Repowering this thing.....from I/O to outboard....

Discussion in 'Powerboats' started by Devildriver, Apr 27, 2009.

  1. Devildriver
    Joined: Apr 2009
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    Devildriver Junior Member

    First off, I'm new and have found some good info here already!!

    And now...

    I got a Hawiian jet boat, which had an OMC unit and a 350 chevy for power. The lower unit was bad, I thought I rebuilt it right, blew it again, then sold the motor, and it has been sitting for a year or so.

    I just discovered those brackets for mounting an outboard to the back end of it, so that seems easy enough.

    The kicker is I found a pair of 90's mercury 200 hp motors......yeah :cool:

    But I'm not sure if the transom would handle it, even if I ran braces down to the 6x6 the 350 was bolted too, or if I replaced them with some 4x4's and ran them up towards the center of the boat.

    Or, if I could fill the big hole, and mount one directly to the transom.

    My major concern, other than the transom holding up, is that the boat doesn't sit very high out of the water, until it planes off.

    Anyway, I've found a few threads about setting up an I/O with an outboard, and was looking for some general opinions about mine.


    So here's some random pics I took, with the bright sun, so they may not come out too well... I gotta hood form a dodge in there, so don't let that throw you off....

    Thanks!!

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  2. pistnbroke
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    pistnbroke I try

    I would beef up the transom and fit one 200hp . The jet unit is so inefficient you should get more than enough performance from 200hp.....simpler ..less fuel ...weight reduction...spare parts

    Clearly look at similar boats with outboards to judge the power and performance to be expected.
     
  3. Devildriver
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    Devildriver Junior Member

    I'm not sure if I could get just one outboard, but I could get them, try to sell the other....or keep looking for something more manageable, like a 150-ish hp...

    So would you use the bracket (flotation or regular), or modify the transom for the mount to slide down over, and bolt directly to it?

    Oh, and too clarify, I thought the boat manufacturer was called "Hawaiin Jet Boat", even though there's no "jet" on the name plate....it was a regular stern drive....Told ya I'm a noob!!

    Thanks!!
     
  4. pistnbroke
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    pistnbroke I try

    Clearly you can have the two at a good price ....so why not sell one on to fund the project???

    I would fill the hole at the rear with ply and fibreglass after you have re inforced the transom with ply and resin in full width sheets .most people would do this on the inside if they have access.....then perhaps some brackets again in ply or metal down to the floor supports. The floor where it buts to the transom takes a lot of the load....the bottom of the transom is pushed forwards and the top bent backwards remember ,,You can also adjust the transom height to match the outboard you have ...,,,With regard to the 200 hp ... you can always turn the throttle back but it wont go faster than WOT !!
    If you take the pitch of the prop and multiply by 3 that will give you an idea of the speed in MPH ( prop revs assumed to be 3000 rpm ish) so a 21 pitch will give you about 60 mph...Two motors will NOTgive you 120 mph !!!
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Yep, you can mount an outboard on the butt of that boat and she will sit very low until up on plane. You could use a bracket, but why go to the expense if you don't need to.

    You'll need to reinforce the inside of the transom so that's it's at least 2" of core, encased in 'glass, well tabbed into the hull shell. You can "tie" the transom to the stringers with a splash well (which you need anyway) structure, that can be made from plywood, again covered in 'glass and tabbed into the hull shell and stringers.

    A belly mount fuel tank, forward of where the engine use to live would be a good idea to help the CB.

    This isn't an easy under taking, you've got a good bit of fabrication to do. You'll need to work out steering, engine controls, engine electrics, tilt and trim hydraulics (or electrics), fuel delivery, etc., plus the structural considerations.
     
  6. Devildriver
    Joined: Apr 2009
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    Devildriver Junior Member

    I've had some practice this winter at rigging up controls.....we clean up oil spills and do quite a bit of on the water work, so we got a boat from NJ that had got flipped onto some rocks so I had to make a console from another boat work with this one, then get the controls and steering to work with a newer motor....so far so good!!

