Plywood transom thickness

[WBC]

I am working on a design and I am a little stumped. I am looking for a formula for calculating thickness. The boat is 14' x 5'6" with a 20" transom. Max. 40 hp. Boat built out of plywood 3/8" bottom 1/4" top sides. I have been through boat strength and did not see it there. Please help.
Bruce

[Gilbert]

I don't have the formula you are looking for but I will tell you what I like to do for outboard motor transoms. I like to put a vertical piece of hardwood (say 1 inch thick and wide enough to clamp the motor to) where the motor will mount. This piece will go from just above the bottom frame to the top of the transom. Then I make a stiffener which runs clear across the boat that will lie perpendicular to the inside of the transom and just low enough to clear the motor clamps nicely. Make it about one and a half inches thick and four to five and a half inches wide depending on how large the motor is. The stiffer the wood used for this piece, the better. Through bolt it to the transom frame at the sides and through the vertical piece. Use two bolts in the vertical piece. If you leave a gap between the plywood and the stiffener except on the pieces where you through bolt it, you will have more strength for the same width stiffener than if you notch it in so it lies on the plywood. Elsewhere on the vertical piece you can use glue or screws or both to fasten the plywood to it. It is a good idea to countersink the nuts and washers on the inside edge of the stiffener. This seems to work really well. And then it doesn't much matter how thick the transom itself is. Of course you don't want it to be so thick your motor clamps won't bite it.

[WBC]

I'm thinking 1" should do it but just want to make sure

[PAR]

Outboard transoms are pretty much standardized at 1.5". Most use two layers of 3/4" ply, but a stronger method is three layers of 1/2".

Transoms are typically reinforced with knees, usually a couple at the rail and two or three on the bilge stringers, to transmit torsion loads to the bottom.

If using solid wood, the same applies, 1.5" total thickness (typically two layers of one by stock on edge)with support knees.

[WBC]

Thank you very much PAR I knew you would be around soon. I thought I was skimping at 1". Thank you very much sir, hope everything is well in Eustis

[Wayne Grabow]

Check with the manufacturer of the outboard you plan to use. I just checked on a Yamaha outboard and was given a installation diagram which shows that the transom should be 1.5 to 2.6 inches thick. Also transom angulation varies between manufacturers.

[PAR]

It's 77 degrees and lovely here in Eustis, maybe some rain tonight, we need it and hopefully it put out some of the brush fires that have started to crop up.

If you have the option, set your transom rake a 15 degrees. Some will say less, but trust me it's better at 15 then half of this, as you can easily decrease the angle, but increasing is more problematic.

1.5" is the standard transom thickness for 100 HP and less. If using plywood (highly recommended) and two sheets of 3/4", then cant each sheet at 12 degrees to each other. I know this sounds weird, but trust me again. Have the grain run athwartship, with the one piece canted up 12 degrees and the other canted down when they get bonded. This increases the strength of the two joined panels and doesn't cost anything more then some setup. If using three pieces of ply, do the same thing except the middle layer of ply is left straight, just the inner and outer pieces cant.