Plan to reduce the size & weight of the transom kneebrace.

Discussion in 'Boat Design' started by the brain, Jan 28, 2025.

  1. the brain
    Joined: Sep 2016
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    the brain Senior Member

    Plan to reduce the size & weight of the transom kneebrace.

    I think my kneebrace is way overkill for my transom. According to Armstrong (the manufactor of my outboard engine bracket) they say for upto a 200HP outboard 2” of

    Plywood transom is stronge enough.

    My engine is 012 Evinrude ETEC 175hp weight 418LBs. I built the kneebrace extra stronge just incase I opted for more power. It turns out 175hp is plently.



    So now I’d like to reduce the transom weight. Currently the brace weight is 45LBs.

    Plan to trim the lower section about 7” across an about 1” on top plus the lower section of the diangle brace.I estitmate the cuttings will reduce weight by 10LBs.
    POD weight reduction side veiw.jpg

    POD weight reduction.jpg
    I may even grind some overall weight maybe 2LBs. What do you Guys think the sections I’m cutting off won’t lessen the strength of the brace?

    Vessel isn’t stern heavey but I’m planning on a transom livewell which will add some weight.

    Thanks for advicing.
     
  2. messabout
    Joined: Jan 2006
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    messabout Senior Member

    Tell us more about your boat. Saving 45 pounds is a big deal on a small boat but does not amount to much on a larger one.
     
  3. comfisherman
    Joined: Apr 2009
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    comfisherman Senior Member

    What about some time with a hole saw making lightening holes?
     
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  4. BlueBell
    Joined: May 2017
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    BlueBell . . . _ _ _ . . . _ _ _

    So you're going to reduce the boat weight by 10 lbs?
    That's not going to make a significant difference in boat performance.

    EDIT: Missed that you're going to grind 2 lbs more off.
    So you're up to 12 lbs removed and then you're going to add a livewell...
     
  5. the brain
    Joined: Sep 2016
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    the brain Senior Member

    1977 22' starcraft Cheiftain express inboard converted to outboard bracket.
    hull is approcimently 1900LBs engine little over 400LB, 40 gallons of fuel, half of which is aft of midship. two groupe 27 lead batterys at midship.basicely she's a 22' shreched 20" very light weight pocket cruiser.
    explodedview.jpg
    ten year years ago I removed the 715LB mercruiser inboard engine an outdrive, replaced w/ a 383LB 04 Johnson 175hp, pod 45LB, kneebrace 45LB total 473LB however
    even though this weight is significantly less than the mercruiser this weight is 20" aft of transom.
    the water line at rest seamed to be the same, notice the side chin thats the water level. been using the vessel configured this way for over ten years.
    SPLASH2.jpg
    I've recently repowered w/ another 175hp 012 Evinrude ETEC which weight is 418LBs haven't splashed her w/ new engine. but am thinking the extra 35LBs of new engine will increase stern weight. trailering.jpg plus the desired livewell proballey 13 gallons additional 91LBs. so I calculating weights.
    BRACE.jpg BRACEBOLTED.jpg
    the lighting holes in addition to the cutting of lower/upper an small bit of side is a good suggestion thanks comfisherman

    I actuely plan two live wells small 13 gallon placed on inside of stern aft seating..slitely larger 20 gallon near midship
    reardeck brace.jpg
    I could probley completely remove kneebrace an just have backing plates where the though hull bolts are but since vessel was built w/ a inbroad thought it be a better plan to just reduce braces weight w/out losing any strenght. thanks for any advice.
     
    Last edited: Jan 29, 2025
  6. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    So you have a new-ish 2" marine ply transom, yes?

    If so, I would have done things differently as far as the knee goes.

    The transom should have a decent epoxy fiberglass skin to handle the usual dock rash issues. 1/8" thickness over fir plywood, or a bit less over hardwood plywood like meranti or real mahogany should suffice. The backing plate is designed based on the mounting bolts. It should be 5/8 as thick as the bolt diameter for SS, or 3/4 as thick for aluminum. This goes for the pod material thickness as well. If the motor is mounted with 4 12mm bolts, the mounting plate should be be at least 10mm thick. 8mm aluminum is doubtful.


