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Thread: Newb here with a question on props

  1. #21
    Team Member JohnsonM50's Avatar
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    Quote Originally Posted by ToneDef View Post
    Thanks JohnsonM50, I've got a 1957 Whirlwind 16' runabout with a Johnson 60 Sea Horse as well that's currently in the water, she runs 40-45 MPH, but we've got the engine de-tuned a bit for longevity reasons. Also I forgot to add the engine is a 18" leg, and I've got a jackplate with 6+" of adjustment
    The jackplate Im sure will be useful to determine best run height & the set back might be an advantage. One thing youll have to work out is the 'ride on the edge' but not porpoise [at speed] thing & I dont know what effect the plate will have on that. You would have to consider at some point beyond the height determination that to make the transome that height & eliminate the weight of the plate if its heavy. If it isnt heavy the setback will still effect weight distribution.. for better or not? What Id do is build the transome hi to begin with so when you get set up you have the option of direct mounting at a height close enuff to shim up to fine tune. In adjustments of the motor, the faster you go the more critical small increments will be. As far as racing "chine to chine" as Capnzee wrote theres several directions you can go, all of them cool stuff.

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    Quote Originally Posted by JohnsonM50 View Post
    The jackplate Im sure will be useful to determine best run height & the set back might be an advantage. One thing youll have to work out is the 'ride on the edge' but not porpoise [at speed] thing & I dont know what effect the plate will have on that. You would have to consider at some point beyond the height determination that to make the transome that height & eliminate the weight of the plate if its heavy. If it isnt heavy the setback will still effect weight distribution.. for better or not? What Id do is build the transome hi to begin with so when you get set up you have the option of direct mounting at a height close enuff to shim up to fine tune. In adjustments of the motor, the faster you go the more critical small increments will be. As far as racing "chine to chine" as Capnzee wrote theres several directions you can go, all of them cool stuff.
    The jack plate weighs under 10 lbs, so it isn't bad, But what you said makes sense, I will run with what I've got and make adjustments accordingly.

  3. #23
    Team Member JohnsonM50's Avatar
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    [QUOTE=ToneDef;90589]The jack plate weighs under 10 lbs, so it isn't bad, But what you said makes sense, I will run with what I've got and make adjustments accordingly.
    QUOTE]Yeah 10s no great amount. The 'fishing' gear foot is designed to push a varied load at moderate speed so a light load will usually yield a only little more speed & alot more pickup. The motor will only spin so fast & the prop will never exceed its inches of pitch per R but always fall short. There being your biggest obstacle, the prop. Count on trying a few at least. One thing to know is to find a wrecked prop that fits & salvage the inner hub. This can sometimes be used to adapt a 'righty' speed prop. Ive seen it on another boat & did it on my pals hydro.. run a non thru hub prop. It -can- work with the thru hub, no guarantee on it tho. There's a couple formulas for whats happening with your prop concerning slip & drag.. What I do is take note of accurate speed. Multiply pitch by RPMs [= inches per minute] Divided by 12 [=feet per minute] Times 60 [=feet per hour] divided by 5280 [= mph*]
    * then determine your gear ratio.. Example my OMCs turn 21 times at the motor per 12 at the prop so 12 divided by 21 = 0.571:1. Multiply 0.571 by [mph*] to get potential speed... The MPH you would go if there was no slip or drag 'perfect world'. Compare it to real speed noted before & see what % of efficiency your prop operates at. Ive got 2 cleavers that work at about 90% so Id call them a good style to try but not discount others. The gain you will get out of a little motor work will pale in comparison to a good prop set up optimally. To get 45mph out of a 30mph motor you would have to more than double your output at the trade off of the low end torque you need to get on plane & maintain speed while the water simulates going uphill in sand on motorcycle wheels. Mike

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    Well Bob's Machine was a no-go on the nose cone/low-water pickup. Apparantly the water cavity on the Nissan 15 is to small to drill and tap into. Anyone else make nose cone/lo h2o kits for small outboard?

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    Quote Originally Posted by ToneDef View Post
    Well Bob's Machine was a no-go on the nose cone/low-water pickup. Apparantly the water cavity on the Nissan 15 is to small to drill and tap into. Anyone else make nose cone/lo h2o kits for small outboard?
    Ive dont that a different way but you still have to tap into the motor somewhere before the pump & probably close off most of the hi water pick up. Till you have a surface running prop you wont need this.



    race props also have height limits, at some point they will just cavitate & not grab. You will have to scream the motor for several seconds till it grabs anyway if your set up well enough to get the speed intended from a prop. Very ruff on the motor & important that it get enuff

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    Quote Originally Posted by JohnsonM50 View Post
    Ive dont that a different way but you still have to tap into the motor somewhere before the pump & probably close off most of the hi water pick up. Till you have a surface running prop you wont need this.



    race props also have height limits, at some point they will just cavitate & not grab. You will have to scream the motor for several seconds till it grabs anyway if your set up well enough to get the speed intended from a prop. Very ruff on the motor & important that it get enuff
    That looks like a setup I could try if I end up with a race prop. In the 3rd pic, it appears that there are some small holes drilled in the plate you have blocking off the hi water pickup? Is this so it is not 100% closed off?

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