I know the liner in 338 isn't very thick.
It may be made with only matt, no cloth, and cuts like parmesan.
Played with it when the cabin windows were filled.
Chopped out the dodger's window rabbets with a DREMEL CARBORUNDUM DISK very easy. A bit tedious but it's a lite and accurate tool. Not exactly the same material as gelcoat and matt but it wasn't fazed by the thicker glass fabrics. The disk is a mm thick, 1.25" D. They mount to an arbor supplied by Dremel.
Have access to a Fein Tool? It oscillates rather than spins. One of their thin cutting blades would slice the liner in no time. They have a thin metal spatula shaped cutter that you could run along a batten propped (or double-stick carpet taped) in place. No brainer. And it has a funky vac attachment that helps collect the dust. But it produces less dust than spinners.
[When I look at some of the hefty backing plates some have used for winches and clutches on the roof, I can see propping a plate against the liner, using the FeinTool blade to cut the liner using it for the pattern, and then rubber mount the winch's backing plate directly to the underside of the coach roof. Wooden plates would definitely appear smaller/thinner and custom. If yer worried about the liner being loose, or you want to flatten a bulge, or have an even neater look, use a slightly smaller pattern to cut the liner hole and rabbet the backing block so that it laps over and supports the liner it is going thru.]
No cut approach.
Can you glue the liner to the roof - then glue your strongback to the liner?
Would work if you include some thru fasteners too. Trouble is the cabin liner was laid-up upside down in a mold and the finish layer of matt and polyester might have had wax in it to get it to set hard. That wax might still be there on the roof AND on the liner. I'd probably use LifeSeal after dewaxing and abrading to install. I used thickened epoxy which is perhaps TOO rigid.
I feel the bulkhead and any support going across the roof inside has to be immobilized by frp tabbing or glue or adhesive (or all of these), especially if the bridge is not going to be fastened thru the roof. imco
In the '60s right angle grinders swung 6" and larger disks and weighed 10#. We have those 4" Makita's now.
Seldom use mine inside anymore because the tool high speeds dust into EVERY nook and cranny all over the boat. It uses a thicker carborundom disk making ten times the mess of any other tool. I wouldn't use it!
I had the bulkhead end of the liner exposed when the strongback was removed. Liner was crooked. 338's liner was put in crooked! Clamped in place by the woodtrim and windows it caused stress bumps and hollows and bulging edges under the deck that are still evident. You'd think that after 337 cabin liners the guys at Pearson would have had it down by the time they got to Little Gull
- without having to crap it in place or cut it to fit!!!
Dang farmers!
Success in your endeavor , sir!
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