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Thread: Is my transom design OK? Overkill? Underkill?

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    Default Is my transom design OK? Overkill? Underkill?

    Here's a picture of my proposed transom design for a 12 foot jon boat:

    JonBoat-R 141 Transom design 500.jpg

    The sides of the boat will be 1/4" marine plywood. The bottom will be 3/8" marine plywood. The outboard motor will be a 6HP.

    The back of the transom will be 1/2" marine plywood. I'm using 1/2" plywood for the back of the transom since I needed 1/2" ply for the two bench seat tops. So one 4x8 sheet of 1/2" marine ply. No 3/4" ply.

    The motor mount horizontal will be a 2x6. The 2x6 will be milled flat so will probably end up being 1-1/4" thick. That will result in the motor mount, including the 1/2" ply back, being 1-3/4" thick.

    The 2 side verticals will be 1-1/4" thick x 2" wide. The center vertical will be 1-1/4" thick x 3-1/2" wide. The bottom horizontal will be 1-1/4" thick x 2" high and the top will be tapered to allow water drainage.

    There will be two 3/4" x 2" angle braces from the transom to a 2x4 mounted to the floor. The front of that floor 2x4 will be attached to the lower horizontal back seat frame. Note that floor 2x4 does not extend to the transom. That is to allow water movement to port or starboard along the transom for drainage. There will be two 1" diameter drain holes in the transom.

    The transom quarter knee will be epoxied and screwed to the top of the 2x6 motor mount and to a 1x3 that runs between the transom and the back seat's vertical frame.

    The top of the back seat is omitted to allow seeing the angle brace and floor 2x4. The yellow plywood on the back seat will contain closed cell foam for flotation.

    Is this transom design overkill for a 6 HP outboard motor? Or underkill? Modifications?
    Last edited by DayTripper; 04-12-2022 at 06:55 AM.

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    Default Re: Is my transom design OK? Overkill? Underkill?

    DayTripper, I am not a naval architect. Other folks smarter than me will be along soon to comment.

    For comparison, here is the transom for the 15 foot skiff I am building, so the size is similar to yours. See Post #92 in this thread:

    http://https://forum.woodenboat.com/...ountains/page3

    Here is a photo of my transom:

    resized transom in progress.jpg

    The designer of my boat recommended doubling the 9 mm (3/8") skin for use with an outboard, and that's what I did. Note that my framing is 3/4" thick, not 1-1/2". Fore and aft stiffness will be added by a seat/compartment at the level of the horizontal member about a quarter of the way up the transom. I will have at most a 3 hp engine, and probably none at all.

    2-by framing is probably overkill for a 12 foot jon boat, and will make it heavy. Your entire hull will be stronger if instead of the 2x4 block you run a plank keelson or keel the length of the boat. My 15 foot plank keel is 3/4" x 6".

    It's hard to tell from your drawing, but it looks like the corners of your transom are square. The boat will resist capsizing better if the sides have some flare, making the lower corners of the transom obtuse (> 90*) angles.

    You might want to compare some online plans of boats similar to the one you want to build. It often turns out to be a good idea to use plans from a trained and experienced designer, who has already figured out solutions to all the boatbuilding problems. The boat design thought process is way different from carpentry.

    Best of luck!
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    Default Re: Is my transom design OK? Overkill? Underkill?

    looks fine to me,overkill in construction if anything
    the corner knees are more important than many realize.
    turning sharply at high speed is what torques em out , not the prop thrust
    of course this is "just a jonboat", but if you van add just a little tumblehome , the quarter inch ply sides will be much stiffer from being just a little bent.
    my wee boat is an extreme example of that...bent equals eggshell kind of strength 25hp/40 mph...9mm is the thickest ply on it. most is 6mm and 4mm

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    Default Re: Is my transom design OK? Overkill? Underkill?

