Leaky portlights and hatches are one of the more frustrating projects to face on an old boat.
The best case scenarios are easiest to deal with, and these are usually the ones in which bedding has dried out and a simple removal, cleanup, and re-bed game plan is all it takes. When an acrylic (Plexiglas) or Lexan (polycarbonate) lens is removed, be very careful with solvents used to clean away old bedding because they can destroy the surface of once clear plastic.
To reattach the mechanically fastened lens, use a thick, adhesive butyl-rubber tape or equivalent bedding material instead of conventional tube-type sealants. (Practical Sailor testers have had good luck with Bomar hatch mounting tape.) Place the -inch-wide bedding on the lens like thick tape, and squeeze in the mechanical joint between the lens and the cabin house. It acts like a compressed grommet as well as an adhesive seal. Fastener holes drilled into the lens should be slightly oversize, never chamfered, and the fasteners coated with a sealant when installed.
In all too many cases, the leak is a symptom rather than a problem. The underlying cause likely is that the holes in the monocoque structure create a loss of stiffness, resulting in excess cabin house flex. Rig loads carried to chainplates, mid-boom sheeting arrangements, and genoa track-induced flex can cause significant deflection. Such deformational loading creates energy hot spots that can change shapes or even fracture laminate. In situations where stiffness increases dramatically-like where mechanical fasteners attach a port lens to a lengthy opening in a cabin coaming-a bending moment can twist the fastener(s), cracking the surrounding acrylic or polycarbonate.
The rigid, much more heavily laminated hull, lacks large apertures, such as holes for hatches, ports, and lockers, but the less structurally substantial deck laminate is Swiss cheesed with such penetrations. Better builders add extra reinforcement commensurate with the size and location of these holes, and it lessens the tendency for a port lens to be deformed by sailing loads.
In some cases, the problem can be solved by reinforcing the inside perimeter of the aperture with a stiff metal surround or additional laminate. Without addressing the structural problems that led to the leak, the drip, drip, drip will no doubt start again.
Why did you specify that the holes in the plexi not be chamfered. That goes against what I have been told when using sealant in a bolted connection.
I suspect that it has to do with the low tensile strength of acrylic and polycarbonate materials. Less material can lead to cracks. Chamfering the holes to countersink flathead bolts would be very crack prone. The seal that matters is through the trunk sides and the fiberglass there handles the tensile loads.
Regarding leaking portlights:
I have a 48 year old Contest 33. Removing the portlights has been something I would like to do but there are a number of technical problem in doing so. I have had good luck over the years sealing the old gaskets around the perimeter of the frames against the glass and doghouse using a sealer recommended by Matt Rutherford when I bumped into him in The Gambia back late in 2009 as I recall. He suggested and gave me some of the product. It is called “Dr. Tolley’s Creeping Crack Cure”. I have used it on a number of other applications as well with success. Great stuff! Perhaps you have tried it?
I’m not sure what Matt is up to recently but he is quite a character.
S/Y Luturna II
What should we be doing for windows with a rubber gasket? Our 20+ Capril 25 has a aluminum frame in the cabin top with a cracked and shrunk gasket. Is there a site with different sizes of gasket material for a replacement gasket?
I have had good luck making ports more waterproof using port visors .
We have a cracked port light on the aft port side of our salon, on our 1980 Lippincott 30, SV Vigilance. Suggestions on the best material for replacing the existing portlight? Suggestions as to the method. I had hope to find this somewhere in PS.