Principals of catamaran design.

[larryrose11]

Hello all,
Im beginning a new project, a custom catamaran hull.
Aprox 34 long,
Aprox 13 wide.
Probably set up for a twin drive.
The goal is to reach myself a few tings, and have fun along the way.
As the title suggests, i'm looking for information on the principles of catamaran hull design.

In the end, I will CNC a plug, finish it, and use it to make a female mold. Into this mold, I will vacuum and autoclave a sandwich of CF / Innegra / CF.
I plan on making 3 or more hulls using decorative weave CF on the outer skin, and perhaps the inner skin too.
As for styling, I would LOVE to make somthing inspired by the Cadillac CTS Coupe

https://www.google.com/search?q=Cadalac+CTS+Coup&bav=on.2,or.r_qf.&bvm=bv.53217764,d.aWM,pv.xjs.s.en_US.Es3CeXuAQeM.O&biw=1680&bih=965&dpr=1&um=1&ie=UTF-8&hl=en&tbm=isch&source=og&sa=N&tab=wi&ei=7MpFUsPECoTnqQHP94Bg

Depending on the response, I will make them available to members for parts trade or somthing. I plan on having the finished hulls by summer here in Mi.

Any insight into the design, once I rough it out, will be more than welcome.
The initial goal is so have something that is SAW capable, but wide enough to be stable.

There is w wealth of knowledge in the members here, and I would love to tap into it.

[siberianhusky]

The ones I have followed being developed are simply a matter of experience, building something new, trying it out and making tweaks and changes until it runs the way they want. Really not that many people building hulls unless you count hydros, most people are running the same few hulls that have been well proven in race conditions.
If it were me I would be building my designs out of wood until I get where I want to be then pull a mold from the hull. Easier than building a bunch of plugs and molds for hulls that don't work.
I'm kind of thinking about building something myself, mono hull just shy of 34", would like to try using some ideas from my current monos and some other hulls. No plans on re inventing the wheel, would just like to incorporate a few areas/ideas I like into one hull. Been measuring boats for a few years now and some design features seem to be incorporated in all the good running ones.
Not a big fan of cats but it must be the same for them.

[NativePaul]

Do you have the CNC or would you be getting it done for you? If you have to pay for it I would seriously consider building from wood or wood over foam to start out, as above it is very unlikely that your first one will be right (for your use), I started off knowing the basics having worked in a boatshop with a cat designer, and still it wasn't until my 6th different cat design that I would have wanted to mold one.

Cats like any boats are a series of compromises, if you make it better in one situation its going to be worse in another.
Wider tunnel, +better cornering -a lot more lift
narrow tunnel, +little lift good, for high speed -have to slow down for corners
High deadrise, +good in rough water, better cornering -extra drag
No deadrise, +fast in a straight line -but only on a millpond
One of the few free lunches are ride pads, pads of the right width for your weight and speed can give you all the speed of no deadrise and a little of the rough water handling of high deadrise, not win win, but win and a bit.
Steps, +faster -poorer cornering
34"x13" and SAWs capable is a big ask, a tunnel that wide will invariably pack a lot of lift, you may be able to ditch enough of it by running an air dam/air dams at SAWs or maybe not, how tight do you want it to turn, I would start with a pair of sponsons the shape you want on a sacrificial tunnel the shape you want, making sure to leave as much room on the tunnel floor as you can, then test it in the rough and smooth if it shows no sign of misbehaving take 1cm out of the tunnel with a hacksaw glue and duct tape it up and test again, continue till it does misbehave then add one if the 1cm strips of tunnel back in and test again to make sure it doesn't, you may find that you don't need 13" if so it would make things a lot easier at the SAWs.

That car you linked to looks fugly to my eyes, but it should be easy enough to convert it onto a cat by having slightly inward sloping freeboard and adjusting the width of the ridges in the bonnet to line up with the insides of the sponsons. Easy to make out of wood for prototypes too as it looks to be nearly all single curvatures or sandable from there.

