Converting a Sailboat to a powerboat

I was hoping to use around 25hp.

I think adding wedges to the bottom would be easier than cutting the bottom off and building from scratch.

 

I don't know that adding wedges would be sufficient, that's the problem. They might have to be large enough to offset those rising buttocks that they add so much drag, you have a whole new problem. Unless they're so big that they're basically a new running surface, in which case...cut and replace. But that's very difficult.

25hp to move a 17' boat at that speed isn't an issue if you don't start with one of those hulls, though. I am just wrapping up the build of a 17 foot design that runs those speeds easily. So it can be done, I just think it's very hard if you start with one of those hulls.

 

Would it be possible to do something similar to this?

 

1" foam wedges that are 8ft long and go from nothing to say 6" aft. After being glued to the hull and each other a single layer of glass might be sufficient to create a smooth surface. It doesn't need to be structural since it is in addition to the hull.

Yes and no. I don't think it needs to get wide aft. And your starting hull looks to have pretty flat buttocks on centerline, which on a displacement hull I don't think will be the case.

 

Intuitively I don't think wedges as you describe would be sufficient but if you have numbers on how that would work I'd be interested to see the math.

 

That is why I was telling you that having the body lines plan, or at least an idea of the shapes (sheer line, main transversal section and longitudinal profile), would be important to give a viable solution.

 

I don't have the math, but here is the rough idea

 

I would give them to you if I had them, but unless I go out and buy a boat and a 3d scanner I can't.

 

Okay then, you decide the procedure that best suits you.

 

Well, if you alter the running surface of a displacement boat enough to turn it into a planing boat, it should plane, although obviously you'll give up displacement performance, and of course affect the trim at rest unless you add enough weight to offset the change in flotation - the outboard will help but is it enough? Probably not. The question then becomes "how much power will it take to plane after those changes?"

Hard to say. Lot of factors. Need a fair amount of numbers but if the running surface alteration doesn't radically increase the weight, 25hp...maybe. Definitely doing it the hard way, though. Lot of headaches to end up with a kludge.

 

I am hoping that the above would be an exaggerated case. There are (32) 2" strips. The largest one being 9" x 8ft. Bouncy calculates out to 490#s minus the weight of the materials. 150# for the outboard so it's still 300# off. If I were to scale it back to 4" x 4ft. I think it would be 25% of the volume or 120# about the weight of the outboard. The Buttock might have a little bit of an S shape which wouldn't be best for proposing etc, but I don't think it would be much of an issue at 16 knots. But I'm guessing. There are reports of people planning out these boats while sailing so I don't think it will take a lot.

Below is an interceptor option. One benefit being it doesn't add much bouncy. Not sure how effective it would be.

 

The interceptors make me think of driving your car with the parking brake on.
The high volume foam wedges may yield a planing boat but it will float bow down in addition to other unanticipated evils.
The glass is half empty.

 

The problem with outboards is they are designed to run with the leg largely out of the way of oncoming water, in a planing hull the deep transom or pod fulfills that role, with a displacement hull the transom is almost out of the water, putting the prop too high, where it is prone to ventilating in certain situations. You can get extended leg motors, with streamlined legs, but not easy to find in sizes away from the lower hp end of the range.

 

Is that your own brain-child ? Quite novel, if so. The problem I think is that the lack of added buoyancy is more of a handicap than a help. You don't get much dynamic lift at say 8-10 knots, the boat will still squat like crazy, and have quite a bit of drag as well from that. Don't forget that under sail power, the line of propulsion is quite high, and that tends to counter squat.

 

It might be worth testing that idea with a model.