glassing new transom

[dcobbett]

I’d like comments and opinions on my proposed glass layup for the new transom I’m installing in my 1976, 20’ SeaCraft SF.

I’m doing the job from the outside and I’ve raised the transom from 20” to 25”. The new core is 2 layers of ¾”, 24# Penske Board. I’m using epoxy throughout, and I have 1708 (45x45) and 1808 (90x90), and some 1208 as well.

I took all of the old plywood transom and discovered bonding issues with the inner layer of matt and WR and the ends of the stringers and the bottom and sides of the hull, so I removed the old matt and WR to a line about 2” below the bottom of the inner liner. That means the lower portions of the inside layer of Penske is exposed and can be ‘glassed directly to the stringers and the bottom and portions of the sides of the hull. I’ll also install knees while I’m working in that area.

My plan is to lay on 4 or 5 layers of 1708 and 1808 (alternating) on the outside face of the transom (not carrying over the top of the transom), largest layer first, in accordance with the West booklet on repairing glass hulls. That should be close to duplicating the old skin and will let me add a layer of 6oz cloth as a finishing layer. I’ll do the same on the inside face, again, not covering the top 1-1/2” wide cut out line. Note that the exposed portion of Penkse on the inside face

SeaCraft tells me that their current transoms are 2 5/8” thick, with a 1-1/2” Coosa core, and I’d like to get more thickness of glass on my project, but I don’t want to go past the edges of the transom and on to the sides of the hull. I’m thinking about another 4 or 5 layers of glass up and over the transom cutout, feathered into the existing glass on the inside just above the splashwell drain holes, and into the outside of the new transom glass, roughly out to (between) the red lines I’ve added to one of the attached pic’s. That should give me ½”+ of new glass on each side of the main portion of the transom, plus I’ll get a box beam effect along the line of the engine cut out. I did a test with 4 layers of glass over poly over a scrap piece of 2”x6” on edge, and I was amazed at how strong the piece was when I took it off the 2”x6”.


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[hermco]

Sweet!

[dcobbett]

HermCo,

Thanks for the comment. I was wondering if the second layer(between the red lines)was over kill. Do you think I can (or should) cut back on the extent of that layer at all?

[oldbluesplayer]

dcobbett

overall, the layup plan sounds really good -

I guess my thinking would, if I follow your description correctly:

1- re-building, re-inforcing the inner transom glass to the hull and sides would be a critical component

2- I'm a strong beleiver in knees - a good strong knee from the transom, as high as you can get it, just under the splashwell tub would be good, down to the center stringer, and out along the center stringer about 14" - 16" from the inner face of the transom. When I did mine, I had cut the same hole in the splashwell as you, and I put an extra piece of plywood, bedded in glass and covered by glass, on the inside face of the inner transom glass - this giving me a fresh surface to mount the knee, extra central re-inforcement, and improving the load bearing / load distribution, of the knee.

3- then the outer face of the transom doesn't need to be built up so dramatically - a good, clean layup resulting in +1/4" or more of glass would be fine.

Total transom strength is not the result of any one component, but the sum of all parts - outer face, core, inner face, connection to hull/sides at perimeter, and central supports (knees and-or support from stringers).

Hope that gives you some food for thought.

Bill

[dcobbett]

oldbluesplayer,

Thanks for your input.

In response to your first comment, one thing I didn’t mention is that I took out the inner skin from about 2” below the lower edge of the liner, out and down to the sides and bottom of the hull. When I removed the wood core, I found that the layer of matt and WR were not attached along the bottom of the hull, or to the end of either major stringer (note that the picture was taken before I removed the matt/WR from the end of the starb’rd stringer).
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So, essentially, I will be glassing the lower portion of the inside face of the inner layer of Penske Board to the sides and bottom of the hull. I am also planning on a knee between the center stringer and the transom, and, perhaps, 2 more to connect the sides of the main stringers to the transom.

Regarding the glass, I was under the impression that the core really only acted as a filler (in principal, anyway) and that the real strength came from the makeup and thickness of the skin. Using my planned lay-up of alternating layers of 1708 and 1808, I can probably only get 4 layers of glass on the outside (about 3/16”+) without running into fairing issues. That’s why and when I started to think about a thicker lay-up in the center portion of the transom.

[oldbluesplayer]

dcobbett - re: "I was under the impression that the core really only acted as a filler"

I've heard that said before, but I will stand by what I said, that the outer skin/core/inner skin is a combination package, as far as strength, bounded on the perimeter by both the outer skin and inner skin attachment to the hull, reinforced in the middle by knees (if any).

Think about this - if the core was only a filler, and balsa core has a large history of use in boat building, why don't you see balsa cored transoms? plenty of use in hulls and decks, but not transoms - because it does contribute to the strength and stiffness of the structure - balsa cored structures allow some flex - which is desired in a hull, to provide some give, but not desired in a transom.

The innerskin/core/outerskin transom is essentially a long, thin (narrow), fairly deep, box. Even if that box were made of flat pieces of premanufactured G-10 Fiberglass 1/4" sheet, in that size, it will flex - only the core material stiffness prevents that, and the (motor) force input to the entire structure is passed to the rest of the hull thru the attachment of the inner skin and outer skin to the hull.

Just tryin to lay out the structural dynamics - I do hope you reconstruct the cut-out portions of the inner skin and bond that to the hull/sides.

just some thoughts for you.

Bill

[thehermit]

dcobbett,

Any progress pix to share? I am about to cut the outer skin off my 1970 20' and am taking the approach you and Hammer have taken. How did you piece in the core??