On the older Chap hulls I’ve helped on, composites can work great, but the devil is in the secondary bond and hardpoints.
Resin choice: For bonding new composite to an older polyester hull, epoxy or vinylester both work, but epoxy usually gives a safer secondary bond. If you stay vinylester, grind to clean glass (16-24 grit), dewax thoroughly, big dust-free radius, and hot-coat within 24 hours. I’d avoid polyester for this job.
Transom cores: Coosa Bluewater 26 (or Penske) laminated to spec thickness is the usual go-to. Don’t use honeycomb here. Keep OEM thickness for your drive (MerCruiser/Volvo gimbal spec is critical). Bed core in thickened epoxy/VE putty, then skin with multiple layers of 1708 biax, tabbing 8-10″ onto the hull. Add knees to the stringers. Install solid inserts (G10/Coosa) and compression sleeves for all through-bolts to avoid crushing the core.
Stringers: Foam cores like Divinycell/Airex (H80/H100) are fine, but motor mount zones need solid inserts. Typical schedule I’ve seen hold up: 2 layers 1708 per side, capped across the top with another 2-3 layers; generous fillets; no exposed foam edges; seal all limber holes.
Weight/trim: Expect 40-100+ lb saved aft versus wet plywood. That can raise the stern slightly and change planing attitude. Some folks had to tweak prop pitch or move a battery forward to get the same bite. If the boat had waterlogged foam, removing it will amplify the change.
Common gotchas:
- Transom thickness off by a few mm = alignment pain. Make a drill/alignment jig for the gimbal housing before glassing the inner skin.
- Poor radii at the hull/transom corner leads to air and weak tabbing-use ½-¾″ fillets.
- Over-tightened hardware crushing foam around swim platforms and trim tabs-use hardpoints/sleeves.
- Print-through from heavy 1708 if you glass up to the cap-peel ply or a light cloth veil helps.
Longevity: The boats done with Coosa + VE/epoxy and proper tabbing have been “set and forget” (no rot, noticeably crisper transom feel). Failures I’ve seen were always bond or hardpoint related, not the materials themselves.
If you can share your exact model/year and drive, target transom thickness, and whether you plan to vacuum-bag, folks can sanity-check a layup schedule. For examples, search “Chaparral transom rebuild Coosa” or “Coosa knees vinylester 1708” on TheHullTruth, BoatDesign.net, and ChaparralOwners-there are a couple threads with step-by-step photos and tabbing schedules worth bookmarking.