Portholes Revisited

The overall movement is about a milometer for this size port hole. Plus I'm building completely epoxy encapsulated teak frames that will fit around it, and the tape will have tight seams.

 

The frames also need to provide that millimeter of space all around, if that's the calculated expansion. Epoxy on teak is a waste of money, teak is used because it is naturally resistant, epoxy will not increase that, plus you then have to keep up the varnish or paint to protect the epoxy.

 

I disagree about epoxy changing it. I have a life time experience with teak. There are a lot of myths about teak, people have warned me that you can't get reliable joinery, the list goes on. Epoxy encapsulation completely stops all movement. The epoxy is a much better surface for the tape to stick. And, of course, you have to varnish the epoxy. This boat is getting trimmed out with teak, teak with varnish.

 

I did a test of the 3M VHB .045 X .95
Two pieces of 1 square inch of acrylic sheet was bonded to a larger sheet of acrylic.
1. A single application of VHB tape.
2. Two layers of VHB tape.

The theramal expansion is 1.5 mm, overall so .75mm on each side.
Both test pieces were able to be moved more than needed, with the double layer providing additional margin and probably reduced repetitive load degradation.
The double layer has a .090 gap which is going to provide better caulking flexibility during cyclical expansion loads.

There is only one caveat, 3M does not recommend doubling the tape, because it has not been tested. I'm going with the double layer, I'm betting that the tape will stick to itself better than it will to the acrylic.