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Transom: Solid Fiberglass or Cored

Nov 30, 2009
Oday 28 Lake Michigan
Truly like all the features and dimensions of my 1984 O'Day 28. Very manageable for solo sailing. Interior simple and roomy. Would like to add a swim platform to the stern for future boarding convenience (mooring) and the grandkids (swimming) . Got all the required dimensions for ordering before winter storage, but forgot to "check" the structural integrity of the stern. Is the 36 year old transom solid glass (transom should be sound and not a concern), or is it cored (and needs to be checked for water intrusion)?
Sep 25, 2008
Alden 50 Sarasota, Florida
Our son used to have a 27 same year and it was solid, however, it wasn’t think enough to support the stress of a boarding step without some bracing regardless of the core material
Jul 19, 2013
Pearson 31-2 Boston
If you can see the inside of the transom panel, you can determine if the panel is cored by visually inspecting the edges to see if there is a ridge around the panel edge, which indicates coring, as the thicker coring will not extend all the way to the edge of the panel.
Jan 24, 2017
Hunter 34 Toms River Nj
most transom's on boats are not cored as decks are. Decks have to be stiff and support loads therefore the core them for structural support. Regardless if you are adding a swim platform I would highly recommend a wide backing plate to support and distribute to weight load.

If the transom is cored, you should be able to hear the diferance by tapping on the fiberglass.
Oct 22, 2014
CAL 35 Cruiser moored EVERETT WA
The manufacture of transoms is all over the place. Sometimes there is core material (I would expect plywood not balsa), sometimes not.

If you find wood core you just need to seal the core with thickened epoxy and move forward with the project.

Your swim platform will present challenges to your sailing experience. It is like adding a second trunk to your car. You need to be careful when you go to park it. The new caboose is sticking further back than before. Then you have the idea of waves in a following sea. It will want to lift the swim platform up and off the boat. Perhaps a platform that folds up when not in use should be a consideration.

The strong points on your transom will be along the outer edges. Using good backing plates at the connection points for the swim platform will be critical to providing a strong addition to your boat.
Sep 24, 2018
O'Day 25 Chicago
I believe it's glass. I can't say this is how it should be done but my swim ladder is held on by six screws with fender washers on my O'Day. It's been there for at least 20 years. No Fiberglass damage was noted last time I was there
Nov 30, 2009
Oday 28 Lake Michigan
Gentlemen, thank you for the feedback. After thinking about traditional boat building, I thought decks should be cored and the hull should be solid glass. Hearing tht you are thinking that too is reassuring. I had viewed some of the archives and read repeated reference to rebuilding the transom due to rot. After checking it a second time every one of the postings I read were of smaller sailboats powered by outboards.
The plan is to have generous backup plates fabricated.

Ward H

Nov 7, 2011
Catalina 30 Mk II Barnegat, NJ
On my 1980 O'day 25 there was plywood coring in the transom on the port side where the motor mounted and in the center where the rudder was mounted. These were separate pieces, you could easily see the outline from the interior. The starboard didn't have any coring.

If it were my boat I would plan for no coring and adding reinforcement, I like 1/4" G10 fiberglass board epoxied in place.
Sep 24, 2018
O'Day 25 Chicago
It appears as if the inboard 25's had a glass transom (except by the rudder) and the outboard had a cored backing on the port side by the outboard