SailNet Community banner
  • SailNet is a forum community dedicated to Sailing enthusiasts. Come join the discussion about sailing, modifications, classifieds, troubleshooting, repairs, reviews, maintenance, and more!

Replacing windows

7K views 30 replies 15 participants last post by  bljones 
#1 ·
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
 
See less See more
#2 ·
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.
 
#3 ·
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.
 
#11 ·
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.
Gary,

The only ones advising against a silicone, like Dow 795 for acrylic, clearly don't understand the how and why of using it for the appropriate application. I hate silicone as much as anyone and could argue against its use till the cows come home but even I can admit there are certain applications where it is appropriate and acrylic bonding is one of those areas.

The 1/8" gasket will give you a nice thick bead of silicone which is exactly what you want. I would inset the gasket at least 1/4" from the outer edge so you get enough 795 under there. You could also use 1/8" thick by 1/2" wide butyl and have a very nice seal. Butyl allows for tremendous expansion contraction.

For the bolt holes I would over drill the acrylic and fill with 795 before tightening down. Consider truss head screws as they have a nice wide and low profile head to hide the over drill. Obviously I would suggest not bolting them but you already have the holes.
 
#4 ·
Gary
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.
 
#7 · (Edited)
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")
 
#10 ·
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.
 
#12 ·
The bolt holes in the old windows were drilled quite a bit oversized. They also had EPDM gaskets under the bolt heads. The bolts weren't leaking. The Butyl tape had failed in many places. It was no longer adhered to the plexiglass or fiberglass in lots of places. I plan on putting the EPDM gaskets under the bolts again. that makes them easily resealed without removing any sealant. My whole boat is built this way, EPDM gaskets under everything.

Gary H. Lucas
 
#13 ·
Dick, the rule of thumb is that acrylic will move about 1/16" per linear foot, so at about 3-1/2" long you could see the edges shifting nearly 1/4" in position.

Check the specs for the specific grade of acrylic you are getting--they will differ. Then check online, GE (Lexan's papa) and Rohm & Haas (Plexiglass) and their major competitors all have technical papers available online for free, showing different "proper" ways to bed and seal their materials.

It doesn't matter if you bed it, tape it (3M some some wicked new tapes), or bolt it--as long as you follow the correct procedure, it will work. If the plastic can't move freely, it will tend to craze from pressure building up in it. So, simply brute force isn't the answer.
 
#14 ·
Dick, the rule of thumb is that acrylic will move about 1/16" per linear foot, so at about 3-1/2" long you could see the edges shifting nearly 1/4" in position.
I would guess, about 1/8" per each side (1/4" for the entire length but, I suppose it shrinks/expands somewhat uniformly). 1/8" is not too much (and then underlying frp probably expands/contracts a tiny bit too, reducing this amount some). Presumably as long as screw holes are not too tight and sealant has enough flexibility, this should work out just fine.
 
#15 ·
When I replace my ports in the spring I am tempted to use Dow 795 with no fasteners at all. After reading Dow's tech papers last night and looking at a few blogs where it has been done (with Dow 791) I think it would work. My 6 ports are all the same size at 21" x 6" with a slight curve horizontally. I think with an overlap of about 1" all around and an inner seal of 2 sided foam as a sealant stop that would leave over 1/2" for the 795. I would leave the appropriate thickness of 795 for expansion, 1/8" plus.
Maine Sail - comments please.
 
#18 · (Edited)
That sounds like a good plan but... Tony D'andrea is the marine plastics guru. He advises ABYC and works hand in hand with Lewmar on many projects. He owns Select Plastics in CT USA. For years he worked for Cyro Plastics, makers of ACRYLITE GP, and now owns one of, if not the largest hatch warranty & repair facility in the world. In short he knows marine plastics and is not shy about giving his opinions on issues such as screwing/through bolting, acrylic vs. polycarbonate and what sealants to use..

I have had lengthy discussions with Tony on all things marine plastic and he is a walking encyclopedia of plastics knowledge. I will simply default to him as he can say what needs to be said better than I....;);)

Here are some quotes from Tony D'andrea. I through in the acrylic vs. polycarb stuff just because it's good info..

Tony on Acrylic vs. Polycarbonate:


"Consider the Following:


  • All major hatch, portlight and window manufacturers use Acrylic in offshore / bluewater marine products.
  • Acrylic is more scratch resistant than standard (9034) polycarbonate.
  • Acrylic is significantly more durable when exposed to Ultra Violet radiation (sunlight).
  • Acrylic is less expensive than Polycarbonate.
Don't misunderstand my preference for acrylic. I buy, use and sell a significant amount of both products and each has its application. In my humble opinion Acrylic is more durable, versatile and cost effective in the hands of a skilled craftsman than polycarbonate.

