a custom catamaran hull

[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=Cada...PECoTnqQHP94Bg

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.

I started this thread in the Q and A forrum, but I realized it will morph into a build thread, so what follows in the 7 posts from the previous thread.

[larryrose11]

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.

Copy post#2

[larryrose11]

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.




copy of post 3

[larryrose11]

copy of post 4

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 :cool2:
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?
http://forums.offshoreelectrics.com/attachment.php?attachmentid=105868&d=1380482733

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.

[larryrose11]

copy of post 6

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/showthread.php?36985-Little-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.

[larryrose11]

copy of post 7

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.

[srislash]

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.


Ahh,there you go. Learned something today :-) Thanks Paul
Best to get our terms straight

[keithbradley]

Interesting Larry.
You mentioned that you're thinking of using decorative fabrics on both inside and outside, but if you are vacuum bagging it in a female mold only the outside will have a nice glossy and flat finish, right? (I am assuming that you're not using gelcoat on the outside.) I may be missing something with this so forgive me if I am, but if this is the case I would personally just use whatever is easiest on the inside and then do an inlay with a decorative fabric after joining the hull (maybe that's what you already plan on doing).

In regards to your goal of SAW capability while retaining stability, which side are you leaning most heavily? The cheetah is fun to run and carves out corners pretty well, but I think it's absolutely worthless as a SAW cat. I found that with the Cheetah you start seeing VERY diminishing returns around 75mph. They seem to be best suited to run 60s. I see HPR being on the other end of the spectrum, with great straight line efficiency but not a lot of use as an oval boat.
In my personal experience, the best compromise between the two would be:
-Leaning toward sport/ovals: Something like the TFL cats (Apparition/River, Zonda/Fantasm). These cats are capable of decent straight line speed, and while not as aggressive in the corners as a cheetah, they don't hook and spin like some of the SAW cats and are very easy/forgiving to set up. Perhaps with some changes this design could be improved.
-Leaning toward SAW: The MHZ mystic. Low speed handing is poor due to the lack of ride pads, but once they get out of the water these cats can cut an impressive corner. I've made sharp turns at over 100 without crashing. They are touchy in the turns though and require a more careful driving style (rudder input has to be different at different speeds). This would be a good design to study but in my opinion difficult to improve upon.

Sometimes I struggle to get the right tone in forum posts and my interest is perceived as criticism. I'm not being critical here, just interested in what you're doing and trying to offer what little info I have. This is something that I have wanted to do for a while, but just don't have the time. If there's anything I can do to help, please feel free to email me at keith@keithbradleyboats. com
I'm not sure where you're at in Michigan, or if you already have everything you need to do this or not. Time is always in short supply for me, but I may be able to help with some materials and will gladly share any info I have that may help. I have some Pro-set 145 epoxy resin and a couple different hardeners that I don't use that often and could donate some to your cause if it helps. It's a fairly thick viscosity resin but I've used it for vacuum laminations a few times and it works well.

By the way...I love the idea of using the Cadillac design as an inspiration. If you can capture that look in your hull it would look incredible!

[larryrose11]

Thanks Keith,
YOu put a lot in here, so, Ill break it up.

Interesting Larry.
You mentioned that you're thinking of using decorative fabrics on both inside and outside, but if you are vacuum bagging it in a female mold only the outside will have a nice glossy and flat finish, right? (I am assuming that you're not using gelcoat on the outside.) I may be missing something with this so forgive me if I am, but if this is the case I would personally just use whatever is easiest on the inside and then do an inlay with a decorative fabric after joining the hull (maybe that's what you already plan on doing).

I would rather not get away from the vacuum / autoclave approach for the hull construction. There is just too many opportunity to get bubbles, poor adhesion, and such.
As for hr decorative weave on the inside, I have had good success with a fill and sand approach once the hull is cured. I would brush an additional epoxy layer on the peel ply side, and gently block sanding it down, then paint on another layer of epoxy, block sanding down until it is smooth, and not sanding through the layers. Final sanding is 400 grit wet sand, and an automotive clear coat. The CF still shimmers, and has depth to it. This is what I was planning on the inside.



In regards to your goal of SAW capability while retaining stability, which side are you leaning most heavily? The cheetah is fun to run and carves out corners pretty well, but I think it's absolutely worthless as a SAW cat. I found that with the Cheetah you start seeing VERY diminishing returns around 75mph. They seem to be best suited to run 60s. I see HPR being on the other end of the spectrum, with great straight line efficiency but not a lot of use as an oval boat.
In my personal experience, the best compromise between the two would be:
-Leaning toward sport/ovals: Something like the TFL cats (Apparition/River, Zonda/Fantasm). These cats are capable of decent straight line speed, and while not as aggressive in the corners as a cheetah, they don't hook and spin like some of the SAW cats and are very easy/forgiving to set up. Perhaps with some changes this design could be improved.
-Leaning toward SAW: The MHZ mystic. Low speed handing is poor due to the lack of ride pads, but once they get out of the water these cats can cut an impressive corner. I've made sharp turns at over 100 without crashing. They are touchy in the turns though and require a more careful driving style (rudder input has to be different at different speeds). This would be a good design to study but in my opinion difficult to improve upon.

