Doral 164 with rotten, well um, everything!

Thanks, I will try the boil test and see what happens.

 

So the first 4 hour boil test was just done and for the hell of it I wanted to see how everything was fairing. So I pulled my pocket knife out and started filleting the outer skin off. I could split the glue joint with moderate effort with my knife but when I grabbed the piece of wood and tried to pull the remainder of the tag of wood off, it wood stop splitting at the joint and tear the woods fibers. Seems good so far, but I have no baseline because I have never test an exterior plywood to see how it fared. Next is to put the test piece right on my houses boiler which runs at 180*-190* for the night then I will reboil and see what happens after. Also after 4 hours of boiling I would have thought the piece of ply would have been quite flexible, but it is still very stiff.

 

So here are the results of my test. What I should do is tell you how I did the test first. As per PAR advice I,
1-boil for 4 hours
2- dried for 20 hours at whatever temp the wood got to sitting on a pipe of my boiler (was real warm to the touch)
3- boiled another 4 hours and then let the wood cool in the water to room temp.

I then grasped the ply with a large crescent wrench on the end of the piece and mildly flexed it a good number of times back and forth on the floor maybe to about 10-15 degrees of deflection and then I put a large amount of force to it crushing it to the floor. I did this to both ends of the piece.

Outcome is in the pics. The piece, from what I would say, sheared across the fibers for the most part. Maybe you guys see something different than I do but after the amount of twist I put on this piece of wood trying to get it to fail, I would say it was acceptable. Your Thoughts??

 

Might be time for the freeze test....nah, just joking. You look pretty safe with that.

 

Ha ha!
Glad to hear someone else thinks it will be OK to use. Well I guess that means it is the itchy time!! Time to break out the baby powder and Tyvek suits.

Question for you all. If I am almost doubling the amount of stringers in the hull and halving the distance between them, would it be unreasonable to think that I could drop the thickness of the center stringer from 1 1/2" to 3/4". FYI the center stringer is 12" tall.

here is the way I would do if OK;
Scarf 2, 8' X 12" pieces with a 8:1 scarf then add 2 layers of 13oz Uni per side. cut stringer to shape and then tab to hull with 2 layers of 18oz double bias. Is this a scary thought?

 

Sounds reasonable to me.

 

Your tests are flawed, not in the boiling/cooling part, but in the load portion. Samples are tested in shear, which is a fixture that holds both sides of the sample and then applies an opposing force on the plywood along the glue line axis. I've done it by simply using a bearing press or even a vice and a set of offset spacers, placing the veneers in an unfair loading situation.

What you did by bending repeatedly was create a "rolling shear" failure, which is quite different. What happens with plywood in particular, but opposing stacked laminates in general is, the fibers physically roll over as they're bend through various arcs or radius.

This image clearly shows what's happening. The outside of the curve created by the bending moment, has to conform to a larger radius than the furthermost inside layers. The cross grain fibers aren't bent, but try to conform, while the longitudinal fibers do bend. As the cross grain fibers attempt to conform, they literally roll next to their neighbors, shearing in place, from the longitudinal counterparts. This is the primary reason plywood doesn't make a good rudder or centerboard blade, except on small, lightly loaded boats.

All this said, it does appear the plywood will do. Encapsulate the hell out of it, tab it in good and move on, you got some itchy stuff to contend with. I don't see a need for the extra unidi, before tabbing in. Slather it up with thickened goo, insert into the old tabbing, lightly clamp in place, then apply more tabbing until satisfied with its bulk. I also don't think you need to double the amount of stringers, nor the athwart partitions. This is just unnecessary weight and effort. Stringer height is important, but so is width, to prevent it from buckling under load. The centerline stringer is the most loaded of them all. Simply put you can actually make a hull that's too stiff, which is nearly as bad as one that is too lightly built. From the looks of your previous pictures, the centerline stringer failed from buckling, so it can stand some improvement, which I'd do with more laminate, rather than more wood. In other words, don't make more work for yourself, just put back what was there lumber wise and make sure you've done a lot better job, of tabbing it all into the hull shell than the manufacturer originally did. This is the easier and cheaper route to pursue IMO.

 

Thanks PAR for taking the time to analyze this and giving your professional advice, I really didn't know how to test for shear in the piece of wood so I tried to figure out away that I could get the ply's of wood to slide along each other and either break at the glue joint or in the fiber.

 

So the transom is ground and almost ready to accept the new plywood, I first have to rebuild about 1 1/2 sqft of delaminated CSM that looks like it was never stuck to the transoms outer skin in the first place.

Also I will soon need to be bedding and glassing the new stringers in. But I am wondering about the way the stringers curve near the bow and how I am going to duplicate this curve with the new 3/4" ply. The pic doesn't really show it but it is a pretty tight radius and the stringer gains height rapidly, which will make bending it there very hard. Any ideas? Does it need to have the curvature or could it be run straight down the hull ending somewhere on the flat between the 2 strakes?

 

You'll need to make a jig to hold them in place, I'd say, and weight it down till it sets up. Some notched cross pieces should suffice to locate them.

 

Instead of focusing on repairs, work on getting the hull shell straight, with athwart supports along it's length. You might have to screw it down to these supports to remove any twists.

Once you've checked the hull for square and a good, clean straight run, you can move onto repairs. The longitudinal stringers can be simply traced to get their profiles. Simply suspend a taut string at the height of the top of the stringers and measure down, every 1/3rd of a meter or so, to pick up a rough profile. With these measurements you can spring a batten around the points and rough cut some template material to fine tune the same.

 

One assumed that the need to have a reasonable resemblance to the original shape, was understood from earlier discussions.