Additional considerations may include polycarbonate with UV and scratch resistant coatings. While these products are heavily promoted by several manufacturers and carry 5, 10 even 15 year warranties the following information has been reported in "real life"applications:
  • Polycarbonate is impact resistant. When its new it is almost impossible to break.
  • Small quantities (less than a 4 by 8) in gauges over 1/8th inch are difficult to to find in the uv/scratch resistant grades.
  • Colors are limited. Only two standards (gray and bronze). Try and find anything thicker than 1/4 in UV/ scratch resistant!
  • Polycarbonate foreshortens when subject to static or dynamic loads. What this means is if you replace your hatch lens with polycarb, seal it and then step on it the ductile material will deflect (bow) in the center. One of two things may happen. 1st you will surely break the watertight seal, 2nd you may end up with a leg in your galley.
  • As for the warranty: The original owner is warranted against failure subject to the material being submitted to the distributor for evaluation with the original invoice subject to actual replacement cost at the time of purchase. I guess this means they sell you a new square of material and apply the old payment to the new cost. How about the labor to fabricate the part, install it and sealant? Why take the chance?
Polycarbonate is a great material, The US Air force uses it for fighter canopies! I sell Polycarb to the USCG and US Navy. Remember they don't mind using it because we are paying to replace it every three years.
Both Acrylic and Polycarbonate have specific uses and installation requirements.

Cast Acrylic (of a specific thickness) is in accordance with CE and ABYC guidelines, and installed on virtually all of the big blue water sail boats produced on both sides of the pond. Polycarbonate is commonly used as a replacement due to its ease of fabrication and incredible initial strength. The USCG and USN require Polycarbonate on their vessels but they also have a PM cycle of 36 to 42 months for change out. My Tax dollars at work...

Due to its ductility Polycarbonate it is more challenging to install. I have seen Sika Flex 295UV with primer and Dow 795 both mentioned. I use and recommend both. Dont go over 4 ft continuous length with a fixed portlight. Remember the coefficiant of thermal expansion for Acrylic and Polycarbonate is in the neiborhood of .000039 per inch per degree F. That means an 8ft plastic port will expand and contract up to 1/2 of an inch from the coldest day in Feb to the hottest day in summer. WOW!! Compartmentalize the job. It will be easier to install and less prone to leaks.


Tony on Screwing or Through Bolting:

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. Been to a boat show lately? Seen any screws?
"

Tony on Sealants:

"I hear the question as to which sealant to use when bonding Acrylic, or Polycarbonate to aluminum, stainless or FRP over and over and over.....

Well here goes... The only three adhesives I would consider using are Sika Flex 295 UV with the primer, GE SG-4000, and Dow 795. Using the correct adhesive is only 1/2 the battle. Do not apply the sealants below 50 degrees F. The temperature must maintain at least 50F during the entire 21 day cure cycle. Cut this corner and your finished before you start. Preparation of the bond area is also very/ very important. DO NOT TOUCH THE BOND AREA WITH YOUR BARE HANDS! Contamination from the dirt and skin oils will make a solid cure impossible. You may clean the FRP and metal with acetone to prep the area but if you touch the Lexan or Plexi with harsh solvents you will ruin the portlight. A 50/50 mix of isopropal alcohol and distilled water will work well to clean the plastic if needed. Remember that clean enough is not clean enough."



Select Plastics (LINK)
 
#17 ·
braidmike
In this instance, gluing plexiglass to fiberglass without fasteners, I wouldn't have it any other way. By the way there is a 20 year guarantee on 795 used properly in architectural installations. I have yet to hear anything bad about it. A lot of high rises have their glazing adhered with it and it seems to be the best structural adhesive available. Aviation uses it extensively as well. And Maine Sail likes it for the ultimate recommendation.:D
 
#19 ·
I'd love to do away with the bolts. However my windows sit on the surface, not in a recess. They aren't rectangular, they sit on an angled surface, and they are bent to a slightly curved surface. Anybody got a great idea how to hold these suckers in place until the sealant cures?
 
#20 ·
Thanks Maine, that confirms many things. I plan on acrylic and not polycarbonate. A neighbor has a 1966 Spencer 35 and his ports are original smoked plexiglass. Except for a few marks they're still like new. They are sandwiched against the hull side by bronze rings with all fastenings through the bronze. And they don't leak! not bad after 43 years. Dow does however state a wider temperature range for 795 than Tony does. My concern would be the temperature dipping below 50 f during cure, but Dow states it can be applied as low as -10 f (I'm not likely to do this though:D ). I would like to avoid the expense of having frames fabricated if it makes sense. Thanks for your quick reply.
 

