plywood transom- sheer strength of 12mm meranti

Discussion in 'Fiberglass and Composite Boat Building' started by container, Oct 8, 2020.

  1. container
    Joined: May 2019
    Posts: 18
    Likes: 7, Points: 3
    Location: new zealand

    container Junior Member

    hey y'all got any more of that wisdom?
    I'm rebuilding an old fkd 1980's 14ft NZ javelin sailing skiff into an outboard cruiser (dont judge, small boats are more fun)

    The hull is 10mm foam, polyester & chop strand layup however appears it was infused in a female mold so is pretty solid for a 38 year old dinghy. I have installed a new 12mm meranti BS1088 plywood transom. it is the lower grade 5 ply, I ordered 9 ply but this is what turned up and didnt realise until too late. the verticals- outboard well sides and longitudinals- are 9mm gaboon and the floor will also be 12mm meranti. the dimensions are as follows-

    transom beam OA - 1480mm
    outboard well width at transom- 300mm
    longitudinal frame spacing- 630mm apart
    transom height- 350mm
    cockpit floor height (top of longitudinals)- 180mm
    total number of horses swinging off the back- 15

    The aft face of the transom inside the well will have a 12mm ply doubler as per the drawing to allow for a greater range of outboard trim. the cavity will be filled with foam and timber wedges where the outboard sits to take the compression. the transom structure is bonded to the hull with 300g carbon biax, everything else is stuck in with 300g glass biax. all 100mm tape. There will also be scuppers immediately outboard of the well sides, these will be 100 x 50mm glass box section tube running (at cockpit floor height) from the main transom out through the transoms on the squat boxes. cutting these holes will obviously remove a lot of structure as there is only 150mm between the c/pit floor and the deck.

    My concern here is the sheer load on the transom at the hard point where the well sides intersect.

    I was thinking either a foam and carbon knee as drawn in red (I have lots of scraps laying around perfect for the job) this is the lightest option but more work. I also want to eventually use the space immediately in front of the transom for an under floor fuel tank so the knee would complicate that and eat up volume. the other option is another ply doubler on the front of the transom to spread the load out to the longitudinals. I have 4, 6, 9 and 12mm ply to choose from. easier but heavy and if i take this approach with everything then the boat will get very heavy very quickly.

    OR
    I am grossly over estimating the power of the mighty 15hp yammy. Its not much but I want to be able to load the boat up with **** and power out of a trough when downwind in a breeze. it gets pretty lumpy where i live and i dont like to not go boating just because its a bit windy so seakeeping is #1 priority

    any educated opinions on the matter would be much appreciated. i dont want a heavy slug of a thing, but also dont want it to fall apart half way to great barrier full of camping gear fresh water and 12 dozen beers. but most importantly, i dont want to do any more than absolutely necessary

    cheers!
     

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    Last edited: Oct 8, 2020
  2. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    Don't forget to give your transom some rake, at least 12-13 degrees, outboards assume that much.
     
    fallguy likes this.
  3. container
    Joined: May 2019
    Posts: 18
    Likes: 7, Points: 3
    Location: new zealand

    container Junior Member

    Yes you can see in the sketch, the ply doubler on the aft face of the transom has angle of 10-12° to allow for outboard trim
     
  4. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    It doesn't look quite enough to the eye, you might find yourself wanting a little more tuck-under, to lift the stern.
     
  5. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    My first thought was not enough angle, too. fyi. It could be a camera thing, but I am assuming you finished the knees square and that is not what I'd have done.

    I'd have run the knees to the transom. Transom would be 3x12mm laminated together with light sheathing...

    I don't like the doubler thing at all. How the heck are you gonna bolt onto that?
     
  6. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    Your ob mounting bolts must sit flush. The transom must be one solid unit.

    see this pic! I have finger sketched bolts. You can see they will not be washered or backed up properly due to the angles...yes, I exxagerated the angles to highlight the issue

    Get rid of the doubler idea. Run the knees on the angle of the transom. Build the transom at 1-1.5" thick. Glass wrap the top or it WILL separate.

    Also, made thick enough and with some glass; you really don't need to run the angled knees. Unless that dimension above the sole is great. You can also glass tape the sole to the transom.

    E4604950-BE91-4D63-A488-91D12216B1D1.jpeg
     
  7. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    Revise the concept. You must modify the work on the stringers and hull to accept an angled transom.
     
  8. container
    Joined: May 2019
    Posts: 18
    Likes: 7, Points: 3
    Location: new zealand

    container Junior Member

    copy that mr. efficiency, it is drawn at 11deg assuming of course the (structural) transom is plumb when the boats floating, which is anyones guess. i'll build it at 15 degrees just to be sure.

    fallguy- thanks for the advice but that was not really the information i was after. a 15hp does have bolt holes but will most likely just be attached with the usual screw clamps found on any small outboard. if i were to bolt it on in the future then making some glass/carbon wedge shaped backing plates to accommodate the angle of the hardware will be very easy. the void in between the 2 pieces of 12mm ply will be solid timber where the motor sits- so no issues with compression, either side of that the cavity will be filled with 80kg/m3 pvc foam. very solid and a fraction of the weight of solid wood. light sheathing is for surfboards! you are better off with a thick coat of resin if that is the case, everything on this boat is 300gm biaxial glass, including the external laminate of the hull extensions/transom

    re the plywood splitting, i realize this, i am a boatbuilder! i also realize that simply gluing 3 pieces of 12mm ply together will be bomb proof. it will also weigh about 20kg and be completely over kill for a 15ft dinghy where weight is critical. there
    is currently no knees, only the 2 longitudinals which the cockpit sole will sit on and the 2 sides of the outboard well, everything else is speculative at this point. i want it to be engineered for the task/loads at hand. no more and no less. i
    im thinking the single large foam/carbon knee in the centre will be the go (as drawn in red on the original sketch) as my only concern here is the shear load on the 12mm ply where it is intersected by the outboard well sides.
     

  9. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    Even your clamps will not work and will try to roll toward the top where the angles open. But do not heed the advice and find out on your own. You can always build a shim.
     
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