    Here is a Jim Michalak jon boat design on Duckworks. Note the flare at the transom. Spending $30 to download the plans would easily save $30 worth of time, frustration, and materials.

    https://duckworks.com/jonsboat-plans-instant-download/
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    Default Re: Is my transom design OK? Overkill? Underkill?

    As always, the more complete a picture we are presented with, the more on-target the advice will be.

    At first glance, I'd say your sketch is hella stout for a boat that size. But you don't say what size outboard you intend. Nor what sort of use you intend. Nor what sorta skipper you'll be: caveman or ballet dancer? Etc. Etc.
    David G
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    Default Re: Is my transom design OK? Overkill? Underkill?

    I like Bruce's suggestion of a bit of tumblehome at the transom for strength as wells visual interest. A Jon Boat is the most basic hull form (IMHO) and the tumblehome would raise it to another level..... wait, how about some at the bow as well. I want to see some more pics as you go along. You have a chance here to make history! The tranny design you have drawn looks plenty strong for a 6 hp. Not sure why you want to plane it down to 1 1/4?

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    Default Re: Is my transom design OK? Overkill? Underkill?

    Quote Originally Posted by UCanoe_2 View Post
    DayTripper, I am not a naval architect. Other folks smarter than me will be along soon to comment.

    For comparison, here is the transom for the 15 foot skiff I am building, so the size is similar to yours. See Post #92 in this thread:

    http://https://forum.woodenboat.com/...ountains/page3
    Unfortunately, the link doesn't work for me.

    Here is a photo of my transom:

    resized transom in progress.jpg

    The designer of my boat recommended doubling the 9 mm (3/8") skin for use with an outboard, and that's what I did. Note that my framing is 3/4" thick, not 1-1/2". Fore and aft stiffness will be added by a seat/compartment at the level of the horizontal member about a quarter of the way up the transom. I will have at most a 3 hp engine, and probably none at all.
    Thanks for taking the time to post the picture of your transom. I see that my design is "kind of" similar to yours.

    2-by framing is probably overkill for a 12 foot jon boat, and will make it heavy. Your entire hull will be stronger if instead of the 2x4 block you run a plank keelson or keel the length of the boat. My 15 foot plank keel is 3/4" x 6".
    I was wondering about using 2-by framing for the transom being overkill. I decided not to run a keelson the length of the boat because I want the bottom to be flat without bumps to bare feet when moving about on the boat. I do totally agree that a keelson running the length of the boat is good for strength. Also for transferring power from the motor to the entire hull. But my thinking for my simple jon boat is that 3/8" marine plywood bottom 46" wide will be sufficient to do that for my 6HP motor.

    It's hard to tell from your drawing, but it looks like the corners of your transom are square. The boat will resist capsizing better if the sides have some flare, making the lower corners of the transom obtuse (> 90*) angles.

    You might want to compare some online plans of boats similar to the one you want to build. It often turns out to be a good idea to use plans from a trained and experienced designer, who has already figured out solutions to all the boatbuilding problems. The boat design thought process is way different from carpentry.

    Best of luck!
    Last edited by DayTripper; 04-12-2022 at 03:26 PM.

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    Default Re: Is my transom design OK? Overkill? Underkill?

    Quote Originally Posted by wizbang 13 View Post
    looks fine to me,overkill in construction if anything
    Thanks for that vote of confidence. Makes me feel better that it's not underkill.

    the corner knees are more important than many realize.
    turning sharply at high speed is what torques em out , not the prop thrust
    of course this is "just a jonboat", but if you van add just a little tumblehome , the quarter inch ply sides will be much stiffer from being just a little bent.
    My design is using flat, 90 degree sides because I want to store the boat on one side to save space.

    my wee boat is an extreme example of that...bent equals eggshell kind of strength 25hp/40 mph...9mm is the thickest ply on it. most is 6mm and 4mm
    Nice boat! Thanks for the pic.

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    Default Re: Is my transom design OK? Overkill? Underkill?

