Flat bottom with weak transom

huntingtheworld

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I have a older flat bottom boat that has a weak transom right below the wood.it flexes.any advice on what I should do.it has a 55 on it:( image.jpg
 

Woodonglass

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Re: Flat bottom with weak transom

If you mean the aluminum is flexing on the bottom half of the boat below the wood, I'd guess it's because you may have to large/heavy of a motor on the boat. The transom wood looks to be in good shape. How big is the boat? What size motor is it rated for? At the very least I would think you would need to have a full cover transom installed from top to bottom of the stern. But hey, I'm not a TIN BOAT guy so, I'll go away and have the Tin Boat guru's take over!!! Good luck with your project!!!
 

huntingtheworld

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Re: Flat bottom with weak transom

If you mean the aluminum is flexing on the bottom half of the boat below the wood, I'd guess it's because you may have to large/heavy of a motor on the boat. The transom wood looks to be in good shape. How big is the boat? What size motor is it rated for? At the very least I would think you would need to have a full cover transom installed from top to bottom of the stern. But hey, I'm not a TIN BOAT guy so, I'll go away and have the Tin Boat guru's take over!!! Good luck with your project!!!

16 foot and not sure about raiting.it don't say...55hp and what's a full cover transom?
 

greenbush future

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Re: Flat bottom with weak transom

The transom in there now looks a little short to me too. But maybe that's how it came from the factory. Also make sure that the wood that's installed now is not pressure treated wood. That will cause a chemical reaction with your alum. 55 seems rather big for the boat, but you didn't say how long or big the boat is either.
 

huntingtheworld

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Re: Flat bottom with weak transom

I said 16 foot 3 times lol I'm wondering if it's not showing up.wide at the bottom is 4 foot and at the top very wide idk.also my drain plug was leaking.it was because the tube is bent and the hole in General is in bad shape.the plug seems to fit now without the tube.do I need the tube? It seals good without.
 

MikDee

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Re: Flat bottom with weak transom

The transom looks pretty good to me, but it also looks like it was rebuilt better then original. It all depends on how good the wood is? Try pulling outward on the transom, and see if you get any movement, or how much? That 16ft Alum. boat seems like it's overpowered with a 55hp motor on it?
 

jbcurt00

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Re: Flat bottom with weak transom

I have a older flat bottom boat
Any more details then a flat bottom 16ft boat? Maker unknown? In a formerly no titles for boats so it never had 1, now it's registered as a home built? Any more pix, possibly showing the interior of the boat from above, the outside shape of the transom and the profile of the entire boat. Flat bottom jon boat w/ the flat bow, a slight semi-vee w/ very little V, mostly flat? Welded aluminum boat (based on the corner gussets or riveted boat? HEAVY gauge vintage aluminum boat, or lighter weight/thin gauge aluminum?

Knowing some of that ^^^ will help determine what to do about this:

attachment.php


The 3 2x4 'knee' braces are NOT original, the 2 painted white probably are. Based on that pix ^^^ I suspect 2 things:
The current transom is NOT original, AND it's not adequately supporting the torque of a 55hp motor.

I haven't seen every old aluminum boat made, but I've seen a bunch, and I doubt that the original layout of the 2 metal knee braces was done so that they WEREN'T fully in contact w/ the transom plywood. Following that logic, it sounds to me like the torque of the motor is putting pressure against the bottom edge of the piece of plywood, and w/out enough vertical surface area of the knee brace foot in contact with the plywood, it's allowing the aluminum skin to flex inward below the transom plywood, if only slightly. The pix isn't really clear, but it also appears that the 2X4's are only in contact w/ the transom plywood across an also very small cross section.

W/out more info about the actual boat, I won't hazard a guess as to whether the 55hp is in excess of the boat's actual hp rating, but it's clearly in excess of the current setup of the boat & it's transom.

I'm not sure full cover transom is the correct use or term, WOG, but I completely agree: transom plywood that more fully covers the interior side of the transom skin would, IMO, certainly help resolve the current problem of the aluminum flexing. It wouldn't have to extend all the way to the inside corner of the transom/hull bottom, but certainly could extend further down the aluminum transom skin's height.

By the way, the knee braces may also be transferring the flex from the transom down thru to the bottom of the boat's hull. Constant movement of the aluminum hull, in about the same place regularly over time isn't good. IMO, it's extremely NOT GOOD. I'd remove the motor, existing transom plywood & existing knee braces and extremely carefully examine the transom skin & hull bottom. If those braces tie into stringers, how do they tie in, and is that attachment substantial enough.