My main gripe is about boats flipping in the corners. The cheetah does the corners well. What, in your opinion, is limiting the Cheetah hull? Have you tried the mod for the last step, to flatten it out?
On the other hulls you mentioned: can someone / post pics of the bottom of Apparition/River, Zonda/Fantasm, MHZ mystic hull so we can all discuss?



Sometimes I struggle to get the right tone in forum posts and my interest is perceived as criticism. I'm not being critical here, just interested in what you're doing and trying to offer what little info I have.
I'm a perfectionist too. Professionally, I'm also an engineer / mad scientist. So, I can totally relate. I have a tough skin, and I'm looking for constructive criticism. This forum is pretty BS free, and that why I hang here.



I'm not sure where you're at in Michigan, or if you already have everything you need to do this or not. Time is always in short supply for me, but I may be able to help with some materials and will gladly share any info I have that may help. I have some Pro-set 145 epoxy resin and a couple different hardeners that I don't use that often and could donate some to your cause if it helps. It's a fairly thick viscosity resin but I've used it for vacuum laminations a few times and it works well.

How clear it is? Is it compatible with West?



By the way...I love the idea of using the Cadillac design as an inspiration. If you can capture that look in your hull it would look incredible!
I think something like this could help with some of the clearance issues with twins, and I love the look too. This is going to be cool.

[larryrose11]

I found an image of a MHZ Mysic back end, and you can see some of the hull details.
-no ride pads
-it looks as though each of the 3 sponon section on each side are flat, except where the step is.
106055

106056

[keithbradley]

I found an image of a MHZ Mysic back end, and you can see some of the hull details.
-no ride pads
-it looks as though each of the 3 sponon section on each side are flat, except where the step is.
106055

106056

No ride pads on the Mystic. The most capable SAW cats aren't going to have flat ride pads, and they are not going to corner very well under power. It's a conflict of hull attitude to accomplish the same two tasks. It just depends if you are leaning more toward the sport/handling side, or the SAW side.
If you made a 39"-40" Cat that ran like a MHZ Mystic, I would happily make it the proud owner of a Q-cat SAW record. :thumbup1:

The mystic is my kind of cat, but not everyone would feel the same. It's not something you see at oval races. Here's a vid to demonstrate handling:
http://www.youtube.com/watch?v=dzFd6DeciFw

[Cooper]

Kind of funny I just saw this thread after a post in another pertaining to ride pads and such. I have always been interested in hull dynamics and some areas needing redefining. I'm more for the saw cats and eager to see this envelope our interests.

[larryrose11]

srislash,
I would love to learn from the issues you had with the Komodo design, in particular the tunnel. Did you base the Komodo design on an existing one, or strike out on your own?
As of now, I'm leaning towards something with SAW capability. Sponson design something like the MHZ Mystic may be in order.

[srislash]

So I wonder what would happen with ride pads up front and blended back to MHZ style on the rear? Just a thought...

So we had started the Komodo as a larger Geico with flat pads.

[keithbradley]

So I wonder what would happen with ride pads up front and blended back to MHZ style on the rear? Just a thought...

So we had started the Komodo as a larger Geico with flat pads.

What would be the expected advantage?

[srislash]

I was thinking it wouldn't be such a knife edge going thru the water resulting in better corners with pads up front but if you really get it going(aired out) it would not have much in the water. Just a thought Keith, without innovation we get nowhere.

[keithbradley]

I was thinking it wouldn't be such a knife edge going thru the water resulting in better corners with pads up front but if you really get it going(aired out) it would not have much in the water. Just a thought Keith, without innovation we get nowhere.

I am definitely not against innovation. :thumbup1:

Just trying to see where you're going with this...
What would the advantage be of just a ride pad in the front, vs. a normal configuration where the rides are in the rear?

[srislash]

All I have had so far Keith is the flat ride pad for experience but have read over and over about the poor handling of Daytona's,HPR's and the like. Now look at the narrow flat pads of the Sprintcats or R-42. They can handle the "race" water. These both also have tall tunnels. However, when the tapered sponson is aired up there is just a bit slicing thru water as opposed to flat sponsons which have a tendency to "slap" the water resulting in control issues. Like my Shocker,great on perfect glass,but a little chop...

[larryrose11]

Keith,
Could you do an experiment with your Mystic? Put ride pads on the first step. Something thin on the inboard side, and thicker on the outboard side. Does it improve handling, as srislash suspects?