    Quote Originally Posted by David G View Post
    As always, the more complete a picture we are presented with, the more on-target the advice will be.

    At first glance, I'd say your sketch is hella stout for a boat that size.
    Thanks for sharing your opinion. Very important to me.

    But you don't say what size outboard you intend.
    Guess you missed it. I did say that I will be using a 6HP motor.

    Nor what sort of use you intend. Nor what sorta skipper you'll be: caveman or ballet dancer? Etc. Etc.
    The jon boat will only be used for close to the shore fishing on the ocean. Probably 200 yards max. Only taken out on reasonably calm days. Traveling will be at trolling speed or drifting. We want to catch fish, not speed around ..... . Flat bottom design so can also be used to go "torching". That is going out at low tide at night on really calm days with strong head lights. Spearing or using a net on a long handle to scoop sleeping fish on the flats.

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    Default Re: Is my transom design OK? Overkill? Underkill?

    Quote Originally Posted by Oldad View Post
    I like Bruce's suggestion of a bit of tumblehome at the transom for strength as wells visual interest. A Jon Boat is the most basic hull form (IMHO) and the tumblehome would raise it to another level..... wait, how about some at the bow as well. I want to see some more pics as you go along. You have a chance here to make history! The tranny design you have drawn looks plenty strong for a 6 hp. Not sure why you want to plane it down to 1 1/4?
    Thanks for your vote of "plenty strong for a 6 HP". I will be planing the 2x6 down to 1-1/4" to flatten it. The 2x6 that I bought has a slight cup to it in the 6" direction after drying it in our carport for 3 weeks. I'm going to flatten that 2x6 so the 1-1/2" thickness will probably end up as 1-1/4" or 1-3/16". I think that might be better anyway for a bit of weight saving .

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    Default Re: Is my transom design OK? Overkill? Underkill?

    More than strong enough to my eye. But I see lots of dirt getting trapped around the bottom end of those braces. And trapped dirt = rot. I would favor one big fat traditional standing knee. Simpler and it would look better, too.
    -Dave

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    Default Re: Is my transom design OK? Overkill? Underkill?

    Quote Originally Posted by DayTripper View Post
    Unfortunately, the link doesn't work for me.


    Thanks for taking the time to post the picture of your transom. I see that my design is "kind of" similar to yours.


    I was wondering about using 2-by framing for the transom being overkill. I decided not to run a keelson the length of the boat because I want the bottom to be flat without bumps to bare feet when moving about on the boat. I do totally agree that a keelson running the length of the boat is good for strength. Also for transferring power from the motor to the entire hull. But my thinking for my simple jon boat is that 3/8" marine plywood bottom 46" wide will be sufficient to do that for my 6HP motor.
    DayTripper, try this link:

    http://forum.woodenboat.com/showthre...ountains/page3


    Here is a photo of my keel, just a plank on the bottom of the boat outside (not yet sawed off flush with bow). No interior keelson to stub your toes on or collect dirt.

    keel and skeg installed_resized.jpg
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    Default Re: Is my transom design OK? Overkill? Underkill?

    Quote Originally Posted by Woxbox View Post
    More than strong enough to my eye. But I see lots of dirt getting trapped around the bottom end of those braces. And trapped dirt = rot. I would favor one big fat traditional standing knee. Simpler and it would look better, too.
    Thanks for your valuable evaluation of "more than strong enough".

    I did consider one big fat vertical knee at first. But the two 1x3 angle braces would be lighter and "much" easier to make so I decided to go that route.

    As far as dirt being trapped around the bottom of the angle braces, we will be rinsing the inside of the boat with fresh water after every (or most) trips to get any salt off. Spraying down the inside should keep the angle braces clean.

    Thanks.

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    Default Re: Is my transom design OK? Overkill? Underkill?

    Quote Originally Posted by UCanoe_2 View Post
    That link works. I'll browse it later this evening. Thanks.