Repeated flexing of the hull against a brace, stringer, rib, or rivet can lead to fairly minor or very major cracks in any of the components. We see LOTS of rib end, rivet hole & hull cracks in the Starcraft rebuilds. Many are so minor to have been indetectable until the boat was completely stripped of it's interior and the exterior hull had all traces of paint & primer removed.

Minor cracks tend to become larger over time & much more difficult to repair.

So, who made your boat & lets see some more pix of her
 
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jbcurt00

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Re: Flat bottom with weak transom

Ummm, is this the same motor as these motor threads:
[h=3]I need help I have about had it with this motor![/h]
[h=1]Need help mismatched motor[/h]
[h=1]mismatched motor[/h]
[h=1]Card help[/h]I think it's supposed to be Carb Help ^^^

If the motor is a cobbled together mess w/ multiple years & model parts, I'd suggest you treat it as the larger displacement 60hp for the purposes of these discussions. A 60hp motor will impart more load & torque to the transom then a 55 will. I suspect it's over rated w/ the 55hp so 60's just more so.

Is the boat ACTUALLY 16ft long, or is it 15' 6", 15' 9". NOT what it's listed as on the title or registration, but actually measured transom to bow tip. That may help ID it as well.......
 

huntingtheworld

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Re: Flat bottom with weak transom

Any more details then a flat bottom 16ft boat? Maker unknown? In a formerly no titles for boats so it never had 1, now it's registered as a home built? Any more pix, possibly showing the interior of the boat from above, the outside shape of the transom and the profile of the entire boat. Flat bottom jon boat w/ the flat bow, a slight semi-vee w/ very little V, mostly flat? Welded aluminum boat (based on the corner gussets or riveted boat? HEAVY gauge vintage aluminum boat, or lighter weight/thin gauge aluminum?

Knowing some of that ^^^ will help determine what to do about this:

attachment.php


The 3 2x4 'knee' braces are NOT original, the 2 painted white probably are. Based on that pix ^^^ I suspect 2 things:
The current transom is NOT original, AND it's not adequately supporting the torque of a 55hp motor.

I haven't seen every old aluminum boat made, but I've seen a bunch, and I doubt that the original layout of the 2 metal knee braces was done so that they WEREN'T fully in contact w/ the transom plywood. Following that logic, it sounds to me like the torque of the motor is putting pressure against the bottom edge of the piece of plywood, and w/out enough vertical surface area of the knee brace foot in contact with the plywood, it's allowing the aluminum skin to flex inward below the transom plywood, if only slightly. The pix isn't really clear, but it also appears that the 2X4's are only in contact w/ the transom plywood across an also very small cross section.

W/out more info about the actual boat, I won't hazard a guess as to whether the 55hp is in excess of the boat's actual hp rating, but it's clearly in excess of the current setup of the boat & it's transom.

I'm not sure full cover transom is the correct use or term, WOG, but I completely agree: transom plywood that more fully covers the interior side of the transom skin would, IMO, certainly help resolve the current problem of the aluminum flexing. It wouldn't have to extend all the way to the inside corner of the transom/hull bottom, but certainly could extend further down the aluminum transom skin's height.

By the way, the knee braces may also be transferring the flex from the transom down thru to the bottom of the boat's hull. Constant movement of the aluminum hull, in about the same place regularly over time isn't good. IMO, it's extremely NOT GOOD. I'd remove the motor, existing transom plywood & existing knee braces and extremely carefully examine the transom skin & hull bottom. If those braces tie into stringers, how do they tie in, and is that attachment substantial enough.

Repeated flexing of the hull against a brace, stringer, rib, or rivet can lead to fairly minor or very major cracks in any of the components. We see LOTS of rib end, rivet hole & hull cracks in the Starcraft rebuilds. Many are so minor to have been indetectable until the boat was completely stripped of it's interior and the exterior hull had all traces of paint & primer removed.

Minor cracks tend to become larger over time & much more difficult to repair.

