A tail of 2 Cats

Hi all, I’ve been lurking for awhile reading and learning, and finally joined up.
I’m in the very early stages of designing a home built boat project for myself and am looking for opinions and advice from people with more boat design and building experience than I have (I have just a bit more than none). What I do have is 30+ years in the automotive prototype and design business with several concept builds under my belt, the last being the Cadillac Converj. My last “home project” was to re-body for low volume production, a C5 Corvette (97-04) as “57 Corvette on steroids” that died an untimely death do to my customer’s lack of funds. In short complex projects, working with most common materials and styling above the water line I can handle, below the waterline I’m out of my depth so to speak.
From where I live in SE Michigan there is close to a 100 inland lakes, many interconnected by rivers, as well as lake Erie and lake ST Clair within a hour drive from my house
So my basic requirements are:
1) Shallow draft for inland waters (24” or less)
2) Reasonable to trailer
3) 25’+/- OAL 8’6” beam max
4) 40 mph min
5) Clear bow and stern deck space for fishing/camping
6) 6-10 passenger
7) Be able to handle 3’ chop
I started out with the thought of modifying an existing pontoon boat to suit my intended use, but decided a clean sheet might be the better way to go. At this point I've roughed in 2 different Cat hulls based on what I've seen and would like some input on the pros and cons of each configuration.

Thanks Drew

 

The type with symmetrical sponsons has met with greater acceptance here in Australia, where seaworthiness in open sea conditions is the foremost consideration. At least some of the "tunnel hull" type acquired a reputation for directional instability in some sea conditions, having an unnerving tendency to veer off course when stern quartering seas. It is understandable how this could occur, whether tweaking the lines can get rid of the trait is the sixty-four dollar question, and I'd be keen to hear the expert opinion on that.

 

25 feet +/-, 40 MPH min. Gonna take a bunch of horsepower. OK if your thing is Vettes and such then you can find the power. Operating costs will be more than mere lunch money. Three foot chop and 40 mph are not often enjoyed at the same time.

Why a Catamaran??????

 

Most folks who get to owning a cat don't go back to monos.......if they can afford the extra $.

 

You need to determine how much weight you will be carrying with the finished product - that will determine the size of your hulls. You may be better off going with a single barge-style hull, esp. if you want to go 40mph and maintain shallow draft, although you will have a wetter, rougher ride when the waves pick up. Are you calculating draft with the engine(s) tilted free of the water, or trimmed all the way down? If trimmed down, then you will need to either go with a single outboard between the hulls, or a barge style with a well-designed pocket tunnel, or something on the order of a Hamilton jet, although 40mph will be more difficult to obtain with the jet.

 

http://www.multihullsmag.com/magazine/articles/cathulls/cat hulls.htm and much more to read up on, any pics of that 57 Corvette ?

 

Designed for speed?

Using the traditional criteria of a house on a hull (barge or catamaran) there are just so many trade offs.

Before you get to the speed issue, you must deside on the sea-state you need to operate in--generally a catamaran is more sea kindly, but has deeper draft and less load carrying capability for its size.

Bigger issue is when you have a narrow boat with vertical sides going out to full hull width--it's difficult to dock with no side decks. Wider hulls create issues with towability and slip width--how much will you actually be trailering? If only once to a place where you keep it, or seasonally--everything get's better with more beam and, to some extent, more length. (Generally most marinas can handle hauling and storing, or a trailer can be designed for ramp launching for these shallower draft vessels.) Consider, perhaps, opting for a boat that needs a wide load permit, but not a super wide load permit--about 12'--look at the states where you will be operating for their rules.

These shoal type hulls are generally not going to be so seaworthy and to make compromises to achieve high speed may just not be worth the trade off. (Bigger engines mean more fuel, more weight, more volume is then required just to accomodate more fuel (wight), more volume means you need bigger engines to achieve high speed (and other design considerations), and round and round you go)...

Settle on a speed of up to about 10-12 knots (typical trawler speed) and things get easier and more practical.

Suggest you look at solutions by production builders to point the way to realistic compromises. Aqua Lodge, at 42' probably demonstrates many good solutions for a Cat. Take a look at her at our web site: http://www.bayacht.com/houseboats.htm

The worst investment is to have to do it over. The best--get it right the first time, even if you spend a little more!

 

2TS:
1. You don't need split hull for this speed.
2. Symmetrical hulls will work, though they should have enough lifting surface and chine flats; transom deadrise - up to 20...24 degrees.
3. Check hull chine beam at full load condition: V/(2*Bc)^3 < (0.5...0.7); where V is volume displacement in m^3; Bc is chine beam in m. If boat is heavy and does not possess enough lifting surface, it won't plan.
4. Horizontal clearance c (distance between inner surfaces of hulls) is not critical issue at that speed; c=(0.1...0.2)*L is good reference point.

 

The idea for doing a Cat hull started out as modified pontoon, but by the time I make all the changes I want, starting with a clean sheet of paper seemed worth exploring. I whole heartedly agree that 40mph in 3’ chop sounds less than fun in the boat I’m contemplating; I meant 3’ chop off plane. HP wise I’m thinking twin 150hp motors but that’s still open to change.

 

This is what we tested last week, with 2x170HP:
http://www.youtube.com/watch?v=hOemakPfG6E
On first test with brand new engines delivering 38kts on SS2.

And this is sistership with 2x140HP, delivering up to 36kts with half load (on flat water):
http://www.youtube.com/watch?v=A0lbpsddgVA

Yes, acceleration level on these cats at 34kts is 0.3...0.5g at 3' chop that is not really comfortable, but acceptable if we are talking about 10...20 miles passage to fishing or diving site. For monohull of compatible capacity accelerations would be at least 30% higher.

 

Thanks Yipster, that was very helpful. I'll post a pic or two of the Vette later.

 

Vette pic for Yipster

 

thanks for that timeless looking f/vet pic claydog, here's one i did in '85

 

Did you ever finish it? I see the beginings of someting cool. Judging by the wheels and lower facia it looks Vdub based, a Karman Geia maybe?

 

Boy is that the truth, been there, done that, didn't want the Tshirt.