    Here is a photo of my keel, just a plank on the bottom of the boat outside (not yet sawed off flush with bow). No interior keelson to stub your toes on or collect dirt.

    keel and skeg installed_resized.jpg
    Very good solution substituting a keelson with a keel to keep the inside bottom clutter free. I'll think about that solution.

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    Default Re: Is my transom design OK? Overkill? Underkill?

    Here is a rather elegant transom knee. It appears to be fastened through the boat's bottom and probably also through the external keel.

    https://artatkinson.blogspot.com/200...som-build.html
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    Default Re: Is my transom design OK? Overkill? Underkill?

    I'd add a drain in one of the corners. If it were a vee bottom I'd center the drain in the lowest point, directly under the knee.
    There is no rational, logical, or physical description of how free will could exist. It therefore makes no sense to praise or condemn anyone on the grounds they are a free willed self that made one choice but could have chosen something else. There is no evidence that such a situation is possible in our Universe. Demonstrate otherwise and I will be thrilled.

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    Default Re: Is my transom design OK? Overkill? Underkill?

    Quote Originally Posted by JimD View Post
    I'd add a drain in one of the corners.
    Mind reader. That's exactly my plan for the drains.

    One drain in the starboard corner for drainage while rinsing the inside of the boat. The boat will be tilted in the starboard direction while rinsing. Chose the starboard corner, (since the fuel tank will be on the port side), in case the starboard corner drain plug needs to be pulled while underway for drainage. ..... Though it's not obvious from the picture I posted, there will be drain notches in the frames running along side the chine logs. That will allow superior drainage while rinsing the inside of the boat with the boat tilted to starboard. Those chine long drain notches will be on both sides of the boat. Also two drain notches midway in the frames, that can be seen in the picture I posted, for drainage while underway.

    The second drain in the transom will be on the port side closer to the center of the boat. That will be the main plug to pull while underway for drainage.

    If it were a vee bottom I'd center the drain in the lowest point, directly under the knee.
    Last edited by DayTripper; 04-12-2022 at 07:01 PM.

  18. #18

    Default Re: Is my transom design OK? Overkill? Underkill?

    your design is plenty strong for much more horses and heavy use.
    One first princple point though...the short longitudinal that takes the angle braces.
    Framing should not end in the middle of a panel, but should run from frame to frame, in this case from last frame to transom.
    It sets up a hard point in the panel.
    In this case the rest of the overengineering will make this a moot point.
    Is Buoyancy included in this design?
    I like a big chunk back there to float the motor and of course level buoyancy when fully flooded.

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    Default Re: Is my transom design OK? Overkill? Underkill?

    Quote Originally Posted by Wayne Poulsen View Post
    your design is plenty strong for much more horses and heavy use.
    One first princple point though...the short longitudinal that takes the angle braces.
    Framing should not end in the middle of a panel, but should run from frame to frame, in this case from last frame to transom.
    It sets up a hard point in the panel.
    In this case the rest of the overengineering will make this a moot point.
    Is Buoyancy included in this design?
    I like a big chunk back there to float the motor and of course level buoyancy when fully flooded.
    I don't read the forces that way. I see the stringer that the knee lands on, p&s, transferring and distributing the load that would - without it - as you say, create a stress riser in a relatively flimsy location.
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    Default Re: Is my transom design OK? Overkill? Underkill?

    Quote Originally Posted by DayTripper View Post
    My design is using flat, 90 degree sides because I want to store the boat on one side to save space.
    Just make up a couple of wedges at each end of an 8' 2x to rest the flared sides on, holding the bottom vertical or just past. If putting the top against the wall, a screw eye or hook can secure the boat to prevent accidental damage.

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    Default Re: Is my transom design OK? Overkill? Underkill?

    Another space saving storage idea, for a smaller boat and a 10 foot ceiling.

    resized flying dinghy.jpg
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    Default Re: Is my transom design OK? Overkill? Underkill?