    Most people make the backing plates too large. They only need to extent about 1 bolt diameter past the bolt hole. You want about double the bolt cross section on the pod-to-hull joint as you have on the motor to pod joint. And it helps if the footprint of the pod is bigger than the motor plate. The main reason you need more bolt area is because the transom is wood and you need to manage both compressions and shear in the transom. I'd use 8 12mm SS bolts of decent provenance to mount the pod.

    This leaves the problem of the backing plate. All the bolts need are about a 3" circle of 10mm aluminum each. But you might as well reinforce and stiffen the whole area. My first idea would be to make a frame of 3 x 3 X 0.25 aluminum angle and then use heavy-duty 12 mm x 24 mm x 2.7 mm SS washers to land the bolts.

    As far as the knee goes, the one you have is ten times stronger than the stringers it is attached to - so it isn't doing anything except wallowing out holes in the stringer. I'd add a frame around the top of the transom and bracket it to the sides of the hull. Small knees connecting the transom to the bottom stringers, sized like the stringer material, would help stop the transom from pounding down on the hull. But they would weigh all of a pound or so and can be fastened with #10 sheet metal screws or small bolts. Think Simpson Strong Ties in aluminum.
     
  7. the brain
    Joined: Sep 2016
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    the brain Senior Member

    So you have a new-ish 2" marine ply transom, yes well ten years ago rebuilt transom w/ marine grade plywood covered w/ fiberglass resin?

    If so, I would have done things differently as far as the knee goes.

    The transom should have a decent epoxy fiberglass skin vessel is aluminum transom skin is 3/16” aluminum to handle the usual dock rash issues. 1/8" thickness over fir plywood, or a bit less over hardwood plywood like meranti or real mahogany should suffice. The backing plate is designed based on the mounting bolts. It should be 5/8 as thick as the bolt diameter for SS, or 3/4 as thick for aluminum. This goes for the pod material thickness as well. If the motor is mounted with 4 12mm bolts, the mounting plate should be be at least 10mm thick. 8mm aluminum is doubtful.
    I used the same size bolts to mount the pod which normaley mount the engine to transom, pod so I reamed out 4 holes on pod
    cutaway.jpg


    So I have the pod bolted to the transome w/4 bolts IIRC 5/8" SS.

    Most people make the backing plates too large. They only need to extent about 1 bolt diameter past the bolt hole. You want about double the bolt cross section on the pod-to-hull joint as you have on the motor to pod joint. And it helps if the footprint of the pod is bigger than the motor plate. The main reason you need more bolt area is because the transom is wood and you need to manage both compressions and shear in the transom. I'd use 8 12mm SS bolts of decent provenance to mount the pod.

    This leaves the problem of the backing plate. All the bolts need are about a 3" circle of 10mm aluminum each. But you might as well reinforce and stiffen the whole area.yes this is why I over built the backing of the knee brace My first idea would be to make a frame of 3 x 3 X 0.25 aluminum angle and then use heavy-duty 12 mm x 24 mm x 2.7 mm SS washers to land the bolts.

    As far as the knee goes, the one you have is ten times stronger than the stringers it is attached to - so it isn't doing anything except wallowing out holes in the stringer. I'd add a frame around the top of the transom and bracket the hull is basicley built like you describe from the factory
    reardeck brace.jpg

    it to the sides of the hull. Small knees connecting the transom to the bottom stringers, sized like the stringer material,stringers are are very thin about 1/16” diangle brace this thin would be way to flimsy would help stop the transom from pounding down on the hull. But they would weigh all of a pound or so and can be fastened with #10 sheet metal screws or small bolts. Think Simpson Strong Ties in aluminum.

    It will be interesting to see if indeed the diangle brace has hollowed out the mount holes in the stringers.will report back on this when pods is removed for reduction.

