hardware, etc.
Fortunately my theoretical vessel has a D:L >100, a L:B =10, a LOA <50', and a speed of <45 knots.

Thank you.

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Again, I apologize for not being clear that I am looking for inexpensive estimates only.
I promise not do anything substantive nor put my nor anyone else's life at risk based on these estimates

You bring up good points.

Thank you.

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### TANSLSenior Member

I am convinced that you will not take risks that endanger anyone and I am convinced that you (like me and like anyone else), if you find the right tool, can make the estimates you need.

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Thank you.

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### TeddyDiverGollywobbler

I've compared Gerr scantlings to Schionning catamaran schedule. Gerr scantling create 50% more core and skin thickness so bet it's on the safe side. But as TANSL mentioned if they must be verified it gets complicated.

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### TANSLSenior Member

If we only go to the thickness, we could put a laminate, with a very large thickness, that was totally insufficient to support the loads to which the laminate is subjected. It is necessary to analyze, it is obligatory, the loads in each one of the layers individually.

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### TeddyDiverGollywobbler

It's not obligatory for DIY boats, anyway not in most EU countries. Besides if 10mm core with 600 (outside) and 400 (inside) gsm is sufficient so why 20mm core with 1500/600gsm skins is not (materials being the same)

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### TANSLSenior Member

TeddyDiver, you are right, nothing is mandatory "per se", you can always do what you want but (this is what I should have specified), when you want to do something that is based on some regulation, it is obligatory to do the calculations of the loads in each layer.
Nothing is mandatory in any EU country. You can do what you want, but when you wish, for whatever reason, that some official body accepts your boat for a certain purpose, you must show that the laminate meets certain minimums and one of them is that, with thickness independence, none of the layers is overloaded.
The normal, I think, even if it is a DIY project, that the builder tries to fulfill what, in every moment, is considered "good practice".
Captious question, without answer. I think that neither you nor I, nor anyone else, is capable of answering it. The only thing I can say is that you can NOT answer affirmatively to that question.

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### TeddyDiverGollywobbler

However done the calculations you get the minimum schedule, which by definition, must allways be met or exceeded more or less (my point). Another matter for DIY builder is the cost of inspecting body as they watch over also the building process and environment according the regulations and that in most cases is prohibitive. That is unless the inspector in question happens not to be your son-in-law or nephew..

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### TANSLSenior Member

What calculation do you refer to and what is the "minimum schedule" for you? I could agree with you. But I'm afraid this is not going to be the case.
On the other hand, the question that you raised in post # 10 leads me to think that you have not reconsidered much about how you should consider and calculate a panel of composite material, and even less of sandwich. As you know perfectly well, that is what I deduce since you have been arguing on this subject, such an "unbalanced" laminate as the one you are setting as an example, "20mm core with 1500 / 600gsm", is quite incorrect. Would you like to comment on this aspect?. But first, if you are so kind, answer my questions on the first line. Thanks a lot.

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### TeddyDiverGollywobbler

Any calculation which defines the minimum schedule for panel loading. Numbers I presented were just from my memory what Schionning had calculated for one catamaran. The second numbers were what Gerr scantling defined as schedule for a same hull length and about the same weight monohull. That was from 12yrs ago so I'm not going to find those anymore.
What comes to "unbalanced" schedule the outer skin is thicker to protect against puncture loads as flotsam or handling heavy gear on deck..

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### TANSLSenior Member

I am sorry but I still do not know what is for you the "minimum schedule" (minimum number of layers, minimum weight, minimum thickness, minimum hours of work, ...?). As for the rest, although you have your explanation, which of course I respect, I do not agree at all. And another thing, loads on a panel (panel loading) do not depend at all on the eschedule (whatever that may be).

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