# Pontoon Boat Conversion to Sailboat

Discussion in 'Multihulls' started by lowe210, Nov 15, 2008.

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### sharpii2Senior Member

Pontoonmaran

I just got done doing some meatball math.

I calculate a 20 ft pontoon boat should have a righting moment of around 9,500 ft lbs max.

I came up with this number by the distance between centerlines of the pontoons multiplied by the max weight of the craft.

16 ft * 3.14 sf * 62.4 lbs/cf * 6 ft Beam / 2 = 9405 ft/lbs

The 6 ft Beam came from the pontoon boat being 8 ft wide, to be trailer legal in all states, minus the two foot width of a pontoon, which equals half the width of two pontoons.

If its loaded down any more, the lee pontoon will go under. If it's loaded down less, the windward pontoon will come out of the water.

Using stays to hold the mast up, I calculate the mast will have about one ton of compression at the step.

The two windward stays will have around 1500 lbs of tension each.

Its fascinating to see how fast these numbers build up.

Just for the fun of it, I sketched a pontoon boat rigged with a single gaff main. Getting a jib to stand on anything this loose could be more than a challenge, so I did away with it altogether.

Part of the fun of all this is to see if I could come up with a real sailboat that could, by definition, make decent windward progress and be able to shorten sail, if need be.

For lateral resistance, I drew a centerboard case placed under the deck, next to the motor well. The case is lengthened provide a place to hang a rudder on. The whole business would bolt onto the deck from underneath and hopefully re enforce it there under the mast. Up front it would be kept upright with a pair of large gussets. Aft it would be supported with a pair of guy straps.

The odd looking rudder has a disk like rudder extension which is supposed to 'roll over' any logs or stumps it might meet. The pintles would be arranged so the whole thing could drop about 6 inches deeper.

My guess is that it could do maybe 6 to 10 knots reaching or running and maybe half that, made good, going up wind.

The range of conditions it could sail in would be quite narrow. About 12 to 24 knots of wind. More is too much. Less is simply not worth the trouble.

A 6 to 10 hp outboard would be used when the wind is light.

When the wind is too strong, it stays tied to its slip.

See attachments below.

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2. Joined: Aug 2011
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### silverbearNew Member

I just joined this forum due to this thread. I am no sailor, but have lived on lakes in northern Minnesota for the past sixty some years. Next summer I intend to rebuild a 20' pontoon boat into a kind of cabin version of a houseboat... able to go for week long cruises for a week or so at a time with my dog Aaniimoosh (Ojibwa for 'dog')... Huck Finn days on the lake, fishing, swimming and lolling about. Since I live on social security I want to use little gasoline for getting around. The cabin will be small, 8' wide X 8' to 10' long depending on the length of the boom and steering setup... will know about that next summer after rebuilding the deck. Cabin will be at the rear of the deck with access to the motor from a hatch at the rear of the cabin. The heavy 40 hsp motor now on the boat will be replaced with an old 10 hsp Evinrude from the 1960's which will be modified by removing the two stroke engine and in it's place will be a flat plate with a vertical shaft 4 stroke 5.5 hsp motor bolted to the flat plate and it's crankshaft joined to the original lower unit by means of a coupling. Exhaust will go out the lower unit under water as the original engine did. The motor comes up on sale now and then at Harbor Freight for around a hundred bucks. Steering and gear selection will be normal. No electric start, just pull start with a kill switch at the controls forward of the cabin. At the bow will be the mast and sail from a Grumman canoe sail rig. This will be used mostly for downwind sailing to save fuel. Slow is fine as I will already be where I want to be, on the lake, and going to another anchorage for some different fishing can take all day for all I care.
Some years ago I had an 18' pontoon boat and rigged up this same sail, again using it downwind. I just used the motor for steering, which was not very effective. No, it was never fast, but what great fun that was running silently with wind power. I recall a day when several bald eagles were riding the thermals directly above me... my sail was full and it seemed like I, too, had a "wing" and was flying with the eagles. Beautiful memory. Different boats have different purposes. The one I will be building will suit my needs and budget, allowing me to spend much of my summer on the water and at least part of the time moving along by wind power. Perfect.
From reading this thread I'm already getting some ideas on how to improve the sailing aspect of this boat, allowing me to use the sail more often and more effectively. All I know about sailing is that it is fun.
Why couldn't I add more rudder area onto the outboard motor and steer that way? Should be easy enough to make up an aluminum plate affixed to the lower unit. I would not be using this rig in rough water and in a big wind would use the motor to get behind an island or into a protected bay to wait it out.
Many thanks for past comments and for sharing your knowledge with an old newbie.
Silverbear

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### sandy daughertySenior Member

This is an intriguing challenge. Here's my WAS (wild-assed suggestion. Since your cabin structure will have a lot of windage, I suggest you attach a rather large rudder surface to your outboard. Perhaps a 1.5 square foot flat plate. At the bow you will need a centerboard or daggerboard of the same size that can be raise or lowered from the helm. Consider letting it drop down on a hinge pin to keep you vessel amphibious, at least the bow, for those morning visits with Dog (english for Aaniimoosh) This will allow you to use the sail in a larger range of wind directions. But don't go for much more sail area; there would be stability issues.
I suggest the forward board go near the bow, Like a Chinese Junk because your trim under sail will vary wildly because of the cabin. being able to vary how much lateral resistance it produces will help a lot.

