Joining plywood

[Hobbyman2]

I did a 1966 14' starcraft mustang fiberglass hull we made new and installed all new floor beams and floor,, was a fun project, it took a whole year to finish .

When you put the plywood in running across the boat not length way ,,,if ya can ,,,put all the seams on the beams ,again,,,if you can ,,,, that way the seams were all reinforced .

Back then I never took pictures of any builds ,,LOL,,,,I dont even think I had a cell phone ,, so I cant show the details ,,,but I fiberglassed the floor right to the sides like the factory had done.

Be sure to seal the wood back by the sump well , thats usually where the rot starts.
Adding the foam actually added floatation.

Sounds like you have a very fun project.

[sawmill]

I did a 1966 14' starcraft mustang fiberglass hull we made new and installed all new floor beams and floor,, was a fun project, it took a whole year to finish .

When you put the plywood in running across the boat not length way ,,,if ya can ,,,put all the seams on the beams ,again,,,if you can ,,,, that way the seams were all reinforced .

Back then I never took pictures of any builds ,,LOL,,,,I dont even think I had a cell phone ,, so I cant show the details ,,,but I fiberglassed the floor right to the sides like the factory had done.

Be sure to seal the wood back by the sump well , thats usually where the rot starts.
Adding the foam actually added floatation.

Sounds like you have a very fun project.

On this one the strings are aluminum and made fast to the hull. The foam from the factory is the closed cell foam so I don't have to mess with it. I have a 27 gal gas tank in the middle of the boat at the rear so I have to make a cutout in the floor to allow an access point to get to that if needed. Today the cleanup begins and then I will make the new floor to the measurement I took from the old floor. I plan on cutting the floor to size then using my tractor and loader to drop the floor into the boat to check the fit. Once that is done I will remove it and add carpet and reinstall the floor to be fastened down. I marked all the stringers using tape on the side of the boat so I know there location when screwing the new floor down

[videobear]

What about a floating tenon?

Use a trim router and cut a 1/8" grove down the center of the pieces say 1/4" deep , then glue in one solid long piece of hard wood strip just a little under 1/2"w x 1/8" thick to join them ? " tong and grove" ,,Marine plywood ,,,,I have done this on a old restoration years ago ,,, also fiberglass on both sides and all edges of the flooring will help keep moisture out .

I drilled 3/8 holes in locations and filled the main cavities with expanding foam once the floors were in ,,,, every now and then a few 1/2" holes to allow the foam to expand ,,trim it off after it dries,,,,except the center of the boat , I left it open for drains.

This is what I will do. I bought a 18 foot Lund Pro V boat that needed a floor and the price I paid for boat motor and trailer was about a 1/4 of what the motor alone is worth, besides I like to do things like this. I am using marine plywood 5/8 thick which will be coated with fiberglass to make it 3/4 inch when finished. The floor is 64 inches wide so I need to join the plywood and there will not be any fancy cutting required. Just one end need an angle cut 6 inches back from the end and 3 inches wide
The old floor is coming out in chuncks as the glue let loose on it so I have a lot of cleanup to do after it is out. I had one of the best days Sat as my 2 youngest grandsons were here and helping. I gave each a drill and let them drill the rivits out of the floor. They worked with me all day and of couse we had to stop for a popcicle every now and then.

I like the spline idea, but I'd orient the grain of the spline crossways to the seam, or maybe at a 45 degree angle. Busting a 1/8" thick spline lengthwise along the grain wouldn't take a whole lot of force. Of course, unless you have a mighty wide board to crosscut your splines from, you'd need several of them per plywood edge.

How about making the spline itself out of 1/8" plywood?

[Hobbyman2]

Nice,, nothing like a fun project that brings the family together

[Hobbyman2]

This is what I will do. I bought a 18 foot Lund Pro V boat that needed a floor and the price I paid for boat motor and trailer was about a 1/4 of what the motor alone is worth, besides I like to do things like this. I am using marine plywood 5/8 thick which will be coated with fiberglass to make it 3/4 inch when finished. The floor is 64 inches wide so I need to join the plywood and there will not be any fancy cutting required. Just one end need an angle cut 6 inches back from the end and 3 inches wide
The old floor is coming out in chuncks as the glue let loose on it so I have a lot of cleanup to do after it is out. I had one of the best days Sat as my 2 youngest grandsons were here and helping. I gave each a drill and let them drill the rivits out of the floor. They worked with me all day and of couse we had to stop for a popcicle every now and then.

I like the spline idea, but I'd orient the grain of the spline crossways to the seam, or maybe at a 45 degree angle. Busting a 1/8" thick spline lengthwise along the grain wouldn't take a whole lot of force. Of course, unless you have a mighty wide board to crosscut your splines from, you'd need several of them per plywood edge.

How about making the spline itself out of 1/8" plywood?

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That should work to if ya an find it in a marine plywood it would be even better, ,I seen a metal channel once where you just seal and but both edges together , I never used it and have no clue where to buy it .

[sawmill]

I like the spline idea, but I'd orient the grain of the spline crossways to the seam, or maybe at a 45 degree angle. Busting a 1/8" thick spline lengthwise along the grain wouldn't take a whole lot of force. Of course, unless you have a mighty wide board to crosscut your splines from, you'd need several of them per plywood edge.

How about making the spline itself out of 1/8" plywood?

=============

That should work to if ya an find it in a marine plywood it would be even better, ,I seen a metal channel once where you just seal and but both edges together , I never used it and have no clue where to buy it .

I have done this before with 1/8 plywood. But after yesterday I got info overload which don't take much anymore. I bought some marine carpet from a place north of me that buys excess stock from the boat companies and they have a shop where they do this kind of work. The owner showed me on a boat which is the same model as mine but longer. What he showed me was that Lund makes the one stringer on each side that goes from bow to stern so that that no mater which way you put the plywood in the seam will always be over this stringer so no support is needed in this joint, but he said they use biscuts spaced about 12 inchs apart and glue so they can add the carpet before they install the floor. He told me so much about this boat that when I got home my head was still spinning. Now to get time to go work on it before I forget it all

[shipwright]

Industry standard (for full structural continuity) is an 8:1 scarff, preferably glued with epoxy. This can be used with no backer piece and no fastenings for any application. In your case, any of the above suggestions will likely do because it doesn't sound like a structural issue.
If when you say fibreglass you mean glass fabric (mat, roving, cloth) and polyester resin, don't expect much of a bond with the wood. If you mean glass fabric and epoxy it should work welL