Converting a Sailboat to a powerboat

I'm interested in converting a 17-21 centerboard dinghy to a powerboat.

Ideally, I would like to make 16 knots, s/l ratio ~ 3.5-4 so it would be mostly planning, but most of the boats in this size range are displacement or semi-displacement.

So my hypothetical question is... What modifications would you make to the hull would you make to maximize performance and minimize cost and complexity. For the purpose of this thread assume all other boats have been burned to ashes and you will be shot for suggesting starting from scratch. (I know it is a terrible idea, but I don't need you to tell me)

I'm thinking trim tabs, interceptors, or wedges to flatten the affective buttock angle, but I want to hear your ideas.

 

Do you have a body lines plan that you can show us? It would be important to be able to propose valid ideas / solutions.

 

Tragically, no.

I did a little research for candidates on sailboatdata.com list included;
Scandia Boatyard Ski-Breeze 18.3
Buccaneer 18
American Fiberglass Corp. Discoverer 18
Sailstar Boats Explorer 17
O'day Daysailer
Gulf Coast Sailboats Gulf Coast 18
Newport 17
Macgregor Venture 17
O'day Jollyboat
Cape Cod Knockabout 18
Essex One-Design 18

If anyone has line drawings for any of those I would be interested.

 

Most sail boats don't like to plane. And it takes lots of extra hp. If you want a planning boat, start with the best planning hull you can afford.

 

Similar concept. super-size 15'4" Grumman SportBoat into 23'6" SportsUtilityVessel? https://www.boatdesign.net/threads/super-size-154-grumman-sportboat-into-236-sportsutilityvessel.66042/

Maybe find a beat up MacGregor 26 designed for big outboard then cut it down.

 

No doubt if you trawl through enough of the "prospects" you have listed, you will find one that is "least bad". Wider and flatter at the stern would probably help, but one aspect you have to clarify, is what is the motive power to be supplied by, inboard or outboard ?

 

I have converted a 14 ft O'Day Javelin sailboat to a planing inboard motorboat. I wold have preferred the 17 ft O'Day Davsailer, and both of them are planing sailboats. With a 14 x 17 three bladed prop driven by a 13 horsepower inboard IC engine I got 15 MPH, and the boat behaved well, in spite of several "nay-sayers" who thought the conversion would not work.

While far less than ideal, I am satisfied with the conversion, which required about two weeks spare time to cut out the centerboard well, and install a 1 inch diameter propeller shaft, plus two aluminum bilge keels for stability, mated to wood engine stringers running the length of most of the hull. A rudder clamps onto the transom, much smaller than the original. I see no reason why an outboard would not work well here also.

 

Why do you say wide and flat? I thought narrow would be better and flatness seems less important than buttock angle. No?

I thought an outboard would be easier

 

You got shot. Sorry

How did you learn that the O'day daysailers were planing sailboats? 17 O'day Daysailer was on my list, and it's one of the most common ones.

I think an outboard would be an easier conversion. Did you have to add trim tabs, wedges, or interceptors?

 

Narrow stern will kill any chance of speed, I would say, it will squat horribly well short of your target speed, but these boats will all be similar shape unless they were designed to plane in the sail mode. That sort of boat would be the best prospect I think.

 

Buttock angle causes squat. Round bilges on sailboats in combination with narrow sterns would cause a high buttock angle aft. I guess I make the distinction because that is not the case in powerboats. If there is a hard chine narrow stern boats run great. Case in point, Bartender boats. It almost looks like they have a spray rail looking thing aft on some models.

 

You are getting the double whammy, a narrowing stern and rising buttocks, with non-planing sailboats, those bartenders actually might have been a better low speed conveyance than a typical transom-ended planing boat. As for the outboard power, there is the problem of not being able to run the prop lower without a lot of drag on the leg, and also interference from any keel in front of it, that all needs to be allowed for.

 

What is the benefit of running the outboard leg lower?

I was thinking of a centerboard boat so there is no keel to deal with. It also makes it lighter and easier to trailer. Either remover the centerboard entirely or leave it in and only deploy it when at anchor or drift.

 

I don't think I saw a power requirement mentioned...what's the max HP? Or planned engine, if there is one?

I haven't been through all the proposed boats but just looking at three or four, I see a lot of hulls that will squat like crazy and dig deep holes if you try to get them to that speed. The rising buttocks especially seems very, very difficult to address. The modification I'd suggest would be to cut the latter half of the running surface off, and replace it with a running surface designed to plane, whether monohedral or "warped" plane.

 

LOL!! For not using a screw driver for a chisel. ;-)