View Full Version : transom thickness

12-26-2003, 10:54 AM
I'm building the Gardner's 14'4" Outboard Flatiron Skiff from the Dory Book.

The plans call for the transom to be constructed of 2 pieces of 7/8" white pine 10" wide, running horizontally, one above the other, fastened together with 2" oak cleats. The plans also call for an oak 'backer' board where the engine would be mounted, 7/8" thick and 12" wide.

I plan on using ply instead of the pine, I also plan on having some sort of backer board installed. So my question is: what thickness of ply would have the same strength as the 7/8 pine?

I was thinking 1/2" would be needed, or perhaps 3/4? I will be powering the skiff with at least 10hp, possibly 15hp(haven't got the motor yet).


Jack Heinlen
12-26-2003, 11:00 AM
Seat of the pants, the 1/2 would be roughly eqivilent in strength, but I think I'd use 5/8 or 3/4 for the added stiffness. 1/2 would be liable to flex and vibrate, neither good, methinks.

12-26-2003, 11:04 AM
Using 3/4" plywood transom and 3/4" plywood motor board will work just fine. It's what I have for my Glen-L out board and is about what similar size plywood boats seem to call for.

12-26-2003, 11:04 AM
It never ceases to amaze me how many people choose to deviate from the plans. You have your safety and those of others to take into consideration. Why change something that is tried and proven?

Jack Heinlen
12-26-2003, 11:10 AM

Shifting from solid wood to ply is okay, he's just asking after scantlings. Prudent. smile.gif I don't imagine there's much good white pine in Richmond.

And I agree with Jim, 3/4. How are you going to fasten into it/hold it all together?

12-26-2003, 11:39 AM
WOW! almost instant response! Thanks! I'll be on the safe side and go with the 3/4....

Regarding deviation from the plans.... Gardner provides 2 ways to do the bottom of this boat; planks or ply.... I chose to do ply for the bottom. Considering he gives an option on the bottom, I don't think it's really that much of a deviation to use ply on the transom in this case......

Regarding fastening... there are also 2 1/4" x 7/8 oak cleats edging the sides and bottom of the transom, these will be epoxied and fastened with SBronze screws. The side to transom will also be epoxy and SBronze screws. The side to bowstem will also be epoxy and SBronze screws. Most everything else will be epoxy and SBronze ring shank nails...

thanks for the help!

Gary E
12-26-2003, 03:02 PM
"The plans also call for an oak 'backer' board where the engine would be mounted, 7/8" thick and 12" wide"

If this is where your outboards clamp screws will seat on the inside of the boat, I would sugest you keep the oak, it's stronger and will not fall apart as softer wood will.


12-26-2003, 03:16 PM
Is 10-15HP the power the designer intended?

12-26-2003, 03:55 PM
The shift to ply over "real wood" is OK, and strength-wise 3/4" ply is certainly the equal of 7/8" planks. I'd make sure that the ply "end grain" at the top is well sealed because this is usually where ply transoms begin to fail by delamination. As for the "backer board", most outboards have a mounting bracket that is engineered for a transom thickness of min/max 1.5 to 2.0 inches, so the backer board is mostly to attain required thickness, and minorly to distribute the point load of the mount over a larger area of the transom.

Good luck with the project, it is a fine little boat.

12-26-2003, 04:11 PM
Originally posted by mmd:
[QB]I'd make sure that the ply "end grain" at the top is well sealed because this is usually where ply transoms begin to fail by delamination.
QB]Wouldn't this be a good place for a "cap strip" (sorry if that's the wrong terminology) of solid wood to cover the open plies? I think it would more decorative than epoxy gooped ply too.

MMIke - any idea what 10" wide 7/8" thick white oak boards cost? ;)

12-26-2003, 05:10 PM
Yes, Meer, the cap strip is a very good solution to sealing the end-grain of the ply transom, but if the transom is to be finished bright, the challenge of properly fitting a cap to the convoluted shape of a transom with standard cut-out for an outboard and retain proper aesthetics is no small task of joinery and stock selection. Personally, I prefer a strip of light (3/4 oz) 'glass matt, merely as wide as the transom top edge, set in epoxy and richly wetted out. It forms a hard plate over the endgrain that can take the abuse of mounting/de-mounting the motor, is almost invisible, and is readily repairable.

12-26-2003, 05:16 PM
Meerkat sez: MMIke - any idea what 10" wide 7/8" thick white oak boards cost?

You tell me. I usually don't deviate from the plans. Especially on a critical piece like the transom. MMD's the designer. He said it's OK so it's OK.

I build as it is affordable. In other words, I wait till I can afford to buy materials and build a little at a time. That's what is great about it. You don't have to buy everything all at once.

[ 12-26-2003, 05:17 PM: Message edited by: Memphis Mike ]

12-29-2003, 03:10 PM
[ 12-29-2003, 04:18 PM: Message edited by: stevenj ]

George Roberts
12-29-2003, 04:05 PM
stevenj ---

I am not certain that 3/4" ply is equivalent to whatever 7/8" wood is specified, but you can look up the numbers on the APA website and compare them to the numbers found in the National Design Standard for Wood Construction.

But them I am not sure what the designer had in mind for the strength, so looking up the numbers would do me no good.

12-29-2003, 04:35 PM
The boat has a flat bottom, and is designed for a 10-12hp engine.

On a related note, I'm not having any luck finding decent ply with local sellers. Anyone Virginia builders have a good source?


John E Hardiman
12-29-2003, 04:51 PM
I would like to second what Gary E said. I've seen a lot of ply motor mounts that needed to replaced because the motor clamps ate up the softwood. I like a metal sheet screwed and glued over where the engine lands to prevent this.