low wake design

Long slim hulls, very little power, very little wake. Can be inflatable.
Bay Area entrepreneur to roll out floating bikes on the San Francisco Bay https://www.mercurynews.com/2019/04/27/mill-valley-entrepreneur-to-roll-out-floating-bikes-on-the-bay/

 

Aerodynamically, a canard will work for flea powered watercrafts. Just a glimpse but at 1.00, you can see the small wing up front. At the back is a large high aspect ratio wing. This works like a two step hull.

 

Here is a better view of the "wing" arrangement.

 

Phil Bolger had many flat bottomed square hull boats slightly rockered that were well know for their low wakes

 

Coaching boats/launches used by coaches and referees to accompany competitive rowers require low wake. Catamarans with narrow hulls appear to be the current design choice. Is that the intended use of this project?
Westport Jolly Boat http://www.westportrowing.com/Coaching%20Boats.html
https://www.rowingsolutions.com/

 

Those are a good sanity check. The catamarans require twice the power that the OP wants, and they weigh twice as much. Notice that the little monohull is under half the weight and requires 1/3 less power at the same capacity and same performance.

 

1. OP says stitch and glue is okay
2. OP says pieces must be car toppable
3. OP is considering that motor
4. OP has given a max speed but no indication of cruising speed

 

Good advice all, thanks. Cruising speed will typically be about 10 mph but it still needs to go 15 mph top speed and I don't care how much fuel I burn at that speed. Needs to be really stable side to side so I can stand up, lean over, etc.- hence my desire for a trimaran or catamaran.

Kind of leaning towards the rowing shell idea with sponsons on the ends of the outriggers. The really small outboards only weight about 30 lb so that should be good enough for 15 mph.

Would have to think about how stable they are to stand up in though.

 

A single rowing scull is 27 feet, but you could cut off 5' to make your 22' bogie
and have a flat transom to mount your tiny motor on.
Variable width sponsons, amas, pontoons, floats, outriggers, whatever-you-want-to-call-them could be hugely useful.
OR
Two surf-skis (20') would make a nice catamaran with ample standing room.

Are you really wanting to stitch and glue or modify used parts?

 

Angus rowboats has plans for a build it yourself racing shell. It's the boat I had in mind. A pair make a catamaran.

 

Yup, two of those would certainly work.
Replace the cockpits with bridge deck
Cambridge Racer https://angusrowboats.com/pages/cambridge-racer
I know Colin, he lives here in town.

I can't believe these words are coming out of my mouth but have you considered electric?
A 2.5 HP would move this at 15 mph.

 

If your first question can be within the following assumptions :
- slender monohull Lw/Bw >7
- Froude V/(g Lw)^0,5 in the range 0,4 - 0,9 (in metric units)
, then (as already mentioned by members in this thread) the residuary (wave) drag can be considered mostly in function of the lenght displacement ratio Lw/D^(1/3) , (Lw in m, D in m3) , a global result
based on various model series tests and back by good authors.

>>> I did myself this investigation from the model series tests available and average/smooth them in the figure 1B page 17 of the attached document, giving the residuary drag Dr in % of the displacement Mg (Mg in weight = D in volume), in function of Froude and Lw/D^(1/3)

The other two components of the total drag (on calm sea) to consider in your project is the friction drag (you need to know the wetted surface at displacement D) and the aerodynamical drag (you need an estimation of the aerial crossed sections)

With the total drag x the design speed = the net power

Net power / propulsion system efficiency = the engine power

 

used small Hobie cats are a dime a dozen, but I hear they aren't suited to easy dismounting and car topping. Don't know the weight of smaller Hobie cat hull. Might be doable to butcher up a used free/cheap (because its lacking decent sails, rigging and trailer) Hobie and create an easy to dismount and car-top power-cat. Smaller Hobie cats have asymmetric hulls to aid sailing on beam wind, which isn't ideal for power-cat but not a deal breaker.

 

I think the OP needs to quantify "very little wake". He should post the maximum wake height at a distance from the stern.

 

Correct, however it was their first priority and minimal would mean as near to zero as possible.
It's going to be an inherent trait of this vessel anyway as there is no extra power for wake at this speed.