[srislash]

Not alot of time here Larry but I do think I have a little insight after the Komodo design/build. Definitely things I would do different. Shape of the tunnel is my biggest concern but that area greatly takes speed into consideration. How you compress air in that area effects lift in the nose or rear. Height of the rear of the tunnel effects how the hull slips through the air by letting air escape easy or not. I'm along for the ride here on this thread
Shawn

[larryrose11]

Well, I really like th way my cheetah handles. I have mod the last step to make it even and flat with the others. It rode nice and flat under all power conditions. No diving when you let off the power. The strut adjusted flat, about 1/4 of an inch of rise when the hull is on a flat surface. I have yet to prop it up to full potntal, but Ill get there.
I was planning on making a 3D scan of the bottom of the hull with my mods in it as a starting point.

NativePaul, What is deadrise in a catamaran?
The ride pads are not flat left to right, when viewd from the back. They have an slight angle to them. I suspect that this helps with the rough water. Is this deadrise in a cat?


Also, in the pic, the sponson has hook in it on the tunnel-side face, on the left. What is the purpose of this hook, other than to make the tunnel width narrower?

srislash: what would you do differently?

I deff see the huge advantage in a test, mod hull, and test. To be honest, I would prefer to avoid making multiple hulls, I just don't have the time for that.
I'm part of our local Techshop, and am in the process of learning CNC. The experts at TechShop have a lot of expertise in transferring surfaces from Inventor into the CNC software. I plan on making a plug out of hardwood, so all the dimensions will be tight when I make a mold.
The only cost to me will be the CNC courses which I want to take anyway, tooling (bits), and materials used.
Also, If you want to participate in this design exercise on a more technical level, all the files I generate in AutoCad Inventor will be available, just ask.

[srislash]

I think Paul speaks of that first square step up from the ride pad Larry. I spoke with Andy (ACboats) at our last gathering and he doesn't like any rise there,just rolled up(no square edge). As far as the pads,a Daytona has a pretty good angle on theirs and that makes it want to go "just straight" as they cut into the water as a long line where as a flat pad wants to run on top.I had a Shocker hull that has very wide flat pads and a square edge(deadrise). It would start to hop around (slapping surface)on rough water(even straight) and trip easier in corners. The Cheetah seems to be a good compromise in a narrow pad as are the Sprintcats.

The Komodo we designed ended up with the tunnel "too steep" at the forward section. I would've preferred (and wanted) a more swept back tunnel roof, Like my Shocker which was very "wedge like. The Komodo packs too much air too quickly(my theory) and just won't stop lifting the nose and letting it down. I have tried everything. But over about 45mph it starts and I have decided that is where it will stay,the girlfriend can run it.

The "strakes"?? on the inside of the tunnel I used to believe helped with initial planing.Just a bit more surface to "get er up".

FYI, I am no engineer but have been messing about with mods on cats for years dating back to MRP Fast Cat and Graupner Systems 7. My latest was the pad mod to my Geico twin http://forums.offshoreelectrics.com/...e-Twin-G/page2
I would still like to mod the pads on a Shocker to narrow them and raise the tunnel and roll that deadrise.

[NativePaul]

Yes that angle you marked out is deadrise, and what you have pictured is very high deadrise for a ride pad, although it would be considered very low for an un-padded sponson.

I also think that the hooks on the tunnel side face are to get it out of the water faster, and as such I don't fit them as a FE boat should be planing near instantly anyway. they could be spray rails but as long as you have sharp edges I dont see the need for spray rails on vertical surfaces, and the last thing you want is extra lift on the inside sponson during turns.

The Daytona does not have pads, it has a constant medium/high deadrise from keel to chine, with 1 lifting strake and one spray rail.

Free access to a CNC mill and 3D scanner is awesome, and if you are just copying an existing design and doing a few mods to it, I would guess your first attempt will be pretty decent, most of my new boats were a step in the right direction and I only had one that flat out didn't work.

[larryrose11]

I shoul;d have moved this over to the build threads. Ill start one now, and copy the posts over for notes.

[larryrose11]

I moved the thread here
http://forums.offshoreelectrics.com/...d=1#post529641

[Steven Vaccaro]

thread closed by Larryrose request. See his new link
http://forums.offshoreelectrics.com/...d=1#post529641