New to me boat with rotting bulkhead

[Timm R Oday25]

HDPE does not have the compresive load ratings as wood . It will bow out or "flex" under pressure . Probably the highest loads on our boats is the mast compression post . As for using it as a replacement bulkhead ,it will flex or twist . It wasn't designed for this application.

 

[Project_Mayhem]

Aside from lack of structural support that Timm mentioned, If the 23' has a bulkhead of similar size to the 25', you'll need an entire sheet of material for the port bulkhead. Marine grade plywood runs $80-120. A sheet of HDPE runs $200-300 and often times needs to be freighted. Your original lasted 40 years. I'd say that's pretty good

 

[G Newboat]

Well, you might be right, but I already bought a sheet from a local plastics company in town for $230. I can bend it a little by pressing hard in the middle, that is true. Over time, I'm sure the boat could bow it out if the compression post weren't doing its job. At this point, I am going to try bolting a couple of steel strips to it vertically to give it support. That plywood might have lasted 40 years, but 35 of them were dry. After a few years wet, it melts away like cardboard. Yes, I do want to fix the leak, but I already have the HDPE sheet, so I am going to see what happens. Worst case scenario, I have a nice template for a replacement piece of marine plywood.

 

[G Newboat]

I decided to follow your guys' advice and get a sheet of 1/2" marine plywood. I traced the old one, cut the new, then clamped them together to drill all the holes and pilot holes. I will take everything off the sheet again before I stain and polyurethane it. Do you think I should leave the top unfastened, as found on later models, or put all the screws back in the way it was?

 

[Timm R Oday25]

I'm not quite sure what you mean "unfastened " . As I am also a huge fan of polyurethane ,apply several coats . Where the chainplates attach, I would slightly enlarge the holes and use fiberglass epoxy to seal the hole from moisture if your deck would ever leak. The main reason I replaced my bulkheads was not because of rot. The chainplate holes had become enlarged after 35 plus years of supporting the mast stays .
I added a stainless steel strap on the other side of the bulkhead to sandwich the plywood . I figured more surface area would prevent the bolts from pulling upwards .

 

[G Newboat]

Maybe I'm misunderstanding the terminology, but what are they saying in post #13- etc. when they talk about the bulkhead being "tabbed to the hull", if not fastened with screws? (Fasten to the deck, not the hull.) In any case, I think I will put it back in with screws at the top and bottom the same as it was originally built. That worked for 40+ years.
I also picked up some 3M 5200 marine sealant for the chainplate. A second plate for the other side of the bulkhead might be a good idea. Now I'm just waiting for the weather to get a little nicer. The are predicting possible snow tomorrow.

 

[Timm R Oday25]

G Newboat, in general terms ,"tabbing " refers to fiberglassing a wooden partition ,in this case a bulkhead to the hull .
Rereading the posts in this series shows Oday changed how they put our boats together . I would definitely put all the screws back in that I removed . By the time spring finally rolls around ,I'm quite sure you are going to be mighty proud of a boat you know you can trust .