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  1. #1
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    Default Show me your transom's knee brace (tinny) jon boat transom rebuild

    I need to add a knee brace to my aluminum jon boat so transom flex is reduced. Need some ideas of how they are installed, height up transom, length extending on hull, fastened to hull, etc. I have a 3/16" thick aluminum plate that should work nice, just need to figure out the best way to make the knee brace.

    Here is my original post with a few pics, guess I didn't ask the my question right the first time around.

    http://forums.iboats.com/boat-restoration-building-hull-repair/beefing-up-transom-cracked-welds-288510.html

  2. #2
    Honorary Moderator Emeritus tashasdaddy's Avatar
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    Default Re: Show me your transom's knee brace (tinny)

    this was my duracraft.

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  3. #3
    Vice Admiral 5150abf's Avatar
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    Default Re: Show me your transom's knee brace (tinny)

    I was gonna jump in on your original post but got busy.

    Ya that transom is pretty weak, can't believe they didn't design one in it at the factory, that is what happens when price point is more important than design.

    As you said in your original post it would help alot just to get a proper transom in it, double up 3/4 ply gunnal to gunnal and 12-14'' wide with maybe some angle aluminum bolted to the gunnal and transom to tie everything together and take all the load off of those 2 corner peices.

    The problem with a knee brace is you are going to have to attach it to the bottom of the boat for it to work right which means holes.

    I wouldn't weld it as the constant flexing would eventually crack the weld.(aluminum welder for 21 years)

    I went out and shot a pic of mine for what is worth.

    do you have access to aluminum and a brake?

    If it were me I think I would redo the transom first and see how much that gets you. I think just that with 6-8 thru bolts should get you along way.
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  4. #4
    Moderator Bob_VT's Avatar
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    Default Re: Show me your transom's knee brace (tinny)

    This is a before picture and the wood was so rotted I removed it almost completely by hand. This is a 14' Starcraft that is rated for a 20 hp max for reference.

    Look at the size of the wood.




    Now this is the after. I made to wood extend just a bit deeper into the boat and actually cover a portion of the brace.

    This is a great link to boat specifications http://boatspecs.iboats.com/
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  5. #5
    Ensign Shizzy's Avatar
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    Default Re: Show me your transom's knee brace (tinny)

    Nice work Bob! looks like you also added a little metal to the top edge of the transom?

    the brace in my Alumacraft is a cast aluminum piece, so Im no help there. It also has the "aqueduct transom" which adds bracing all along the top. its a 14' tinny and is rated for 40hp.

    my first thought when looking at the pics Is that the transom wood should extend further down. there isnt really a lot there.
    '68 Alumacraft FD, '65 Johnson 6HP "The Sea Monkey"

  6. #6
    Ensign Shizzy's Avatar
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    Default Re: Show me your transom's knee brace (tinny)

    here are a couple of pics of my FD anyway. this transom is kind of over built. It has a knee brace as well as two braces on the top side of the transom. it has whats called the "Aqueduct transom" which is essentially a tray made of the same material as the rest of the boat that attaches to the sides as well as the transom bracing the whole works. it does have holes that allows it to drain any water that comes over the transom. the wood on the inside of the transom is 1.5" or so thick and it has a much larger piece of 3/8" thick plywood glued to the outside of the transom. most boats I see without this type of transom are rated for 15HP max but This little boat it rated for a 40HP motor.







    its hard to see in this pic, but it shows the wood on the outside of the transom. Again its 3/8" and its glued solid and screwed to the transom.

    '68 Alumacraft FD, '65 Johnson 6HP "The Sea Monkey"

  7. #7
    Moderator Bob_VT's Avatar
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    Default Re: Show me your transom's knee brace (tinny)

    The aluminum "cap" that was on the boat was just turning to dust. I just used some 2" aluminum angle and used a rubber mallet to contour it and then welded it. It really has no other purpose then covering the top of the transom. The boat is a '63 and in great shape.
    This is a great link to boat specifications http://boatspecs.iboats.com/
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  8. #8
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    Default Re: Show me your transom's knee brace (tinny)

    Thanks guys,

    The way it is right now, the exterior plywood is about 8" shy of the hull's bottom and is pushing in-out at that point. I'm thinking of extending the exterior piece of plywood lower and attaching a knee brace. Also integrating the trolling motor mount into the exterior transom plywood, kind of a wing. I like 5150's idea of adding angle braces to the ends of the interior transom plywood, adds rigidity and rivet holes are out of the water. I have scrap aluminum (1/16, 1/8, 1/4, 2x2 angle) to work with, but don't have access to a brake, brake sure would make this job easier...

