glassing new transom

[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??

[dcobbett]

TheHermit
Yes, I have made some progress. I'll put some pic's together and try and get them posted this evening.

[thehermit]

dcobbett,
What did you do with your original transom eyes? How did you cut them out or get to them? Thanks. TH

[dcobbett]

TheHermit,

I managed to get a crow's foot wrench on the nuts through the louver openings. Took a while, but I got them out in one piece without bitch'n up the threads and I hope to reinstall them.

Here's is a follow-up on the transom work.

When I laid out the joint lines for the first layer, I tried to have as few pieces as possible, making sure each piece could be wedged and forced against a firm surface of existing glass/liner material. I also wanted to avoid joints along what I thought would be potential stress lines. Same ideas with the second layer, but I also wanted to one large central piece in order to keep the transom as flat as possible.

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I used Mas epoxy (slow hardener) throughout. I’ve used it before and it has no odor or blush (at least from what I’ve seen. I’m working 100% outdoors so I mixed the epoxy in my cellar (cool) during the hot weather and poured it into roller pans as soon as I brought it outdoors.

My initial lay-up was 4 layers (outer skin plus inside the splash well) alternating 1708 and 1808.


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I used black duct tape to define an area that I wanted additional glass on and repeated that lay-up, this time bringing all of the fabric up and over the transom cutout and tying it into the sidewalls of the well. After it had cured, I could set up off of the tapeline and run a 45^ bit in a router to cut off the waste. I still need to take the hard edge off of the outer edge of the 45^ cut line, and I’ll probably try and work in a small fillet where the bevel meets the transom face.

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I also need to get inside, under the splash well and form a fillet around the perimeter of the Penske Board, then glass the surfaces and tab into the sides and bottom of the hull. I’m also planning to add a knee between the small, centerline stringer and the inside face of the new transom.

[RS]

Nice work Dan. Glad to see you're making significant progress.
Hope this hasn't cut into your fishing too much.

[thehermit]

Nice! Just what I neeed to see. If I read it correctly it took 4 layers of 1708/1808 to bring the cutout up to the original thickness of the hull. One question though. Did you consider making the initial transom cut up to the cap, including cutting the cap on the top, to allow dropping a piece (core) down into the transom in one whole piece? Do you think that would work?
Thanks

[71kidcasper]

dcobbett, I hope that I am not asking a stupid question, but why did you build up the transom more in the isolated area in the center. I am not questioning your work cause I think it looks great. I just didn't understand it. I'm doing the same job on my 71 20.