Original SeaCraft transom design ???

[FLexpat]

Didn’t want to derail anyone’s rebuild thread but I want to make sure I understand the engineering behind the original SeaCraft transom as I design a layup to replace it…

When I disassembled the transom on my 23 Sceptre I/O, I found that the outer glass was ~1/4”, the plywood core was 2 layers of ¾” stapled together and apparently wrapped with thin cloth (8-10 oz?) along the bottom and sides (tape?), and the plywood assembly was glassed into the hull with woven roving – something like 24 or 36 oz. This created an inner skin of about 1/16” or .0625”. I didn’t dissect the exterior skin but it appears to be a combination of mat and roving. It was definitely a hand layup.

When I attempt to reverse engineer the design loads, it appears to me that the strength comes from the 2 layers of plywood rather than a sandwich or cored construction. The inner skin does an ok job of attaching the plywood to the hull along the edges but does not seem to do very much structurally other than form a weak horizontal tension member. The inner skin is marginally effective at sealing the plywood transom from water in the bilges, but mine was open at the top (so not a vertical tension member other than from adhesion to the plywood). If it didn’t have a zillion holes letting saltwater in, it might have still been a little dry.

The inner skin of .0625” is MUCH less than what you would expect from Gerr’s scantlings for cored construction; he indicates an inner skin of ~75% (or more) of the outer skin thickness. Following that, the inner skin should have been at least 3x thicker or about 3/16”.

So am I right in thinking that the original transom strength came primarily from the compressive, tensile, and flexural characteristics of the plywood rather than the ‘cored’ construction?

[flyingfrizzle]

Originally Posted by FLexpat

Didn’t want to derail anyone’s rebuild thread but I want to make sure I understand the engineering behind the original SeaCraft transom as I design a layup to replace it…

When I disassembled the transom on my 23 Sceptre I/O, I found that the outer glass was ~1/4”, the plywood core was 2 layers of ¾” stapled together and apparently wrapped with thin cloth (8-10 oz?) along the bottom and sides (tape?), and the plywood assembly was glassed into the hull with woven roving – something like 24 or 36 oz. This created an inner skin of about 1/16” or .0625”. I didn’t dissect the exterior skin but it appears to be a combination of mat and roving. It was definitely a hand layup.

When I attempt to reverse engineer the design loads, it appears to me that the strength comes from the 2 layers of plywood rather than a sandwich or cored construction. The inner skin does an ok job of attaching the plywood to the hull along the edges but does not seem to do very much structurally other than form a weak horizontal tension member. The inner skin is marginally effective at sealing the plywood transom from water in the bilges, but mine was open at the top (so not a vertical tension member other than from adhesion to the plywood). If it didn’t have a zillion holes letting saltwater in, it might have still been a little dry.

The inner skin of .0625” is MUCH less than what you would expect from Gerr’s scantlings for cored construction; he indicates an inner skin of ~75% (or more) of the outer skin thickness. Following that, the inner skin should have been at least 3x thicker or about 3/16”.

So am I right in thinking that the original transom strength came primarily from the compressive, tensile, and flexural characteristics of the plywood rather than the ‘cored’ construction?

Sounds like yours was laid up on a Friday at 4pm before go time. Much thinner than most seacrafts. I would build it back so the inner was as thick as the outer core is or go by Gerr’s scantlings for best results building it back.

[CHANCE1234]

My inner skin was the same way. Very, very thin. I added 4 layers to it alternating between mat and roving to beef it up

[FishStretcher]

In my opinion:

Compressive strength: yes the plywood helps more than foam.

Bending: Not likely. The extreme outer fibers take the brunt of the load. Outer fibers being the skins. The math gets complicated because of the different stiffness of the fiberglass versus the ply, the fiberglass might be quasi isotropic- it doesn't care which way the strain goes. But the plywood definitely cares and has different stiffness. And extreme outer fibers of ply aren't necessarily oriented correctly. (But it helps that the shear stiffness is higher than foams, generally.)

The higher (but variable with orientation and moisture and condition) shear stiffness of the the plywood generally is also what probably allows the lightweight inner skin, as the load is transferred to the inner skin better, when the plywood is dry.

So the math on a dry transom might work out fine as is. A foam core probably will need more laminate on the inner skin. But the foam should never loose its shear stiffness. Plywood will as it gets moist.

I can't say on pure tension. I think the tensions loads are usually a load that goes with bending.

Executive summary: More glass on the inner skin with a foam core. And no weight penalty and no rot.

Or: More glass on the inner skin with plywood. It will be overbuilt and heavier until the plywood gets damp. Then it will be less underbuilt. And even heavier.

In either case, I would be interested in a solid laminate around the I/O penetration.

Originally Posted by FLexpat

Didn’t want to derail anyone’s rebuild thread but I want to make sure I understand the engineering behind the original SeaCraft transom as I design a layup to replace it…

When I disassembled the transom on my 23 Sceptre I/O, I found that the outer glass was ~1/4”, the plywood core was 2 layers of ¾” stapled together and apparently wrapped with thin cloth (8-10 oz?) along the bottom and sides (tape?), and the plywood assembly was glassed into the hull with woven roving – something like 24 or 36 oz. This created an inner skin of about 1/16” or .0625”. I didn’t dissect the exterior skin but it appears to be a combination of mat and roving. It was definitely a hand layup.

