Raising Transom Height from 20in to 25in?

Discussion in 'Boat Design' started by KismetFI, Sep 3, 2020.

  1. KismetFI
    Joined: Sep 2020
    Posts: 5
    Likes: 1, Points: 3
    Location: Fire Island, NY

    KismetFI Junior Member

    Currently rebuilding an 18' V-hull. I have the boat completely broken down and it is basically a bare hull. I recently rebuilt the transom and bonded three 3/4 marine grade pieces together, glassed between each layer with 1.50 oz chop and added a poly peanut butter. Came out solid as a rock and bonded that solid piece into the transom. I bonded her in with a poly peanut butter, have tabbed the piece in with strips of 1708, one layer of 1.50 oz chop and two layers of 1708 thus far. I let that cure and I am about to go back in and glass a few more layers of 1708 to finalize the transom...

    BUT I am having second thoughts here. I am now debating going in and ripping that piece out (I know...a very timely timely timely and kinda costly mistake). I wish that I made the transom without the cut out so she could hold a true 25in length two stroke. Currently the transom lip measuring to the bottom of the V is at about 22 1/2 inches. I have lined up a 200 2.5L 25in two stroke and plan on adding a jackplate. The jackplate could take care of the height difference but my worry is that the boat will sit very low in the water and the engine well/ transom will be very close to the water. If I raised the transom height and took away the cut out, I imagine that I could eliminate the engine well and the transom height will be higher/ out of the water. I would have to make a few knees to support the added transom height and adjust the stringer support.

    If I kept the transom as is, I would have to build an engine well like the original design, see below pic.

    Any thoughts here or input on raising the transom? Thanks for the input!
     

    Attached Files:

  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Welcome to the Forum Kismet.

    Don't be too hasty about ripping the whole transom out - that would be rather drastic.
    I presume that you will be fitting a pair of new longitudinal stringers in the same locations as where the old stringers were?
    If so, then I think it should be feasible to simply put in a 3 layered plywood insert into the cut-out in the transom, and then glass over it.
    You could even scarf the outer and inner layers where they join the existing transom.
    And with the middle layer, if you have a router you could maybe rout out some of the existing middle layer of the transom so that the ends of the new middle layer insert are effectively 'built in' to the transom?
    And you could fit a couple of 'knees' to connect the transom to the stringers - if the knees go all the way up the transom, then it should be pretty solid?
     
  3. KismetFI
    Joined: Sep 2020
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    Location: Fire Island, NY

    KismetFI Junior Member

    Thanks bajandailor! Long time thread reader here, glad that I joined!

    Yeah this is my first boat build and it has been quite the learning experience...it is fun though! I have about 4 days into building that transom. The shaping, glassing, sanding all the above. It is very solid, I could only imagine how long it would take for me to rip that out. The wet wood that was originally there took a few days to dig out.. Again my first boat building project so its been a bit of a learning curve. I will be fitting a pair of longitudinal stringer in where the old ones were. To make the stringers which are 14' in length, I will be using one 3/4 marine ply piece that is 10" high for each stringer. Also plan on using 3/4 marine ply to make the bulkheads and separate transom knees (apart from the stringer connections to the transom). I will certainly be butting the stringers up and tabbing/glassing them to the transom (planning on using two layers of 1708 to cover them up for good), should be a solid support system. I really like that idea of glassing three pieces of 3/4 ply together and scarfing them into that cutout. Butting some knees all the way up to support the cut out and then finishing off with several layers of 1708 and 1.50 chop. I figure that only two engine bolts would go through the cut out section and the others would be in the main transom. Thanks for the advise, its a great idea! Any additional words are wisdom are certainly welcomed!
     
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  4. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    So the cut-out at present is 22.5 inches ? That looks like quite a steep vee for an 18 foot boat, at least 22 degrees. I would not trust that to be deep enough without a well, I'm not even sure I would trust 25" without the well, it would be enough 99% of the time, but it is the one percent that gets you. So I think you need some kind of splash well, with either shaft length. You can bolt on a 25" motor to a 22.5" transom, with or without jacking plates, I have owned a boat that was built with 22.5", and if people wanted to fit a 20" motor, it was just cut down. The 25" just sat higher above the 22.5" high ledge. No dramas. Sure, there will be a little more water splashing into the well from time to time, than with the 25" height, but you have the option of making the well to suit the 25" height engine anyway, so less water can be held in the well, if you are sworn never to go back to 20". That is a decision to make now, because if you did revert to 20", you may not be able to tilt the motor all the way up.
     
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  5. KismetFI
    Joined: Sep 2020
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    Location: Fire Island, NY

    KismetFI Junior Member

    Thanks for checking this out! Yeah it is rite about 22.5" from the top of the cut out to the V. Its a unique boat with a very deep V, it honestly rides amazing. Last I was on it was about 20 years ago, she flew and was real stable in some chop. I have been mulling this over and I also agree with the splash well input. I could certainly design the well so it drains really good and isn't a water trap. Depending on how she sits in the water with the gas tank (having an aluminum tank built that will hold about 35 gallons, will be 69" long x 16" wide x 8" in height) and engine. I could always glass on a 3/4 piece of ply to block excess water from entering. I thought this was a lost cause but this is some great input. Thanks!
     
  6. possum
    Joined: Aug 2020
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    Location: Ga

    possum Junior Member

    I’d be interested to know what the hull is, if it was a production boat. I’m no help resolving your question though.
     
  7. KismetFI
    Joined: Sep 2020
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    Location: Fire Island, NY

    KismetFI Junior Member

    Possum, she is a homemade 1977 18' deep-V Clam Poaching boat....The hull was based off a Donzi, I am told. But she is genuinely homemade, a hand laid glassed boat. A few of these hulls were built in Oakdale, NY in the 70's. They were designed for poaching clams out of the Great South Bay, out running the cops with a loaded down hull and the hull is built thick for breaking ice during the winter months to access the clam grounds. It is a really cool boat!
     

  8. KismetFI
    Joined: Sep 2020
    Posts: 5
    Likes: 1, Points: 3
    Location: Fire Island, NY

    KismetFI Junior Member

    Pic of her before I broke it down....
     

    Attached Files:

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