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Thread: Need help designing my transom

  1. #1
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    Default Need help designing my transom

    I'm designing a 12 foot flat bottom Jon boat and need help designing the transom so that it will be strong enough for a Tohatsu 6hp short shaft motor. Here's a picture of my transom design so far:

    Mo Jon Boad D900 b.jpg

    Although I've read recommendations for a 1-1/2" thick transom, those were for much larger size outboard motors. I want to keep the weight down to a minimum since the boat will be car topped on our van. So I want to use a 1" thick transom made out of two 1/2" marine plywood epoxied together. I'm open to adding a 12" square piece of 1/2" plywood on the back to make the mounting area for the motor 1-1/2" thick if that's necessary.

    All references to glue and resin will be epoxy.

    The sides and bottom will be glued and screwed to the transom. I know that screwing into end grain of plywood is not recommended but the outside of the hull will be fiberglassed using 6 oz cloth and the inside joints of the sides and bottom to the transom will be reinforced with fiberglass tape.

    Will this transom design be strong enough for a 6hp outboard motor?

    BTW, there will be a rear bench seat 2 feet from the transom and a front bench seat aft of where the frame is. The bench seat frames will be 1x2 and will have 1/4" marine plywood on one side for strength. Two frames per bench seat.

    The sides will curve inward from the frame to the bow about 3" on each side. The amount of curve will be determined by how much I can bend the sides with the chine log and upper 1x2 on the top edge of the sides.

    Thanks for any help,
    DayTripper
    Last edited by DayTripper; 05-05-2021 at 05:04 PM.

  2. #2
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    Default Re: Need help designing my transom

    The transom is thick enough as-is. Beware that the transom knee may not be able to be that tall - check to see what clearance you have re: the motor when clamped on the transom. If there is severe clearance problems, install two transom knees far enough off centre so the engine & pad fit between the knees. I would prefer this anyway, because it places the unsightly transom pad on the inside of the transom.

    Will there be a deck or washboards extending to the transom? If yes, secure it well to the transom top edge. If not, I would like to see horizontal knees in each top corner of the transom and hull sides to tie everything together.

    The thickness of the transom pad will be dependent on how wide the engine clamp jaws are. I haven't done many 6-hp boats, but I usually build up the transom in way of the engine clamps to be 2" thick. Check what your engine needs are before you commit to epoxying the pad on the transom!
    Hope for the best, but plan for the worst.

  3. #3
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    Default Re: Need help designing my transom

    Quote Originally Posted by mmd View Post
    The transom is thick enough as-is.
    That is the main answer I needed.
    .
    .
    Beware that the transom knee may not be able to be that tall - check to see what clearance you have re: the motor when clamped on the transom. If there is severe clearance problems, install two transom knees far enough off centre so the engine & pad fit between the knees. I would prefer this anyway, because it places the unsightly transom pad on the inside of the transom.
    There's enough room for the transom knee to go between the motor clamps but I did forget about having clearance to twist the clamp levers when tightening the clamps. I will investigate using two knees as long as there's room for the gas tank to fit between a knee and the side. Shouldn't be a problem though.
    .
    .
    Will there be a deck or washboards extending to the transom? If yes, secure it well to the transom top edge. If not, I would like to see horizontal knees in each top corner of the transom and hull sides to tie everything together.
    Yes, my design does include horizontal knees at each top corner to support the transom. I failed to show them in the drawing I posted.
    .
    .
    The thickness of the transom pad will be dependent on how wide the engine clamp jaws are. I haven't done many 6-hp boats, but I usually build up the transom in way of the engine clamps to be 2" thick. Check what your engine needs are before you commit to epoxying the pad on the transom!
    Would it be worthwhile to add a 12" square piece to the outside of the transom to get the motor mounting area 1-1/2" thick?

    If I use two vertical transom knees I could put the piece between the knees. Maybe that would be better to have that piece just screwed on to the transom to be replaceable if the round clamps eat into the wood over time.
    .
    How much would the motor clamp on the outside of the transom eat into the transom. The transom will be fiberglassed. I'm guessing not much since the outside of the clamp has a lot more surface area than the round clamps on the inside.
    .
    .
    Thanks for the help,
    DayTripper
    Last edited by DayTripper; 05-05-2021 at 07:25 PM.

