Calculating dynamic loads on a Catamaran?

[dustman]

Originally Posted by Tupaia

This is a complex issue and using aluminum adds to this by introducing rigid connections and fatigue factors that are different from composite materials.

Loads, forces and fatigue are discussed in this article. Considerations for Seaworthiness refer to the section near the end of the article Construction and Fatigue - Integrated Structure

You mention a ketch rig, are you planning a bi-rig one mast in each hull or one behind the other on the centre line?

I just bought that book, haven't gotten far into it yet.



My thought is that the fatigue can be more easily quantified because the properties of aluminum are well known. Trying to figure out a composite structure with various materials and build it without inconsistencies seems like it introduces a lot more uncertainties. There are other factors that make it a much more pragmatic choice for me.



The rig would be on the centerline, attached to the crossmembers, with further reinforcement. The bi-rig seems interesting and would make some things simpler but from what I gather that setup has interference issues. At one point I had fancied a thought or two about a quad rig.

[Tupaia]

Originally Posted by dustman

The rig would be on the centerline, attached to the crossmembers, with further reinforcement. The bi-rig seems interesting and would make some things simpler but from what I gather that setup has interference issues. At one point I had fancied a thought or two about a quad rig.

A conventional centre line ketch rig is a very poor choice for a catamaran. Such rigs perform badly to windward and will not be able to take advantage of the a multihulls speed that brings the apparent wind forward.


There are some very successful bi-rigs check out Kelsall (Cool Change) and Schoinning (Radical Bay).

[NevilleCat]

Originally Posted by dustman

Yes, the basic structure would be aluminum beams, with fine foam and fiberglass hulls, very small cabin on deck, ketch with freestanding masts. Thinking 32'x20'. To me, an aluminum frame seems much simpler and like it would have greater longevity. Also, far fewer variables as the properties of a basic aluminum structure would be much easier to predict than a complex composite structure with many variables. Would be much easier and faster to build as well. I am not a trained engineer so I'm going to stick with something I can manage.



Your cat had solid wooden beams? What diameter?


Thanks for the feedback.

No, my boat had standard mast section aluminium extrusions but I can't remember exactly what size. also they were oriented so they lay on their side (aerofoil section chubby side forward) but I think they would have been more resistant in a vertical orientation. My boat was the opposite of what you are designing; open bridgedeck with the accommodation in flared hulls. 31' with 23' beam but the hulls were over 6' wide inside at shoulder height.
I think the engineering of an unstayed ketch rig an an aluminium beamed cat would be as much of an issue as anything.
If you are considering a bi-plane rig that would be way easier but on small hulls you would lose much of the accommodation.

[zstine]

As an engineer, I'm always surprised why people want to reinvent the wheel. You don't need to create a loading analysis, just reverse engineer an existing boat that is similar in size/displacement. If I were designing a laminate schedule for my own Cat, it would cost sooo much time and money to develop the loading and CAD models and FEA analysis.. F' That! Just copy the laminate schedule from an existing/similar catamaran and be conservative... unless you are designing a very high performance race boat out of exotic and new materials, you don't need to do the calculations.


P.S. we called this "qualification by extension" when I was in the submarine building business. If the old design was similar to the new and it passed a test as rigorous as the new criteria, (the test of time for you comparing to existing cats) than the Navy accepted the new design qualified 'by extension'. The 2 other methods of qualification is "testing" and "analysis". Both are more expensive than extension and therefor extension was the go-to... DON'T OVER ANALyze

[MIRELOS]

you might want to ask a naval architect experienced in catamaran designs, for a fee you'll have an answer

[thinwater]

Originally Posted by zstine

As an engineer, I'm always surprised why people want to reinvent the wheel. You don't need to create a loading analysis, just reverse engineer an existing boat that is similar in size/displacement. If I were designing a laminate schedule for my own Cat, it would cost sooo much time and money to develop the loading and CAD models and FEA analysis.. F' That! Just copy the laminate schedule from an existing/similar catamaran and be conservative... unless you are designing a very high performance race boat out of exotic and new materials, you don't need to do the calculations.


P.S. we called this "qualification by extension" when I was in the submarine building business. If the old design was similar to the new and it passed a test as rigorous as the new criteria, (the test of time for you comparing to existing cats) than the Navy accepted the new design qualified 'by extension'. The 2 other methods of qualification is "testing" and "analysis". Both are more expensive than extension and therefor extension was the go-to... DON'T OVER ANALyze

^^ This! An engineer only starts from first principles when he has no other choice. The goal is to include the body of experience.

[Montanan]

Enough said, Nike Time. Just Do it!

https://www.youtube.com/watch?v=GsKIRkFEDwA

[zstine]

here is a short design article on beam sizing for Trimaran design. may be of interest. There's a lot more info on this site as well....

https://smalltridesign.com/Trimaran-...crossbeam.html

[catsketcher]

Zstine is right on the money.

There has been a huge amount of real life testing done on catamarans, its called "use and experience". Clever builders and repairers work out what works and what doesn't.

Reverse engineering and copying similar boats is fine, if you know what you are doing. Dynamic loads on something like a foam cat are hard to work out. As was stated in the thread it seems that other loads are higher - such as a dinghy hitting the topsides. So laminates don't get much lighter than 600gm on the outside of a cruising multi - even if they get below 30ft.

If you want to build a boat, talk to builders. Don't start with a blank sheet. Stand on the shoulders of giants. Better yet, talk with Richard Woods, or Schionning or Hughes, give them some serious money and get them to modify one of their designs. You will save thousands and thousands by spending money up front. Then you get proper input and you don't make stupid and really expensive mistakes.

Advice from someone who wishes he could go back in time and swat himself around the head for design issues that needed lots of time and money to fix. After I had spent 20 years sailing multis.

When I launched Kankama (my 38ft Chamberlin) it had all these wrinkles I built in, like a vertical tiller, pop top dodger, rotating davits, single outboard, no furler, wishbone rig and more. In the 20 years since I launched her MOST of the "clever" things I designed and built have been removed. I still have the wishbone, but that was in the original design. And I had modified and cruised on multihulls before. Like in natural selection, most deviations from the normal are unhelpful. The traits nice cats have built within them are the successful ones that have undergone lots of evolution. What you have to do is work out what type of successful cat you want - a performance type one or a condocat.

cheers

Phil