    Is it bad to have the exterior transom plywood extended all the way to the bottom of the hull? Just wondering if the transom flex will stop if I extend it all the way to the bottom, and not cause other stresses and cracked welds,

    Here's my plan for the knee brace; rivet/JBweld 2x2 aluminum angle to 1/4" (it's 1/4 not 3/16) aluminum plate, rivet this to the hull with backing plates on the exterior, thru-bolt it to the transom and plywood.

    What do you think of this idea,,, have the knee brace bolted to the transom, extend forward to the rear seat and then bolted to the seat bulkhead and hull at this point? The hull is made from pretty thin aluminum and I'm a bit leary of adding something that's going to cause metal fatigue. It's either this idea or a conventional knee brace, something like 12" up and 8-10" out.

    Here's a pic of what I have in mind, criticize away


  9. #9
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    Default Re: Show me your transom's knee brace (tinny)

    Oh BTW,

    I'm going to be the Iboat's guinea pig for Arauco plywood. If you guys haven't seen it, it's made with very thin plies (thinner than marine ply) and exterior glue. Much stronger than normal exterior ply, but the down side I hear, it's more susceptible to rot than doug fir. Going to soak for a few days in Jasco Termin-8 wood preservative, home-grown pressure treating I'm not going to seal it at all (outside of the wood preservative) and let it breath like like the boat is built right now.

    If it falls apart, oh well, I'll just make new pieces in a few years...

  10. #10
    Honorary Moderator Emeritus tashasdaddy's Avatar
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    Default Re: Show me your transom's knee brace (tinny)

    take a notch out of the brace, where it meets the transom and hull bottom, for drainage.
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  11. #11
    Ensign Shizzy's Avatar
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    Default Re: Show me your transom's knee brace (tinny)

    what TD said. All the bracing in mine has spots built in to let the water drain towards the plug or the drain holes on the transom.
    '68 Alumacraft FD, '65 Johnson 6HP "The Sea Monkey"

  12. #12
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    Default Re: Show me your transom's knee brace (tinny)

    Yup, drain holes will be built into it.

    I was at Sportsman's Warehouse today and looked at (Alumicraft it think?) jon boats to see how they incorporated the knee brace. Pretty sad how my boat is constructed compared to the Alumicraft, even their 7hp rated 1236 jon boat had a knee brace ,,, and the transom of their 1436 was built like a tank compared to my Tracker 1436.

    Alumicraft and Tracker 1436 boats,,, same price

  13. #13
    Captain Tim Frank's Avatar
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    Default Re: Show me your transom's knee brace (tinny)

    You could check the pics in my post on replacing the transom on my tin boat (search "orlando clipper) for another angle.
    I had to "disconnect" the knee braces (2 on either side of the motor) to get the old transom ply out....and was intimidated with the thought of messing with the through bottom rivets; I used part of the originals and just sistered new aluminum plate and angles to the existing stubs. In hindsight and after a few comments by others in this forum, I will probably go back and redo that with proper bucked aluminum rivets eventually.

    I would think that you could use a web of the 3/16" plate and attach it at each end with two pieces of 1/8"x1" angle as I did. Solid as a rock.

  14. #14
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    Default Re: Show me your transom's knee brace (tinny) jon boat transom rebuild

    Finally got it all together and don't think I'm going to need a knee brace with how I have it put together, feels real solid.

    Here's what I started with,



    These are the new transom plywood pieces made out of 3/4 marine plywood, the old ones are sitting on top for comparison. Decided not to use Arauco plywood since I had enough marine plywood scraps. Had to use the B side for the exterior piece (notice boat patches in ply?) Oh well...

    The plywood was treated with Copper Green. I made a "vat" out of a few scrap pieces of plywood lined with a garbage bag and then filled with Copper Green. I let the plywood soak for 24 hours on each side and edges.


