# Advice on Rabbet in Marine Plywood...



## Nickp (Dec 4, 2012)

We are replacing some of the bottom on a flat-bottom wooden boat at our yacht club. It was suggested that I rabbet the ends of the existing 3/4" marine ply to join the new section. The new section, 42" long, will be 2 pieces of 3/8" marine ply as it needs to bend around the bow (tough to bend a 3/4" section). The intent being one of the 3/8" would butt against one part of the 3/4 and the second top piece of 3/8" would overlap to the inner part of the rabbet. 

I'm thinking I will rabbet about 1 1/2", 13/32's deep, into the 3/4" to allow for screws and glue/silicon. My approach would be to rabbet about 1/4" at a time by moving the clamped guide till I get to 1 1/2".

The two 3/8's will be glued together by laying the first piece and then bending the second 1 1/2" longer piece over that and the rabbet. I'm thinking Titebond III for the two 3/8's and caulking for the rabbet'ed section. Router is the Rigid handheld and am thinking 1/2" straight bit nice and slow...

...and yes, the end of the 3/4 to be rabbeted is a slightly delaminated (maybe about 1/4" into the board).

Have I learned anything from you guys or shall I modify my approach...?

Thanks in advance...Nick


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## Cherryville Chuck (Sep 28, 2010)

I would never use a water resistant glue on a boat, I would use a water proof glue approved for use below the water line like an epoxy or fiberglass resin or possibly a polyurethane. Another member who was more versed in boats than I suggested a sulfonyl caulk. It too is approved for use below the water line. Can't remember the full name of it now but a boat supplies shop would know it. I wouldn't use silicon.


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## JOAT (Apr 9, 2010)

Interesting. Personally, I would not rabbet the plywood, instead I would use 1/2" X 1/2" glue strips - which would basically accomplish the same results. 

IF the boat was taken out of the water after every use - lives on a trailer perhaps - then I would quite possibly use water resistant glue. HOWEVER, if the boat lives in the water, water resistant is unlikely to cut it - unless covered with fiberglass sheathing, and epoxy.


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## Nickp (Dec 4, 2012)

Cherryville Chuck said:


> I would never use a water resistant glue on a boat, I would use a water proof glue approved for use below the water line like an epoxy or fiberglass resin or possibly a polyurethane. Another member who was more versed in boats than I suggested a sulfonyl caulk. It too is approved for use below the water line. Can't remember the full name of it now but a boat supplies shop would know it. I wouldn't use silicon.


Good point, Charles. Intending to use 3M 4200 sealant at the seams but not between the 3/8's...guess I should re-think that. Reason behind the Titebond was to keep the cost down...I suspect, in the long run, we should use something better. Thanks for the nudge...


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## Nickp (Dec 4, 2012)

JOAT said:


> Interesting. Personally, I would not rabbet the plywood, instead I would use 1/2" X 1/2" glue strips - which would basically accomplish the same results.
> 
> IF the boat was taken out of the water after every use - lives on a trailer perhaps - then I would quite possibly use water resistant glue. HOWEVER, if the boat lives in the water, water resistant is unlikely to cut it - unless covered with fiberglass sheathing, and epoxy.


Thanks, Theo...boat is staying in the water. I guess so far the Titebond is out...

On your other point...I don't understand the application of 1/2 x 1/2 glue strips. Can you elaborate ? Thanks


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## jw2170 (Jan 24, 2008)

I would use WEST Epoxy..


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## Stick486 (Jan 4, 2013)

jw2170 said:


> I would use WEST Epoxy..


anther vote for West Systems...


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## JOAT (Apr 9, 2010)

Nickp said:


> On your other point...I don't understand the application of 1/2 x 1/2 glue strips. Can you elaborate ? Thanks


What they are is reinforcing strips. I've been using them for so long pretty much forgot how to explain them. This will probably be a better example then I could give. Sometimes I use short blocks, sometimes long strips. They really increase strength in a joint, and I seldom, if ever, use nails.
http://www.diynetwork.com/how-to/how-to-make-a-reinforced-butt-joint/index.html


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## DaninVan (Jan 1, 2012)

The standard against which all others are compared, for waterproofness, is resorcinal.
Whether you can find it is another story.
Resorcinol Marine Glue
If you're concerned about toxicity, keep in mind that epoxy is highly carcinogenic as well. Whatever you use just be sensible about protecting yourself from both skin contact and vapours. We don't want to lose members _that_ way...


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## Cherryville Chuck (Sep 28, 2010)

Living on the Sunshine Coast, you probably have a much better handle on that sort of thing than I do Dan. I will say that I have used autobody fiberglass resin as glue on an occasion or two and it has excellent holding power as well as unsurpassed gap filling capability but I wasn't using it for anything that would be submerged.


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## Garyk (Dec 29, 2011)

West epoxy is a favorite for building experimental aircraft (wood of course). If it will hold up @ 5000 feet @ 120mph it would be great for your boat. Incidentally the pilot/builders buy it from Marine dealers. It's strength & durability is well proven.


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## DaninVan (Jan 1, 2012)

Cherryville Chuck said:


> Living on the Sunshine Coast, you probably have a much better handle on that sort of thing than I do Dan. I will say that I have used autobody fiberglass resin as glue on an occasion or two and it has excellent holding power as well as unsurpassed gap filling capability but I wasn't using it for anything that would be submerged.


That's the catch, Charles. A lot of extremely effective glues fail when they're subjected to water saturation. Silicon gets recommended a lot, but in fact it releases from wet wood. That was a huge contributor in the Condo rot nightmare.
Items, flashings for example, that had been 'sealed' to woodwork in fact were letting water in behind the seals...large amounts of water.
In the case of polyester resin (fibreglass) the marine industry has a big problem with water ingression through the fibreglass, causing delaminating and worse.
Oh no! My boat's got blisters!


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## Cherryville Chuck (Sep 28, 2010)

Excellent article Dan. Thanks. I lost my love affair with silicon quite a while ago as I noticed it pulled away from what it was supposed to be protecting just as mentioned. I use other caulks instead now and rarely bother with silicon.


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## DaninVan (Jan 1, 2012)

The dad of a friend of ours had a beautiful Cheoy Lee sailboat that he had bought used. After a few years of enjoyment, he wanted to sell it and had it surveyed.
Rot, from moisture penetration under the decks, was so severe that he basically couldn't give it away. Enough to make grown men cry...


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## JOAT (Apr 9, 2010)

An interesting read.
House paint vs. marine paint


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## Nickp (Dec 4, 2012)

Thank you all...Theo, Dan, Charles, Gary, Stick and James...

Here's where I am so far...rabbet will stay to accommodate the two 3/8's mating to the 3/4...Titebond goes...West comes in...3M 4200 for the seams.

We have 5 of these boats (old crab boats) that we'll refurbish (one a year). I'm sure each will have its own peculiar challenges. Thank you for letting me off-topic...your comments and reference to other forum were really helpful...

...and the constant reading of all the woodworking on the forum has given me enough confidence to keep these old boats from the burn pile...

Nick


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