cockpit build...

hi all... i think ive got my head around this but need some details filled-in. 35' fiberglass sailboat of unknown pedigree. i like everything about it except the lack of a traditional cockpit. theres just a footwell where the wheel is mounted and the rest of the back of the boat (10' behind the cabintop) is flush to the toerail. the space under this structure is open with no bulkheads. so the plan is to build a traditional T-shaped cockpit with raised coaming, pedistal, drains, storage etc.
i know its a big job. ive been involved in big jobs before and know my way around a sailboat, raceboat, woodshop, and chandlery. since the space below is open now, tabbing in bulkheads and floors will only stiffen things up. and it gives me the opportunity to lead the lines aft and fit a dodger.
so heres the plan: build in place, from the bilge up, in 1/2 inch marine ply (im a cabinet maker by trade). every individual piece gets two coats of CPES including the ends before assembly with thickened epoxy and tacked together with SS brads. then fillets or tabbing depending on the piece. all the wear surfaces will get two layers of glass and epoxy. glassed in carlins will attach the deck and coaming, and gapped bulkheads glassed to the hull will support the floors. then lots of wood for make-pretty.

what would 'best practices' be for a tabbing schedule at the hull? cloth type and weight for encapsulation? should all surfaces be under cloth or is the CPES enough on the undersides of the ply? im concerned about weight as this could add 300 pounds relatively far back in the boat. is there a structural foam that could be substituted for some or all of the ply and would i have to increase the amount of glass over it? anybody remember a thread i could look up, or a good and up-to-date book with the details?

thanks in advance..now let me have it.

 

Your tabbing---- I would go with three layers of 9 oz tape--- 6", 4", and 2" or three layers 4" staggered centered over a 3/4" radius fillet of thickened epoxy. The bulkheads can be pre-coated with three layers of neat epoxy beforehand.
Cloth is only required at joints but can be added to the deck/sole/seats if desired. One layer of 10 oz cloth is enough. Two layers is overkill. No cloth is required belowdecks except to reinforce joints.
Even if you add 300 lbs, unless this is an ultralight 35 footer, the weight will be well inboard and considering the overall displacement (12-15,000 lbs?), shouldn't effect performance noticeably.
Rather than use foam, which requires glass both sides and tons more work, use plywood with only the aforementioned epoxy coatings for ease of construction and practicality (you can attach things to it with screws, etc.).

 

thanks alan. and i think know the answer but ill ask anyway. is marine ply really necessary when its going to be encapsulated with epoxy? wouldnt A/B exterior be adequate at about 1/3the cost?

 

Yes, standard AA or AB exterior plywood can be a good choice IF your edges especially are well sealed. Keep the bulkheads at least 1/8" away from the hull by spot-gluing first (using Popsicle sticks or paint mixing sticks cut to 3/4" long or so for spacers), cure, fillet from one side with thickened epoxy and it will squeeze out the other side, then fillet the second side right away. Your tools will be a homemade tongue-depressor-shaped stick 1 1/2" wide and rounded to a 3/4" radius at the end and a 2" or so putty knife to scrape excess after filleting. Your grinding should show green resin in the old hull before the fillet is started.
Coat with three coats neat (100%) resin all surfaces as well (though again, this is best done to the plywood on the flat atop horses beforehand).
This is exactly the place for cheaper plywood. Up in the ends of a boat there's no standing water issues and no weather issues.
Good thinking.

 

Skip the CPES robwilk37, it's just soaking up your money. Encapsulate with straight epoxy, not penetrating epoxy. If you want to improve epoxy penetration (not necessary, but some like to do it) then use the "hot on hot" method or select a particularly thin resin like Raka. The trick is sufficient film thickness of the correct resin. This typically requires 3 coats to be sure, though 2 coats usually gets darn near all of it, but the end grain usually is still being pissy.

The cheap plywood you use should be a BS-6566 sheet. The big box stores don't carry anything near this quality and good luck finding a double A or AB sheet at Lowe's/Depot. The resason you want a marine quality sheet isn't rot resistance, but strength. A BC sheet marked ground contact (APA 1-09 exterior) from Lowe's/Depot will be 3 veneers at 1/2" which just isn't worth the trouble of applying epoxy to. The internal voids alone will be on the order of several dozen linear inches. These form weak spots and will be the place a panel will suddenly fail under load.

On the first layer of tabbing to the hull shell use 45/45 biax. In fact, this is my choice for all the tabbing. Stagger the overlap as Alan directed, but error on the side of too much. If you buy a roll of biax, this is your most economical way to go and the strongest too.

1/2" plywood is fairly heavy stuff and likely not necessary except for the cockpit sole and seat tops. Using 3/8" else where will save a fair bit of weight and money. 1/4" and 3/8" would the racer's way to go, which is more the strong enough, though the sole will probably feel mushy in some locations.