plywood transom question

Discussion in 'Wooden Boat Building and Restoration' started by john mac, Apr 14, 2012.

  1. john mac
    Joined: Apr 2012
    Posts: 31
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Ireland

    john mac Junior Member

    hey everyone how thick would i need to make the transom for a 22 ft long and 7 ft beam plywood on oak cabin cruiser. The original boat had an inboard engine which took up a lot of space, so i'm going to use an outboard initially, but am rebuilding it with the option of converting her back to an inboard setup later. The new boat is also two feet longer. The transom was made up of 3 layers glued together, a half inch outer layer and two three quarter inch inner layers that stop half way up, the upper section replaced with a frame screwed and glued to the outer layer. (sorry no pics as camera battery flat)
     
  2. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 476, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Transom thickness is dependent on outboard size and target speed. A 1" (25 mm) transom is my minimum, which is good up to 8 HP. 1.5" (38 mm) is used up to 50 HP, 2" (51 mm) up to 100 HP and 2.75" (70 mm) over this. Very light, high speed craft should consider thicker transoms, as well as other heftier scantlings. There are other considerations besides a transom for outboards, like knees and braces to transfer loads to the hull bottom.
     
  3. john mac
    Joined: Apr 2012
    Posts: 31
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Ireland

    john mac Junior Member

    Hey Par thanks, going on power source, and the old transom, 2" seems to be about right. There are 1" thick stern knees attached to the keel and stringers, do you think making up metal L shaped brackets to connect the transom to the stern knees is a bit over the top? cheers
     
  4. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 476, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Without seeing what you're working with I can't tell honestly, but another set of knees at the sheer clamp/shelf to transom would be wise too.
     
  5. john mac
    Joined: Apr 2012
    Posts: 31
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Ireland

    john mac Junior Member

    Hey Par, forgot to say it already has sheer clamps, another issue is the transom to keel and transom to stringer knees are a fair size, which was fine when they were hidden in the engine compartment. But i'm planning on using an outboard to free up space which will unfortunately expose the knees. Is there any way of constructing them smaller but just as strong? cheers
     
  6. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 476, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

  7. groper
    Joined: Jun 2011
    Posts: 2,467
    Likes: 123, Points: 73, Legacy Rep: 693
    Location: australia

    groper Senior Member

    Composite...
     
  8. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 476, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I'd use composite on a more modern boat, but this is a wooden transom, that sounds built in a traditional manner. You can mechanically fasten a composite knee or brace, but they're much better suited to bonding and tabbing. Metal on the other hand doesn't have this issue and needs fasteners to do a great job. Lastly, I'm not sure of materials availability on his emerald island, but a hunk of metal is pretty commonly acquired and very likely much less costly too.
     
  9. john mac
    Joined: Apr 2012
    Posts: 31
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Ireland

    john mac Junior Member

    cheers guys, metal is no problem, i've more experience with metal than i have with wood. if i can get a few pictures of metal knees i can fabricate a few up in no time. cheers again.
     
  10. tunnels

    tunnels Previous Member

    What part of Ireland you in ??
    :confused:;)
     
  11. john mac
    Joined: Apr 2012
    Posts: 31
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Ireland

    john mac Junior Member

    hey tunnels, up north, Ballywalter County Down.
     
  12. tunnels

    tunnels Previous Member

    Close to Belfastbut onthe coast !!
    Me ancesters can from Sligo some where !!
    ;)
     
  13. john mac
    Joined: Apr 2012
    Posts: 31
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Ireland

    john mac Junior Member

    hey tunnels, yeah about 40 minutes from Belfast, i'm on the east coast looking at Scotland, Sligo is a beautiful part of the country, great for surfing but it can be a bit scary in a small boat when a storm sweeps in from the atlantic!
     
  14. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 476, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Metal knees and braces look like wooden ones in profile, except they usually are "flanged", which means they cut the basic shape from plate, then weld a top, back and possably an angled flange (more flat stock) to the plate to stiffen it up. Lets say it's a 8"x 8" (~200 x 200 mm) triangle looking thing. Along the edges some1.5" to 2" (38 to 51 mm) flat bar will be welded to the triangular plate, making an "I" beam of sorts out of it. It can also be done as a "C" section instead of an I section, which provides more room for through bolts too.
     

  15. john mac
    Joined: Apr 2012
    Posts: 31
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Ireland

    john mac Junior Member

    cheers PAR, i should be able to fabricate the knees out of metal as you described, but i have a bigger problem. The only importer of hardwood in the north of ireland doesn't have timber longer than 16 ft, my boat is 22 ft. None of the saw mills have anything either. i know a single keel timber is stronger than two joined together, but i don't have the option, has anyoe any ideas on making a keel from multiple timbers?
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.