BYYB Forums

I just finished attaching the deck to the hull bottom/bulkhead assembly. The deck on my boat, the Triad, is 12 feet long and 50" wide, at the widest. The deck was built from a sheet and 1/2 of marine ply. The front 1/3 is from the 1/2 sheet and the back 2/3s are pieces cut from the whole sheet. The 3 pieces were joined using fiberglass butt joints. Before attaching the deck to the bulkheads I attached stringers to the deck bottom, faired the butt joints and applied a layer of epoxy, a layer of fiberglass and 2 thin layers of epoxy to the bottom of the deck. The top of the deck is still unfinished. The deck does curve over the length. The curve is about 4" deep, if you lay a long board from the front to back.

To attached the deck to the bulkheads I put thickened epoxy on the bulkhead top edges, positioned the deck and began drilling and screwing the deck to the bulkheads. After the first 2-3 screws I heard a load SNAP/POP noise. As I added more screws I heard a second SNAP/POP and then after completing the attachment, was I was walking around the boat there was a third load SNAP/POP. I have looked under, around and felt with my fingers but can not find any cracks or split joints?

Is this something others have experienced when joining preassemblies? I hope there is enough detail for those reading to get an accurate picture of the deck at the time I attached it to the bulkheads?

Any time you force plywood into shapes it wouldn't normally want to live, you'll hear some noise as it bitchs about the stress you've imposed on it. The cheaper the plywood, the more the complaining you'll hear, some making very disturbing bangs and kaboms, followed by a visible distortion in the panel, which is a sign of failure within the plywood sheet.

A 3/8" panel of good quality will have a minimum of five veneers, which will tolerate these stresses pretty well, 1/4" the same, but not so much tolerance. Lesser quality sheets of plywood have fewer layers, plus inconsistencies in the manufacture process, making them weaker by a substantially margin. These will pop and moan quicker and show signs or deformation under less strain.

Eventually the plywood will settle down and accept it's fate in the new, twisted environment. Unless you have a visible buckle, pucker, bulge or other distortion, you may be okay to continue. I'd press on the suspected area with a shoulder to test and see if it's at the breaking point, before continuing. If it survives this test, then you'll probably be all right.

Thanks Paul! I was sure I had made a critical error by glassing and epoxy coating the underside of the deck assembly before attaching it. I never considered the plywood itself. The plywood I am using for the deck is 5 ply Merenti. It is very stiff and the deck pieces are joined in a slight curve from starboard to port and when in place then forced to bend somewhat from stem to stern. I had dry fit it several times and it seemed to take the curves just fine, however it was not screwed down and it was during and just after screwing that the snap/pop noises occurred. I should have mentioned in my first post the snap/pop noises were not the cracking of wood. I have heard that many times from many different types of board. I am sure it was a bonding/joint snapping a bit. It also seemed to come from the overall deck piece and not one area or joint so the plies separating/moving a bit would fit. I will recheck the sides and deck front with a bit of pressure to make sure they are sound. I wouldn't want the kid to climb out there to sit in the sun only to have her fall through.

I have said it before and probably will again; I am sure I could have built this boat without the aid of this forum, and particularly Paul, but I would not have had the confidence or abilities their knowledge and ideas have given.

Greetings Terry,

The hull bottom on the weekender is cut from two sheets of plywood and includes in its entirety a front half, a back half, port and starboard extension strips, a butt block, two combined side butt blocks / stringers, four lengths of stringers, and about one hundred screws. The panel is assembled upside down so that you can drive the screws through the bottom into the butt blocks and stringers. I was careful to spread lots of glue over all of the mating surfaces of the parts including the ends where the stringers butt up against the port and starboard butt blocks. The background of the following photo includes my weekender hull bottom leaning against the wall. One of those stringer to butt block joints is visible at the top-center edge of the photo.



After the glue had set up, I went to lift the panel from the floor by grabbing on to just the bow, and I heard a very load pop, almost like a .22 gun shot. The sound was caused when one of those stringer to butt block joints failed in tension from the stress caused by having the panel sag "the wrong way". I suppose this was to be expected ... using an eight foot lever to try to open a 3/4 inch square joint. Oh well, live and learn.

To fix this, I set the panel right-side-up on some saw horses and let the bow sag again to open up the joint a little (POP, there goes the other side), dribbled glue into both of the open joints, then closed the joints by blocking up the bow. I don't know why I bothered as that tiny edge joint will be covered by the side panels and wrapped in fiberglass.

I do not know the details of a Triad's construction, but I suspect if the panel includes longitudinal strings that butt up against an athwart-ship butt block, then the very load popping sound you heard was one of those tiny joints letting go.

Cheers,
Tom

I never have liked wood butt blocks after I had a few fail under stress. I now use plywood butt blocks. The only place I used the butt blocks on my PC was on the deck joints. The rest of the butt joints I used tripple fiberglass coating.

I never use butt blocks except in restoration work where I may be forced to. Butt blocks make an inherently weak point and induce a new set of stress risers, especially on materials that "flow" or are bent into position.

The Payson butt joint (gouged out areas filled with fabric and goo) also work very well and leave a smooth, seamless joint. I'm usually inclined to scarf, which I find easier then the Payson butt, though I've never had either fail, but seen plenty of "sprung" butt block over the years.

When inspecting a planked boat, the first place I check for weeping leaks are the butt blocks, which often are a common source for these issues.

When I did the front two butt joints I used Paul's suggestions for planing/sanding the adjoining edges so the bump was a lot smaller then it would have been had I just layered on the fiberglas and epoxy on each side. Still if I had it to do over I would try using scarf joints. I think mostly because I distrust the butt joints and from what I have read there is no real reason for the distrust. They were the first place I looked when I heard the noises.

I have applied preasure to the deck and can't find any movement or weaknesses. I picked up the back of the boat and gave it some small twisting action also. No new noises and no movement. I think the pops came from inside the plywood or from the stringers. Both are places that would not show any gap.

The stringers were stretched a bit by the curve of the deck being on the outside of that curve. Before attaching the stringers I measured the added length that the bend gave the deck bottom surface, where the stringers are glued, and the added distence was about 5/8 inch over the 12 foot length so there could be some movement there. The stringers had coats of clear epoxy then a liberal amount of silica/wood flour mixed epoxy and finally screws to attach them. I think if I had it to do over I would put them on after the deck was bent into position. I did the hull bottom after adding the keel because I felt the curve of the hull bottom was severe and it would have compressed the stringers and epoxy used to attach them. The deck stringers are stretched not compressed.

I will make addtional inspections when I flip the boat over to work on the sides and bottom but until then I think the source of the noises will have to remain conjecture. Thanks all for your support and ideas.