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post #1 of 9 Old 04-11-2013 Thread Starter
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Bedding Deadlights w/ Butyl tape, DC795 and Screws

Bedding Deadlights w/ Butyl tape, DC795 and Screws

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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, Acrylic or Polycarbonate for windows

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

s/v Siren Song 1994 Precision 28
If it ain't broke, it will be soon
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post #2 of 9 Old 04-11-2013
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Re: Bedding Deadlights w/ Butyl tape, DC795 and Screws

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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post #3 of 9 Old 04-11-2013
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Re: Bedding Deadlights w/ Butyl tape, DC795 and Screws

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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post #4 of 9 Old 04-11-2013
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Re: Bedding Deadlights w/ Butyl tape, DC795 and Screws

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.

Ron

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post #5 of 9 Old 04-12-2013 Thread Starter
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Re: Bedding Deadlights w/ Butyl tape, DC795 and Screws

Quote:
Originally Posted by Faster View Post
I do have the deadlights standing off the cabinside surface 1/16 or so, allowing the 795 to create a full contact gasket.
This is the purpose that I thought the butyl tape would serve, spacer as well as sealant, for both the screws and acrylic.

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Last edited by mmazour; 04-12-2013 at 09:09 AM. Reason: poor grammar
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Re: Bedding Deadlights w/ Butyl tape, DC795 and Screws

Quote:
Originally Posted by mmazour View Post
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...

Ron

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post #7 of 9 Old 04-12-2013
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Re: Bedding Deadlights w/ Butyl tape, DC795 and Screws

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.

Quote:
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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post #8 of 9 Old 04-12-2013
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Re: Bedding Deadlights w/ Butyl tape, DC795 and Screws

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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post #9 of 9 Old 05-18-2013 Thread Starter
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Re: Bedding Deadlights w/ Butyl tape, DC795 and Screws

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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s/v Siren Song 1994 Precision 28
If it ain't broke, it will be soon
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