Converting a Sailboat to a powerboat

Straighten up the curved sides of the wedge and extend the planing surface either side of the outboard, but by this point its just about easier to build a plywood hull designed for the purpose. Im currently rebuilding an old 1980s NZ javelin 14ft sailing skiff into an outboard powered cruiser, these boats are designed to do 25kts under sail so the hull form is perfect for a 10-15kt cruise speed although hull volume is quite low. Honestly its barely worth the work, i should have just started from scratch!

 

I don't disagree what you are suggested wouldn't work, but it is a lot more work. The wedges I modeled could be glued, glassed and faired in a weekend. Adding an extension is a different game.

In your case I might agree.

 

I know that this boat is not really related to what fpj is looking for, but I saw it mentioned on the Tiny Liveaboards (under 33') group on Facebook, and thought that she is rather neat.
She was converted from an O'Day 20 sailing yacht, and she just has the one outrigger /ama.
Apparently her owner has been living on board her for 3 years in Key West.

 

I think the canvas/vinyl enclosure provides a cheap solution as well as easy ventilation. On the flip side I don't think that it looks as nice, nor protects from the spray as well.

 

This is an older post in a thread referenced above. I'm just getting a chance to read through it all. Doesn't seem out of the world of possibilities.

 

I tried to go back and find this when this thread was first started, but didn’t have any luck.

Today I happened to come across it again, so here it is.

Login • Instagram https://www.instagram.com/reel/CSddBxdDRh1/?utm_medium=copy_link

 

The term trawler used to suggest a round bilge, pressure recovery style hull, similar to a sailboat. But many of recreational trawlers tend to be hard chined hulls with enough power to plane. The Mainship is one of these as the picture suggests
is one of these "hard chined below- trawler looking above" boats. The Ranger series of tugs are the same.

Bayliner had issue with a high bow up angle due to their round bilge stern cross section and prop pockets which took away some stern lift. A company, I think out of Seattle, did up a mold and offered a foam filled set of "chine extensions" which could be fibreglassed in to harden the chine, and widen the chine to over come the prop pocket bow up tendancy. I could not find a picture of this though several years ago, there was a You tube link. I found a link but no pictures as yet. North Harbor Marine, Anacortes

If you want this boat to plane, you basically need to flatten out the rear stern ( target a deadrise of say 16 degrees) and add a hard chine. Pick the widest point of the hull near the water line and try to keep the chines parallel to the back.

 

I agree that in order to plane the hull will need more lift aft. I might disagree that that means it needs to be done in the manner you describe. There are many round bilge boats that plane fine. Albury brothers and Giesler are two examples. Also, I don't think it needs to be widest at the transom. There is lots of study about planning boats being faster with reduced transoms. I think 16 degrees of deadrise would be more than necessary and make the boat slower. But if you were mentioning that because that would be the easiest to produce, then that makes sense. Lastly, I think there is a possibility that the hull could plane without any additional planning surface, but rather by adding interceptors.



I think the question would be whether I would want the hull to plane, run well, or run at peak efficiency.

For now, I have put this project on the back burner. I bought a smaller hull that already planes. Maybe I will revisit this in the future.

 

People purchasing boats don't care about fuel efficiency. I wish that wasn't the case but it is. The average boat owner uses their boat for 80 hours a year. The amount of fuel that a boat will burn in a year equates to less than 1% of the purchase price of the boat.

When talking about converting a sailboat to a powerboat I'm not talking mass production. If the boat runs 20mph with a 20hp it's a success. If I spent more time and money maybe I could get 22mph, but that's not that big of a deal to me.

"Transom chine beam to be 66% or preferably less than the maximum chine beam." - David Martin's Naval Architects Notebook Vol 2.
Not a study, but an anecdote. I didn't feel like spending a lot of time looking. Scaling off the PT Elco 103 below I get transom chine beam at 68% of max chine beam. Series 62 hull by Clement and Blount also below scales to roughly 65%
Most of the modern sportfishing boats (my industry) have 90% or less for transom beam. I don't know why. Call Donald Blount and Associates and I'm sure they can reference some papers.

If I had to guess I would say there are a few reasons we don't see the reduced transom beams I'm referring to.

• On very fast boats it's not helpful. Any go fast has LCG very far aft and will be fastest with max planning surface all the way aft.
• On boats of a reasonable speed, the efficiency gain isn't worth the loss in static stability, reserve buoyancy, or cockpit space.

