Calculating dynamic loads on a Catamaran?

[dustman]

How to go about quantifying the various loads on a catamaran? Static loads are easy. Twisting loads? Waves crashing into the side of the hull? Dropping off the back side of a wave at speed? What is the largest load that a catamaran structure will be subjected to?



How do designers go about determining just what these loads will be? Any good resources out there?

[seandepagnier]

It's really difficult to calculate without more information. Most boats don't break from waves but from hitting something solid.

[Chotu]

There is a rule of thumb that makes a lot of sense to me. The biggest loads a catamaran will experience are on the ground.

Take your catamaran, prop it up on all four corners. Now remove one of the props. Nothing should happen. If nothing happens when you do that, your boat is in good shape for the water. This is one of the tests I did before launching mine.

Remove one at a time, leaving the other three, and do it at all four corners.

[Dsanduril]

I know that Woods, for one, advocates this as a litmus test, but it really doesn't make any sense. A very stiff boat could still rotate about the diagonal. If you remove the support under one stern the other bow may raise. The boat could still be perfectly stiff, rotating about the diagonal line between the supported bow and the supported stern. Whether it will or not depends entirely on the weight distribution, not on the stiffness/strength of the vessel. Since I haven't tried it on any vessels, it's only a guess, but I suspect in most cases you'd have better luck with this removing a bow support than a stern.

From a calculation perspective you can look at the weight of one hull floating the entire structure, the other in air and including the sail forces required to get you there. That gets close to the worst case loading of the entire structure, in any other case (both hulls with some buoyancy) the loading on the structure is less. There are some cases where the torsional load might be greater, and those are much harder to assess.

Dynamic loading from a wave hit is an entirely different question, as it is a concentrated load that happens on a section of a boat. At the same time there are tons of formulae for calculating scantlings to deal with that question (see Skene's, for instance), and it doesn't really matter if the boat has one, two, or three hulls.

[dustman]

Originally Posted by Dsanduril

I know that Woods, for one, advocates this as a litmus test, but it really doesn't make any sense. A very stiff boat could still rotate about the diagonal. If you remove the support under one stern the other bow may raise. The boat could still be perfectly stiff, rotating about the diagonal line between the supported bow and the supported stern. Whether it will or not depends entirely on the weight distribution, not on the stiffness/strength of the vessel. Since I haven't tried it on any vessels, it's only a guess, but I suspect in most cases you'd have better luck with this removing a bow support than a stern.

From a calculation perspective you can look at the weight of one hull floating the entire structure, the other in air and including the sail forces required to get you there. That gets close to the worst case loading of the entire structure, in any other case (both hulls with some buoyancy) the loading on the structure is less. There are some cases where the torsional load might be greater, and those are much harder to assess.

Dynamic loading from a wave hit is an entirely different question, as it is a concentrated load that happens on a section of a boat. At the same time there are tons of formulae for calculating scantlings to deal with that question (see Skene's, for instance), and it doesn't really matter if the boat has one, two, or three hulls.

The hulls I am creating will be bulletproof basically, what I am concerned about is the loading on the aluminum crossmembers during extreme events. Useful information would be something like maximum expected force from a wave strike per square foot of surface area. How many times stronger than baseline common loads does a structure need to be to withstand thousands of cycles. Even how strong a length of weld is compared to the material.

[Dsanduril]

Not my favorite site, but there is an interesting paper on Academia.edu that addresses your specific question. If you have/can create an account could be an interesting read. If you can’t access I might be able to download and send, but not before tomorrow earliest.

[dustman]

Originally Posted by Dsanduril

Not my favorite site, but there is an interesting paper on Academia.edu that addresses your specific question. If you have/can create an account could be an interesting read. If you can’t access I might be able to download and send, but not before tomorrow earliest.

Alas, that article is beyond my mathematical ability to decipher. Thank you though.

[thinwater]

Originally Posted by dustman

Alas, that article is beyond my mathematical ability to decipher. Thank you though.

You're asking for a degree in engineering or marine architecture through forum posts. Like every other engineer, you need to start with structural and dynamic basics, and work your way up. Or you can look at other boats of similar size and copy. Those are your options.


I'm not being mean. You asked a big question. Even experienced engineers have to estimate values based on assumptions about gusts and sea states. They calculate and they build of experience.

[dustman]

Originally Posted by thinwater

You're asking for a degree in engineering or marine architecture through forum posts. Like every other engineer, you need to start with structural and dynamic basics, and work your way up. Or you can look at other boats of similar size and copy. Those are your options.


I'm not being mean. You asked a big question. Even experienced engineers have to estimate values based on assumptions about gusts and sea states. They calculate and they build of experience.

That's what I'm asking for, is the best estimated values, which I can apply to my design. There must be commonly accepted values out there. Aside from wave impacts I believe I can roughly estimate other variables, then build in a sizeable safety margin.


I guess a good question would be whether a beam on breaking wave impact would be the greatest stress that would likely be imposed on a catamaran's structure. In my mind the answer is yes.



If I knew the greatest probable breaking wave velocity and associated water column volume for a given area I could calculate the force. I need numbers though and not sure where to get them.

[cal40john]

Originally Posted by dustman

That's what I'm asking for, is the best estimated values, which I can apply to my design. There must be commonly accepted values out there. Aside from wave impacts I believe I can roughly estimate other variables, then build in a sizeable safety margin.


I guess a good question would be whether a beam on breaking wave impact would be the greatest stress that would likely be imposed on a catamaran's structure. In my mind the answer is yes.



If I knew the greatest probable breaking wave velocity and associated water column volume for a given area I could calculate the force. I need numbers though and not sure where to get them.

While not catamarans, the Around the World racing boats, particularly when they started using canting keels broke a lot and badly. Big money went into those programs presumably with lots of engineering. Did they underestimate the dynamic loads? So they're still having to guess?

[NevilleCat]

Is it an aluminium cat? Or otherwise why are you using aluminium beams rather than box ply beams for instance.
I had an open bridgedeck 31' cruising cat using mast section beams and there was noticeable flex in a seaway. Not much and probably not a problem mechanically but, with that and the three wire rotating rig, it did mean the lee shroud could get pretty slack and whipped a bit going to windward.
I would prefer the boat rigid I think.

[dustman]

Originally Posted by NevilleCat

Is it an aluminium cat? Or otherwise why are you using aluminium beams rather than box ply beams for instance.
I had an open bridgedeck 31' cruising cat using mast section beams and there was noticeable flex in a seaway. Not much and probably not a problem mechanically but, with that and the three wire rotating rig, it did mean the lee shroud could get pretty slack and whipped a bit going to windward.
I would prefer the boat rigid I think.

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.

[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

[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.