11 HP briggs outboard conversion

Discussion in 'DIY Marinizing' started by Dave T, Oct 28, 2011.

  1. ben2go
    Joined: Jul 2008
    Posts: 168
    Likes: 4, Points: 18, Legacy Rep: 13
    Location: Upstate, South Carolina,USA

    ben2go Boat Builder Wanna Be

    I got an old Ted Williams 5hp Game Fisher OB.Mine is the one without F-N-R and spins 360 for what ever direction I want.If I can get the lower housing seals and weld the cavitation plate,I plan on using a Briggs 6.5 with electric start with pull start back up.They come on those high end fancy self propelled mowers.LOL The stator puts out a couple amps to charge the lithium ion battery pack, so I maybe able to power a couple LED nav lights.

    My plan is to connect the exhaust so it runs out of the lower unit as it originally came on the OB.The water tube for the exhaust cooling comes up high enough that I can plumb it into the exhaust pipe right after the 90 degree bend to go down into the lower unit.I expect this engine to run quite compared to the original power head.

    The Briggs 6.5hp maxes out at 3600rpm,so I am considering a possible chain and gear set up so the lower unit will see close to 7000rpm like the 5hp 2 smoke Tecumseh.

    The 5hp came with a two blade prop.The identical Game Fisher 7.5hp had an optional three blade prop with the same diameter and pitch.I don't know if one extra blade will make any difference,but if I find the correct one for my motor, I will buy it.
     
  2. pistnbroke
    Joined: Jan 2009
    Posts: 1,405
    Likes: 33, Points: 48, Legacy Rep: 404
    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    you will loose efficiency by going up to 7000 rpm both at the prop and in the gears. you just need to balance the pitch of the prop to hold the engine down to 3600 at WOT on ;my 6.5 lifan long tail I geared it down 3:1 ...you will already have about 2:1 in the gearbox so leave it alone ...remember the trick is to weld the end of the old motor crankshaft onto your new engine to make the coupling easy.
     
  3. ben2go
    Joined: Jul 2008
    Posts: 168
    Likes: 4, Points: 18, Legacy Rep: 13
    Location: Upstate, South Carolina,USA

    ben2go Boat Builder Wanna Be

    I am selling the power head so I will make my own coupling.2 smokes have been banned on the lakes here.I know I have 2:1 in the gear case.The problem is,there was only two props made to fit my lower.At 2:1 with a 3600rpm 4 stroke,the engine will turn the prop at half the rpm than the 2 smoke.That is very inefficient.Regearing will keep the prop speed inline with engine power and rpm.
     
  4. pistnbroke
    Joined: Jan 2009
    Posts: 1,405
    Likes: 33, Points: 48, Legacy Rep: 404
    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    Re pitch the prop then..why make life hard
     
  5. ben2go
    Joined: Jul 2008
    Posts: 168
    Likes: 4, Points: 18, Legacy Rep: 13
    Location: Upstate, South Carolina,USA

    ben2go Boat Builder Wanna Be

    Can't be done on the cheap thin cast aluminum props.
     

  6. pistnbroke
    Joined: Jan 2009
    Posts: 1,405
    Likes: 33, Points: 48, Legacy Rep: 404
    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    make one one in stainless then....boss blades silver solder...
     
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