Flush Thruhull/Seacock Installation

Got two seacocks installed for the cockpit drains. The Westsail originally had two seacocks for the three drains in the cockpit (the two starboard drains combined into one secacock). We have four cockpit drains (the three original, plus one “port aft” drain we added). The general rule with cockpit drains is “the bigger, the better”. If you catch a wave in the cockpit, you want it to drain as fast as possible. This especially important for aft cockpit designs, center cockpit designs, one could argue, not so much (less likely to take a wave). We originally contemplated installing four seacocks for each cockpit drain. But that makes for more holes in the hull. So, we stuck with two. The port and starboard drains will combine into two thruhulls, one for each side. We could still change our minds and install two more.

As for the installation, here is how it went. Note this is a “new build” not a “refit”, we don’t have existing thru-hulls to deal with, its all a blank canvas.

  1. Create a backing plate. Maybe an inch larger diameter than the seacock. Drill a center 1/4 inch hole in the plate.
  2. Spot the location for the seacock, where the backing plate will fit. Mark through the center hole.
  3. Drill a 1/4 inch hole at the mark, through the hull.
  4. Goop up the backing plate generously with 3M 5200 adhesive/sealant. Position the plate and insert a long 1/4-20 bolt, with fender washer, though the plate and hull. From the outside, thread a wingnut with fender washer and tighten to clamp firmly.
  5. On the inside, cleanup any excess 5200 that squished out from the clamping.
  6. Let the 5200 cure (at 30-40 degrees it takes about two weeks).
  7. From the outside cut a hole with a holesaw that matches the diameter of the thruhull. Cut through the hull and backing plate.
  8. This is a flush thruhull (not mushroom), so the hole needs to be beveled. On the outside. with a router and a 45 degree bearing bit, gradually route out the hole till the thruhull fits nice and flush. Best to cut small bits at a time. If anything its best if the the thruhull is a bit recessed rather than proud.
  9. Goop up the mating surfaces of the thruhull and seacock with 5200. With two people, one person threads the thruhull from the outside into the seacock on the inside held by the other person.
  10. Tighten and voila.

The backing plate here is made from Coosa board. If using wood, use solid white oak (resists rotting). Plywood is not advised (coat it generously with epoxy if you do).

I could have used the “fast cure” version of 5200 and not waited two weeks, but I am not in a hurry.

Note, the hull is solid glass and is about 1 inch thick where the thruhulls were installed. If this were a cored hull, there would be a lot more work (removing the core and building up glass in the area of the thruhull).

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