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jarcher 04-10-2009 06:32 PM

replacing dead lights - gasket or sealant?
 
Quote:

Originally Posted by 39512 (Post 472736)
I think the gasket idea needs further thought.

This quote was taken from another thread, so as not to take that thread off topic.

I am in the process of replacing the dead lights on my Scampi. I could put back the existing ones with new beding, but I am leaning toward replacing them entirely with 3/8 acrylic, smoked, for a nicer look. Not only will it make the look newer, it will cover some gel coat stress cracks that are around the edge of the frame.

This topic has been discussed quite a bit here, and I have taken some advice from various threads. I am planning to over size the holes the bolts go through to leave room for expansion and contraction of the plastic, use stainless barrel nuts, countersink the holes to make room for sealent, and probably use 3M 101 under the bolt head.

There is one thing I have not decided, and that is weather to use sealant or a gasket between the plastic dead light and the fiberglass.

I am leaning toward a gasket for these reasons:

Its easier to replace when it dies, as there will be no mess to clean.

There is evidence of silicon having been used to bed these dead lights in the past and I won't have to worry if an adhesive will adhere to the fiberglass.

if I use bolts every few inches, then that should be sufficient to produce a strong mechanical bond so i won't need an adhesive.

I won't have to worry about sealant (3M 101, butyl, whatever) not sealing so well around the shafts of the barrel nuts, although the same issue may exist with a gasket as well.


The disadvantages of a gasket are:

May not last as long as a good sealant.

I will have to fill in a number of large gel coat chips around the edge of the opening, to make sure there is a tight seal all around. Sealant would probably fill this easily.

I would appreciate any thoughts...

Thanks!

Jim

Maine Sail 04-10-2009 08:25 PM

Your should read..
 
You should read Tony D'andrea's opinions on his Select Plastics web site. This guy has spent his entire career in the plastics industry including working for Cyro one of the largest manufacturers of cast acrylic.

One of them is to not through bolt acrylic or polycarbonate lenses.. ever...

Select Plastic's (LINK)

Which Sealants To Use (LINK)

Acrylic vs. Polycarbonate (LINK)

jarcher 04-10-2009 08:45 PM

Thanks MaineSail, now that you mention it, I had read his site. I sent him an email seeking advice but he never replied. I'll phone him next week.

Here is what you referred to:

Quote:

Never ever bolt a plastic portlight in place. Screws are fine to hold a lens till the adhesive cures. Take them out asap and fill the holes with the afformentioned products. Both of these products are rated at 700 + percent elongation before tear. Strong flexible and UV resistant. Kinda like me!
So his position is that if I use adhesive to hold an acrylic dead light in place, this will be strong enough? My concern was that these are 45 inch dead lights, and I was thinking that they add to the structural integrity of the deck. I don't have much experience with adhesives, what if I want to remove it later?

If I end up using adhesives, the project may have to wait for several weeks until we have temps continually exceeding 50 deg F :(

sww914 04-10-2009 09:48 PM

There's no question that he knows more about plastic than I ever will but I can't help but doubt his statement about never screwing down any plastic. I've seen so many kinds of adhesive in so many different applications fail. I've also seen about a gazillion racecars with plastic windows screwed in. A very different but also very harsh environment.
It is necessary AFAIK to drill the screw or bolt holes oversized and install grommets in the holes to insulate the plastic from any shock or concentrated pressure from the fasteners. I've used screws and bolts with shoulders to install plastic windows to prevent over tightening cracks in the plastic.

sailingdog 04-10-2009 09:58 PM

If you're going to through-bolt the ports, I recommend that you chamfer/countersink the holes on both sides slightly, since that will help prevent stress cracks from forming at the holes.
Quote:

Originally Posted by sww914 (Post 473661)
There's no question that he knows more about plastic than I ever will but I can't help but doubt his statement about never screwing down any plastic. I've seen so many kinds of adhesive in so many different applications fail. I've also seen about a gazillion racecars with plastic windows screwed in. A very different but also very harsh environment.
It is necessary AFAIK to drill the screw or bolt holes oversized and install grommets in the holes to insulate the plastic from any shock or concentrated pressure from the fasteners. I've used screws and bolts with shoulders to install plastic windows to prevent over tightening cracks in the plastic.


jarcher 04-10-2009 09:59 PM

Quote:

Originally Posted by sww914 (Post 473661)
I've seen so many kinds of adhesive in so many different applications fail. I've also seen about a gazillion racecars with plastic windows screwed in. A very different but also very harsh environment.
It is necessary AFAIK to drill the screw or bolt holes oversized and install grommets in the holes to insulate the plastic from any shock or concentrated pressure from the fasteners. I've used screws and bolts with shoulders to install plastic windows to prevent over tightening cracks in the plastic.

From reading his site I think his concern about using screws involves twisting forces. As to the race car, well the windows need only last for the length of the race, these need to last through summer/winter for years. Still, I see many screwed down plastics. I'll ask him when I call.

I like the gromet idea.

NOLAsailing 04-10-2009 11:00 PM

DO NOT USE 101 ON PLASTIC!

Polysulphides do bad things to plastic. Use either BoatLife LifeSeal (my preference) or butyl caulk


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