    Quote Originally Posted by Wayne Poulsen View Post
    your design is plenty strong for much more horses and heavy use.
    Thanks for sharing your insight. I've wondered if using a horizontal 2x6 for the motor mount plus the 1/2" plywood back is too much overkill. Since I'll be flattening that 2x6 anyway, do you think I should flatten it to 1" thick to have a total motor mount thickness of 1-1/2"? Fattening the 2x6 from 1-1/2" thick to 1" thick will reduce the weight of the 2x6 by 1/3. And consequently the weight of all of the rest of the framing lumber on the transom will also be reduced by 1/3. A lot more work flattening the 2x6 to 1" thick but it's just a one time job.

    One first princple point though...the short longitudinal that takes the angle braces.
    Framing should not end in the middle of a panel, but should run from frame to frame, in this case from last frame to transom.
    It sets up a hard point in the panel.
    Yes, I originally had that longitudinal floor 2x4 going from the transom to the seat frame. But I really want water that collects at the bottom of the transom to be able to move along the transom to the drain hole in the starboard corner.

    Strength wise I'm thinking that the force created by the outboard is trying to twist the top of the transom backward. There will be tension on the angle braces trying to hold the top of the transom in place. Plus any pushing force from the motor would be pushing the angle braces, which in turn pushes against the floor 2x4 which in turn pushes against the bottom frame of the seat, which in turn distributes the force horizontally on the floor plywood.

    In this case the rest of the overengineering will make this a moot point.
    OK, then I guess flattening the horizontal 2x6 to 1-1/4" thick will be better than 1" thick for "over engineering".

    Is Buoyancy included in this design?
    I like a big chunk back there to float the motor and of course level buoyancy when fully flooded.
    Yes, in my first post, I mentioned that the yellow plywood around the back seat will be filled with closed cell foam for buoyancy. I calculated that filling that much foam will offset about 85 lbs of sea water. The engine weighs 55 lbs so there will be 30 lbs of additional flotation for miscellaneous stuff. I will also be filling a smaller compartment in the front bench seat with closed cell foam to offset about 40 lbs of sea water. I'm assuming a small anchor and chain+rope rode will weigh about 8 lbs. So the front seat will provide about 32 lbs of additional flotation. That should float the boat level (I think )
    Last edited by DayTripper; 04-12-2022 at 10:49 PM.

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    Default Re: Is my transom design OK? Overkill? Underkill?

    Quote Originally Posted by ulav8r View Post
    Just make up a couple of wedges at each end of an 8' 2x to rest the flared sides on, holding the bottom vertical or just past. If putting the top against the wall, a screw eye or hook can secure the boat to prevent accidental damage.
    Good plan. But that will also involve building two dollies with the prescribed angle for transporting the boat on its side to the storage location and not have the boat slip off the dollies.

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    Default Re: Is my transom design OK? Overkill? Underkill?

    Quote Originally Posted by UCanoe_2 View Post
    Another space saving storage idea, for a smaller boat and a 10 foot ceiling.

    resized flying dinghy.jpg
    Our carport's roof is much too low for storing a boat like that. Good idea though for a car topped boat except that the boat should probably be stored upside down. Then just lower the boat on top of the car .

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    Default Re: Is my transom design OK? Overkill? Underkill?

    Have another transom design idea. I don't want water to collect on top of the 2 quarter knees. So with that in mind, I've decided to angle the quarter knees downward to allow water to drain off while the boat is at rest in the water.

    DESIGN:
    • I will notch the top of the 2x6 motor mount where the quarter knees will go, so that the top of the quarter knees are flush with the top of the 2x6.
    • The quarter knee and the top of the transom's 2x6 will angle downward to allow water to drain off while the boat is at rest in the water.
    • The 2x6 will only be angled down up to the notch in the 1/2" plywood back. A 90 degree top will be retained on the 2x6 where the motor will be mounted.


