Engine for 9' Speed boat

[win_wood]

I am building a 9' speed boat that calls for a 250cc motorcycle type engine. I have a line on a polaris 2 cylinder, 2-stroke engine that will need rebuilding. Is there anything that I should know before I purchase the engine? The engine is a pull start the configuration as far as where the exhaust and shaft are seem like they will be ideal. I was wondering if I would need some kind of clutch or can it just be a direct drive? Any info would be appreciated.

[win_wood]

http://www.imagestation.com/mypictures/inbox/view.html?id=4190062680&url=http%3A//www.imagestation.com/picture/sraid102/ p17df154fcb8b1241295d0b23bd793307/f9bf4858.jpg&caption=Too%20small%20of%20a%20boat%20project%20fo r%20such%20a%20large%20 shop!&album_id=4287818789&from_album=1
Here is the link to a picture of the boat.

[Tom Robb]

No clutch means you must be ready to rock-n-roll as soon as the engine fires. Quite an inconvenience at times. Is there a transmission? Forward and reverse might be nice.
Rebuilding 2-stroke motors is relatively simple - few parts - of course experience is always handy and a shop manual will be helpful. No manual and no experience with the type and I'd pass on it, but that's just me. I hate expensive guessing games.
BTW the picture link didn't seem to work.

[Cuyahoga Chuck]

A few topics for your consideration.
The current crop of snowmobile engines are much larger than 250 cc. How many cc is the motor in question?
A larger displacement engine will probably weigh more too. Is your hull up to carrying you and a heavier motor?
Motorcycle engines of the required size all have integral clutches and transmissions. Are these things required for the boat you are building?
Snowmobile engines are usually equipt with a variable speed belt system sometimes called a "Reeves Drive". It acts as both a clutch and a transmission. Are you certain that it will work in your boat?
Charlie

[Del Lansing]

The biggest question that comes to mind is; will the snowmobile engine run cool enough at boating temperatures? They are designed to run when there's snow out, maybe under 35 degrees F. My snow-blower has a caution on it not to run when _over_40F so it won't over heat. I would see if that model has a similar warning, then decide.

[Cuyahoga Chuck]

Snowmobile engines are all concealed inside the body of the machine. They must have a cooling fan and shrouding to avoid overheating. They may cool
better than a comparable motorcycle engine which has nothing to aid cooling when not at speed.
Some of the new cheapie 4-stroke air-cooled outboards have an inclosed fan system and they are inclined to overheat when run at low RPM's where the fan is inefficient.
All this pondering has brought me to the conclusion that I don't know the answer to your question. We'll have to wait for a snowmobiler to stumble onto this site.
Charlie

[win_wood]

Here is the engine that I am refering to. http://cg i.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2486706146&rd=1&sspagename=STRK%3AMEWA%3AIT (http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2486706146&rd=1&sspagename=STRK%3AMEWA%3AIT)
I am still trying to remember how to put in a picture.
I am not familiar with the type of clutch that you are refering to but I can kind of picture a belt around the cylindrical housing on the shaft that I see in the pictures. The reason I like this engine is that it has the configuration that I need with the pull start in front, the exhaust out to the side and the shaft coming from the back. Would this engine work without a clutch? Or would the boat want to always be moving when it was running?
http:www.imagestation.com/picture/sraid102/p17df154fcb8b1241295d0b23bd793307/f9bf4858.jpg
Pictures still not working. Can't figure out what I'm doing wrong.
http://media5.hypernet.com/ubb/ultimatebb.php?ubb=get_topic;f=1;t=005533
Here is the link to my original post. I now have most of the hull completed except for the seat back.

[ 08-15-2004, 09:01 PM: Message edited by: win_wood ]

[Cuyahoga Chuck]

win_wood,
If you have plans, don't they address the question of what kind of motor this thing is designed for? How big. How powerful. Whether or not you need a clutch.
What about a gear reduction? I don't think you can operate a boat propeller at motor speed. There's some kind of rule of thumb that a boat prop at 6 grand is all but useless for motive power.
Yea, it would be nice if you could start the motor without smashing into the dock.
Stow the pictures. We need the engineering details. They will be the arbiter of what works and what doesn't.
Charlie

[win_wood]

These plans are from Clark Craft about 1983. They are not at all clear about what type of gearing is involved. The only spec is that it is to be a 2-stroke, 250cc motorcycle engine. I don't see any type of gearbox in the plans. Just a direct drive.
Thanks for your input Charlie.

[ 08-16-2004, 07:22 AM: Message edited by: win_wood ]

[win_wood]

I guess I don't have to worry about using the engine from E-bay. The guy cancelled the auction at the last minute because he only had a minimum bid even though it was a no reserve auction. That kind of burns me up. It will be interesting to see if it goes back up for auction.

[Rema1000]

Motorcycle engines have an integrated transmission and clutch. In fact, they are bathed in the engine oil; if you remove them, you would have quite a bit of engineering work to do to get the engine to hold oil pressure. The clutch is a simple cable-operated clutch; you should be able to make the cable as long as needed.

You may not need frequent access to the gear selector; just pick the gear you're likely to need, and use as much clutch as necessary to get going (don't worry, you won't wear it out like a car clutch).

One concern I'd have is that 2-stroke engines are tuned with the exhaust. You can't just lop-off the exhaust system and expect it to run well. You could make your own, and experiment with exhaust length and a turnable baffle.

Another concern would be that 2-stroke engines usually have a very non-linear power curve: no power delivery, up until a certain "sweet spot", where suddenly they produce ridiculous amounts of power.

A single-cylinder air-cooled 4-stroke engine, such as off a 4-stroke scooter, would be heavy, and produce less power, but have a better torque curve, and would be more flexible with the exhaust system.

A 2-stroke is still simpler, but you may want to use the original exhaust (which will be long and awkward).