2.5hp Catamaran - power delivery issue

Kerosene nailed it. Do what he has suggested and the boat will have more of the essential elements that allow it to go faster.

 

In a stiff headwind, it will be going half a knot backwards, if you are lucky. These ideas are hardly practical, and can become dangerous. The last thing you want, is an under-powered planing craft, being flogged along at continuous peak revs. You don't get a medal for "going the fastest, with the smallest engine", any more than there is a prize for seeing who can whisper the loudest. It's kid's stuff ! Oh, and one more thing, a sailboard is not designed to travel up-wind, just one more snag to contend with, the thing would likely become awash in a steep chop, head-on.

 

Novice Joe,

PM me when you're ready (learned).

"Good grief Charlie Brown!"

 

Myself thinks that the #28 tertiary hull might possibly introduce more drag, turbulence, debris collection, and control issues than the benefit it produces. There may be other ways to do the #28 pedestal function that introduce less drag. Maybe it's better better to extend the tertiary concept to complete coverage as kerosene posted in # 29, although it will it will be greater drag than the original uncut sailboard had. The tiny free board available might require almost perfect calm water and wind conditions for safe operation, as pointed out by others.

 

You might like to check this out about page 3.
Catamaran tender to replace rib? https://www.boatdesign.net/threads/catamaran-tender-to-replace-rib.50834/page-3
There was a video link later but it's no longer working.
Jeff

 

That seems to me a pretty bad shape to start with for planing. You've mentioned a sharp corner at the bottom-transom transition allowing the water to break away from the boat with the least effort, I think the same applies to the chines and inside edges also. Being rounded like that would seem to create drag or suction as water is dragged along and can't get away easily. If there's rocker of the bottoms, that wouldn't help. The tapering inward of the sides, at the back, looks to be anti-plane. The rough inside edges of the cut are no help and the interacting waves between the hulls can't help.

If I was trying this 2.5 hp planing catamaran thing, I would try lightweight constructed catamarans in the shape of vintage water skis. Flat bottoms, very smooth with no rocker and no side/width taper and clean breaking edges on all 4 sides and both transoms. And no fins. You might try the floats farther apart to cut down on wave interference between the hulls. Sometimes in small boats with low power shifting weight forward gets it up on a plane and then moving back for running.

 

This concept gets close, saying it will plane a 165# person 15mph with a 3 hp motor. Not sure if 3 hp was a different beast back then. Not sure if you can find a 165# person now.
HydroPlanes minimax http://www.svensons.com/boat/?p=HydroPlanes/minimax

 

I built a minimax a while back, powered by an old 1980s 4hp Nissan. It clocked just over 18mph on the GPS averaged two ways. I probably weighed about 190lbs at the time.

With an additional 150lb passenger it wouldn't get up on a plane and top speed was just 5mph or so.