ohno I screwed up

[ohno]

Need some help here guys.

I am building a 23 plywood boat. I am using marine fir plywood, and vg fir for chine, sheer, and all frames. The plans call for a single layer of 1/2" ply for the bottom of the boat. I wanted to put on two layers of 3/8" for a little thicker bottom. The problem is this; the plans call for screw every 3" in the bottom batten (which are 3/4" thick) and every 2" in the transom and stem. Well I did that and it is fine, but now when I go do it again with the second layer of 3/8" ply the chine, stem, transom, and sheer will have twice as many screws in them as they are suppose two. I am affraid these extra hole will weaken the all those longitudenals. I am thinking about just fastening the next layer of ply with evenly spaced 3/4" screw that will not penitrate any of the longitudenals. What would you do?

[paladin]

A sugggestion.......
if you have an air compressor (I assume all nice boat shops have one)....acquire a staple gun...
Remove the driving nib and machine off about 1/32 to 1/16 th of an inch to countersink the staple. Use plastic staples. If you want to remove the staples after the epoxy sets forget modifying the staple gun......and proceed gluing down the plywood....

[mmd]

Please excuse the cynicism, but I've had a frustrating week. :mad: What we have here is yet another case of an amateur builder presuming to change a design without full appreciation of the designer's forethought and planning, nor due consideration of the pre-planning necessary for a seemingly inoccuous alteration. Fer God's sake, folks, if ya wanna make a change, call the designer! We love to talk to the folks who like our designs, and we can usually offer pretty good advice on the effects of the changes you want to make. We're not all primadonnas who throw hissy fits if you propose to change something.

OK, OK, rant is over, I've vented. Now to the solution.

Without knowing how the boat is constructed, it is a bit difficult to advise on specifics. Your thought on using short screws is reasonable, although using 3/4" screws will expose the screw tip on the inside due to the screw head going below flush with the outer ply skin. Another option may be to back out every second screw of the original set and stagger the second layer screws to land between the first layer screws (this presumes that the screws are reasonably evenly spaced). Paladin's proposed method of stapling is probably best, though. The outer skin is bonded to the inner skin, which carries the load to the hull structure via the existant screws. Be aware that one of the really tricky things to do in a double-skin hull construction is to achieve a bond between skins that has NO voids. A bubble between the ply layers will show on the surface no matter how much you fair the outside, and will eventually fill with water and cause delamination and rot. Paladin's suggested method of staples (shot thru a nylon packing tape) and then removed after curing enables you to drive the second skin down on the first without being constrained to following the widely spaced stringers.

[rkrough]

Been there ,done that about two weeks ago, My designer is dead and not much help.
On my boat the first layer was screwed and glued with thickened epoxy. I waited a few days to make sure it was set then I backed out nearly all the screws and puttied the holes. I laid the second layer in thickened epoxy put on with a 1/16 by 1/16 notched trowel, Screwed 2" on the borders and 3" in the field On the forefoot I cut the second layer into 6" strips to make sure it stuck to the 1st layer. I used 3/4x 3/4 strips of scrap that I sprung between the ceiling of the shop and the plywood strips to act as clamps.
If I were to do it allover again I would use 3/4 for the aft 2/3rds of the bottom and save a day of labor

Rich