CONTINUOUSWAVE

On my son's 2001 Super Sport [13] the [molded fiberglass] thwart [seat] is cracked right across the middle, just to the port of the helm console. This happened because a previous owner attached something heavy to the underside of the [molded fiberglass] thwart [seat] with screws and did not seal the screw holes, so water got into the foam [interior of the molded fiberglass thwart seat].

The gelcoat and fiberglass on the top [surface] of the [molded fiberglass] thwart [seat] are apparently in good shape.

REPAIR METHOD
I purpose to grind out all the [fiber]glass [and] foam [on the] under[side of] the [molded fiberglass] thwart [seat], leaving the top [surface fiber]glass and gelcoat [layer] intact. Then I will [use] epoxy [resin to seal and adhere] either the closed-cell foam or [newly added] marine plywood, [bonding it] to the underside [of the molded fiberglass thwart seat], and then cover [the repair] with [some sort of woven fiber]glass [cloth].

Q1: am I on the right track [with the repair method described above]?

Also, any suggestions given will be welcomed.

I am confused by your use of “the thwart” to refer to a part of the boat. Please clarify.

Q2: is the damaged area on the deck of the cockpit of the 2001 SUPER SPORT of undesignated length?

Q3: or, is "the thwart" a seat that is made of molded fiberglass?

By "thwart" I mean the [forward] bench [seat] the console is mounted to. The boat is a 2001 SUPER SPORT 13. [The thwart seat] is [constructed with] molded fiberglass and with what appears to be a foam core.

[Moderator’s note: these further clarifications have been added to the initial post and to the thread TOPIC line.]

dockthedog wrote:Q1: am I on the right track [with the repair method described above]?

Also, any suggestions given will be welcomed.

You may benefit from reading the factory advice on making repairs to Boston Whaler boat hulls. In this case, the foam core molded fiberglass thwart seat is similar to the Unibond hull's construction. See the article in the REFERENCE section which reproduces in HTML the factory printed instructions that were distributed to dealers at

INSTRUCTIONS -- HULL PATCH KITS
https://continuouswave.com/whaler/refer ... tions.html

The appropriate repair instructions in this situation will be the advice give at

4. Structural Repairs (Holes larger than a dime)

The advice offered there is similar to your narrative of your plans, but you will find more details in the factory instructions.You need to restore the strength of the cracked area with proper fiberglass mat cloth. Leave DRY original foam intact, as shown in the illustrations, but remove any wet foam.

Also, the underside of the molded fiberglass thwart seat will not be particularly visible, so you really do not need to go to the trouble of making the top layer of the repair using a color-match gel coat patching resin. You could just use some white acrylic artist pigment to tint the epoxy resin to a shade of white. This repair won't be in direct sunlight, and therefore the epoxy resin color shift due to exposure to ultraviolet rays should be minimal; if there is a change in hue it won't be visible with the seat in place unless someone crawls under the seat and looks upward at the repair.

If you are worried about the strength of the repair then adhering a wood board on top of the thwart seat’s underside to bridge the repair area would be a good way to ensure the thwart seat won't crack again. And, as before, the repair won't be in view and its appearance won't be objectionable.