Help with setback bracket dimensions

Discussion in 'Outboards' started by aussiebushman, Jan 19, 2015.

  1. aussiebushman
    Joined: Oct 2009
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    Location: Taralga NSW

    aussiebushman Innovator

    Would someone with relevant experience please look over the dimensions of the proposed setback bracket (see attached PDF file) and advise if I have got it right before starting fabrication (NO way can I afford to buy a branded version, also I have contacts to make one at low cost from high tensile alloy):

    Boat is 19" speedboat with solid glass hull, timber fore and side decks, no transom cutout or splashwell. Transom is heavily reinforced 50mm thick ply plus epoxied glass. 100 litre fueltank is midships

    Proposed engine is an elderly used 90 to 115 Hp (haven't got one yet). Research suggests weight will be around 300 pounds

    The PDF drawings are NOT to scale, but I have drawn up a scale version to check that the lengths of sides all fit together. For those who only think in inches - please see the last page

    Key questions are:

    1 Is a 22" setback enough?
    2. If so, is 2.5" rise about right?
    3. Would it be best to enclose the bottom of the bracket for flotation reasons and put a plate in the top for bolt access?

    Thanks

    Alan
     

    Attached Files:

  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    The bottom should be closed, except for small drain holes. Otherwise it may hit the water and "catch". It is difficult to say if the dimensions are ideal, but they are similar to other boats. A good idea is to have slots for the bolts to be able to fine tune the position of the motor.
     
  3. michael pierzga
    Joined: Dec 2008
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    Location: spain

    michael pierzga Senior Member


    Yah...box it in. Less spray and wave slap...and the box volume helps to offset the engine aft weight at rest. Engineer your setback to be as light as possible.

    Perhaps first mock up a plywood unit to get the detailing correct before welding or laminating the stand

    Google for pictures

    [​IMG]
    hosting imagenes
     
  4. aussiebushman
    Joined: Oct 2009
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    Location: Taralga NSW

    aussiebushman Innovator

    Good advice thanks guys

    Comments on the dimensions from other also welcome

    Alan
     
  5. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    Make sure you have a stainless prop, in good condition, on your old engine, alloy is hopeless in "skinny" water, that is where the slighest hint of water starvation will have them letting go (over-revving).
     
  6. aussiebushman
    Joined: Oct 2009
    Posts: 283
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    Location: Taralga NSW

    aussiebushman Innovator

    base added

    Incorporating your advice, the sheet layout has been changed to include a bottom plate, also an access hatch in the top. Please see modified drawing,

    The lower mounting tab is not missing. To make it possible to cut most of the shapes on the power guillotine (the rest will have to be done with an angle grinder in the absence of a plasma cutter), that tab will be 50X50 mm angle welded to the base.
     

    Attached Files:


  7. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    Judging height Vs setback is an inexact science, you should make it so that you have the freedom to drop the engine lower without it bottoming out on a pod ledge, still too high. That will give you more freedom to experiment with heights.
     
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