Attachments

#28 ·
Gary
I have this issue as well. My ports are 21" fore and aft 6' high and the cabin side has a slight curve - less than 1/2" over their length. Any ideas welcome.
I was pretty concerned about this when I made my first set of windows. Even though I through bolted (wish I hadn't), I thought the curve should be set before installation. I bought the materials a week or so before I made the windows. I left the plastic leaning against a wall until I was ready to use it. One week later, presto! curved plastic. You gotta love an accident like that.
 
#22 ·
I think Mainesail addressed that issue earlier... use screws then remove them post-cure and fill the holes.

I used DC795 and screws, have not removed them but don't believe they are really holding anything anymore... Reviewing Maines/Tony's info I don't think I cleaned the bonded area of the acrylic as well as they recommend so I'll leave the screws in place.
 
#24 ·
I watched a guy replace a glued-in portlight on a C&C 29 mkII.. he used the toerail and lifelines and a series of cut sticks to hold the acrylic in place during the cure... looked like it worked OK.

Another friend with a C&C 35, same vintage, made SS frames and held the new portlights in place with those. Polished up they looked OK too.
 
#25 ·
That's what I'm thinking. Bracing them to whatever is handy, the rail, lifelines or whatever. On a previous boat I has stainless frames made (expensive) and built out the cabin sides around the plexiglass. The stainless frame was bolted through the raised section and the plexiglass floated in sealant. Looked good but a lot of work and expense. If I can find a way to brace it until the 795 sets I'll go for the clean look.
 
#26 ·
I've been planning to do my portlights this summer on my Grampian G30. The side lights are about 51" x 13". I've read all the advice I can find. Considering my skill level (low) and cheapskate level (high), I'm going to take this approach:

This is one of the best videos I've found on the subject:
Gemini 105Mc | How-To Series | Replacing a Window on Vimeo (This movie takes you step-by-step through the process of installing a Lexan window onto a Gemini 105Mc Cruising Catamaran)

Like suggested in the video I'll use GE Silpruf SCS2000 Series Sealant.

I'm going to use 3/8" grey acrylic.

I'm going to use Chicago Bolts (aka Binder Bolts aka Sex Bolts) to secure the portlight. These have a low head profile (1/16"). I'm going to use anodized aluminum on the exterior and white nylon on the interior. (strictly for looks).

I'll over size the window holes by about 50% to allow for expansion. I use rubber o-rings as spacers between the window and hull to guard against over compression.

Wish me luck.
 
#30 ·
I'm afraid you will need more than luck!



There are some good ideas on the GrampianOwners.Com website for replacing your portlights.
After viewing your movie link, I don't think that's the way to do your Grampian. You can contact Holland Marine in Toronto for accurate materials and instructions on how to do them properly. They owned a Grampian for many years, and stock the exact materials you need to do the job correctly.
 
#27 ·
Without knowing the exact plastic you are using, or the exact expansion numbers for it, I'd guess 1/16" oversize isn't enough. Acrylics often move 1/16" per foot of length, so a 51" long piece will change approx 4/16" (1/4") in length from expansion. Assuming it moved evenly, that would mean each end, each bolt hole, might need to move 1/8" back-and-forth across the bolt, if I'm remembering the instructions correctly.

Doublecheck some manufacturer's tech sites on that. Silpruf also might work against you, IIRC that bonds to the acrylic and won't let it move freely when there's a piece that long involved. The good news is that every reputable plastics supplier has tech support available, usually by phone once you get the call routed to the right people. They can tell you "for sure" what will be right for installation.

Then there's also 3M's "VBT" tape. At least I think it is called "VBT". Looks like a black double-sided adhesive butyl tape, but it is used to bond glass into skyscrapers. No bolts or holes needed, and yes, some folks have used it for portlights as well. 3M is very very good at answering the phone and eventually getting the call connected to their experts--no matter where they are to be found. I'd be real tempted to do it that way, rather than the old-fashioned way. I like bolts, but this stuff works on skyscraper glazing, and that's fairly difficult environment.
 
#29 ·
To hold the ports in place on an uneven or curved hull, I would highly recommend using straps and wedges. If you can get spectra webbing, rather than nylon or polyester, it will not stretch noticeably.

As for holes in acrylic-they should be countersunk slightly so as to prevent stress cracks from starting at them.
 
#31 ·
to provide EVEN clamping action to maintain the curve while the sealant curves, I don't like the idea of battens wedged against the toe rail. I would use a fenderboard or a 2x6 plank fastened to the stanchions with u-bolts to provide an outboard surface to act as a stop.
Then use reversible bar clamps like these:

in spreader mode, as opposed to clamp mode, to provide some gentle, even consistent pressure to the window surface. one end pushes against the window, one end pushes against your stanchion board, you get secure even pressure.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top