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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 12-25-2009
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Replacing windows

I am in the process of replacing the ports in my Islander Freeport 41. After much soul searching I have decided to use 1/4 acrylic, just like the originals. I have searched the posts on this subject and the major issue seems to be expansion. The largest window is 40 inches long.

Lots of people have used butyl tape. Others swear by Sikaflex. I have used butyl a lot in the auto industry to affix trinkets, nameplates and such, and it is strong and lasts for years in the exposed environment of a car's outer surfaces. Others swear by Sikaflex. Everyone seems to agree, no silicone.

I have 22 windows to replace so I want to do it right the first time. Here's the current plan. I would mount them directly on the deckhouse with adhesive. The overlap of the original ports is 9/16 inches. I plan to increase this to 3/4 inches. Then apply Sikaflex primer to the outer 1 inch, both for priming and to provide a blackout strip. Following that, 1/4 inch in, would be a strip of butyl tape. Then apply the window to the deckhouse and mask the outline on the deckhouse. Finally, a bead of Sikaflex to fill the 1/4 gap.

Any comments would be appreciated. Compatibility of adhesives with acrylic, effectiveness of sealing, application method...all are fair game. Please let me know your experience.

Thanks, best regards and Merry Christmas,

Dick Pluta
AEGEA
Coral Gables, FL
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Old 12-25-2009
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Actually silicone is the best sealant in the case of acrylics. Dow 795 structural should be the first choice. The choice of Boeing - they use no fasteners. Many sealants are not compatible with plexiglas, but butyl is. Which Sikaflex product are you planning on using? In any case the holes should be overdrilled and not countersunk to avoid cracks due to expansion. Acrylic will expand 1/32 of an inch per foot for each 20 degrees F increase in temp. In Florida that could mean from a 50 degree night to a 90+ degree day temp. Over 40 inches that is nearly 1/4". The perfect solution is no holes in the acrylic and a stainless or aluminum plate to sandwich them against the cabin side. I'll be doing the same thing as you soon but my ports are only 19" long so I won't have to allow for as much expansion.
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Old 12-25-2009
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I'm doing two bolted on 38" long 1/4" acylic windows right now. They leaked, and now that I have them off I can see how the butyl tape failed. I'm going to bolt them back on with 1/8" thick EPDM washers under the windows so there is a 1/8" gap all around. Then against everyone's advice, I'm going the use a bead of silicone all the way around. With the 1/8" gap the silicone can take up + or - 1/8" without breaking loose. In about 10 years, I'll report back how it worked!

By the way, the 1/4" expansion over 40" assumes the fiberglass didn't expand at all, which is not the case. Itdoes expand less than acrylic, but it is still quite a bit.
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I agree fiberglass expands but not as much as acrylic. The thermal expansion of acrylic is over 3 times as much as reinforced polyester. I was pretty conservative with my temp range though. My suggested low is close but in the sun the high temp in a climate like Florida could easily hit 120 f or higher. If this is the range the sunny side of the boat has to contend with the expansion could reach higher than I suggested. 1/32" is .03125" for easier math. Multiply by 3.3 for the 40" and that's .103". Multiply that by 3 1/2 for the 50 to 120 degree range and you get about .36". Now the fiberglass is expanding at a bit less than 1/3 as much so multiply this by .66 and you get .24" so in the extreme temperature that could be encountered the expansion would be about 1/4" over the 40". And that is why ideally acrylic should not be bolted if avoidable.
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Old 12-26-2009
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I agree that bolts through acrylic are not ideal, but once you have all the bolt holes there isn't much you can do to change it!

Gary H. Lucas
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Old 12-26-2009
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Fill them with thickened epoxy like the other holes I am not using anymore. Then touch-up paint.
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Last edited by mitiempo; 12-27-2009 at 06:01 PM. Reason: correction
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Old 12-27-2009
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Very timely post ,I was thinking about using 15mm acrylic as permanant storm shutters over the overlarge sallon windows on my Hudson Seawolf. I had intended to make rebated hardwood frames to sit them in and fix using silkaflex ,had missed the effect of thermal expansion (must be getting senile) anybody got a better idea ,thanks Keith. (largest window is 40" X42")

Last edited by mintcakekeith; 12-27-2009 at 06:41 AM.
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Old 12-27-2009
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Storm shutters don't have to be permanently fixed. I'd just install vertical tracks on either side of the existing ports and slide them in when necessary. Allow for expansion and you'll be fine.
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Old 12-27-2009
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Check out this thread on the WoodenBoat forum related to your question. I posted my own experiences near the bottom.

Fitting perspex windows experiment - The WoodenBoat Forum

Good luck and let us know how it goes.
Jeff
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Old 12-27-2009
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I replaced non-opening ports on my boat using 1/4" acrylic (same as original) and sikaflex (they have a specific product for acrylic port installation).
So far I have not found the expansion to be an issue, though it's only been a year (that said old windows were there for 36 years or so).

The largest windows were over 4 feet long.

They were installed "on the surface" on the topsides, held by screws and with sikaflex gasket/sealant.
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