Calculating Required Transom Thickness

Discussion in 'Boat Design' started by Mat-C, Sep 5, 2007.

  1. Mat-C
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    Mat-C Senior Member

    Few of the scantling rules or boat design engineering texts that I have cover the topic of calculating transom thickness for a sterndrive or outboard powered boat.
    A couple of the older ones give required thickness for engines up to something like 50hp - obviously the biggest available back when they were written!
    I suspect that the defining requirement will be the engine manufacturers required thickness - at least for sterndrives, but I'm not sure of this, and I'd still like to check it to be sure...

    Any help much appreciated....
     
  2. Greybarn
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    Greybarn Junior Member

    CE has a requirement for this. I don't really remember the ISO reg, but it may be ISO12217, or somthing like that.
     
  3. Guillermo
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    Guillermo Ingeniero Naval

    It is ISO 12215, but I'd rather use any of the Classification Societies' Rules for the time being (Lloyds, ABS, etc, etc). But probably it's easier for you to consult an easy to get book on the matter, as it is Gerr's "Boat Strength for Builders, Designers and Owners"
    Cheers.
     
  4. Nojjan
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    Nojjan All thumbs...

    What you are looking for is ISO 12215-5 (you don't need all sections).

    I don't remember Gerr addressing the transom reinforcements with specific regards to outboards, did he?
     
  5. Mat-C
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    Mat-C Senior Member

    No - unless I've missed something, Gerr suggests the Transom thickness as the same as the topsides - which clearly isn't sufficient for a panel with the weight and thrust forces from an o/b or sterndrive.

    And again, I may have missed it, but ABS - at tleast the Rules for High Speed Craft - doesn't seem to address it either....
     
  6. alan white
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    alan white Senior Member

    It would probably make more sense to calculate or test a finished boat based on ft lbs applied to lever the transom either forward or backwards without deformation, something that could and should be engineered based not on thickness alone, but on a variety of significant factors such as transom height, width, thickness, curvature, angle, material, deck, well, and sole structural contributions, etc..
    Thickness alone is such a wild-assed engineering generalization. It is possible to build a thin transom of enormous stiffness by incorporating ribbing, gussets, etc., or to build a relatively weak thick transom due to extreme width (a pontoon boat that has a ten foot wide transom but no gusseting at all to provide triangulation to a horizontal member).

    Alan
     
  7. raw
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    raw Senior Member

    Australian Standard AS4132.3 I assume you are looking at grp? If not what?
     
  8. Guillermo
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    Guillermo Ingeniero Naval

    Applicable ISO norm is not ISO 12215-5, but ISO/DIS 12215-6, point 5.5.4 "Transoms for outboard engine and sterndrive installation", where a table in kW-thickness is given. But this norm is still under discussion, that's why it has to be taken with care.

    I've checked Gerr's and I agree with Mat-C a topside-like panel may be not enough, depending on power.

    Det Norske Veritas Rules for vessels under 15 m, chapter 6, points 6.532 and 6.533, gives specific reccommendations for transoms for outboard and stern drives engines, with a table in kW-thickness.
     
  9. gonzo
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    gonzo Senior Member

    O/Ds and outboards are all build to accomodate a transom of 1 5/8" to 2". You don't have much of a choice.
     
  10. Willallison
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    Willallison Senior Member

    Volvo's Duoprop leg requires a transom thickness of 51mm +/- 6mm. The Mercruiser Bravo 3 specs 51 - 57mm.

    For those who don't have them - me included;) - it would be nice if rules like DNV were freely available....
     
  11. Guillermo
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    Guillermo Ingeniero Naval

    I'll see on monday what can I do for you, Will. ;)
     
  12. Willallison
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    Willallison Senior Member

    Thanks my friend...:p
     

  13. Mat-C
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    Mat-C Senior Member

    Thanks everyone - sorry for my delayed response - I'be been away.

    I've seen, though no longer have, the DNV chart. From memeory it only really covers transoms for power outputs up to 8o kW (?), and anything over this remains the same thickness....
    And what about construction detail? I mean a 50mm transom could be made up of two 25mm layers of ply only, or it could be 1 layer of ply and 25mm of FRP laminate!
    I guess you could always use a program like Vectorlam to calculate the 'strength' etc of a given laminate schedule - but how to determine the required values? I don't think any of the rules give this?....
    All rather lacking.... still, I guess we can always fall back on the tried and tru method... the manufacturers require 50mm thickness, and the transoms don't (often) fall apart....
     
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