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Bedding Deadlights w/ Butyl tape, DC795 and Screws

Specifically the long title:
Installing new Acrylic Deadlights w/ Butyl tape, DC795 and Trusshead screws. Is this a good, ok or poor idea?

I know this subject has been addressed a number of times but I have not seen this particular method suggested or addressed.

On my new, to me, Precision 28 I managed to crack one of my portside deadlight. I tightened a loose screw on an interior trim piece and well...

The current dead lights are once piece 68" long x 11" high tapering down to 9" high in a trapezoid shape, covering two portholes. They are screwed to the cabin with panhead blind screws and sealed with an unknown a clear sealant (silicone?).

I am having new 1/4" acrylic dead lights cut by a local glass shop using the old dead lights as templates. However I am having the new dead lights cut as two piece one for each porthole with a matching gap with a similar overlap betweeen the two. So I will have two dead lights approximately 27" x 10" and 33" x 9".

I believe this will make them easier to deal with during the installation process as well as dealing with potential expansion issues.

So next week I will be installing my dead lights and plan to use the following procedure (bear with me here):

First to deal with the acylic expansion issues I will be drilling .218 dia holes and mounting with SS #8 Trusshead screws. A #8 screw is a .16" dia. screw this will give me .058" clearance for expansion. If acrylic expands at a rate of .000039/"/degree F and my screws are spaced at 4-5" and a temperature fluctuation between New Years and July (100 degreees) I should see an expansion of .0176. So that should give me plenty of room. With Butyl tape in each of these screw holes under a truss head of .375 I should have plenty of coverage. The trusshead should also distribute the load holding forces a little better reducing the chances of deformation.

Surface preparation:
Wetsand the mounting/sealing surfaces around each port light followed by wiping down with Interlux 202 Fiberglass Solvent Wash.

Mounting:
Apply butyl tape(1/2 x 1/16 from MaineSail) on deadlights 1/16" in from the outer edge. Wrap a small bead of butyl under the head of each screw to ensure that it fills the drilled hole. Fasten the deadlights to the cabin with the trusshead screws and gently tighten them over the course of a few hours until the tape has compressed about 50%. Leaving a a nice little 1/32" sq gap to accept the Dow Corning 795 Sealant.

Sealing:
Mask the top surface of the deadlight and around leaving a 1/8" gap. Apply the Dow Corning 795 around the deadlight making a point of working it into the 1/32" gap. Lay one more bead around in the 1/8 gap and smooth it out a nice radius/fillet. Remove the tape and allow the DC795 to cure.

I have seen the 3m VHB tape and dc795 method but I like the idea of the dead lights being mechanically fastened and I like MaineSails notes and demo of the long service life of butyl. As far as the appearance of the screws I think it looks just fine. Here are Fasters, http://www.sailnet.com/forums/gear-maintenance/59692-acrylic-polycarbonate-windows.html#post540852

So, is this a good, ok or poor idea?
 

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I'm gonna watch this thread... because I liked to replace my aged framed windows on my boat, in a similar fashion. I've considered a bunch of ways of approaching it, but to my eye, mounting with screws as well makes sense. I hadn't thought of using the butyl as well.
 

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My CS27 had all deck hardware bedded with butyl and while I am certainly a fan of butyl, I would not use it for acrylic ports. Dow 795 is the best product for this, in conjunction with VHB tape. Few fasteners are needed and after the 795 sets they can be removed and the holes filled with 795. If it is possible to install without any fasteners that would be even better.

I believe that the pressure required to compress the butyl will deform the acrylic. Even with other methods as you walk the docks you see many owner installed ports where the acrylic is closer to the cabin side in the way of the fasteners, leading to a scalloped effect that looks terrible - and that is without the problem of compressing butyl.

Using 2 pieces of acrylic instead of one is a good idea for the reasons mentioned.
 

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I've never used butyl in combo with DC795... but have had success with straight 795. I do have the deadlights standing off the cabinside surface 1/16 or so, allowing the 795 to create a full contact gasket. I followed it up with a bead around the outside as you described.
 

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This is the purpose that I thought the butyl tape would serve, spacer as well as sealant, for both the screws and acrylic.
I think the major difficulty with your intentions (and what you plan may well work fine...) would be the assurance that the 795 would indeed penetrate fully under the acrylic up to the butyl to create that 'gasket'.

One other issue would be aesthetic.. despite the darkest smoked acrylic you will still 'see' any lack of uniformity in the material between the cabinside and the acrylic. I basically 'troweled' the 795 on so that we had full coverage of black 795 under the port light, tightened panel gently to avoid dimples as much as possible, then removed the excess before running the outer bead. Alternately you might consider painting the under-portlight area black first.

I'll be interested to see how this all works out if you proceed with the butyl/DC795 combo...
 

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I would add one thing. Any holes you drill in the acrylic i would machine/sand in a small radius on both sides to help avoid any cracking once tightened. Leaving sharp edges anywhere can lead to cracking down the road.

I basically 'troweled' the 795 on so that we had full coverage of black 795 under the port light, tightened panel gently to avoid dimples as much as possible, then removed the excess before running the outer bead. Alternately you might consider painting the under-portlight area black first.
I think this may be the best coarse of action. Acrylic can have some movement.
 

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Common issue on C&c many people use two sided automotive tape and 795. No need for butyl for this job.
 

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Discussion Starter #9
Update - deadlights are installed and do not leak!
However my initial above noted installation method was slightly altered. While screwing down the deadlights the butyl compressed and extruded out beyond the edges of the deadlights. So I cut around the perimeter of the deadlight with a plastic razorblade and cleaned it up using the stab and pull method. This left me with no gap for the DC795 so I skipped it.
 

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I'm looking at doing this job in the spring. A thought - your calculation of expansion is for the distance between two screws, whereas the greatest amount of expansion will be between the two screws that are furthest apart. That is about 33 ins not 5 ins. So the movement will be about six times your calculation. However, the substrate (the deck) will also get hot, almost as hot as the acrylic, and expand also but at a different rate. But as there are no leaks, what the hell, I'll copy you!
 

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Butyl is great for this job, but you will find that over time more butyl will ooze out the edges. I did some of mine over a year ago and every so often I give the edges another clean up. No leaks though!
 

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I owned a j/30 for twenty odd years it was a good boat. Like C&C and Cals of the time it had those fixed side ports bedded in some sort of goop that failed after x amount of years. Removing the ports cleaning up the old sealant was arduous to impossible to remove. I talked to other owners of j'S C&c's and the like and found that it was an on going problem that no matter what you use or how you affixed them in x amount of years they ALL leak. So from the very beginning I took the regular maintenance and up keep look about it just like you do on chain plates. So every third year without fail I popped those suckers out cleaned up the old and re-bedded with tub and tile silicon and never ever had a leak. It was maybe hour and a half job and did the chain plates at the same time. I figure all that goop is just as bad as the next and it will fail and chose the one easiest to clean the old off with least amount of damage.
 

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What you're talking about is what we in the industry call a "structural glazed joint". Skip the screws. With VHB they are not required. Set the VHB 1/4 inch back from the edge, adhere to the cabin, mask all surfaces, motor in the 795. Now TOOL the joint. Never listen to anybody that says they don't or that it isn't required. Remove masking tape immediately. Go to any big city and look up. Much of that glass is held in this way.


I just brushed over the process but if you search for it on any of the sailing forums, you'll find me preaching about it on many occassions. Another guy posted a real nice page with photos and everything. Made me jealous.
 
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