    I believe the majority of the preasure the pod see’s is from the top being pulled backward.

    Very little stress on lower pod.

    Additionally this older Armstrong pod (early 90ies) has rod bars that attach to top mount holes of pod just above the top engine mount holes is designed to bolt to ransom.since I reduced pods lenght when reducing rods I may have to tap the threas I doubt the threads are the lenght of rod.
    . ROD1.jpg

    These bar’s are used for higher horsepower engines over 250HP. I haven’t used them but would like to.
    topbar brace hi HP.jpg
    To farrer complicate things I modified the pod from 31” setback to 20” then I modified the transom by cutting it down from 46” an incorporating a splashwell(depth of 10”) for several reasons.

    1.didn’t want engine 31” off transom. An was still able to tilt engine completely out of water at rest.
    pod welded.jpg

    fully tilted.jpg
    There’s a few other reasons for the diet stern weight reduction an additional floatation cambers) I have another thread on relocating the stern mounted fuel tank an moving forward the saddle tank) in addition to the smallish livewell, I.

    1. want a 120LB 9.9hp kicker engine.
    kicker w pods.jpg

    Plan to reuse the 11”cutoff piece of the modified pod as the stationary kicker bracket mounted on port side about 7LBs.
    Top brace bolted to POD an transom.jpg


    2. a rollcage engines protector stretching the width of the transom.probaly 25LBs.here's a example image.
    engine protector4.jpg
    3. add smallish sealed floatation pods to each side of the bracket pod. Think of a single engine pod w/ a large floatation chamber a little less the size of a dual engine pods.
    pod stage1 2.jpg
    pod stage1.jpg
    4. install a modern telescoping ladder (these are very hard to find mine needs a bit of fabbing to surport the platform) which is suitable for dog to stand on,an facilate landing extra large fish on plateform example cobia tap them on head before bringing into vessel.10LBs
    telescoping ladderfull ladder.jpg
    5.Yamaha 1600watt portable generater 50LBs mounted ontop of transom.
    my yama1.jpg
    I've been planning all my transom area mods for over ten years. it not like I just rolled outof bed the other day an came up w/ all this stuff.thanks for any advise or constructive critizen.
     
    Last edited: Feb 2, 2025 at 12:42 PM
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  8. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Okay, I have a lot better feel for what you are doing with the additional pictures and explanation. It looks like the added stiffeners on the outside of the transom handle that issue. The transom is strong enough to deliver all the engine loads under all conditions to the hull aft perimeter. You just need to make sure the transom and the hull stay bonded all the way around. There isn't a serious moment problem here that requires a knee like that. The bigger issue is all the extra weight slamming up and down. Judicious use of that 1/16 stinger material to reinforce the transom-to-hull joint is where I would concentrate my effort. I'd rather see 20 little #8 screws than a few big bolts. The hull has no way to distribute big point loads without oil-canning.

    As your pictures showed, the original 31 inch pod used 12 bolts plus two optional rods to extend the footprint on the transom. You really do need more than just 4 at 20 inches extension - even if they are 5/8". And they need a footprint that is 3 inches bigger in all directions than the motor mount footprint. So I would pull the bracket, bog the (original?) 4 holes, retool the bracket with a 10 mm flange to take 8 new bolts, make a matching backing plate from plate or flat stock, and ditch the knee.
     

  9. the brain
    Joined: Sep 2016
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    the brain Senior Member

    I have been considering adding 2" plate verticaly on both sides of pod an have though hull mount bolts on outside rather than inside also seal bottom an add a drain plug.
    https://www.amazon.com/gp/product/B0829VC919/ref=ox_sc_act_title_8?smid=A1A4WAEGJNR9ED&psc=1

    I'll probably just replace w/ another pod 20" setback is hard to find.

    I acutely have three 1/2" bolts on each side of pod, look closely at the top of pod in additional floatation pod images only used the outside holes.
    in 2nd image of initial post the 3rd bolt on star side can be observed just above battery an oil resovior.
    thanks
     
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