    I'm heading outside right now to clean the leaves and garbage out of it and start with some cardboard templates for the hole and braces, too see what I can come up with....

    Thanks!!
     
  7. stormin
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    stormin New Member

    you could even mount the outboard on a gill bracket once you have beefed up the transom that would spread the load get away without haveng a draining well
     
  8. pistnbroke
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    pistnbroke I try

    I think you mean by a gill bracket what the power boat people would call a pod .
    for 200Hp you would need something welded up in at leat 3/8 plate . these things usually push the engine back 18 inches which means you lift the cavitation plate on the outboard up 6 inches...( 2 inch lift or each 6 in back) The pod does not come to the bottom of the transom but to within about 3 inches.. there may be some merrit in a pod of say 6 inches ..it would save a lot of cutting of the rear.fibreglass....being silly it could almost be solid wood and use long ( 10 in ) bolts to attach the engine . like a glorified spacer ......could post a sketch if you are interested...we dont have a good photo of the full transom and rear deck.
    Transom height would be usefull to know and what transom is the outboard intended to fit short 15 in or long 20 or extra long 25 but check carefully a 15 inch usually goes on a 17 in transom in my experience ....
     
  9. Devildriver
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    Devildriver Junior Member

    In the 4th pic where the trim is, right at that line the wood stops and its just a thin fiberglass layer from there up (or just above the bolt centered above the big hole in the first pic below). So the supported area is about 22.5" from the very bottom in the center.

    I can get my hands onto a newer 115 hp honda 4-stroke for mock-up, but its not for sale and I'd hafta return it to work :(

    And I'm not sure of the length of the 200hp mercury, but should probably find out :D

    Anyway, here's some more pics of the transom....dunno if it'll help or not...

    Thanks!!

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  10. pistnbroke
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    pistnbroke I try

    If its 22 inches supported then a 20 inch motor would be ideal...I would get all the crap out the inside ..cut back the floor stingers ,take those steps off the back and re inforce the rear with two sheets of at least 22mm ply with plenty of glass and mat in between . on the inside ...pullit all together with screws when wet. I think the idea of a short pod /spacer is a good idea and you can make it to suit the engine you end up with and it will save cutting the back about ...you can have some nice seats in that rear compartment ...

    Extra thought...
    Looking at the photos I think the transome is at 90 deg to the bottom of the hull....now for an outboard the transom leans back 15 deg and the outboards are designed with this in mind .so you are going to need an angled spacer to put the motor on ...say 24 in wide and high enough to suit the motor .....1/4 plate exterior and a triangular wedge magbe 4 inches at the top tapering to zero at the bottom (have not calculated it )..case all the wood in glass and bolt right through from engine to inside ...a lot less trouble than an engine well and it will cover that hole....you still need the ply inside the transom..hope you got an electric plane .ha ha ...laminate it from ply..
     
  11. Devildriver
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    Devildriver Junior Member

    So you suppose I could safely and without effing myself later cut a notch down to the reinforced part, about the same width as the hole?

    And I'll get a side shot, but the transom does angle back, actually where the reinforcement is, then straightens out (up?) about where that trim is.

    Thanks for all the help so far!!
     
  12. pistnbroke
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    pistnbroke I try

    dont cut any of the origninal away if its in good shape only add to it ...what angle is the transom to the hull bottom ....if you cannot tuck the outboard under its not easy to get onto the plane without the front comming up too much ...15 deg

    You are only going to re inforce it up to the joint between hull and deck..then a small pod on outside to angle and hold engine ...all engines are same spec for mounting holes ...cavitation plate level with bottom of hull...
     
  13. rasorinc
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    rasorinc Senior Member

    You can always use an angle block on the outside of the transom for the right angle the manufacturer reccomends. Done all the time. Don't go into the transom if you do not have to. As pistnbroke says just add to it.
     
  14. Devildriver
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    Devildriver Junior Member

    Thanks guys.....no cutting just filling!!
     

  15. pistnbroke
    Joined: Jan 2009
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    pistnbroke I try

    like this
     

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