Need advice on deck core replacement

I'm in the process of replacing the core on the deck (fuel tank cover) of a Boston Whaler 17 Outrage II. The original core was comprised of plywood pieces maybe 8" x 12", with some kind of composite in areas requiring greater compressive strength, such as T-top and leaning post mount locations. I assume the relatively small plywood pieces were an attempt to prevent moisture migration, but it failed; the entire 4' x 5' deck was saturated completely. The topside is fiberglass and gelcoat; the underside is fiberglass.

My plan it to use 1/2" 6061-T6511 aluminum in place of the composite, but should I try to emulate the small piece-by-piece construction with marine plywood or fit larger pieces?

 

Welcome to the forum.

The plywood was used as a local stiffener and "hard point", not so much to improve compressive strength. Plywood takes fasteners, far better than a sandwich core, plus it's easy to shape, machine and is low cost - this is common in the industry. Your idea of a 1/2" aluminum plate is good, though pretty darn heavy and costly comparatively. You might consider a less costly alternative, such as another sandwich panel, but with an inert set of hard points, like G-10 or maybe StarBoard or HDPE, etc. to replace the plywood. HDPE doesn't bond well, nor does it like screws, but through bolts, nutserts or blind nuts can solve this issue and is becoming more common than plywood, if a bit more costly. I'm not sure how old your boat is, but even a rebuild of what you originally had should be considered, of course depending on how much service it provided, before it took a soggy dump.

Without seeing an image of the panel, it's difficult to suggest the hard point arrangement, though often the plywood was placed as "off cuts" (scraps) to save every ounce of material, bonding it all together to form the whole. Once covered in 'glass who's to know, until it was well out of warranty. A crappy attitude I know, but these are built to a price point, not any particular level of quality, once standards are met.

The typical removable "panel" would have a perimeter of plywood, just inboard of the actual edges. Next it would have some "longitudinal" stiffeners of more wood, to help control panel flex in the "unsupported areas" of the panel. This might be just some strips running lengthwise across the longest dimension or could simply be a hodgepodge of strips, all mashed and bonded into the laminate. Both methods work, though the hodgepodge is heavier.

Post an image and we'll have a look. There's lots of different ways to build panels.

 

The piecemeal plywood and phenolic coring although removed in this photo, can be imagined from the impressions in the outer skin. Phenolic (at least that's how Boston Whaler refers to it) was used in high stress areas, such as T-top and leaning post mounts.

 

Here is a photo of the outer surface:

 

As PAR said, the plywood pieces are typical construction. The ply scraps are from their own waste in making other plywood shapes, plus they can be purchased as scrap from other business for almost nothing. The small pieces also lay down easier into the wet chop than a larger piece, and while not a bad method, the execution can get very sloppy in a busy production shop. Products like Coosa Board work very well as a core, but compare the cost of ply to Coosa and you'll see why people use plywood.

 

Thanks Fellows. I'll be laying this deck up over the next couple of days. Already wondering if I bought enough materials, but fortunately more are available locally. The one thing that still bothers me is the access hatch in the center of the deck. I'm afraid to remove it and cause additional damage, but I'm afraid also to trust it to be leak free. I'd hate to go through all this work only to put a leaky deck back on my skiff. It seams to be glassed in around its periphery, so perhaps just a good cleaning and a bead of 5200 around the visible joint between hatch and glass?

 

Boatworks Today, aka Andy Miller up in Huron?, Wisconsin, has a great video on repairing the seals on a boat hatch. I watched part of it once upon a time.

I used to be a paying member of his group, but he didn't respond to a polite email or two, so I left. I think he got mad about some candid remarks I made, but he still does a lot of good information for the boating world and I respect that about the guy.



As for your boat build. 1/2" aluminum sheet is awfully heavy, not sure why no one here questioned you on it. I think PAR understated the badness of the idea/plan.

I think the comparison would be roughly
square feet of aluminum (using 32') ~ 7# x 32' or 224#
versus corecell High Density 1/2" ~ 40#, plus say 18ounce biax each side and double with epoxy or roughly 72 ounces per square yard, let's call it 4 yards or 288 ounces or let's just say 20 pounds more for glass and epoxy for total of 60 pounds. But I think you are keeping the top by my read, so this add is only half or so or 50 total. The difference between aluminum and high density core is about 175# for 32' feet, or for your 20 square feet about a one passenger reduction on the rating plate and the boat would ALWAYS use more fuel, be slower to plane, etc.

The reason the deck rotted is rather simple. Water ingress, of course, either from not impregnating the plywood on the underside with resin, or from screwing stuff down through the plywood without overboring and filling with epoxy. To think you will fix this by just adding almost 200 pounds to the boat bugs me. Given the fact you were replacing the middle section with the aluminum means the glass is still going to be on top, so my numbers even for 20 square feet are not far off.

And corecell would be so much easier to put back in place than torch cutting aluminum.

Maybe I'm confused about the approach here, but at 4x5; you ought to do the entire top with a single composite board and a gallon? or so of epoxy and some fabric and cab and balloons. The whole job would be done in 8 hours of single man work-an hour of piecing the core, and hour for putting down some filler if needed, and hour to make an epoxy bed, an hour to lay the core down into the wetted epoxy, an hour to wetout the core, and an hour to laminate with glass and two hours for beer drinking. you'd need plastic and a sheet of ply for weighing it down

I think you need to step outside the comfort zone. The only thing aluminum has over corelite is the water absorption rate of corelite is under 1% versus the strict zero of the AL. I'd say you are taking things too far if that is the rationale. AL will corrode and could cause galvanic issues in the boat.

https://www.corelitecomposites.com/files/CoreLite Board Data Sheet.pdf

 

Hi there. I know this is quite old but i wondered if there was an update on how this went and what materials you used. I am about to undertake the same thing to exactly the same type of boat, an outrage 17 mk II 97 built. I wondered what thickness material you used in the end. I think it is 3/4 inch but haven't yet pulled the deck up. what type of fibreglass did you reseal with and what did you reinforce with for seats etc.

 

You can get 25 years from marine plywood with a few sealcoats of epoxy.

 

Just layed down 8 sheets of coosa on a commercial deck that used to be ply core. They tried to do glass over divinycell and kept crushing sections when pots landed funky. The coosa 20 and 26 hold up to abuse like glass over ply. I really do like it, I'm a fan of divinycell for lots of things as well... but coosa is my favorite of the wonder cores. I've used cases of 1/2, 1 as well as some 3/4 and 3/8, for holding screws and t tops it would be my first choice.

The easy button is blue water 20 same thickness as the ply with same glass layers.