  4. #4
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    Default Re: Need help designing my transom

    using 1/4" marine 5-ply w/ NO FOOTBALLS will buid up w/ more plies than 2x1/2" 7-ply

    this is a ruff dwg of what my remembery tells me as how my CLC pram transom was assembled to a total of 1" for a smaller HP w/ a bench seat filleted to the transom instead of using a knee

    IMG_5210.jpg

    BON CHANCE

    sw
    "we are the people, our parents warned us about" (jb)

    steve

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    Default Re: Need help designing my transom

    Quote Originally Posted by DayTripper View Post
    Would it be worthwhile to add a 12" square piece to the outside of the transom to get the motor mounting area 1-1/2" thick?
    I don't know how fancy you want your finished boat to be, so I will assume fancy, and you can down-grade from the following as suits you. Assuming that the transom will be 'glassed in clear epoxy and light glass to achieve a varnished-transom look, you could finish the transom, then make a varnished pad of the same wood species for colour and grain matching, and place it on top of the varnished transom with some bedding goop and a few countersunk and plugged screws. That way, when (not if) the pad gets worn and damaged by the motor mount, it won't be an ordeal to make a new one and replace the old.

    Quote Originally Posted by DayTripper View Post
    If I use two vertical transom knees I could put the piece between the knees. Maybe that would be better to have that piece just screwed on to the transom to be replaceable if the round clamps eat into the wood over time.
    That would be my first go-to solution.

    Quote Originally Posted by DayTripper View Post
    How much would the motor clamp on the outside of the transom eat into the transom. The transom will be fiberglassed. I'm guessing not much since the outside of the clamp has a lot more surface area than the round clamps on the inside.
    See my reply at the top of this posting.
    Hope for the best, but plan for the worst.

  6. #6
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    Default Re: Need help designing my transom

    While designing and building your transom it would be wise to build in some sort of solid tie-off point for a short line between the boat and the motor. It's funny how even small motors sometimes want to twist off of the transom and go swimming (which they don't do well). Funny at least until it happens to your motor.

  7. #7
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    Default Re: Need help designing my transom

    Add two quarter knees.

  8. #8
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    Default Re: Need help designing my transom

    quarter inch is thick enough for the transom, if you brace it right.

  9. #9
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    Default Re: Need help designing my transom

    Quote Originally Posted by swoody126 View Post
    using 1/4" marine 5-ply w/ NO FOOTBALLS will buid up w/ more plies than 2x1/2" 7-ply

    this is a ruff dwg of what my remembery tells me as how my CLC pram transom was assembled to a total of 1" for a smaller HP w/ a bench seat filleted to the transom instead of using a knee

    === sketch not shown for simplicity ===

    BON CHANCE

    sw
    Thank you for taking the time to draw and post your sketch. I really appreciate all of that effort. I will only use marine plywood for my build.

    I planed to use 1/2" ply for my transom because I need the 1/2" ply for the top of the two bench seats. So my plan is to buy 1 sheet of 1/2" ply for the transom (2 pieces for 1" thickness), 2 bench seat tops, transom to bottom knees, knees at the top corners of transom and sides.

    Thanks.

  10. #10
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    Default Re: Need help designing my transom

    Quote Originally Posted by mmd View Post
    I don't know how fancy you want your finished boat to be, so I will assume fancy, and you can down-grade from the following as suits you. Assuming that the transom will be 'glassed in clear epoxy and light glass to achieve a varnished-transom look, you could finish the transom, then make a varnished pad of the same wood species for colour and grain matching, and place it on top of the varnished transom with some bedding goop and a few countersunk and plugged screws. That way, when (not if) the pad gets worn and damaged by the motor mount, it won't be an ordeal to make a new one and replace the old.
    No, not fancy. My goal is just to have a functional 12 foot jon boat that is as light as possible for car topping but structurally sound. Looks is not important. Will fiberglass the outside and then paint. Will use the boat one day a week in the beginning and probably one day every 2 weeks later. Rest of the time it will be stored outside on a side but covered with a tarp.
    .
    .
    "If I use two vertical transom knees I could put the piece between the knees. Maybe that would be better to have that piece just screwed on to the transom to be replaceable if the round clamps eat into the wood over time."
    That would be my first go-to solution.
    I'll go with the two transom to bottom knees with a 1/2" piece between the knees on the inside (for a total of 1-1/2" thickness for the motor to clamp on).

