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  #1  
Old 09-30-2004, 12:36 PM
rlewis rlewis is offline
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help with design of 36 foot cat hull

I have been working on a 36 foot cat hull, here it is:
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help with design of 26 foot cat hull-cat36_wavebreaker.gif  

Last edited by rlewis : 09-30-2004 at 03:06 PM. Reason: oops wrong size, I meant 36
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  #2  
Old 09-30-2004, 12:52 PM
rlewis rlewis is offline
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Why a 36 cat? I want something big enough for a liveaboard, for 100 years from now when I want to retire and cruise the carribean into eternity. So it is basically a dream boat. It is a cat for efficiency, stability, and space. I also want it to be able to handle rough weather. Ideally I'd like to be able to ride out a storm up to a cat 1 hurricane, and lets face it they go up to cat 5 and it could happen, I'd like the boat not to sink in this case.

I designed it as a displacement simply because I do not know how a planing hull works, but efficiency is a big driver, so I added a small wave breaker as an afterthought. Seems like a lot of people here REALLY know this stuff, so is a planing hull for a 36 foot cat a silly idea? I do not want it to be particularly fast, I want to get from point a to b with minimal fuel consumption. Planing might help but you need to build up more speed to do that so maybe overall it is not a big plus on economy.

I designed both cat hulls and the wave breaker using the "tuna" shape. I did this because I know tuna move very fast in the water so they must be doing something right.

I also am considering design types. I think steel is the strongest but fiberglass is easier to work with and has nice strength to weight characteristics. I also considered using fiberglass over a tubular steel frame and bulkeads, or even steel reinforce fiberglass. I read soemthing about using steel reinfoced concrete and that was appealing whne I saw a picture of a good sized hull layed out in rebar and basically you just plaster over that, so why couldn't you do the same thing in fiberglass? Hoever, I think it would be hard to get the fiberglass to bond nicely with the rebar. Also, where 2 or more pieces of rebar cross you get a big disconfomity, I think in real life the fiberglass would begin to seperate and it would become a big pile of crap. But the idea is apealing, if only you could make it work, you could combine the strength of steel with the weight of fiberglass. Maaybe I'll try som small scale experiments and see if I can make it fly. But, that is when I came up with the tubular steel frame idea, as sort of a compromise. However, the steel frame might be needless overengineering, when plywood bulkheads can provide plenty of strength and are much cheaper and easier to work with for a do-it-your-selfer. Could you put in enough bulkheads to make it handle a cat 5?

Any and all opinions sugestions etc would be apreciated.
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  #3  
Old 10-03-2004, 09:41 AM
eckmuhl eckmuhl is offline
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hye, you right about displacement vs planning hull. There is no interest at all to design a planning hull if you don't want a high peformance cat cause at low speed the drag of this kind of shape is really high so you gonna use fuel just to move water... In view of your sketch you should be careful about the stability , your boat seems a bit high for a 10metre . For the stability you should be aware of traditional stability consideration as weight centre of gravity but also the effect of the wind or seaworthiness...
For the construction I don't really know prize in US but in Europe the cheapest and the easiest way to built personnal boat is ti use fibreglass and wood . Once more it's quite easy to repair... Make a welding in the middle of Carribean could be a good challenge sometimes. If you want you can use steel for the engine bed. For all the other part FRP, foam and basla core is perfect. To reduce the weight of your structure you can laminate all your plywood use for the interior on the side shell. For the bottom shell traditional framing is the best. For structural calculation ask a naval architect or at least take a look to ABS or other regulation
Good Luck in your project
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  #4  
Old 10-04-2004, 03:54 PM
rlewis rlewis is offline
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Thanks! I am approaching this from a very limited amount of knowledge, but the idea of GRP over a marin plywood frame seems like it is both simple and cheap. Thanks for your input. Now, I need to figure out how many blukheads, how thick, and how many layers of GRP to use. I think I will use C-flex, that looks like a good method to me.

I am sure I can solve the blukhead problem, that is a simple engineering problem of load. The frame needs to support a iven load, so I just need to calculate how much plywood is needed to carry that load. The GRP is a little bit tricker, I am not sure hoe many layers of GRP I'll need.