Unless you have access to a fairly sophisticated machine shop, I would advise against your engine plans. The cooling issues alone would require some very expensive trial and error. Far better alternatives exist.

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### WavewackerSenior Member

Yes, I think a junk rig would be more appropriate.

We make more pontoon boats within a hundred mile radius of me than any place on earth! Bass Pro, Lund, and a bunch of others that slap different brands of a plate. Few such boats weigh anything near 9,000 pounds, if they do they are high dollar I/Os. All round pontoons are mounted with a flange at the decking that is not designed to hold the boat at an angle beyond about 20 degrees. I have seen pontoons snap while sitting on the hard sideways at a steep ramp. With a 2' tubes you have 4 to 4 1/2' distance between the tubes and the decks are usually wider extending beyound the tube and all of that is within 8 or 8 1/2 feet. Some are fitted with planing hull plates on the bottoms, not just round tubes and I doubt you can take them off without messing up the pontoon, it would be alot of work and welding to remove them. And some have skeggs as well.

If you must use aluminum tubes, take them off the pontoon and build a proper frame, hanging the deck lower, widen the beam and provide support for what the sail rig might require, then you'll have a cat that might get a few scratching their heads instead of laughing.

You can put a tarp up on the awning and catch enough wind to move a pontoon boat, they are really bad anyway with air draft with the deck fence. You can't take the fence off a pontoon boat as it is a required safety feature, unless you replace it with safety lines, that is if there is any kind of motor on it. And, you can't take the seats off unless there is a cockpit, or with hand holds somewhere for passengers, again, if there is any motor on it.

I'd agree with others, pull a sailboat and sail when you want to sail and leave the barge as it is.

But try it out, good luck with it, I'm no expert.

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### sandy daughertySenior Member

My inpression of Silverbear's question is that He isn't going to spend more than a couple hundred dollars on this lake raft, and sailing qualities aren't even on the list of considerations. A Huck Fin raft with a canoe sail and a motor is a hundred years ahead of the technology of Mark Twain's youth, and that places us somewhere in the Fifties, I think. I believe we are talking about a floating camp that is somewhat self propelled.
I doubt Silverbear is worried about deck fences or lifelines, or even going to weather. If he wants to go somewhere he will probably wait for the wind to change.

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### WavewackerSenior Member

Yep, and that's why I said go for it, it should be fun and funny too. Infact, I'd do it too, put on a straw hat, some Big Ben's wear my army boots and get a corn cob pipe! We know how to do things up right in the Ozarks!

Tires off an old lawn tractor make great bumpers at the dock.

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### silverbearNew Member

Thank you guys for your responses. Yes, it would in fact be something like a glorified raft with some creature comforts. I think of it as a pontoon 'cabin boat'rather than houseboat. I do want for it to look respectable even if it is a little on the oddball side of the lake. I'll polish up the pontoons some to make them new looking. I did that on a sidecar I made for a light motorcycle. The sidecar was made from a former 17 ft. Grumman canoe (much modified) and I was astonished at how well it shined up.
The cabin will be sided in cedar shakes and there will be shutters inside the windows which will have stained glass panels of cat tails, wild iris, that sort of thing. I had a stained glass studio onceupona and still love the stained glass. So a little bit of elegance mixed in with funky. Heat will be from an elegant Petit Godin wood stove I used to have in an old Airstream. It is air tight, has a small footprint and can burn either wood or coal. I'd like to be able to be on the water until snow is in the air around the beginning of October.

Regarding the power for the boat. The sail is only about 50-55 square feet. I've given a lot of thought to the air cooled engine conversion and think it will work fine. I do have a friend who works in a machine shop if something special needs to be made up in a coupling. But I think something can be made up from standard parts.

Yesterday I picked up a 177cc 4 stroke Predator vertical shaft engine from Harbor Freight for \$99.00 on sale and paid another \$20.00 for an extended two year warranty beyond the two year warranty that came with it. It is intended to replace 6 hsp engines. I imagine it will put out more than that by a bit by opening up the air intake with better breathing. There are also better carbs available from the go kart racing folks which will fit on there if I want to goose the power, which I don't anticipate. So, if the engine blows up I return it with no questions asked. But I don't expect it to blow up. I don't see a cooling issue, either. The engine shrouding is designed to cool the engine from the flywheel blades and it is designed to either run a lawnmower all day or a stationary industrial type of use. Also, northern Minnesota tends to be on the cool side even in the hottest part of the summer. And I don't foresee running the engine for great lengths of time continuously. That's where the sail comes in to play. Speed is not an issue, as slow is good. Heavy winds will drive me to a secluded bay or the protected side of an island. This boat will never see big water. I'm mostly interested in the motor power to get me where I want to go and for being able to idle way down for trolling while fishing.

