Transom Angle?

Discussion in 'Boat Design' started by ddrdan, Sep 2, 2010.

  1. ddrdan
    Joined: Mar 2009
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    ddrdan Junior Member

    I did a search on here and was unable to find the answer to my question. The answer may be so simple only an idiot would have to ask. So here goes:

    Is there a standard or "industry typical" transom angle, relative to the bottom of the boat?
     
  2. Olav
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    Olav arch. nav.

    12° is quite common as most outboards and sterndrives are designed to match that angle.
     
  3. Robbo
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    Robbo Junior Member

    the angle (whatever it is-12°?) will be measured against the waterline rather than the vessels bottom.
     
  4. Petros
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    Petros Senior Member

    of course on sailboats, they are all over the map. It was once considered racy to rake it way back as a steep angle, now all the racers like it raked forward (or no transom at all!). And there is always the near vertical transom (or slightly forward, or aft rake too). With sailboats I think it is just current fashion, there appears to be no reason other than appearance.

    for outboards the angle of the transom sets the approximate thrust angle, so there is much less variability.
     
  5. Vulkyn
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    Vulkyn Senior Member

  6. marshmat
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    marshmat Senior Member

    If we're talking about the angle at which the transom is raked aft (as I believe ddrdan is asking):
    12 degrees is fairly typical for outboards and sterndrives. I prefer a bit more, 12 to 15 degrees, as you'll occasionally need to trim the engine in when running stern heavy. A typical outboard or sterndrive has about 16 to 20 degrees of usable trim range; on a 12 degree transom, that usually works out to something like -5 to +15 (but almost nobody runs above +10 except at idle, and many boats could really use -10 or so when climbing to plane)
    Jet drives typically take 0 or 5 degrees, although there are exceptions.
    If it's not for an outboard, sterndrive or jet, you can do pretty much whatever looks right.
     
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  7. rambat
    Joined: May 2002
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    rambat Member at large

    Transom

    One thing for sure, there is no absolute angle. But 12 is what I used to think was the average. With the bigger drives and more power many seem to favor 13. Here is what Mercury say's:

    Transom Construction
    Requirements:
    • Transom thickness ‑ 51 mm (2 in.) minimum to 64 mm (2.5 in.) maximum
    • Transom surfaces must be parallel ‑ Within 2 mm (0.078 in.) measured at top and
    bottom of the cutout hole
    • Area covered by the inner transom plate ‑ Must be flat within 2 mm (0.078 in.)
    • Area covered by the outer transom assembly ‑ Must be flat within 1 mm (0.031 in.)
    • Transom angle ‑ 10° to 16°
     
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  8. messabout
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    messabout Senior Member

    You have some leeway because the outboard has a range of adjustment. I almost always use 14.04 degrees. The reason for the odd angle is simple. The tangent of that angle is 0.25 which is an easy number to work with. It is simply a 1 on 4 slope.
     
  9. tunnels

    tunnels Previous Member

    12 used to be the norm for the older motors that didnt have power trim 15 is better these days Power trims are more efficent and being able to tuck the motor can sometimes be able to get the bow down when the stern wants to dig in !. Nothing like lots a control in the trim department !!:p :p
     
  10. ddrdan
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    ddrdan Junior Member

    Thanks for all the answers and information. It's greatly appreciated!
     
  11. jimbo2010
    Joined: Sep 2010
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    jimbo2010 Junior Member

    I'm just starting to build a 10' Lawton tender, it has a beam of 45"

    [​IMG]

    I'll be laying up the transom next week and will be using a 5hp Merc, I know it's a bit bigger than I need but I already have it.

    I was going to use 7/8 mahogany planks to make up the transom.

    Any thoughts on strength and thickness?
     
  12. PAR
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    PAR Yacht Designer/Builder

    7/8's is plenty for your engine. Place a knee or two at the bottom of the transom to transfer loads.
     
  13. jimbo2010
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    jimbo2010 Junior Member

    Do you think I should just go edge to edge glue up of the planks or biscuits or splines?
     
  14. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    That type of transom construction isn't well suited to outboards, particularly ones bigger then necessary. A typical outboard transom would be double planked with off set seams, glued and screwed together. I've repaired plenty of this style of transom and they don't hold up all that well either, but would be fair superior to a single planked transom. A better solution would be tongue and groove planking, though this still doesn't address the real problem with solid wood planking in a transom.

    Biscuits are useless an only work with water based adhesives. The water based goo causes the biscuit to swell up and wedge the joint tight, but if you use epoxy or other adhesive that doesn't swell the biscuit, they are useless. Spines add more fabrication effort for questionable qualities. If the spine is the same species as the transom planking, then they can be useful, though the rabbits or grooves they live in, should be deep. Honestly it's better to mill tongues and grooves on the stock and not have to screw with spines all together.

    When I have to rebuild a double planked transom, I use a plywood core and glue and screw to this. The result is a stable transom, that doesn't leak. It's a fair bit more work, but it insures you don't have to do it again, which is what most clients desire. On a single planked transom, like yours, I haven't any good ideas, other then what's mentioned.
     

  15. jimbo2010
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    jimbo2010 Junior Member

    Ok but I can't see doing double 3/4 planks on a 70 lb boat?



    How about since I'm doing a strip build anyway I do one two or even three layers of strips with bead and cove?
     
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