When I attempt to reverse engineer the design loads, it appears to me that the strength comes from the 2 layers of plywood rather than a sandwich or cored construction. The inner skin does an ok job of attaching the plywood to the hull along the edges but does not seem to do very much structurally other than form a weak horizontal tension member. The inner skin is marginally effective at sealing the plywood transom from water in the bilges, but mine was open at the top (so not a vertical tension member other than from adhesion to the plywood). If it didn’t have a zillion holes letting saltwater in, it might have still been a little dry.

The inner skin of .0625” is MUCH less than what you would expect from Gerr’s scantlings for cored construction; he indicates an inner skin of ~75% (or more) of the outer skin thickness. Following that, the inner skin should have been at least 3x thicker or about 3/16”.

So am I right in thinking that the original transom strength came primarily from the compressive, tensile, and flexural characteristics of the plywood rather than the ‘cored’ construction?

[Entourage]

I think if you are running a outboard you are going to be putting more stress on the transom especially with a bracket, and the weight and torque of these newer outboards. So if you think you will ever go this route build it up. Ive done a 1979 and a 1973 the 79 was an outboard model and had alot more glass on the inner skin and outter. The 73 im doing now is an i/o and dooesn't have as much glass on both the inner and outter which shocked me cause everything else on the 73 is built a little stronger like the deck. And i know for sure that the 73 transom was original.

[Terry England]

Flexpat, We recently removed a couple of sterndrives in an old 25 Bertram and need to fill in and replace the transom for outboards. (I know this is a Seacraft site) The old Bert's, like the 23' Formulas had a slightly curved transom with a flat spot in the center 1/2 for the transom housings to bolt through so the engines sat in there straight. Since we were concerned about too much flex with the transom plywood not going the full width of the boat we switched to three 1/2' marine plywood rather than two 3/4" which is still approximately the same 1-1/2". We used a 15% styrene thinned wash coat of Polyester on each face of the plywood and laminated 2 - 1.5 oz. mat layers in between the two laminates of the three laters of 1/2" marine plywood. The styrene wash coat only seems to sink in a couple of veneers of the plywood, so with the 1/2" although it doesn't penetrate any further than it does with the 3/4" it comes much closer to complete saturation because the face surfaces are 1/4" closer to each other. We drilled 1/4" holes about 6" O.C., both ways, in the "field" to let any air out that might be trapped when laminating the whole mess together. We put some plastic down on the concrete floor, wet out and stacked the three pieces on top of one another with the glass and mat layers in between then stacked and layer of cement blocks on top as the polyester squished out through the 1/4" holes. We cut some narrow (2") by deep (24") plywood "horseshoes" the next day and after having cleaned up the inside face of the transom fiberglass we squeezed the new transom toghter with two layers of 1-1/2 oz. mat and ployester in between. We shoved the horseshoes down and wedged them tight and clamped a bunch of vice-grip welding clamps everywhere else. This is what we came up with, but I'm sure there are many other theories. There are some pictures on an album on my member profile that might help you see this set up. Good luck on your project.

[FLexpat]

What I can tell so far is that the O/B versions seemed to have a thicker inside skin than the I/Os - maybe to make up for the notch and lack of a full width cap. As long as the plywood was dry it is really stiff and strong. When the plywood gets wet both the stiffness and strength seem to crater. Adding rot, termites, carpenter ants makes it sooo much better. Dry plywood is better than Coosa on every number. Wet plywood seems to be worse on stiffness. Wet plywood appears to fail more gracefully - incrementally. To get the Coosa to perform comparably to the plywood it needs a much thicker inner skin and all the through bolts need to be overbored and filled - just like what you would do to protect plywood. This gives the Coosa an advantage on the weight side but dry plywood will always be better in performance.

One of the big differences to me is that if the glass on a wood core ever lets water in - from screw holes or damage of whatever sort - it will need to be ripped out and replaced. Plywood wins on cost big time - unless you have to replace it again. I never want to have to do this again on this hull. I also want to make it strong enough to handle a bracket if I change power a couple of years from now.

So here is my current plan for going to Coosa:
Pirate FishStrecher's idea of a garolite section around the drive.
Overbore and backfill all the through transom fittings/bolts with epoxy/glass fiber.
Add at least 2 layers of 10 oz cloth on the inside of the outer skin.
The inner skin is to be alternating layers of 1808 (0/90) and 1708 (+/-45) biax for a total of ~0.3 inches. According to Gerrs scantlings this works out to an equivalent solid layup of about .8" but the cored construction (ignoring Coosa properties) makes it much stiffer. Layup is in vacuum bag and targets 40% glass to epoxy resin. This still gets the transom thickness less than the 2.25" limit for installing the drive.

Terry - a 25 Bert w outboards sounds really nice - In a lot of ways I do miss my old Bertram 20 - the curved transom just looked nice to me too. Sceptre rides lots better though.
I heard that the guy that bought my Bertram lost it in Eluthera when Andrew hit.