I think the best way is to find plans for a similar boat in GRP and extrapolate from there.
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  #5  
Old 10-04-2004, 06:01 PM
sorenfdk sorenfdk is offline
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Quote:
Originally Posted by rlewis
I think the best way is to find plans for a similar boat in GRP and extrapolate from there.
Be careful here! A similar design may give you an idea about the GRP layup, but not more than that!
The layup depends on many factors such as panel size, speed, building method etc. and since this is really a matter of life and death, you shouldn't base your layup on what's been used on a similar boat.
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  #6  
Old 10-05-2004, 12:51 PM
eckmuhl eckmuhl is offline
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You should careful about similar design especially about GRP cause most of the time the data given by architect or builder are really simplified so you won't have all the details you need ... You must also be sure that your plywood will be treated for resin bounding (there is lot of issues between plywood and polyester resin) .
I don't know how money you've got for your project but it could be a good idea to ask a professionnel to calculated your structure; because it seems you're not looking for a racer yacht, the layer evaluation according ABS or any other rules won't cost too much &you'll be sure to have a safer boat ;-)
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  #7  
Old 10-05-2004, 02:04 PM
sorenfdk sorenfdk is offline
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Eckmuhl is right - have a professional do the calculations of the structure. This is, as I mentioned before, a matter of life and death.
Only problem is, that not many of the classification societies have rules that cover multihulls like yours. The only rules that come to my mind, are Lloyd's SSC Rules (Special Service Craft). These rules are really for bigger craft, but they can be used in your case.
You don't have to have your design calculated to the rules of any classification society, but most insurance companies tend to like that. If you decide not to have it done like this, then be sure to choose a naval architect or yacht designer with experience regarding multihulls.
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  #8  
Old 10-06-2004, 02:40 PM
eckmuhl eckmuhl is offline
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Lloyd's SSC do it as well as ABS, Bureau Veritas or ISO , the advantage with Lloyd's is that you've got a nice software which help you to do it; ISO is actualy developing is own software...
For the lighter one ISO may be the best cause they actually developing their rules in accordance with what big yachtbuilder and naval architect have done in the past. It was shown that nearly no one of the yacht build before the 90's can pass the rules but because they all working well they have reconsiderated litle part of their safety coefficient. I just speak about small yacht <24metre but it is your case :-)
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  #9  
Old 10-06-2004, 04:12 PM
sorenfdk sorenfdk is offline
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The reason I didn't mention ISO is that these rules are still in the making. Every year we're told that they will be published next year, but informed sources have told me that it will take a couple of years (at least) before they're finished. The reason is that politics has shown its ugly face...
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  #10  
Old 10-06-2004, 10:35 PM
garydierking garydierking is offline
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36' cat design

If I were cruising the Caribbean when hurricanes are about I'd prefer a design with a lower center of gravity than you have shown. 100 knots of wind would have you cartwheeling across the surface of the water.
A better approach would be to emulate sailing catamaran design with long low hulls and no high flying bridge. Long displacement power catamaran hulls are very fast and efficient; forget planing and make it long; length is cheap.
I've built several hulls in C-flex and I must admit that I'd probably not do it again. A better choice would be cedar strip composite with epoxy glass inside and out.
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  #11  
Old 10-07-2004, 05:36 PM
Sean Herron's Avatar
Sean Herron Sean Herron is offline
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MOOSES - TEETHS and GOOSES...

Hello...

See http://www.mooseboats.com/frameset_new.html ...

Oops here is the boss just now...

SH.
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  #12  
Old 10-11-2004, 02:57 PM
rlewis rlewis is offline
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Hmm, thanks again. I'm still playing with designs. Anyone know of free designs for any kind of cruising boat in GRP? I like the O-1 and I think it would be a great boat for me but there are no finished designs. But, anything from a 30 to 40 foot cruiser either mono or multihull, mainly I'm looking for the layout of frames, bulkheads, etc. I really wished someong had finished the O-1 plans, if there were plans I might like to try to build it. But my dream boat is the 36 cat here.

Glen-L marine designs has a BUNCH of deisgns for sale, mostly in plywood which I do not like but some in GRP. There is a 44 foot cruiser I really like. But I'd love to see designs for any kind of power cruiser, inboard, outboard, I don't care. There seems to be a lot more interest in sailing designs, and there seem to be plenty of those available, even free designs. But, I'm looking for a power cruiser in GRP.
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