Portlight Replacement Fail need suggestions

31seahorse · Mar 2, 2019

Hello CYQK, We replaced the large ports on an Oday 272 years ago. The person who cut the panels suggested using a sticky, black sealant that came in a roll with paper between the rolls of material. I believe it was a type of window sealant. It would stick to anything it touched and that made it difficult to use. Perhaps the Butyl tape that is often touted on these discussions would be a good choice. Perhaps in double layer to allow for the movement of the panel as you have accounted for in the oversized holes.

Also, perhaps the uneven tightness of the fasteners caused a pressure that cracked the material. The solution might be to make a frame to cover the panel and push more evenly on the outside of the panel. That will change the look of the port, but might be worth the effort. We did that on an Irwin 31 Citation way long ago. Best Wishes.

Gunni · Mar 2, 2019

Were the OEM panels attached with mounting holes? I thought they were simply adhered into a flange with some kind of adhesive caulk.

dlochner · Mar 2, 2019

Can't speak to Beneteau ports, however, on Sabres the port is held in place by the caulking. There are a couple of screws that support the port while the caulking cures. The caulking compound was not a marine caulk, it was an adhesive caulk used in the building industry to secure plate glass windows. At one point they used Silpruf. There are similar products from other companies.

Harlan · Mar 2, 2019

While I haven't done it, a similar replacement is on the list for my OD34 so I've done a lot of research. Search this forum and sailinganarchy.com for info on replacing fixed ports - there is a lot of info available. The 'sticky black sealant' referred to is likely 3M VHB (very high bond) tape. It is used to adhere windows - even in skyscrapers - without mechanical fasteners. Comes in various widths and thickness. You need a thick version so it will absorb the motion from heat expansion without breaking the seal. Use it to adhere the acrylic, leave a gap around the edge and seal that with Dow 795. 795 seems to be the sealant of choice for this application. Adhering it will be one shot so get alignment exactly right - there's no readjusting after you've started! Based on what I've read 7' is very long for one piece. I'm not familiar with a First 42 but would guess there are 1 or 2 vertical fiberglass supports in the hull in that 7' span. I'd be tempted to cut the acrylic at that point(s) leaving a 1/4" gap for an expansion joint and filling that with Dow 795.

dlochner · Mar 2, 2019

Harlan said:
leave a gap around the edge and seal that with Dow 795. 795 seems to be the sealant of choice for this application

Dow 795 is equivalent to Silpruf and seems to be the current popular choice.

dlochner said:
There are a couple of screws that support the port while the caulking cures.

To be clear about this, the screws do not go through the plastic. The plastic sits on the shank of the screw and the screw heads hold the plastic against the side of the cabin. There are a couple on top and and bottom.

DArcy · Mar 2, 2019

I replaced mine several years ago, the longest is about 30". I didn't use any screws and the frames are aluminum. I'm trying to remember what adhesive I used, I believe it was Sikaflex 291. If you do use screws, do NOT use countersunk heads. The hole for the screws must be oversized to allow some relative movement between the substrate and the acrylic. The big factor is thermal expansion. The coefficient of thermal expansion for polyester fiberglass is about 25e-6 m/m-°C and acrylic is around 70e-6 m/m-°C. If you are in Michigan, you have about a 45°C temperature delta summer to winter, over 7' (3.3 m) this is a difference in length of 8mm (about .32 in). That is challenging to accommodate. Even cutting in half you will end up with 1/8" movement between the acrylic and the mount. That means you need a minimum of 1/16" clearance between the screw and the hole. In order to have enough gap to allow sealant in between you need to at least double this which means the hole should be 1/4" bigger than the screw diameter. Sounds crazy but that is the safe way to do it. Now if the screws are say only 2' apart you can reduce hole diameter accordingly.

CarlN · Mar 2, 2019

With a piece that long, screws will likely eventually cause cracks as the acrylic expands and contracts. Use the screws to hold things while the sealant is hardening and then remove and fill the holes. I was going to use VHB tape but was warned away from it in southern areas. So I went with just Dow 795. As mentioned, it's designed to hold windows into skyscrapers. Very strong. The key to 795 is not to compress the sealant too much. The bead should be at least 1/4" thick both under the acrylic and around the perimeter. This allows for expansion without causing the bond to fail. With your long window, even thicker than 1/4" is probably a good idea. To maintain the thickness, I built a 1/4" high rim of black foam weatherstripping tape on the inside edge of the port. I then 1) put down a thick bead of 795 outside of foam tape, 2) placed the acrylic, and 3) squeezed more 795 around the outside perimeter packing it in with a narrow tool to push it under the lip of acrylic and up against the black foam tape. The result was very neat as the the black foam is all you see inside. Be sure your 795 is well before the expiration date on the tube. Get all old sealant off mechanically and then clean the acrylic with alcohol and acetone on the fiberglass -- being sure to keep using new rags so you don't just smear the contaminant around.

jim9t · Mar 4, 2019

I did 8' long 1/4 acrylic last spring. I used the thickest VHB tape I could find, 3M 4991 1/2" VHB. I put Dow 795 silicone as a fillet on the inside and outside of the window. It is good so far. The thicker tape allows more movement. I think the VHB has better adhesion than the silicone.
Jim

dlochner · Mar 4, 2019

You might find this video from Boatworks Today helpful.

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