Rebedding Portlight Glass

Posted by John

I’ve read a definition that says a portlight is the openable glass flap covering a porthole. It also defines a porthole as a round opening in the side of a ship. Since the windows in Mysticeti are not just round, but also oval and rectangular, I’m not sure what they’re all supposed to be called. So even if I’m technically wrong, I’m calling the glass opening a portlight, no matter what shape it’s in.

We have several portlights that have been leaking rain water. Some of the leaking used to be through the seals where the portlight is dogged against the porthole frame. We fixed that several years ago by replacing the rubber seals. It’s the leaking around the glass itself that’s been the most problematic. Back when we replaced the rubber seals, we also took the glass out of one bad leaker and re-caulked it. For some reason the leaking only slowed down, but did not stop completely. With more leaks in more portlights this winter, we decided to try again, but use butyl rubber instead of caulk.

Some time ago we bought a box of butyl rubber tape. It came as rolls, about 1/2 inch wide, with a paper backing to keep it from sticking to itself. We’ve used it to re-bed deck hardware, including the chainplates we replaced last summer.

Butyl Rubber Tape

Mysticeti has six large rectangular and four smaller oval portlights in the main cabin, four oval in the aft cabin, plus two oval and two round in the head, and one round portlight in the engine room, opening to the cockpit.

Mysticeti Large Portlight Area

The glass in the large portlights measures 17 x 9 inches, 1/2 inch thick.

Portlight 17 x 9 glass

Below is a portlight frame with the glass and old caulk removed.

Portlight Frame

We applied the butyl rubber tape to the inside faces that contact the glass. A cast bronze clamping piece fits on the back side of the frame and screws down with 22 bronze machine screws to hold the glass. Butyl tape is also applied around the mating face of the clamp.

Portlight frame with butyl

The butyl rubber gradually deforms under pressure and evenly seals around the edges of the glass. Any gaps where the tape is not hard against the glass are easily seen through the glass from the other side, and fixed by placing a weight on the glass pane and giving it time for the butyl to deform and even out.

Once the old caulk was cleaned off the portlight frames, the butyl rubber was a neat, clean, easy way to affect a watertight seal.

For an El Nino year we’ve had an unusually high amount of rain this winter in the Seattle area. But since rebedding our portlights, we have had no drips. The best part is we no longer have to put drip catchers under them.

We hope this is a long-term solution. Only time will tell.