Powerboat to Sail

Any sail area would be reasonable, but enough to move you, well, that's a good guess. What is the LWL beam and length? Multiply these two numbers and the result again by 1.2. This will get you a conservative ball park sail area in square feet. So, if your LWL is 16' and your beam 6' 6" you'll get ~125 sq. ft. for your sailboat.

It doesn't matter about your hull shape, you have what you have, lets work with it. Sure, a nice rounded hull would be helpful, but you don't have one so this is a moot issue.

Two leeboards would be better, though conversely, you're not going to heel much with a conservative sail plan and a heavy, flat bottom boat, so one board will likely do just fine.

Now the question of sail plan type and there are many options here, so pick one. I'd stray towards gaff, sprit rig or lug. These have low CE's and will offer the best bang for the buck on the courses you'll sail. Once you decide where to place the stick, figure out the CE (simple geometry) and place the leeboard so it's leading edge is on the vertical line of the CE.

 

While one leeboard would probably provide the lateral resistance, 2 would be easier structurally.
When acting as a leeboard the hull presses onto the board but while acting as a windward board it is being pulled away from the hull and will need a strong stiff connection.

 

Make neither a lee nor a wind board. Just make a hefty board that can be hooked over the rail, maybe so you can side it fore and aft a bit too. Make it a pure tall rectangle and just round over the edges, no foil shapes. This will offer the most adjustment and it can be used on either tack. A hole and pin arrangement would secure it just fine for next to nothing too. Don't over think this.

 

Saw a guy put a mast thwart on a RIB with a sprit sail, steered with the outboard motor, no leeboard. Would almost reach across the wind so maybe 140-160 degrees of useful off the wind sailing.

 

The most irritating thing about this project is going to be people taking the michael on the water, but hell, if you just want to be able to drift along quietly with the wind more or less behind you then why not... Plenty of working craft over the centuries have had what we'd regard as very ineffficient hulls because that was all they needed to do.

I don't know I'd bother with leeboards and things. Lot of space to take up when not in use and a lot of hassle, and if you find yourself wanting to be able to sail with the wind on the side as well as roughly behind you'll probably want a different platform.

Sail wise keep it low and keep it small. The size of sails makes less difference than most people think, especially racers, and you really don't want to get blown over. Don't be tempted by a secondhand dinghy rig, even though its less hassle. These are designed to be very good in directions you can't go, and pay a price for this by not being as good in the directions you can. Size - Pars formula, but I think I'd use an even lower multiple than Pars 1.2 depending n how windy it is where you live. As long as you have enough speed to steer...

Rig wise, a sprit sail is probably good, most of the traditional type things are good. They were all used by people who wanted to pull a heavy load about and didn't much need to sail against the wind. In many ways a square sail would be ideal, but are a lot more trouble to rig and handle, and will almost certainly attract more mockery from the waterside know it alls which doesn't add to the pleasure of drifting along.

 

Most small power boat captains rely on speed to dodge weather...most smaller sailboats rely on an ability to absorb a certain amount of weather and a captain who can handle some weather if they are caught out...you could almost say when it all goes bad..the power boater is praying that the motor don't quit...and the sailor is praying that the radio don't quit cause his rig(motor) is busted all to hell(broken mast etc.) and he forgot to change the plugs on the kicker .At least in your power/sail boat idea if your motor quits you still have a mast/sail to get you home and hopefully a radioif it all blows away which allows you to stay out a bit longer instead of running for cover.This has it's pros and cons...it is a more interesting factor set than just beating it home...some risk involved which alot of us like. Depends on how you use your boat ultimately..I've seen 14 foot skiffs out in the gulf 4 miles out with two guys and a dog in the thing...but they were probably back inside the bay before 2 pm and the t-storm wind and wave time...they knew the limits of the boat.

 

Sprit sail or standing lug.

 

I always wondered how a planing design would perform with an added section to try and ease the transition aft. Perhaps simply a form that raises the transom above the waterline while narrowing to about half the width of the original transom. Then you could unbolt it to use it as a motor boat. You could leave the horse and rudder attached even. Leeboards are a good idea because they can be made to be removable and pivot too. The toughest thing will be mounting the mast. You will have to cut a hole in the deck and reinforce it and the bottom for the step and partner. I would go with a low aspect sail such as a Sprit or one of the Lugs with a lower peaked yard.