So, who made your boat & lets see some more pix of her


Lowe 1978 the white braces are bolted threw the bottom. I have been running the boat weekly but hoping not to blow the back off.i take off slow and do not run wot.it just flexes a little.image.jpgimage.jpgimage.jpg
 

huntingtheworld

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Re: Flat bottom with weak transom

And yes it's a mix matched motor.tape said 16 foot.i really don't want to get rid of the motor I just but a lot into it.
 

sphelps

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Re: Flat bottom with weak transom

I am no expert but it looks to me the boat is not overpowered . But thats just an uneducated guess . Maybe rebuild a full height and width transom and add longer braces that you can tie to the lateral brace just in front of the transom.
Or I could be way off .. :facepalm: :D
 

huntingtheworld

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Re: Flat bottom with weak transom

I am no expert but it looks to me the boat is not overpowered . But thats just an uneducated guess . Maybe rebuild a full height and width transom and add longer braces that you can tie to the lateral brace just in front of the transom.
Or I could be way off .. :facepalm: :D

I can build just about anything with metal or aluminum.i just need some ideas and pics.thanks.
 

Woodonglass

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Re: Flat bottom with weak transom

Well I been doin some invest-a-magashuns and on the Lowe's Boat site a 16' Flat bottom jon boat with a 48" bottom and a 70" beam has a recommended motor size of 30hp. So it seems to me you might be a bit Over doin it with that size of motor. Again, I'm NOT a Tin Boat guy so I can't say for certain just statin' the facts as I found em.;)
 
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jigngrub

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Re: Flat bottom with weak transom

I'm pretty sure this is another episode of "When Bad People Do Bad Things to Good Boats".

I'm betting money that used to be a bench boat (middle and rear benches) and the benches have been removed... along with the structural integrity they provide. Someone has weakened that boat by butchering it and that's why your transom is flexing, and if you could lay a tape measure across the top of both gunwales you would see the side flex out and the boat get wider when you give the motor gas.

So, not only is your boat over powered... it's also very weak from having structural components removed.

When I look at the pics you posted in post #9 those pedestal seats are sitting way to low, and pedestal bases don't bolt to the aluminum hull of an aluminum. They bolt down to a plywood deck, because aluminum isn't strong enough to support pedestal seats.

I see this kind of jack-leg knucklehead modification on another forum all the time and they're all so proud of themselves because they have lots of room to walk around in their boat without having to step over a bench, but they don't have the brains to figure out just how unsafe they've actually made their boat by removing valuable floatation foam and structural integrity.
 

jigngrub

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Re: Flat bottom with weak transom

I'd also like to add that if that boat was meant to be a side console with that big of a motor on it, it would have a splashwell on the transom for additional support. Splashwells on aluminum boats are also structural members and create more support than most people give them credit for.

That boat was designed and built to be a tiller steer bench boat, not an open deck side console.
 

huntingtheworld

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Re: Flat bottom with weak transom

I'm pretty sure this is another episode of "When Bad People Do Bad Things to Good Boats".

I'm betting money that used to be a bench boat (middle and rear benches) and the benches have been removed... along with the structural integrity they provide. Someone has weakened that boat by butchering it and that's why your transom is flexing, and if you could lay a tape measure across the top of both gunwales you would see the side flex out and the boat get wider when you give the motor gas.

So, not only is your boat over powered... it's also very weak from having structural components removed.

When I look at the pics you posted in post #9 those pedestal seats are sitting way to low, and pedestal bases don't bolt to the aluminum hull of an aluminum. They bolt down to a plywood deck, because aluminum isn't strong enough to support pedestal seats.

I see this kind of jack-leg knucklehead modification on another forum all the time and they're all so proud of themselves because they have lots of room to walk around in their boat without having to step over a bench, but they don't have the brains to figure out just how unsafe they've actually made their boat by removing valuable floatation foam and structural integrity.
Ok what would you suggest doing about the transom?
 

huntingtheworld

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Re: Flat bottom with weak transom

Could I not build braces or something? I sure would hate to dump alot more money in it.
 

bigdirty

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Re: Flat bottom with weak transom

....I sure would hate to dump alot more money in it.

you DO know what 'BOAT' stands for right? :lol:

I'm not a tin boat guy, but from whats been posted so far, ya I tend to agree that motor may be a bit much for it.. As for bracing it, sure.. jigngrub made a few good points, which make perfect sense to me.. you need to put the boat back to 'stock' in a way, some way, and add in some benches (or something) that will do the same job. Most 'things' are rated for safe working load at a point WELL below when they will fail (trailer hitches on cars/trucks, OB motor sizes for boats, etc) but if you go over it, double it perhaps, AND loose or delete structural pieces, THEN add in years of service and use.... you're asking for trouble...
 

Woodonglass

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Re: Flat bottom with weak transom

I'd build a "full cover" 1 1/2" thick plywood transom and full aluminum angle braces like this...
 
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