    QUESTION:
    What angle downward in degrees would you suggest for the quarter knee to allow water to drain?

    I know that the angle of the boat at rest in the water depends on the shape and displacement of the hull but just looking for estimates from you experienced people. The hull for my jon boat "that will be in the water" will basically just be a 4' x 8' rectangular box. The 6 HP motor weighs 55 lbs.
    Last edited by DayTripper; 04-13-2022 at 07:14 PM.

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    Default Re: Is my transom design OK? Overkill? Underkill?

    Look at the transoms of inflatables. Not a tug boat. Your plan is way over-built.

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    Default Re: Is my transom design OK? Overkill? Underkill?

    Quote Originally Posted by Ian McColgin View Post
    Look at the transoms of inflatables. Not a tug boat. Your plan is way over-built.
    Would appreciate one link to an example to confirm that I'm looking at a transom that you're referring to. I can then continue research for other examples on my own. The images I've looked at so far don't show the details of the transoms on the inflatables. Just far away pics.

    I do want to keep my transom light but definitely strong enough for my 6HP motor at idle and low rpm. With my motor on an outboard stand, I can see that there is significant vibration at idle and low rpm. The vibrations smooth out considerably at higher rpms. My motor is a 6HP, four stroke Tohatsu.

    Thanks.
    Last edited by DayTripper; 04-13-2022 at 04:18 PM.

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    Default Re: Is my transom design OK? Overkill? Underkill?

    Just pull up inflatable ads. A 6HP might put a bit over or under 20# bollard pull, if moving much less. It's a good thing to have that extra board for holding the clamps and taking the thrust. I don't think you need supports coming up off the bottom.

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    Default Re: Is my transom design OK? Overkill? Underkill?

    Quote Originally Posted by Ian McColgin View Post
    Just pull up inflatable ads. A 6HP might put a bit over or under 20# bollard pull, if moving much less. It's a good thing to have that extra board for holding the clamps and taking the thrust. I don't think you need supports coming up off the bottom.
    Hard to determine transom thickness for inflatables so far. One youtube video shows replacing the transom on a zodiac inflatable as follows:

    Transom thickness on zodiac inflatible.jpg

    Judging from the size of the hand, I estimate the transom thickness at 1-1/4". That's the thickness of the entire transom. Maybe my frame at 1" thickness with the 1/2" plywood back will be about right. Thoughts?

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    Default Re: Is my transom design OK? Overkill? Underkill?

    the actual transom thickness is not a "thing" really.
    it's the knees
    6 mm transom would work with good knees

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    Default Re: Is my transom design OK? Overkill? Underkill?

    Don't think this has been mentioned but is the motor going to stay on the transom during transportation? What I'm getting at is that often the most stress the transom receives isn't from running the motor, it's from the weight of the motor when you're rolling down the highway and hit an unexpected bump or pothole, etc. Either way, what you have looks plenty strong enough to handle that, too. Some boats are damaged more from a rough ride on the trailer than from a rough ride on the water.
    There is no rational, logical, or physical description of how free will could exist. It therefore makes no sense to praise or condemn anyone on the grounds they are a free willed self that made one choice but could have chosen something else. There is no evidence that such a situation is possible in our Universe. Demonstrate otherwise and I will be thrilled.

  32. #32
    Join Date
    May 2021
    Location
    Hawaii
    Posts
    243

    Default Re: Is my transom design OK? Overkill? Underkill?

    Don't think this has been mentioned but is the motor going to stay on the transom during transportation? What I'm getting at is that often the most stress the transom receives isn't from running the motor, it's from the weight of the motor when you're rolling down the highway and hit an unexpected bump or pothole, etc. Either way, what you have looks plenty strong enough to handle that, too. Some boats are damaged more from a rough ride on the trailer than from a rough ride on the water.
    No, the motor will not stay on the transom during transportation. But thanks for the "just in case" info.

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