More deadrise is more comfortable I agree. But there are other factors. Speed, weight, beam and others. Fast, short, fat light boats needing the most deadrise, and slow long narrow heavy boats needing the least.

There are lots of flats boats, lobster boats, and skiffs with little to no deadrise aft.

You are right hard chine is better than round for planning, but round still works. And if the round were easier to produce I think it would be acceptable performance, I guess that is what I was trying to say.

Originally I did say maximize.

 

I still don't see the point of converting a sailboat when real planing hulls are available cheaply. A planing hull intended to survive 16 knots and more. Without knowing WHY you reject this possibility, it's hard to give good advice.

As an example, you could get something like this, toss out the obscenely oversized motor, and have something that would probably be fine on 50 hp, and maybe less.
Sportcraft - boats - by owner - marine sale https://boston.craigslist.org/gbs/boa/d/wrentham-sportcraft/7497781472.html
(I realize this listing will go away after a while, but it's a temporary example.)

In the time it might take you to modify and debug a sailboat, you could probably build one of these:
Workskiff – 18′-0″ x 5′-3″ – H.H. Payson & Company https://www.instantboats.com/product/workskiff-18-0-x-5-3/
or the hull for one of these:
Sneakeasy – 26′-6″ x 4′-3″ – H.H. Payson & Company https://www.instantboats.com/product/sneakeasy-26-6-x-4-3/
Obviously, you'd need something simpler for the deck. I've been on a Sneakeasy, which went surprisingly well with 4 adults on board and a little 10 hp 4 stroke motor. For either of these, if you wanted shelter, you could probably just set up some kind of dodger.


BTW, I HAVE encountered a sailboat that would plane very well. It's the only boat I've sailed in which a passenger asked me to slow down. It would probably be fast enough on 5 hp. It might be a little small for you, and it has so little flotation that, if it swamped, an outboard would sink it. ;-p
O'Day Sprite Sail Data https://www.sailrite.com/Oday-Sprite-Sail-Data

 

In one word, aesthetics. Below is the boat you are suggesting I buy.

I don't care for the looks of that boat personally. IMHO it looks trashy and no amount of work would make it look classy.

I strongly disagree with this statement. My plan was to spend minimal hours modifying a sailboat. Building a boat from scratch is not comparable hours IMO, unless the shape of the hull is changed to a flat sided, flat bottomed boat like you are suggesting. And that is not the aesthetic or functionality I want.

I love the Sneakeasy. I love many of Bolger's designs. In this particular case, I don't think the Sneakeasy is a direct replacement.

Below are some more pictures of converted sailboats. I think they look like small downeast boats. That is the look I am looking for. Sailboat hulls are cheap. Free in some cases. The smallest DEs I have found (14 Holland, Rosborough 18, Eastern 18) are highly desirable and correspondingly expensive.

 

You can spend minimum hours modifying a sailboat, but do you know if it will work the first time?

I don't think that boat looks "trashy", it just looks like it's from another era. In fiberglass construction, that fake lapstrake is not just an affectation. It makes the hull stiffer.

Part of the reason your examples look so nice is the shape of their sterns. When you modify for planing, you get rid of that. If you really want a planing hull, cut the boat at the widest point and keep that cross section all the way back. If it looks crude, you can play games with the shear line towards the stern, which is something I've seen on pictures of real lobster boats.

I was looking at some more boat ads, and noticed that the ones that look like little lobster boats cost several times as much as the others. I guess you probably noticed this, too. But none of the modestly sized outboard powered boats had those cute little cabins.

I notice that some old Herters boats look kind of graceful, but I don't know if any were big enough for your purposes. However, some of them will Hert your eyes, so be careful. Plus at least one of their square stern canoes, as I mentioned before, is terrifying with a small outboard. My taste isn't the same as yours, but I think some older aluminum boats look pretty nice as well.

 

No. Can I run it more than once? Read above; Phil has ran a sailboat without any modifications and it ran fine. Read his suggestions.

Looks are just my opinion.

The stern doesn't have to change for planning. Straight buttocks. That's what matters. Plan view shape has zero affect. Surf boards have two pointy ends and only work when on plane. Nearly all planning boats have fat transoms, but it's not because narrow transoms can't plane.

Did you see the boat I'm running now? The green one above? 16'2" x 52" it's basically a square stern canoe.

 

It's a completely different, significantly wider boat than the Herter's canoe I was talking about.