Outboard radius for Transom cutout.

Discussion in 'Outboards' started by valvebounce, Sep 17, 2013.

  1. valvebounce
    Joined: Dec 2010
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    Location: manchester uk

    valvebounce Senior Member

    Fitting a double thickness 1.5ins transom=2x3/4"
    I have 6" above the outboard clamping point.It has been suggested that I may get away with 26"-28" gap for the outboard,which has tiller control.
    I would like to keep the gap as small as possible to shield from a following sea.I intend to reduce the thickness of the transom on the inner board from the clamping point up,for excess weight reasons.I have bench tested the radius of the outboard,and with the tiller handle it is about 32"The tiller will swivel upwards.Is it neccassary to consider the tiller handle in the radius?
    The outboard is an old 18hp Evinrude fast twin.
    Could do with a bit of advice before I make the cut.
    Any advice will be gratefully accepted.
     
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  2. gonzo
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    gonzo Senior Member

    If you don't consider the tiller handle and your hand/arm in the radius, you won't be able to operate the motor.
     
  3. tom28571
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    tom28571 Senior Member

    Most older motors use a transom height of 15" but that one is about 18" so you gain a couple inches of protection from following waves there. Only way to know how much cut out you need in the transom is to check it with the motor mounted on a board. I can measure mine if I get the chance later.
     
  4. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    Thanks for your reply Tom,the old transom was 15" like you say.I am setting the height on the new transom to 17",which puts the cavity plate online with the lowest part of the hull.The boat is fibreglass and very light in weight.On the bench board the motor clears the starboard side easily at 28"
    but the port side is restricted on the last part of the arc.
    If you have one already setup,and can measure it,that would be very helpful.
    I dont expect to have it on the water until next spring,so there is no immediate rush.
    Changing the subject slightly,could you give me an idea as to what speed this motor will do?It's an old speedboat hull that I have converted to an open boat for fishing.It will be very light in weight.
    Bet regards-V
     
  5. valvebounce
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    valvebounce Senior Member

    Thanks Gonzo,Tom has one already setup and is getting the measurement for me.-V
     
  6. sprit
    Joined: Jul 2013
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    Location: Lexington, MA, USA

    sprit Junior Member

    If you have the original drawings, measure the space you need on the drawings.
    If you have the engine, take the necessary measurements from swinging the tiller.
    BE CAREFUL to measure the necessary space when the engine is raised and at an angle.
    Often there is much less swinging room for the engine and/or tiller (even when the tiller is bent up) than expected.
    Good luck.
     
  7. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    Many thanks Sprit,I have no drawings,I have the engine.I have checked the radius with it clamped on a board,taking into consideration the tiller up and down.I have cut the gap at 32".the boat has been completely re-furbished,stringers,upper and lower futtocks,transom and new gunnels.There was no deck in the boat when I got it,so also a new deck.The project is still ongoing.I am hoping to put it on the water next spring.
    I appreciate your advice,the members on this site have been a great help.
     
  8. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    Outboard motor cutouts are always too big. Go as small as possible if you are concerned about wave wash when stern too.

    Typical outboard dimensiones. http://www.nissanmarine.com/products/tldi003d.html

    Yamaha is also on the internt..do some googling
     
  9. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    Thanks for your info Michael,I have made the cut at 32".which is what I measured the radius at.I had to make a decision because it was holding up progress on the rest of the boat.I didn't want to make the cut with the transom insitu because the job is much neater if done on the bench.There are two laminates of 3/4" glass reinforced ply making up the new transom,so if the gap turns out to be a little wide I should be able to fit peices in without it looking like a bodge up.I have re-designed the boat from a speedboat into a pleasure/fishing boat.It's taking a little time because I am no spring chicken,and the boat is on blocks outside.I am trying to get to a stage where I can flip the boat over and remove the numerous coats of paint that are on it.It's a long job,but I have plenty of time.
    Once again,thanks for your interest.V.
     
  10. sprit
    Joined: Jul 2013
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    sprit Junior Member

    If you are concerned about following waves, you might want to consider a newer motor.
    For example a long shaft Honda 4-stroke 20 HP motor weighs only 105 pounds, and the transom cutout-to-flat-bottom can be 22 inches (even though the nominal height is 20").
     
  11. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    When a boat is moving stern washover is not a problem. At rest or moored is when wave wash is hard on a small craft
     

  12. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    Thanks Sprit,I'm afraid my boat etc.doesn't come first in my budget,so I am sort of stuck with what I have.I have increased the transom height to 17.5"from 15".I am not over concerned about a following sea,but was advised to increase the height for extra safety.

    Michael,apart from the above,I realise a low transom is more vulnerable to quick de-acceleration,so I suppose coming off the gas too quickly has to be considered.
    Once again,thanks for your input guys.V.
     
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