    I added an aluminum plate to keep the outboard and troller clamps from digging up the plywood and through bolted where needed. I noticed there was a touch of movement at the bottom corners so I added an angle brace as 5150 suggested. Fun, fun, fun making those compound angle corner braces, but it really firmed up the corners.




    The exterior piece is extended to the bottom of the hull and just even with the hull so it won't cause drag. I also added a "wing" to support the trolling motor better. The wing also made the trolling motor quieter since it's clamping onto wood on each surface.



    It's going to be 2 weeks before they can weld up the cracked transom welds, but I couldn't wait. Bolted everything together and took it out this morning. Big difference in how sturdy the transom is compared to before. Origionally, when I opened up the outboard I could see/feel the transom flex, not anymore If I man-handle the outboard when it's tilted up I notice a bit of movement, but if it was perfectly solid the rest of the boat would probably fall apart...

    Just waiting for the welds to be done then it's time to paint the plywood, brackets, 5200 bolts, etc...

    I'm still tossing around the idea of adding in a knee brace, but honestly, don't know if I need it anymore.

  15. #15
    Moderator Bob_VT's Avatar
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    Default Re: Show me your transom's knee brace (tinny) jon boat transom rebuild

    Two things.

    The aluminum plate that you added to protect the transom....... if you can get a bead welded across it to capture the brackets...... there is a chance that the motor can slip on that aluminum on a hard turn.

    That copper green is interesting. Will the chemical react with aluminum? Did you epoxy or resin coat the wood after treatment? I am just leary of chemicals playing hell with aluminum.

    It looks great. I doubt you will ever need a knee brace with a 7.5
    This is a great link to boat specifications http://boatspecs.iboats.com/
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  16. #16
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    Default Re: Show me your transom's knee brace (tinny) jon boat transom rebuild

    Quote Originally Posted by Bob_VT View Post
    It looks great. I doubt you will ever need a knee brace with a 7.5
    Thanks! You should have seen the transom flex before I rebuilt it scary amount of movement. But yes, with the way it's built now I don't think I'm going to need a knee brace.

    Good call on the aluminum plate, I'll drill a cup into the plate with a forstner bit so the outboard clamps have a pocket to rest. The picture doesn't show it well, but the transom cap has a lip that would retain the outboard if it did slip. Better safe than sorry though...

    I checked Copper Green (copper naphthalate) and it's supposed to be less corrosive than CCA treated plywood and a ton less corrosive than AQC, so it should be OK with aluminum. I did find a touch of corrosion behind the exterior transom CCA plywood where the outboard was clamped so I treated it with Jasco Metal Prep and painted the bare aluminum. Also painted the bare aluminum anywhere the plywood was touching as I'm not 100% convinced that Copper Green won't cause corrosion.

    Copper Green is petroleum based so I want to wait a few weeks before painting it. If I was going to use epoxy or resin I wouldn't have used Copper Green, too oily and I think it would affect the epoxy or poly resin's bond. I painted a few Copper Green treated pieces with Rustoleum and it leached through on the first 2 coats, 3rd coat hid the green color. There is a water based copper naphthalate product that might work well with epoxy or resin though, Jasco Termin8 H20.

    I'm really not too concerned about sealing the wood though, I live in the desert so the wood will dry out.

  17. #17
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    Default Re: Show me your transom's knee brace (tinny) jon boat transom rebuild

    Yes, I recognize this thread is wicked old but I will hopefully be going through a similar process in the near future and wanted to ask a question about an issue alluded to above. I have a 14 foot jon boat and will be adding a transom to it (none currently). I will eventually put a trolling and 9.9 hp on it, does having wood on both sides of the aluminum stern make for a quieter boat? It makes sense at face value...

  18. #18
    Supreme Mariner Woodonglass's Avatar
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    Default Re: Show me your transom's knee brace (tinny) jon boat transom rebuild

    All boats have transoms. Post some pics so we can see what you're talking about. Double wood will not make the boat more quiet.

  19. #19
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    Default Re: Show me your transom's knee brace (tinny) jon boat transom rebuild

    Cool, thanks. I will be posting pics as I go. I have been gathering a bunch of info on here for my little project. I just have to wait for my broken collar bone to heal before I can implement any of them!

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