A little math

Hi there,

I'm working on my own catamaran design. I'm fairly certain I'll be using foam core construction to make it. I've found some boat calculations but I'm wondering how much reinforcement is needed for connecting the two hulls to the main cabin? I'm planning something in the 30-40 foot range.

Thanks,

 

Welcome to the forum and I hope you find in it an adequate answer to your questions.
I do not want to be rude with you but realistic. Focusing on your question, and with all due respect, my opinion is that if you are able to ask that question, without realizing how difficult (impossible) it is to answer such a complicated issue, it is because, before moving forward, you you need to have the help of an expert in catamarans and structures. In my opinion, and again I say that with all due respect, your question shows a lack of knowledge, about the structure of a catamaran, very worrying.
It is not "a little math" but to assume complicated cases of loads on the structure, and of carrying out calculations that are not simple.

 

TANSL gives a good answer, I think.

 

Yeah. Its not just a little math. There are lots of it. Loads. Bending in all three axises. Torsional loads. etc. etc. etc. People get engineering degrees and spend a lifetime trying to figure out those calculations. Even with software that simulates the materials and automates the maths, there is only so much it can stand in for the practical knowledge necessary to design structures.

But on the other hand.

People designed and built boats and catamarans before there were computers or even... math. But they did so by trial and error and learning from what others built. Your best bet is to look at and emulate existing designs and plans. Bone up on mechanical engineering (you can pretty much get everything but the sheepskin of a B.E. online these days) and you CAN figure out the germane calculations necessary for the kind of thicknesses and reinforcement needed.

If you don't copy an existing design (or structures therein), then you will have to take the care to do your own testing. "Coupons" of your composite layups and even whole structural members tested to destruction to see if what you calculated matches reality. This is why boat plans cost money, not that they are pretty drawings. The designer has already taken the pains to make sure that its tested and found satisfactory.

 

The real problem is, from my point of view, that to pose such an "innocent" question means that you do not have an idea of the general problem existing in the design and construction of a catamaran, and, if you are not able to foresee the problems, I no longer say solve them, but to realize that they exist, it is dangerous to undertake any activity.
Before the existence of computers and CAD / CAM software, even in the time of the Egyptians and their papyrus boats, there were people capable of analyzing situations, detecting problems and solving them or finding someone to solve them. If you are not able to detect that there may be a problem, you can get into big complications thinking that you are capable, with a little help, to build something.

 

Lots.

 

Is that not what the OP is attempting to do?

 

It is likely, but if someone is not able to detect a problem and know that it is a very important problem, it is difficult to give a solution even if it is a bad solution. Let us suppose that someone, with all this data that the OP has given us, is capable of giving the solution on "connecting the two hulls to the main cabin" (what else could be added to what was already said in the post # 6?). The bad thing is that, probably, there are many other problems that, perhaps, the OP can not detect.
And I repeat that what I say is with absolute respect for the OP and that, if he gave more data, I would be the first to try to help him.

 

When a new post comes on to respond you try and work out how much the OP knows for relevance too,..it's natural to express some concern but it can cramp a conversation.

Part of the mystery of taking to the water is comprehending as to what it is going to throw at you and how you are going to go about solving the problems, you want first hand knowledge of each weak point.
To me this is a question on using a monocoque type braced cell type structure or beams to stop twist and shock. At 30/40ft unless it is going to be more a pure racer it's the central hull that does everything, and subsequently gives good access to the outer hulls by getting a good grip on them. This means more potential slamming so attention to this detail can greatly improve the ride with dispersing shape built into the slamming sites. The boats that do this best are the clues as to the end design you need, unless you don't intend to go to sea much at all. I would suggest your own design is , and should be greatly inspired by existing craft. So the plans to a really good one are value for money, if you are on a tight budget customising an existing finished cat that is close to your needs is pretty logical. Especially with what you will have learned working out your set of compromises, which is the biggest part of design.
Tansl is right there is heaps to look into but if you 've got the bug you will.

 

"I'm wondering how much reinforcement is needed for connecting the two hulls to the main cabin? I'm planning something in the 30-40 foot range."

