Doral 164 with rotten, well um, everything!

Thanks Mr E, it will be made from 5/8' plywood. I will be hitting the underside with epoxy to seal it up but I just was not sure if it was beneficial or not to put a lightweight glass on it to resist checking and/or cracking of the epoxy from the stresses above.

 

In the picture i included it shows a bulkhead that the keel stringer runs into and this creates a bilge with the two athwartship pieces tying it all together, what this also achieves is a small pair of knees (kind of week looking) that attach to the transom. What I am wanting to do is remove this bulkhead and run the keel stringer right to the transom and also add the new stringers beside the keel stringer and create the pair of knees as well but the new knees are going to run to the top of the transom and also be tabbed to the bottom side of the splash well(which will also need some reworking). What I will most likely do as well is add a third layer of 3/4 ply between these knees that will be created by the new stringers. I think 1 1/2" is a little thin for the transom for the HP that the motor is making but I don't think the weight penalty of a full 2 1/4" transom is worth it, when I can strengthen the center motor board part of the transom with the 2 added knees. Thoughts??

 

1.5" certainly sounds thin, but of course it depends on other reinforcing structure, it you had a clear span and nothing else, 2" would be mandatory. You could go 2.25" locally, tapering down away from the mounting area, if needed.

 

Light weight cloths (anything less than 18 ounce) is just for abrasion protection and will not add significantly to the strength or stiffness of a piece of plywood. Agreed with above, just coat the bottom with goo and call it a day.

Looking at the above image, it does look like your best option is to start over and partition the bilge up more appropriately. I'm not sure what their idea was with the current partitions, but I'm not sure it's necessary. The whole assembly needs weep holes, so moisture can drain aft, which likely weren't done during the original build.

 

I was walking threw our CGS workshop today and over along the wall was this stack of Ply. It is a 13 ply 3/4", it is what I would say is an A surface ply with what I am unsure of as the inner ply due to there being a couple of voids on the cut ends. Do you think it is an interior plywood and not usable for in a boat?

 

My understanding is that exterior ply has a dark, visible glue-line between the plies.

 

Is a boil test still the best way to determine if the plywood is boat worthy, or is there a better test?
If boil test how long 1/2 hour 45 mins Longer?

In your own opinion which ply do you think would be stronger a 13 ply 3/4"(if applicable) or a standard 6 ply 3/4" Ext

I have read that all plywood produced in north America is built with waterproof glue (Is this true?) and it is only the import ply from South America that is questionable

 

Not all plywood built in the USA or anywhere else uses WBP adhesives. There are essentially two types; urea formaldehyde and phenol formaldehyde formulations. The phenolic resin base makes a stronger bond and leaves the typically purplish/black glue line and is commonly used on exterior and marine grades.

Naturally, the more veneers the more inherently stiffer the panel will be, but a lot has to do with it's construction. There are several tests for determining water/boil resistance. The type 1 test (WBP) is; a sample of 1 x 3 inches is boiled in water for four hours, then dried at 145F for 20 hours, then boiled for four more hours and then cooled immediately and tested wet for bond strength using a shear test on the veneers. If you have questions about the plywoods adhesive, it might survive the boiling and cooling and remain intact, just dramatically weakened, which is why you have to test in shear. Most plywood that aren't WBP rated, will show signs of delamination to some degree and this is a big hint as to how well it'll hold up.

 

W

Thanks for the clarification! I will have to try this. I doubt I will be able to do the test as accurately as you have described but with the boil, dry, boil, cool and try to pull apart the plies, it will be informative either way. It might fall apart in my hands and it might stay stuck together. I guess we'll see.

 

So, a dark visible glue-line does not necessarily mean it is suitable for exterior use, to the extent it won't delaminate ?

 

I haven't tried to verify the glue line color test yet, but generally, the stuff I get, I don't have to question and they all have a dark glue line (damnit).

 

I'd imagined the absence of a visible glue line might indicate a cheap glue like PVA, which certainly is not waterproof.

 

I

You know, I have a want to just use the ply for some reason just because in my head I am thinking it is better and I want to rely on the epoxy to do this absolutely super amazing job and make it absolutely water proof, bombproof and meteor proof(you get the point) leading to it not even mattering if the plywood's glue was made from butter. But the other gremlin in my head is saying no you can't do that because what if it doesn't seal up as well as I think and it rots and delams in a very short period. Obviously I will have to do some more investigation on it. And at $100 a sheet for this stuff and free for the 3/4" ext the cheapskate in me wants to make sure it is usable.
Thanks for all the input on this guy's, I have definitely learned some things today.

 

I took corner clippings from two of the sheets of plywood that is in question and separated them. The glue does have a blackish color to it, does this lead anyone to believe that it is the phenolic resin base glue?

 

It looks like it, but the ultimate test is moisture. Cut another corner and boil it a few times, trying to separate the layers afterwards. If it breaks along the glue line, it's not suitable, if the failure in predominantly in the wood's fibers, it'll work fine.