Cutlass/through hull for simple boat?

Discussion in 'Boatbuilding' started by Speedling, Sep 22, 2016.

  1. Speedling
    Joined: May 2016
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    Location: Cedar Lake

    Speedling Junior Member

    Hi guys!
    Have a 13hp honda air cooled motor sitting in the garage and the wifes permission to make a boat project this winter. I believe i have the simple hull figured out. 2 seater for me and one of my kids. But i can't finish the transom design until i know how to go through it for a prop. I believe this is a cutlass or cutless or through hull?
    Basically this is my point of being stuck so a nudge in the right direction would be appreciated!
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Get the wife to rescind her permission, imo. You really face an uphill battle to make that work properly, it not being a marine engine and all, plus no gearbox or drive line. You'd be better to sell the engine and buy a ready to use, small outboard. Or better still, a complete ready to use boat. Making what you propose work, will not be easy.
  3. rwatson
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    Location: Tasmania,Australia

    rwatson Senior Member

  4. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Generally you can't go through the transom in a small boat. First off, the shaft will be running between the seats, which isn't ideal if something breaks (they do). Second, you have the prop aft of the boat, so where's the rudder going to live? You can do it, but this will require a metal structure of some sort (read, pretty convoluted) extending the rudder aft of the prop. You'll also need an expensive chopper prop, instead of a more conventional one.

    If running an inboard with a straight shaft, the shaft comes through the bottom of the boat and ends somewhat short of the transom, so you can hang a rudder in it's wash, usually under the transom or just inboard of this for best results.

    As to the stuffing box and cutlass, well you have a few options, though in smaller sizes, like you'll need, these will be limited. If you're a reasonable machinist, you could mill one up and there's plenty of cutaway drawings showing how they're done. This is a lot of trouble over just finding one that'll fit.

    Lastly, you might want to consider marrying the gas engine to the lower leg of a suitable outboard. A few pullies, a V belt and you're off and running. You'll get forward, neutral and reverse, plus gear reduction, a way to hang a prop, etc. The lawnmower engine needs to be wide open and not placed in an enclosed space. Not being marinized simply means it's blow up, if you try to stick it in a box. You could run some fans and hope for the best, but invariably, things seems to conspire for most of us, that have tried this sort of thing with non-marinized gas engines. Maybe you'll get lucky, but is this what you'll hope for with your child aboard?
  5. Speedling
    Joined: May 2016
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    Location: Cedar Lake

    Speedling Junior Member

    I should clarify a bit better i guess. Thanks for trying to protect me from stupid mistakes for sure.

    Basically my kids want it to be a jet boat. That is why it would the through transom.
    Two variations for motor placement:
    1. In line with jet pump shaft. This would use a small lovejoy. This would be simple. This would put the motor fairly low in the boat which helps cog but also keeps it hotter.
    2. V drive type of setup using pulleys and/or a go cart clutch. This would put motor up and in wind. Overkill really because the jet pump would control forward and reverse. However since i don't have several impellers to try on my homemade jet pump this would allow more or less rpms with a change in pulleys.

    I am trying to utilize stuff around our shop that i use every day so there is nothing super special.

    We have a grout hopper that has a type of stuffer box hooked to an 11 hp motor. It has 3 large rubber washers with 2 metal spacers between them. You just pump in some grease and it seals up decent. Problem is that after about 40 hours of use it has worn through the rubber and it leaks the grout out. I can only imagine what water could come through if it wasn't even mixed with the grout!
    So with that knowledge i come here but other than telling me to scrap the idea, mount to an outdrive, and the motor will blow up killing everyone, i guess i have my general concept and will perhaps try to better this idea with nylon or brass and get something together.

    Back to fixing mixers for now....
  6. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    What are you using for thrust bearing?

  7. Speedling
    Joined: May 2016
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    Location: Cedar Lake

    Speedling Junior Member

    If i use this grout hopper setup or a variation of, it has one built into the hydraulic pump side and the shaft then just pushes through the sleeve. The shaft that spins inside the drum rests on that shaft so there is side pressure on the rubber washers. The nose of said shaft sits in a nylon cap and needs to be greased every day of use but there is little downward pressure and i have never needed to replace the nylon cap.
    In my variation it will depend on the setup and placement of motor. If motor is in line with the jet pump (which is only 75% designed yet) then there will be a bearing on the inside of the transom, the stuffer box setup going through transom and the shaft continues to the nose of the jet pump where i may just use this same nylon cap or a variant or if i can find a good way to seal a bearing i will do that instead. That is the part that is a bit tricky because while this is only a 13 hp motor i am still trying to make a decent venturi and nose cone. Pump will be either 2.5 or 3" diameter depending on shaft size etc.
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