I will retain deck railings for the open portion of the deck and run the railing right up to the cabin.

Now, I have a question. I have yet to measure the length of the boom, but it must clear the cabin and may mean that I need to extend the front of the deck a bit. I have a good idea how to adequately support the mast and deck extension, especially with the small sail size I don't think that is going to be a problem. I am wondering about the sail filling and wanting to dig the front of the boat down in a strong wind. I'm hoping that the weight to the rear where the engine, gas tanks, batteries (for solar electric) and cabin are will be enough to counteract that digging effect, but I'm just guessing about that. Any opinions? I know this is an odd project and the budget will be low and tight, but just the same, I want to think it out well, have a good and workable plan and end up with a nice pontoon cabin boat which has some sailing ability. I'd like for when a fishing boat goes by for someone to say out loud... "that's cool" and ask some questions about it. Don't want an eyesore.
Thanks for the tips on the size of the rudder affixed to the engine and the forward mounted centerboard to improve handling under sail.
Silverbear

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interesting

9. ### CatBuilderPrevious Member

This is a great thread and great idea. Very interesting. I hope to see pictures of this project at completion.

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### marked77Junior Member

You go for it!!!

How do you think anyone ever did anything? They attempted!!! I am at this thread because I was planning on converting my pontoon to be part sailing and part power. After all, the Macgregor is no sailing guru and not really a very good powerboat. I was planning to make my beam 15 feet rather than it's 8. I also have free standing masts from my cat rigged Beachcomber so that was covered. I figured I would have a boat that sailed ok and powered fine......lotsa fun. However, I also bought last year an old fiberglass pontoon boat and I think I am going to cut the pontoons apart, add 5-10 feet of length (maybe) and using Richard Woods folding cat method.....make my beam 15-16 feet so I can have great sailpower. The hulls are somewhat v shaped and the hull is made to plane.....keeping a 70hp 2 stroke on to keep the planing powerboat and the ability to sail..Not going to win races but what the heck. Someone on this thread knows one of my boats. I have a 37' Sharpie like boat with 2 fiberglass masts. It has a Polynesian flare......canoe like. Anyways let the experiments begin!!!!

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### WavewackerSenior Member

So, where are you guys on this.....any progress?

Could you use the tubes and have a lower slung deck allowing for a small cabin.....and how would it be made seaworthy, at least sat to swing around Florida, in the gulf?

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### marked77Junior Member

sailing pontoon

I have stripped my 20 foot pontoon to just the tubes and crossmembers. It has shed alot of weight. I have a single rudder which I will put on the motor mount and will be adding dagger boards to each side. I already have a free standing sailmast and sail (from a beachcomber 25). I have also been looking for stronger and wider beams so that if this works to satisfaction, I will widen the boad considerably to add more sail area. I believe it will work to satisfaction and also plan on putting on 2 rudders and a 50hp 2 stroke so it will be a pontoon boat at 20mph,and a sailboat when the sail is unfurled. Also I have a 20 foot fiberglass high performance pontoon boat and may consider cutting the pontoons from the boat and even adding length to it as well. Also, I wonder if you welded/brazed 2 aluminum pontoons together with proper structure for length could this be more efficient? If the pontoons are 18 feet apart could you use some real sailpower? I do realize that perfect hull shape is not acheved, but, with enough horsepower pontoons do have a planing effect at about 14mph or less. I could be very wrong but keep it light, give her alot of power, and I do not see why this contraption would not be alot of fun? I did see waterworld...lol Anyways I will never feel embarressed as I will have fun......and worse case scenerio...you have a trolling pontoon which wi;; get you home if the motor stops.....since all my fun will be in the gulf near Tampa! Go for it guys. I built a sailboat at 7 !!!

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### marked77Junior Member

re last post

I was redundant in my posts as I did not know that my first post went through....I had written a good page and my computer decided to wipe it out...anyways...I did find 4 inch aluminum tubes a a junk shop and if all this works, thenext step will be widening it....good luck Mark

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### WavewackerSenior Member

I'd keep the beam at or less than half the length, you may end up with a sailing dock going any wider.

Toons are probably the most versitle boat on the water for a boater, but how would you brace those tubes to be seaworthy? Going light I would think would be the way to go.

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### marked77Junior Member

re last post

Thanks for the input! When I used to fly radio control airplanes...my friend was always fine tuning his motor while I was flying .....I would use duct tape and super glue to stay in the air....he would not even consider. Are you surprised by my posts? lol thanks As an adult I stuck with sailboats because when I was in the air (learning to fly real planes) I said "now what?" I figured I could always throw an anchor and take a nap in a sailboat........and now you know the rest of the story!

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