I would agree there is a lot of yawning nothingness in this question, I would say 3 things:

1) So if by "connecting the two hulls to the main cabin" you mean attaching them to the bridgedeck, well as much as that is a bottomless question, since we have nothing about the overall design to look at, If the bridgedeck is overlapping the hulls, and butted against the beams, it is barely possible to tab it all in without massively supporting it. It kinda comes down to standard details for continuity of materials.

2) Since a lot of boats have already come down the pike, you can often design structures simply by keeping your weight budget very much in mind and designing for ergonomics. So the smallest aft beam on a 30 foot cat that I know of is on the Tiki30. It is something like a 6 inch box of 3/8" ply. When I came to build my much smaller motor cat, I wanted to at least be able to sit comfortably on the aft beam. So I made it something like 8x12. I designed it to be very light. But still sturdy enough for trailering and a motor mount. I have no idea what the numbers say, but let's be serious... The same thing if your 30is cat has an integrated rear beam and cockpit settee...

3) While at this point in multi history there are many yards with design professionals who have been at it for like 30 years, and while a number of the leading lights even earlier were engineers or naval architects. Still, when you get involved with an actual engineer who starts doing a finite element analysis, it tends to end up about doubling the weight of parts. So my feeling is that if people are designing light boats with science, and not using super materials like carbon, they are basically wrapping in, as they should, what the sea has taught. Which you can also reasonably pick up by hanging out in boats.

All else equal, a 40 foot boat is 2.4 times larger than a 30 footer, so a lot will be different.

 

Assuming there are no external load to the join itself, meaning all load and support comes from the joined parts only:
If you use the same amount than is used on the parts being joined and the same fiber orientation, then it is practically not going to fail due to breakage of the joining fibers. It can still brake due to inadequate glue/resin strength/adhesion or inadequate join area.

In case when both parts do not have the same amount of fibers, using the same amount that the part which has less would still work out as long as the orientation is the same.
In some cases you could even use significantly less, if on both parts some of the fibers run exactly along the join line, these fibers are already there in both parts and do not need to be added by the joining laminate for the global perspective at all.

But you also have to consider localized effects. The edges of the joining laminate must remain on the surface of joined parts and not separate from them. As long as the curvature of the area at the join is small, there will be no problems, but if there is high curvature in any direction, the joint can become a critical stress concentration, requiring the edges of the joining laminate to be stiffened one way or the other to keep them from separating. The issue is biggest when there is either high tension on concave surface, or high compression on convex surface. And both only apply when combined with high curvature along .
direction of any of the fibers used in the join.
If the fiber orientation is significantly different in the parts being joined, the joining laminate would better have, in each direction there are fibers, the same amount that is used in the parts. This possibly leads to higher laminate weight than in either part and in that case it is necessary to do so unless you use FEM to determine real load cases or are highly experienced to work it out otherwise. Of course it can always be tested by destructive testing to see what is enough to make the join at least as strong as the parts being joined, which should be the target unless you can prove otherwise by FEM and analyses of load cases.

If there is any significant external load you question can not be answered without knowing what those loads are.

BTW these principles apply to all joins, not just those between hulls and main cabin. They do not guarantee that something won't break, but when applied correctly they indicate the parts being joined break before the join when overloaded. Thus answering to your actual question, not what others have anticipated that you are asking for.

 

It should be taken into account, I suppose, that the transverse beams that connect the two bodies of a catamaran are subject to very different loads (the type of load) to those that support the hulls.

 

I would suggest looking at cats with a similar construction to what you have in mind as a starting point. For instance, will the cabin span from gunnel to gunnel? If so, it becomes part of a monocoque bridge deck. If it is going to sit atop bridge deck beams, that suggests very different structural loading. Find an existing (successful) model as your starting point. Then you can focus on the relevant details, whether you plan to design it yourself or bring in a professional.

What is your end goal for the project? To own a boat or to build one? If you mainly want to own one, go find one on the market that meets your needs and buy it. It’s very unlikely that you will save money designing and building your own, and you risk investing a lot of money and effort on a project that ultimately doesn’t pan out. But if your goal is to experience the creative process, go for it. And have fun doing it. There’s no single ‘right way’ to design/build a boat. Do your best and then build in a large safety factor.

 

Floatything has drifted away...