Stronger Transom

Discussion in 'Boat Design' started by valvebounce, May 9, 2015.

  1. valvebounce
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    valvebounce Senior Member

    I have a 14ft Skipper dinghy,which I don't use as a sailboat,just with an outboard.
    The makers recommendation is a max of 6hp,although they used to sell a kit for strengthening the transom to enable a bigger motor to be used.These are no longer on the market.
    I intend using 20mm grp on either side of the transom,but I suspect it will need
    more support by using ss angle brackets to connect it to the stern bulkhead,with maybe a skeletal frame to attach it.
    Has anybody done similar?
    Kind regards
    V
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    20mm GRP either side ? That is a whole lot of glass. Just looking at a few pictures, I doubt that boat will respond well to bigger engines, it will squat and not get a lot of extra speed in doing it. Not sure why they would be having the option of a kit to strengthen it really. How fast do you want to be going ?
     
  3. valvebounce
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    valvebounce Senior Member

    It's not really speed I am after,just a bit of extra power to make sure I can make way against the wind and tides.
    I will have to check the weather and sea state before I use it,it's stable but very light in weight.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    There may not be an option to fit alternate props to your 6hp, but if there is, a bigger diameter with less pitch might help give it a bit more urge in those situations.
     
  5. valvebounce
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    valvebounce Senior Member

    It's worth looking into I imagine.
    I was thinking maybe an 8 or 10hp would give it that bit extra.
    These 6hp Evinrudes aren't bad reallyI've not had it on the boat yet,I got the boat at the start of winter,I've not had chance to try it yet.They seem to perform ok on Utube,but they are on lakes and rivers,not the Irish sea.
     
  6. tom28571
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    tom28571 Senior Member

    Even 6hp is more than adequate for this boat. I would think 3hp would easily max out the performance in the boat and offer enough thrust for tides and wind. The hull shape will just not allow very high speed under power and large motors will be wasted and perhaps even dangerous.
     
  7. gonzo
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    gonzo Senior Member

    Head winds will create extra resistance and more power will help. However, tidal currents are not an extra force on the hull, but simply water moving over the bottom. Your maximum hull speed will remain the same regardless of power. The speed is measured relative to the water.
     
  8. valvebounce
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    valvebounce Senior Member

    Thanks for the info chaps,every little helps,as Grannie said when she pissed in the ocean.HaHa
     
  9. tom28571
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    tom28571 Senior Member

    A couple of reference points. A 22' 2500# sailboat with a 5hp air cooled Eska was adequate in almost all situations but not all. That motor could not be run at WOT for very long as it was air-cooled and would overheat. A 6hp Johnson was fine for a 22' 2500# sailboat but did have some problem against a 25-30 kt headwind. It made decent headway but when the boat got a bit off the wind, it did spin around. A 3000# 22' sailboat with a 3.5hp Evinrude works but is not adequate in either high wind or rough seas. All of these was more of a windage issue because of the mast, rig and high sides that are not a problem for you.

    That 3.5hp would be a perfect fit for your boat and much more power would be wasted.
     
  10. valvebounce
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    valvebounce Senior Member

    Many thanks for your interest Tom,I have a 3.3 Evinrude,but unfortunately it has no neutral,I have a 5hp Yamaha that runs sweet.
    The boat is double skinned and quite buoyant,so I am hoping the 6hp Evinrude will fit the bill.It can carry 5 adults according to the specs,so maybe
    it will be the right choice.
    I live 40mls approx. from the sea,so maybe I will take both outboards and give them a trial run.
    Kind regards
    V
     
  11. valvebounce
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    valvebounce Senior Member

    Hi Gonzo,
    I seem to have understood things wrong,so what you are saying is-no matter the current or speed of the water the boats speed will not be effected?
    I will probably be using the boat on the river Dee estuary on the welsh and English border,the estuary is quite wide and shallow in places.The tide comes in and out at a fair rate of knots.so by what you are saying,will this not effect the boat speed and it making way.(The boat has a planing hull,and although I won't be using sails,it planes easily under sail according to the specs.
    Last year I was on a boat coming in against an outgoing tide,it was a 16ft boat with a 60hp outboard (planing hull) The boat was reduced to walking pace even at full revs.The wind was minimal.
    If you can clear the point up for me,it will help a lot
    Kind regards
    V
     
  12. gonzo
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    gonzo Senior Member

    The speed over the bottom will be affected by the current. However, your maximum speed over the water is determined by maximum hull speed on boat of the type of design as yours. The current will affect the speed over the bottom but the speed as measured by a knot-meter will not be affected.
     
  13. TANSL
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    TANSL Senior Member

    Valvebounce, the speed is usually measured relative to a fixed point in calm waters (this is the speed that concerns us all). Forget speed on the water, the hull speed and do not know what else. What you care about is that the tide does not take the boat back, compared to a fixed point on earth. Therefore naturally boat speed is affected by the speed of the tide.
     
  14. valvebounce
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    valvebounce Senior Member

    Thanks Tanzl,as much as I thought I might have difficulty understanding,your explanation puts me in the picture perfectly.
    The boat I was telling you about that slowed to walking pace on the incoming had a cathedral hull,and quite a shallow draft,does the draft make a lot of difference ?
    Had a look at your CAT,it looks great,(no pretty ladies on it yet?
    Kind regards
    V
     

  15. viking north
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    viking north VINLAND

    Without photos it's a bit difficult to make recommendations on your transom reinforcement. However I do recall when larger more HP motors came into vogue, the very first mod on our fishing flats was installing more robust transom hanging knees. Many also installed a pair of transom to floor knees. To stiffen and strengthen a hull, wood or otherwise, there's nothing like the almost lost art of installing knees.
     
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