Transom Reinforcement

Discussion in 'Fiberglass and Composite Boat Building' started by PNW, Jun 25, 2024.

  1. PNW
    Joined: Jun 2024
    Posts: 2
    Likes: 0, Points: 1
    Location: washington

    PNW New Member

    Hi everybody,

    I was curious how you guy might approach reinforcing this transom for a full swim step outboard bracket for up to a 250hp 4 stroke. I have a few ideas but wondering if there were some alternate ideas out there. I want to be around the 3" mark when finished. stringers and bulkheads are going to be replaced as well.

    Thanks in advance.
    IMG_7723.JPEG IMG_7724.JPEG IMG_7725.JPEG
     
  2. TANSL
    Joined: Sep 2011
    Posts: 7,610
    Likes: 788, Points: 123, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Welcome to forum.
    Could you show us a diagram, even in pencil, of what you want to do, with dimensions, position of the engine and its weight?. That would help obtain more appropriate opinions on the issue you raise.
     
  3. PNW
    Joined: Jun 2024
    Posts: 2
    Likes: 0, Points: 1
    Location: washington

    PNW New Member

    Tansl,
    Thank you for your reply.
    Here is a couple bad sketches of the basic transom shape/layout and a little more context. I also included a generic google photo of a similar style of flotation bracket I will be using (not built yet)

    Boat is a 1984 Trophy 2260 I/O that is being converted from the original AQ-125 and 270 drive to a 250hp +/- outboard with a full swim step floatation bracket. This will be in a single outboard configuration with a 15hp kicker motor.

    Estimated Weights-
    Main Outboard: 700LBS
    Kicker Motor: 150LBS
    Swim Step Flotation Bracket: 400LBS

    Since this boat was originally designed as an I/O there isn't much meat or structure in the current transom and needs to be reinforced to hold the new outboard and bracket. I have a few ideas on how to accomplish this but was looking for some other alternate ideas and input.

    Sketch Legend -
    Yellow Section: Unchanged from current - constraints for top cap
    Green Section: Current hull fiberglass approximately 1/4" thick - no core material in this area
    Blue Section: 1/2" thick plywood section laminated to inside of transom
    Red Section: Plywood plug for 270 drive laminated to inside of transom approximately 1 3/4" thick

    Option #1 -
    Cut out blue and red sections and start over with glass and laminated plywood to about 3" thick.
    Note: Transom does have a curve to it between red section and gunwale so likely would have to be done in a few pieces or sliced

    Option #2 -
    Add glass to green section to bring up flush with blue sections then laminate plywood sections to bring flush with red section then laminate plywood full width of transom over top of red section ending with about 3" finished thickness.

    Option 3# -
    Leave as is and laminate plywood full width of transom over red section leaving a gap between green and blue sections and new plywood laminate then using a pourable ceramic transom material to fill the gap.

    Option #4 -
    Pretty much the same as option #3 but cutting out blue and red sections first.


    Hopefully this answered some of your questions. What are your thoughts and ideas?

    Thanks.
    transom iso.jpg transom front.jpg outboard-brackets-page-02.jpg
     
  4. TANSL
    Joined: Sep 2011
    Posts: 7,610
    Likes: 788, Points: 123, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    I would do something similar (not the same, but similar) to what is marked in the picture:
    - delete what is marked in red.
    - place a plywood board of, for example, 30 mm
    - laminate a couple of layers of resistant GRP fabric on the inside.
    - place consoles (green lines) that strongly connect the transom with the existing bottom longitudinals.
    Consolas.jpg
     
    Last edited: Jun 26, 2024
    bajansailor and comfisherman like this.
  5. fallguy
    Joined: Dec 2016
    Posts: 7,929
    Likes: 1,786, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    You’d get quite a bit more structural strength with a structural splashwell. It would greatly reduce the deflection from that much weight n hp and would create a set of better knees.
     
    bajansailor likes this.
  6. comfisherman
    Joined: Apr 2009
    Posts: 785
    Likes: 399, Points: 63, Legacy Rep: 10
    Location: Alaska

    comfisherman Senior Member

    I'd laser a line across the transom at at full sheet width high, then grind out all the spots until it's all the same depth. Then I'd probably do a couple layers of csm just to provide a fill and buffer load from the rough ground surface, followed with probably three layers of a combination mat. Then laminate the ply wood in followed by a few layers of combi mat. Would follow it with either a beefy deck to provide some reinforcement near the top of the added outboard/swim platform or some large fish plates to span between the top of the bracket and some stringers.

    But it's hill Billy napkin math, and work exactly what's payed for it.
     
    fallguy likes this.
  7. fallguy
    Joined: Dec 2016
    Posts: 7,929
    Likes: 1,786, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Good advice from the fisherman to get things even first. After plywood done right, I’d use 1708 to tab and finish it all. People don’t like to spec your laminate because when a transom is done poorly; people perish at sea. The structural splashwall should be behind a full height transom. So your drawings need work.

    Here is a link to good copyrighted picture. Look at the “transom width” picture that includes the splashwell and the full height transom. These are good measures. The full height helps prevent a broach and offers strength and the space under can be foamed for buoyancy and provide a home for batteries. For the engine chosen, make sure to allow clearances and consider bolt patterns before structural connections are made.

    shoutout to @Ike

    Safe Horsepower Standards For Outboard Powerd Boats https://newboatbuilders.com/pages/hp.html
     

  8. fallguy
    Joined: Dec 2016
    Posts: 7,929
    Likes: 1,786, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    @TANSL shows a good picture that is a baseline, but full knees would be best in his drawn locations. The full knees are glassed to the stringers and make the splashwell base.

    Everything gets tabbed together. Anywhere you are not using gelcoat; I’d recommend epoxy for the bonds to reduce the amateur factor. The transom is built and tabbed to bottom and hullsides first, then the knees added and tabbed to transom and stringers. Then you add the full height transom. I think 3/4” enough. Glass the inside off the boat.

    All plywood must be precoated before any bonding or glass is done to avoid drysucking the resin from fillers. Ask a lot of questions and disregard underbuilding advice. The full height is 30” I believe. And the transom 20 or 25, the latter keeps the powerhead higher from any breaking wave.
     
Loading...
Similar Threads
  1. BobinUP
    Replies:
    27
    Views:
    1,110
  2. Rranger
    Replies:
    10
    Views:
    1,562
  3. Mcdo2137
    Replies:
    6
    Views:
    1,404
  4. LennyLongbeard
    Replies:
    2
    Views:
    1,079
  5. Cashinhand2
    Replies:
    13
    Views:
    4,847
  6. Toepfer Marine
    Replies:
    7
    Views:
    2,181
  7. motorbike
    Replies:
    11
    Views:
    2,312
  8. container
    Replies:
    23
    Views:
    4,307
  9. Mak1999
    Replies:
    53
    Views:
    8,841
  10. atengnr
    Replies:
    68
    Views:
    6,019
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.