Transom Reinforcement

Discussion in 'Boat Design' started by stylemismatch, Mar 8, 2010.

  1. stylemismatch
    Joined: Mar 2010
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    stylemismatch Junior Member

    I'm restoring a 30 year old Checkmate. The boat came from the factory with a steel reinforcing plate on the transom, something like this sketch:

    [​IMG]


    The original reinforcing plate is kind of beat up so I want to replace it, but it's going to be difficult for me to fabricate a single piece with the 90 degree bends to the exact thickness required to fit the transom. Would I be giving up any strength by using two plates, like this:

    [​IMG]


    Thanks
     
  2. BMcF
    Joined: Mar 2007
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    BMcF Senior Member

    Yes you would..a lot. The top cap section is what provides most of the strength to resist bending of transom in the transverse direction.

    I've made many of those transom plates over the years from stainless steel..three pieces with the top 'cap' welded to the two vertical plates. Most any weld/fab shop should be capable of whipping one up for you that way if you provide them a decent carboard or wood template to go by..
     
  3. stylemismatch
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    stylemismatch Junior Member

    OK, thanks.

    It's my opinion that the plate is overkill considering how beefy the transom is (brace AND knees), but I'm not a structural engineer so I'll keep the plate as close to original as possible.
     
  4. BMcF
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    BMcF Senior Member

    You could be quite right about whether the plate is really required to laterally stiffen the transom to resist bending....you did not specify or show that the transom had any knees or additional reinforcement. And it has been a while since I even looked at a Checkmate transom too.

    Actually, I prefer the approach of using knees that are integrated/connected to main stringers over simply reinforcing a transom laterally. Having owned or worked on many smaller boats with oversized outboards (some extremely oversized in terms of both weight and power..a 235 Evinrude on a 17' Champ for example), I've seen too many that ended up with cracking of the transom vertically at the corners, the only place the transom was 'fixed' at the beam ends.

    It could be that in your case, that plate was not intended as a structural reinforcement but simply as a clamping/compression protection for mounting the engine..if I was to hazard a guess, the max HP rating on that old Checkmate is pretty high like many were back in the 'good old days'. LOL. Heck, I had a 70s vintage GW Invader that was 13' LOA and the hull weighed 'maybe' 300 pounds?..and the rating tag said "Max HP - 80":D
     
  5. stylemismatch
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    stylemismatch Junior Member

    Here's a photo of the new transom showing the brace and knees (it looks much better in person, that photo didn't turn out well). The knees are glassed directly to the hull (btw, the hull has an end-grain balsa core). The stringers hadn't been installed yet, they're inboard of the knees about 6 inches or so. One thing you'll see is how the transom goes down to only 1 inch thick directly behind the splashwell to allow clearance for that, I had been assuming that may have been a reason the plate was used (but I'm not a boat designer).

    (My boat is 18 1/2 ft. and rated at 180 hp. )



    [​IMG]
     

  6. BMcF
    Joined: Mar 2007
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    BMcF Senior Member

    LOL...ok..that looks to be headed towards one very stout transom. IMO you will be fine just adding plate(s) for clamping reinforcement.

    BTW...these days you can order exactly the size plate(s) you want from companies like Penn Stainless. You send them the sketch of your required plate and they neatly waterjet or laser cut it for almost free as a value-added service for buying the stainless steel from them. Most of the major aluminum and stainless suppliers have added that service in the last 4 or 5 years.
     
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