Heightening my transom

Discussion in 'Boat Design' started by valvebounce, Sep 21, 2017.

  1. valvebounce
    Joined: Dec 2010
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    Location: manchester uk

    valvebounce Senior Member

    I have a 14ft f/glass boat with a shortshaft 18hp Evinrude on it.
    I am looking at putting a 1999 25hp longshaft Johnson on it.
    I intend to use three thicknesses of 20mm/3/4" GRP and aluminium angle to raise the transom,which will give me the same thickness of transom.
    The steering will have to be changed because the Evinrude setup is different with it being dated.
    The present steering comes into the splashwell and is at a fixed height because it being fixed by a plate in the splashwell wall.From what I can gather,the steering rod/cable can run free through the splashwell wall totally independant from the wall,and connect to the outboard.
    Looking at vids etc,this seems to be the common practice.
    Just to clarify the boat,it's got a dashboard,two seats and a covered bow and sits nicely balanced on the water.
    It's a planing "V" hull that flattens out slightly towards the stern.It's heavier than an open boat but is still fairly light in weight.The draft is about 6".It has a decent depth of freeboard,and about 6' of space behind the seats.
    Any advice or tips would be appreciated before I start the job.
     
  2. SamSam
    Joined: Feb 2005
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    Location: Coastal Georgia

    SamSam Senior Member

  3. SamSam
    Joined: Feb 2005
    Posts: 3,900
    Likes: 197, Points: 63, Legacy Rep: 971
    Location: Coastal Georgia

    SamSam Senior Member

    .
    Here's some ideas, the third one down with the black motor board looks pretty functional and it's even adjustable. The bottom one is also adjustable and it couldn't cost too much. Those would just bolt on with no alterations to the boat.

    Otherwise just bolting two aluminum angles on the transom that stick up and then the plywood motor board bolted to them would work.

    splicing on a transom piece for a long shaft motor - Google Search https://www.google.com/search?q=splicing+on+a+transom+piece+for+a+long+shaft+motor&tbm=isch&tbs=rimg:CeF5fibB0964Ijh78Tp8m7Cv_1B9YfbxUBikWgvT-pgGAyxKhQubbqcbCOKqiQhlCEmif4oS50vaPNeAnymFqrFrJPioSCXvxOnybsK_18Eesz4wY0Vwb5KhIJH1h9vFQGKRYRs4V6UxQwV-cqEgmC9P6mAYDLEhFPMaL6Kr0y7CoSCaFC5tupxsI4EY5BZrxw6I3aKhIJqqJCGUISaJ8RaJPidqj1PVkqEgnihLnS9o814BFFMNCSLoQCDSoSCSfKYWqsWsk-EaRjY3LGilX0&tbo=u&sa=X&ved=0ahUKEwik8MO5prfWAhXELyYKHeDZB_cQ9C8IHw&biw=1366&bih=628&dpr=1

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  4. valvebounce
    Joined: Dec 2010
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    Location: manchester uk

    valvebounce Senior Member

    Hi SamSam.Thanks for going to so much trouble posting the alternative ideas.
    Because of the steering alteration,I think something similar to the 3rd picture will be the most suitable.
    The top of the transom is lower than the gunnels by about 8".The back of the splashwell (transom) is 5" from the bottom of the drain hole height.
    In effect,the motor sits in a well that is 8" lower than the gunnels.
    My idea is to keep the transom in line with the original and to raise it the 5.5"to accommodate the longshaft motor.
    If I raise the height right across the dip/well this will raise the ransom height which would give me more protection from a following sea.(and when launching)
    I intend to use aluminium angle inside the splashwell,and also on the back of the transom,bolted through with stainless steel bolts.
    I may use three thickness's of GRP where the motor sits,and one thickness for the sides(wings) and fibreglass them in.(all in one lamination)
    The internal angle pieces will be 5.5"+5"=10.5".The outside pieces will be longer,but as of yet I have to determine the length.
    I am using 3/4"x1.5"angle,with the 1.5" face bolted through the transom with two pieces.I'll use two lengths,one each side on the outside of the wings,hopefully to stop it flexing and to give more support.
    With setting a new transom height it will allow me to get the cavitation plate at exactly the right height.
    Thanks for your input,it's much appreciated.
    "V"
     
  5. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    Lots of options.

    Easiest are the sandwiched ply with aluminum inside and outside the current transom.

    Next a single piece of thicker aluminum bolted to the out side.

    I had typed this yesterday and didn't hit send.
     
  6. SamSam
    Joined: Feb 2005
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    Location: Coastal Georgia

    SamSam Senior Member

    Keep an eye on the steering cable hole. On a boat launching on a steep launch ramp, I have seen water come over the transom and into the splash well and then go through the steering cable hole and flood the boat.
    It was a large 3-4" opening and the whole fiasco was a combination of the uncovered hole, a too short trailer that allowed the boat to tip a little steeper and not enough gas in the truck to keep it running on an incline. Later on I saw them going down the highway with one of the crowd sitting on the bow with his legs hanging over to keep the boat from tipping and sliding off the trailer. It's best not to do anything like that yourself, but it's fun to marvel at.
     
  7. valvebounce
    Joined: Dec 2010
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    Location: manchester uk

    valvebounce Senior Member

    Great idea "O" the problem with ally plate inside and outside on the transom would increase the thickness of the transom,and at present the transom is more or less at it's max thickness to take the outboard clamps.It would be great if I could use that method.
    Thanks for your input,it's much appreciated.
    "V"
     

  8. valvebounce
    Joined: Dec 2010
    Posts: 548
    Likes: 11, Points: 18, Legacy Rep: 124
    Location: manchester uk

    valvebounce Senior Member

    Looks like great minds think alike,the hole is about 2" dia.I think raising the transom 5.5" right across the splashwell should reduce the amount of water that can get in the splashwell,and into the steering cable access hole.The boat has a nice shape and design,but because the outboard sits so low on the stern,I think it would have been better with a transom height suited to a longshaft rather than a shortshaft.
    I've considered that when I throttle back in a following sea the pitch forward and then backward would cause the boat to swamp too easily.
    Seeing as I will be using the boat for sea fishing and not on flat lakes (there's a speed limit on most UK inland waters) I think the higher transom will be of a great advantage.
    Most of my launches will be by a tractor because I don't have a 4 wheel drive vehicle,and I'm no spring chicken.
    I'm 70 miles from the sea,so it's at least a 140 mile round trip.Makes it a long day.
    Nice talking to you
    "V"
     
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