transom angle for OB jet boat

Discussion in 'Metal Boat Building' started by Rob Unversaw, Mar 4, 2015.

  1. Rob Unversaw
    Joined: Mar 2015
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    Location: Hamilton, Ohio

    Rob Unversaw New Member

    I am brand new to boat building so I thought I'd plug in somewhere and get a little supervision from people who know what they are doing. Thank you in advance for any input!
    I am building an aluminum 1452 jet jon Powered by an Evinrude 40/35 hp jet.
    The purpose of this boat is for duck hunting a shallow gravel river near my home. There is very limited access so this boat will be hauled in by ATV in most cases which is why it's going to be so small.
    I've been able to find every piece of information I need to do this build except... transom angle.
    I'm under the assumption BTW that 0° indicates a perfectly vertical transom in relation the the bottom.
    considerations: the splash guard will simply be an extension of the roof of the jet tunnel and will extend back 6" or so to cover the leading edge of the intake.
     
  2. Tungsten
    Joined: Nov 2011
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    Location: Canada

    Tungsten Senior Member

    This would be a similar boat that I see around here,40 at the crank yields 28-30 at the pump.This will push a 1448 boat 210 lb boat around.
    I would look at some of the factory Jons transom angle and copy that.Normally a motor that small doesn't have a tunnel,disturbs the flow creating air.
     
  3. Rob Unversaw
    Joined: Mar 2015
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    Location: Hamilton, Ohio

    Rob Unversaw New Member

    The hull will actually weigh in around 400 lbs. The bottom will be 1/8", have 5 stringers, about 10 bulkheads, and have a uhmw bottom. I'm even considering going up in hp but that has yet to be decided. The dead rise will be 9° continuous so I'm not overly worried about cavitation. I'm for sure putting a jet tunnel in to get the jet foot up and out of the gravel. The production hulls that I've seen on regular jon boats appear to be about 10° to °20 depending on the make. What may be typical on a Regular jon boat may not be optimal for a jet boat which is why I'm asking. Do I go steeper or not so much?
     
  4. Tungsten
    Joined: Nov 2011
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    Location: Canada

    Tungsten Senior Member

    Ya sounds like more HP will be needed. Hard to beat a flat bottom Jon for shallow water/ low speed...15 mph
     
  5. yofish

    yofish Previous Member

    Transom angle for pumps on OB's is same as for props. 15 degrees has been kinda the standard for years but I just built a skiff that was powered with a 140 Suzuki and they spec'd 16. Not that anyone would notice. They made or make pumps to work on regular boats, otherwise they'd never sell one! I have too much experience with pumps on 80's vintage OMC 35hp motors, mostly of the nightmare variety. They are cool when they work and are otherwise when they don't. HP lost is phenomenal and more so with every bit of sand you suck through them. They whole idea is shallow, right? Wait until you puke a yard of sand and gravel out of one of them then, you'll have it's true capacity.
     

  6. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    17 degrees gives about as much trim range as can be used, 13 degrees in enough if the unit is not trimmable underway.
     
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