Wet port side deck

I have discovered that the port side deck on my '79 MK1 is wet. This was found through sounding and use of a moisture meter. I cut away some of the fiberglass deck and removed the core. While all of it is wet only some of it is rotted. I will replace all of the core in this area. The question is where did the water enter. In addition to suspecting leaky deck hardware I also have a bent rubrail. I guess sometime over the years a previous owner hit a piling or a dock pretty hard and bent it inward. I think water leaks in at this bend so I want to remove the rubrail. What will I find at the hull/deck joint? Are they screwed together, with caulk at the joint? I can just take it apart and look but I wonder if others have been down this road and can advise me.

I recored both side decks on my 1978 Mainship I.
I did all the work from the inside of the boat so as not to disturb the gelcoat.
I never did anything with the hull/deck joint.
I re-bedded ALL of the stanchion bases and cleats, adding big backing plates.
It was messy, but not all that bad, just one sacrificial sweatshirt and a baseball cap, and lots of gloves.

I'm not on the boat now and I promise I'll go check but if I recall, I think the deck sits on the hull shoe box style. I just don't see how a bent rub rail would let moisture into the deck coring. I'd bet it's the typical Mainship leaky stanchions ,cleats, fuel fills and anything else that wasn't sealed well by the builder and/or rigger.
Like Jay, we rebuilt our decks too. I did it the lazy way and went from the top but I didn't use any core material on the decks at all and very little on the bridge floor. I went with heavier but more stout 21/32" marine plywood. We opted to not replace the stanchions which saved putting a lot of holes in the decks and did away with the single spring cleat and went back with 2- 10" spring cleats on each side,each with aluminum backing plates. The mainship spring cleat was secured with 4 little 1/4-20 bolts and no backing plate...only 4 tiny washers. Not even fender washers! Needless to say, the cleats were loose and a major source of leaks and deck degradation.

Check out marinehoeto.com for an article on how to bed deck hardware, it is excellent.

Thanks for your input Jay. I suspect that for me to do this work from the inside would be brutal. I'm not young and flexible anymore. I cut 9 feet (three 3 foot rectangles) of the port deck open using a Fein oscillating saw, very thin kurf. I hope I can match the current color of the deck with epoxy that will refill the cuts when done.

I agree Shawn that 21/32" marine plywood seems like the right core. I haven't had the stanchions on the deck for a few years now and will also permanently do away with them. It sounds like you applied new fiberglass overtop of the repaired decks, less holes. I noticed the lack of backing also and will correct that flaw as well. I'm not positive the bent rub rail is allowing water intrusion but I suspect it. So I think I have to find out if it is or not. Maybe I will spray water onto that area with a garden house and have a helper observe from the inside.

Yes Comodave, good site with lots of helpful info. Thanks.

tumblehome said:

I agree Shawn that 21/32" marine plywood seems like the right core. I haven't had the stanchions on the deck for a few years now and will also permanently do away with them. It sounds like you applied new fiberglass overtop of the repaired decks, less holes. I noticed the lack of backing also and will correct that flaw as well. I'm not positive the bent rub rail is allowing water intrusion but I suspect it. So I think I have to find out if it is or not. Maybe I will spray water onto that area with a garden house and have a helper observe from the inside.

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I just went down and looked at our 34 and ,unless it's a giant smash, I don't see how a damaged rub rail could allow water to enter the core. The rail is fastened into the side of the boat ,which is solid glass. Above the decks and all around the boat is an 1-1/2" raised fiddle railing, if you will, to add strength to the sides and decks. This "fiddle railing" or whatever it's called (I can hear the sound of everyone opening another window right now to google the proper term. Gotta love an online forum.) is also solid glass. It would be really hard for water to get to the deck core that way unless, again, the damage is severe enough to have cracked the glass on the horizontal deck.
Also, I mentioned 21/32" marine ply on purpose. The balsa coring Mainship used is 3/4". I specifically asked my supplier to order me 3/4" marine ply and I don't know if it was all that was available or it was a mistake, but it'll be hard to use that to match the existing decking without adding an extra layer of glass, or two or three layers. I found this out on our bridge recore. I used 3/4" nidacore in a very few areas and I wished I had used all ply. The glassing and fairing to try to make the 2 different sizes match was a huge pain and it shows on our boat. It doesn't look horrible but it's easily noticeable where the 2 meet. I was out of patience and ready to get the boat back in the water that year!

Mine was leaking thru the stanchion bases and soaked the balsa core. I used regular exterior plywood, I think it was 5/8 or whatever is standard and about that thickness.
I also used that plywood for my backing plates. (I prefer plywood to absorb shock if necessary).
To make the backing plates parallel to the deck and so the replacement screws didn't have to bend like the OEM ones, I used bondo under the outside edge.
Bondo sticks well to polyester resin and works well in compression.

The PO of our last boat replaced some core in the deck and just filled the saw cuts with epoxy and didn’t cover the cuts with fiberglass. Over time the cuts started showing and in a couple of places they started to leak. I had to go back lay some 1708 over the cuts after I ground them out. Then they didn’t work or leak anymore. We painted the deck with Kiwigrip so it was easy to refinish the deck after the glass repairs.

Dave: Unfortunate that the epoxied joints at the cuts opened up and leaked. I will have to study this problem. I'd rather not have to lay-up glass on deck.

Shawn: I'm happy to learn of the shoebox style overlap at the deck-hull joint. I suppose the leaks are only at the hardware bolts. That's easier.

Jay: Bondo between the underside of the deck and plywood backing plates is something I never thought of. Sounds good.