Air cooled motor in 16' skiff - wond'ring aloud

Discussion in 'Gas Engines' started by Homefront, Oct 22, 2009.

  1. Homefront
    Joined: Oct 2009
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    Location: Eastern Pa

    Homefront Junior Member

    Gee, my first post!

    I have a 16' fiberglass skiff with a 40" flat bottom. Sharpie type planing hull (like a miniature Pacific City Dory), low freeboard, approximately 200lbs hull weight. The old (70's) 25 Chrysler it came with is in poor shape (needs at least a carb rebuild and a bearing in the lower unit), and makes the hull sit low in the water at the stern.
    I happen to have a 6hp horizontal Briggs engine, which gave me the thought of mounting a motor amidships, building a shaft log, and running a 3/4" drive shaft to a prop. I'd also build a rudder to steer with. This weight placement would get the stern up quite a bit, and probably help in other ways.
    I'm aware that I'd have to deal with the exhaust in some way or suffer the fumes/noise consequences and I have a few ideas to develop. I'd use a pulleys/belt setup to get the power from the motor to the shaft and build a pivoting motor mount that would use the motor's weight to keep tension on the belt. This would also allow me to rig some kind of mechanism to tip the motor up enough to relieve the belt tension, effectively putting the boat in neutral. I realize there are no hard and fast answers, but my main questions are the following:
    1) Might 6hp get the boat on plane with 2 people aboard? I don't have to fly; 12-15mph would do.
    2) What kind/diameter/pitch prop would I use? I realize that it would involve some trial and error.
    I also found a new Honda 13hp for a good price ($500+); the same questions apply.
    I'd like to keep the whole installation under a thou, which seems doable with either motor.
    All suggestions welcome :p .
     
  2. artisthos
    Joined: Nov 2009
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    Location: Cincinnati, Ohio USA

    artisthos fine artist

    I am an old river rat who just sailed the North Atlantic in a 36’ Pearson with two other experienced sailors. It was a pivotal experience for me since I have always been on the water. Now I am hell bent on getting my own sailboat. Working as a fine artist, painting portraits and other commissioned work never delivered enough income to get a real boat. Once back on dry land in Cincinnati I was hell bent on getting a boat any way possible. A friend told me about FreeCycle an Internet group that offers things free. I joined and posted a message that I wanted a sailboat. Low and behold, a post came back about a neighbor who had a trimaran he wanted to get rid of. Now I have a 9’ long, 6’ beam, 1971, plastic Triumph trimaran with a lateen sail rig. I am replacing the interior foam and wood stiffeners. For power, I want to use a 3-1/2 hp horizontal shaft gas engine with a Thai long shaft, old trolling rig or run the shaft through the hull in the regular manner. I will install oarlocks. Soon I will be on the water. It is never too late. [​IMG]
     
  3. alan white
    Joined: Mar 2007
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    Location: maine

    alan white Senior Member

    It's always refreshing to see someone actually down-sizing the power plant and doing something simple in its place.
    My suggestion would be to use an idler to tension the belt rather than hinging the motor. A simple spring-loaded catch would hold the rig in forward. I at least would prefer the engine to be bolted in solid. It's more easily adjusted for tension, where the micro-adjustment of the catch position allows the belt to be its own "spring" as it is in any car engine setup.
    Prop specs are somewhat open. You can change the pullies and adjust the ratio cheaply, so unlike a normal outboard, your range of props is fairly open.
     
  4. alan white
    Joined: Mar 2007
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    Location: maine

    alan white Senior Member

    That picture is absolutely hilarious. The crew looks very relaxed. The boat must practically sail itself!
     
  5. artisthos
    Joined: Nov 2009
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    Location: Cincinnati, Ohio USA

    artisthos fine artist

    Great Advice

    Thanks for the ideas. I like having the engine mounted using pulleys. The image is from a 1971 Popular Science Magazine. Fiberglass does not stick to the plastic. I am thinking of using liquid nails to fill cracks and holes. Here is another funny image: my flying bycycle, the most popular download on my site:) [​IMG]
     

  6. pistnbroke
    Joined: Jan 2009
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    to be practical I have a 5 hp brigs and stratton outboard I use on a 5m ( 16ft ) canoe with a beam of 1 metre ( 39 in) ....you will do about 15 kts .it has a n 8x8 prop........why not go for the lifan 6.5 clone for $99 from Harbour freight gear it down about 1.5 :1 ( thats what briggs did) and depending where you put the pulley you could turn a left or right hand pitch prop ...electric start and a govener .....
     
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