small inboard boat

[stupidbaker57]

I would like to build a small pond boat for the fun of it. I have already built a Glen L Pee Wee boat for my grandson and would like to built this boat again but power it with a 2 cylinder air cooled engine, V drive style.
The question is, what would be a good prop to start with? The engine would top out at 3400 RPM's and would be a 4 stroke. The V drive that I have designed on paper is 1 to 1 ratio but can be changed up of down if needed.Total wieght of boat, motor, and my fat a** would be around 325 lbs. I hope to use an outboard prop.

[BertKu]

Originally Posted by stupidbaker57

I would like to build a small pond boat for the fun of it. I have already built a Glen L Pee Wee boat for my grandson and would like to built this boat again but power it with a 2 cylinder air cooled engine, V drive style.
The question is, what would be a good prop to start with? The engine would top out at 3400 RPM's and would be a 4 stroke. The V drive that I have designed on paper is 1 to 1 ratio but can be changed up of down if needed.Total wieght of boat, motor, and my fat a** would be around 325 lbs. I hope to use an outboard prop.

Go to www.castlemarine.co.uk and download the prop.calc program.
Play around and enter the information you feel is appropriate to your project.
Then click on "print result" and it will give you sufficient information to make a decission.
Bert

[gonzo]

Is there a reason to build your own transmission? A belt drive directly to the engine has no neutral or reverse. If you have a transmission with gears, then what is the belt for? An outboard is cheaper and easier.

[SamSam]

You need to give the hp. You might have a bit of trouble finding a prop that turns the right way for your rigged v drive.

[stupidbaker57]

I'm looking at an engine with about 12 to 18 hp. As for the rotation, an air cooled engine rotates CCW when facing the output shaft. With the engine in the rear, shaft forward to a chain type V drive, then back through the bottom of the craft, the prop will now be turning CW. I'm hoping that props from 10 to 20 hp outboards will have what I need.
Thanks for the prop chart info. I will look into that next.

[messabout]

Outboard lower units almost always have a reduction gear ratio. Be sure to take that into account when selecting an outboard prop. Gonzo is right. AN outboard is a much easier and perhaps less costly power source. Does not use as much space inside the boat, is less dangerous, does not require bilge blowers, and several more reasons.

[stupidbaker57]

I already have one of these Pee Wee boats with an outboard. Now I want an inboard to challenge my grandson in a race. The drive line doesn't look to hard to build, after all I built a twin engine hot rod.
The V drive ratio can easily be changed by different sprockets.
I went to the prop chart and came out with a 8" 11p prop. I think that will be easy to find.

[PAR]

One way to kill both birds with the same stone is to marry the lower leg of an outboard, to a lawn mower engine's shaft. I've done this more then once in small HP output requirements and it's worked well.

The last was a clever little unit for a centerboard sailboat that didn't have room for a conventional straight shaft setup. It was an electric start, 12 HP vertical shaft engine from a Snapper lawn mower. The cooling was modified to pull in cool air, separate from the carb's intake air, pressurize the engine enclosure then vent over the side. Intake air was ducted in, directly to the carb, letting it breath cool, uncontaminated air. The shaft was coupled to the lower leg shaft with a rubber isolated small tractor link, I picked up from a local tractor supply outfit. The lower leg had reverse and neutral gearing so I had a full setup, without making a transmission and finding a prop to work at the RPM levels I desired was the hardest part.

This arrangement was fixed to the boat's bottom, but with additional engineering could have be arranged for it to rotate, so you could steer with the unit. This would be the ultimate arrangement for a setup like this, a thrust vectoring, sail drive setup. Maximum maneuverability, with a very simple engine/drive configuration.

Heat was a major issue until I arranged a 3" fan in a duct to supplement the engine's cooling air supply and evacuate the engine enclosure. Gas fume build up was also handled by this fan, which went on as soon as the ignition switch was turned on as did a small electric fuel pump. Noise was another issue, as all L head air cooled engines make lots of it, but the enclosure, lined with D mat, worked very well. OHV engines are better, though there's still a fair bit of machine noise associated with them. Without water jackets to absorb the sound waves, the best you can do is insulate the box.

Anyway this will give you a transmission with reverse, offer an exhaust route that doesn't burn your boat, is setup for props of different sizes and is easily capable of tolerating the strains of it's intended use. I trimmed all the external tabs, fins and plates off the lower leg, as they weren't needed any more, so it "cleaned up" nice. I blocked off the water inlet and removed the pump too, though I was dreaming of ways to use this pump to cool and quite things down, but every way I looked it was a lot more complication, so I axed the pump.