    Thanks.
    Last edited by DayTripper; 05-06-2021 at 03:21 PM.

  11. #11
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    Default Re: Need help designing my transom

    Quote Originally Posted by Todd Bradshaw View Post
    While designing and building your transom it would be wise to build in some sort of solid tie-off point for a short line between the boat and the motor. It's funny how even small motors sometimes want to twist off of the transom and go swimming (which they don't do well). Funny at least until it happens to your motor.
    That is definitely in my plan after seeing this youtube video.

    Outboard falls off.


    The outboard falls off at about 36 seconds into the video. It looks to be about a small outboard like I'm getting.

    Here's another youtube video of a small outboard falling off that I just found:

    Outboard falls off rescue boat

    Falls off at 1:27 into the video.
    Last edited by DayTripper; 05-06-2021 at 03:32 PM.

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    Default Re: Need help designing my transom

    Quote Originally Posted by Jim Ledger View Post
    Add two quarter knees.
    I don't know what that means. Please explain to this newbie. Thanks.

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    Default Re: Need help designing my transom

    Quote Originally Posted by wizbang 13 View Post
    quarter inch is thick enough for the transom, if you brace it right.
    That's interesting and I can understand the logic. Though I won't have enough 1/4" ply to execute that method, it would be very interesting to see a sketch of how the transom frame would be built. Thanks.

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    Default Re: Need help designing my transom

    Quote Originally Posted by DayTripper View Post
    I don't know what that means. Please explain to this newbie. Thanks.
    Quarter knees is the proper boat-y term for horizontal knees in the upper corners of your transom.
    Hope for the best, but plan for the worst.

  15. #15
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    Default Re: Need help designing my transom

    Quote Originally Posted by DayTripper View Post
    That's interesting and I can understand the logic. Though I won't have enough 1/4" ply to execute that method, it would be very interesting to see a sketch of how the transom frame would be built. Thanks.
    Basically, Wizbang is being a bit cheeky to try to get a laugh. Essentially he is saying that if you design & build a framework to distribute the loads carried by the transom, the transom skin doesn't need to do anything but keep the water out, so it can be very thin.
    Hope for the best, but plan for the worst.

  16. #16
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    Default Re: Need help designing my transom


    This boat is very curvy..that is for strength.
    A flat bottom/flat sided/flat transom boat will be wobbly without a lotta knees. But putting on a thick transom without the knees is just throwing needless weight aboard.

  17. #17
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    Default Re: Need help designing my transom

    I'm not trying to be cheeky and I'm not trying to get a laugh.

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    Default Re: Need help designing my transom

    The brevity and cryptic-ness of your response in post #8 seemed to be so.
    Hope for the best, but plan for the worst.

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    Default Re: Need help designing my transom

    Quote Originally Posted by wizbang 13 View Post
    This boat is very curvy..that is for strength.
    = picture of unique transom =
    I was hoping for a framing sketch for a jon boat .

    A flat bottom/flat sided/flat transom boat will be wobbly without a lotta knees.
    Wouldn't the bottom plywood, horizontal transom knees, 13" wide bench seat tops (2 seats) and a 2 to 2.5 feet deck at the bow keep the structure from wobbling?

  20. #20
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    Default Re: Need help designing my transom

    Quote Originally Posted by DayTripper View Post
    Wouldn't the bottom plywood, horizontal transom knees, 13" wide bench seat tops (2 seats) and a 2 to 2.5 feet deck at the bow keep the structure from wobbling?
    That's a good start. You should also install gunwale strips along the top of the sides from foredeck to transom quarter-knees, and stiffening longitudinals from bow to stern on the bottom. These latter ones are often put on the exterior of the bottom to act as 'skid plates' to protect the bottom when dragging the boat up on a sandbar.

    Last edited by mmd; 05-06-2021 at 04:27 PM.
    Hope for the best, but plan for the worst.

