Ultra-light Catamaran design questions...

I'm an active Kayaker, but want something that will afford the greater mobility of a motor boat with out the hassles of buying another trailer able boat.

The design I have in mind is a catamaran with twin motors, or a tri-maran with a single engine. The inspiration comes from the efficiency and low weight of a fiberglass 14' Sit on top kayak. I am also considering a modified hydrofoil design. I have started making my 1/3 scale model catamaran for testing.

The key factors are for the final prototype are:
1) Low weight (under 100lbs) for the boat w/out motor
2) Simplicity of power train
3) Off the shelf parts

I have seen boat manufactures that feature Honda air-cooled motors to power their jet boats.

The questions that I have are:

What power train options are out there that weight around 50 lbs or less?
Where can I locate an efficient jet drive for a small hp displacement motor?
Where would the intakes be located, how large would they have to be?
How can I calculate the amount of thrust needed to: reach plane speeds, determine plane speed, and calculate a top speed?


Thanks, Dave

 

I built a small, low power boat once.

I once built an inboard canoe that didn't weigh over 150 pounds, engine and all. It was really low priced too. Used a 2hp Briggs and Stratton edger engine, a couple feet from Echo string trimmers, a couple pullies and a prop I got off an old 9hp engine that I had laying around. Couldn't turn it over or the gas and oil would leak out though. It was a flat transomed 10ft canoe. One person could carry it, but it was not easy.

I put the prop shaft through the transom and the very tips of the prop were sticking out of the water until I got in it. Used a rudder with a steering wheel to turn it.

Top speed was about between 7 and 10 mph. It coulda used a couple outriggers for stability. Nearly tipped it a few times when the asses in the huge boats went by at 50+mph and only 100ft away. Other than that it was a fun boat.

Still have it sitting in the back yard. It fell off it's saw horses and broke the rudder and pushed the shaft against the transom and I haven't felt like rebuilding it. Besides, I got a 40 year old jon boat and 15hp outboard. Don't need it anymore.

 

Dave, we could do with a little more info...
how big do you want the boat to be?
what kind of payload do you want to be able to carry?
how fast do you want to go?
how much $ do you want to spend?

All that side, I'd flick the water jet (unless you have a particular need for a propless configuration) An outboard will probably be cheaper, lighter, more reliable, faster, easier to install...I could go on.....

A tri with ama's for stability would allow the longest waterline length and hence fastest displacement speed...

 

I'm still researching. So far I have come to realize my hull weight goal is very lofty (impossible?). I have also come to realize that I'm looking for an absolute minimum of 20-horsepower. Although I believe that it possible to adapt a small air cooled 4 stroke for the purpose (Honda V-twin) http://www.honda-engines.com/gx670.htm (as it has already been done - http://www.mokai.com/products.htm ), the fabrication and engineering are more than I care to entertain at this time. I have decided to use an existing engine and drive combination at this time.

I'm looking for a small (12'-14') boat that would serve as a fun run-about and as a shallow water fishing platform for one or occasionally two. The layout will be somewhat spartan, similar to a flat's boat.

I like the idea of the lifting qualities of catamarans. The type of material I have decided to use is Kevlar with fiberglass reinforcement. I have found a manufacturer that makes a model similar to what I have in mind. It is 12' long and made of fiberglass; the hull weighs 300#'s. From my research Kevlar should cut about 1/3 the weight off. I have also found vacuum bagging equipment that I think (?) will work, but I need to look into it further.

Here are some examples of boats that I have found after I began my project:
http://www.wranglerboats.com/th12.htm
http://www.islandesign.com/marinedesign/cat_12j.htm

I plan on spending around 400.00 on software, and another 1000.00 to 3000.00 on materials, not including power. I have decided to go with the Catamaran over the Trimaran or V-hull designs for several reasons:
1) Wealth of information available on design
2) Specialized design software available
3) Storage and deck lay-out possibilities
4) Performance possibilities
5) Ease of experimenting with hydrofoils (spanning the pontoons)
6) Less design complexity than the trimaran.
7) Shallow water operation benefits of jet drive
8) I want to get the maximum amount of speed for a minimum amount of HP. I plan on compensating for shorter pontoon lengths with lightweight.
9) Aesthetic appeal of a catamaran

Having said all that my preliminary concept design has a modified center pontoon to serve as jet drive platform.

Recent questions that have arisen are:
1) Possible Kevlar composite hybrid cloth options; Specifically is there something better than what I have in mind but will still keep me in budget (1-2K)?
2) Would it be better to buy a PWC engine and jet drive and adapt it to my purpose or by purpose built drives (like those offered by Merc.)?
3) What is an easy to use/cost effective CAD program for a first time user, that supports plotting for my initial mold mock-up templates? I plan on using separate software to determine hull performance, so I am looking for something that will allow me to create the blue prints, and patterns.
4) How much of a difference does vacuum bagging make????

If your thinking that this is a large investment for a project, on the surface I'd agree. After researching for a new boat that met my criterion I could not find anything. In fact a low end fishing boat that I had considered is 7000.00. Jet boats that are currently available are much more expensive, in the realm of $20k and up. This gives me a fairly large margin with which to work, and to make exactly what I want for my needs. Besides my wife is looking forward to us starting another project together. (She is an exceptional woman).

 

I'll take a look at your other comments when I have more time, but as a start - you're right "she" is an exceptional woman!!