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    Default Re: Need help designing my transom

    Quote Originally Posted by mmd View Post
    That's a good start. You should also install gunwale strips along the top of the sides from foredeck to transom quarter-knees, and stiffening longitudinals from bow to stern on the bottom. These latter ones are often put on the exterior of the bottom to act as 'skid plates' to protect the bottom when dragging the boat up on a sandbar.
    All of what you said is precisely my plan.

    Would 1x2 gunwale strips only along the top of the "inside" of the sides be sufficient without adding rubrails along the top "outside" of the sides?

    What dimensions for the longitudinals on the exterior bottom would you suggest (width and thickness) for a 12' jon boat? And just 3 (center and two sides) be about right?

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    Default Re: Need help designing my transom

    The single gunwale strips on the inside (called - unsurprisingly - inwales) would be sufficient for strength, but on the outside ("outwales") will also protect the plywood sides from damage from bumping into docks, snags, and rocks. On inside and outside makes it bullet-proof. The bottom stringers might as well be the same size as the in/outwales to make milling stock easy. Three ought to be fine.
    Hope for the best, but plan for the worst.

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    Default Re: Need help designing my transom

    Quote Originally Posted by mmd View Post
    The single gunwale strips on the inside (called - unsurprisingly - inwales) would be sufficient for strength, but on the outside ("outwales") will also protect the plywood sides from damage from bumping into docks, snags, and rocks. On inside and outside makes it bullet-proof. The bottom stringers might as well be the same size as the in/outwales to make milling stock easy. Three ought to be fine.
    Since strength will be sufficient I'll just go with inwales. I'll use three 1x3's for the bottom stringers.
    Thanks.

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    Default Re: Need help designing my transom

    Quote Originally Posted by DayTripper View Post
    I'll use three 1x3's for the bottom stringers.
    Maybe I should use the term "runners" for the strips of wood that run lengthwise "under" the bottom. If that's not the correct term, please let me know.

    I've decided on 4 runners (instead of 3 with a center runner) for two reasons:
    - 1. I'll have two vertical transom knees on both sides of the motor's clamps. So having runners under the bottom where the knees are will be stronger.
    - 2. I wonder if a center runner will interfere with the water flow to the prop? Maybe not having a center runner will be a good thing. Discussion?

    HOW TO ATTACH THE RUNNERS?

    A) I plan to fiberglass the exterior of the boat (bottom and sides) with epoxy resin and then add the runners with epoxy glue without using screws. Therefore the runners will just be on the flat portion of the bottom. The runners will have a tapered front that starts at the the upward curve of the bottom plywood. If I just epoxy the runners on, the inside corner formed by the runner and the bottom will be square.

    B) If I use thickened epoxy fillets on the inside corners with fiberglass tape over the runners, the inside corners will be rounded.

    Would the boat track better with square inside corners?
    Will just epoxying the runners on as described in plan A above be strong enough?
    Last edited by DayTripper; 05-07-2021 at 05:18 PM.

  25. #25
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    Default Re: Need help designing my transom

    Runners take a lot abuse, and epoxy is pretty permanent. Might be difficult to repair a damaged one.
    Hope for the best, but plan for the worst.

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    Default Re: Need help designing my transom

    Quote Originally Posted by mmd View Post
    Runners take a lot abuse, and epoxy is pretty permanent. Might be difficult to repair a damaged one.
    I have no idea, nor seen anything on the web on how to attach replaceable runners. Are they screwed on? If so wouldn't that compromise the fiberglassed bottom?

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    Default Re: Need help designing my transom

    Bed them in some goop that is a little more forgiving than 3M 5200 (that stuff just won't let go!), and screw them in place. Fill the countersunk screw holes with putty and paint over. The bedding will keep the water from penetrating the screw holes
    Hope for the best, but plan for the worst.

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    Default Re: Need help designing my transom

    Quote Originally Posted by mmd View Post
    Bed them in some goop that is a little more forgiving than 3M 5200 (that stuff just won't let go!), and screw them in place. Fill the countersunk screw holes with putty and paint over. The bedding will keep the water from penetrating the screw holes
    Sounds like a plan. Thanks.

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