Only For Nutters – Part 20

Window shape can have a huge effect on the overall appearance of the boat, so I had originally decided to wait until I could see the boat from far enough away to be able to get the perfect shapes with bits of black paper.

But as the polytunnel prevents this I thought it might have to be a job postponed until after moving the boat. But edges need sealing and holes need drilling, and the interior had been like a dark cave for long enough, so I took the plunge, measured carefully, looked at it from all angles and before cutting made window templates from hardboard – more dimensionally stable than cardboard or paper. I checked again, compared both sides, even drilled little holes to check where the openings would fall on the inside and finally took jigsaw in hand. What joy to be able to sit in the cabin in the light! Suddenly it seems a whole lot bigger, and at last possible to get a real feel for the boat.

Now some important decisions had to be made. Were the windows to be planted on or inset? Should they be acrylic or polycarbonate? How to fix them? Little did I know what a minefield of conflicting advice I was stepping into!

Which material?

First question was whether to use acrylic (e.g. Perspex, Plexiglas etc) or polycarbonate (e.g. Lexan). Polycarbonate has some outstanding qualities – it is very tough and shatterproof and so can be used in thinner sheets for windows than would be necessary for acrylic. But it also has significant drawbacks. Being quite soft it is very prone to scratches, unless it has a special hard coating applied, and it is not resistant to UV light unless there is another coating, which must go on the outside. Polycarbonate cannot be easily moulded to shape, so it has to be bent around any curves. It has higher expansion than acrylic and it is also more expensive.

Acrylic is used very widely. It is generally resistant to UV, although it does tend to craze and break down after ten years or so. The material is hard and reasonably durable. Cast acrylic is available in a huge range of tints and colours. It can be heat-moulded and easily machined. But it needs to be used in thicker sizes than polycarbonate, so works out heavier. The big dangers with acrylic are with breakout and cracking when drilling and cutting and with cracks after installation due to stresses or thermal expansion.

Planted-on or inset?

I had dreamed about the perfect installation, recessed windows creating a smooth cabin profile, with no fasteners. Sleek and superyottie-style… But it would entail a great deal of extra work just for that ‘wow’ factor: cutting back the balsa, rebuilding and glassing the edges, and sorting out the attachment method. It would be much simpler to remove and replace the windows if they were just bolted-on to the outside.

Fixing methods

Clearly one of the major factors in my decision would be how the windows were to be attached. The more research I did, the more I realised that every person has a different answer! Which usually means that no one has come up with the perfect system.

A wander around a boatyard shows how very far from perfect many installations are. I noticed lots of boats where there was some sort of problem: leaking windows with extra silicone around the edges, star-cracking around bolt heads, green gunge taking hold between the window and the seal, rippling windows where the bolts had been tightened down too hard or the coachroof had distorted.

The challenges

(1) The major problem with plastic windows is movement. This is what breaks seals and causes leaks. Movement (thermal expansion) cannot be prevented, so it must be taken into account. For acrylic it is approximately 0.07mm per metre per degree Celsius. So a boat window going from freezing to 40 degrees C would experience around 2.8mm of expansion for every linear metre. But temperatures might well go beyond this: in the winter they might drop below zero, and in the heat of Mediterranean summer with tinted acrylic perhaps they would get as hot as 50 degrees or more. So the extreme range might be as much as 4mm of expansion for each metre. This is about ten times as much as glass and three times more than aluminium. And multihull windows tend to be large.

But things aren’t quite as bad as all that, because if I imagine my window rigidly fixed in the middle of its length, then the maximum expansion at the edges would be only half the total: 2mm. And if the window is installed at, say, 20 degrees C, then it will expand by 1mm and contract by 1mm at the extreme edges. (The window will actually expand in all directions, so there will also be some movement vertically. The cabin side/roof will also be expanding and contracting…) At least I have a rough idea –around 1 to 2mm of movement per metre of window.

Alkema, the makers of Plexiglas, issue a warning in their guidelines: “Tapes and sealants that adhere to both the acrylic sheet and the frame should be sufficiently extensible to accommodate thermal expansion of both.” Any method using a flexible adhesive alone must be able to cope with a considerable amount of movement. But relying only on sticky stuff at sea? I would worry that if water and wind managed to loosen one corner of the window, it would only be a matter of time before the whole thing came away. Perhaps this is where the inset window wins out. There is no corner to be lifted, and the whole perimeter can be filled with flexible sealant. Except that when the time comes for replacement, it will be the devil’s own job to prise the window out.

(2) Bolts or screws are often a problem. Certainly they can be a source of leaks, especially if they resist the inevitable movement and cause the acrylic to part company with the seal as a result. Plexiglas issue this warning, which seems to be directed at boatbuilders: “Avoid inflexible fasteners such as bolts that do not permit expansion and contraction.” Oh dear! Surface-mounting the windows means that bolts are pretty much essential, so extreme care must be used to ensure that movement is permitted. They must be placed into oversized holes. 1 or 2mm larger holes are appropriate. Countersunk heads are a definite no-no. These may look neat, but the V- shape effectively prevents movement as well as exerting a splitting pressure on the plastic.

You might think that spacing the bolts more widely would improve things, but not so! Point-loading and expansion between fasteners can result in a rippling effect along the window and break the seal. If using bolts, it would seem that more is better as the loads are divided between them so each one needs less tightening pressure.

(3) The type of gunge being used is important. There are a lot of different types, and they are all good at some but not necessarily every job. High modulus means stiff, so low modulus is the stuff we need for sealing very flexible joins. Some of my past problems have been due to assuming I could use one basic tube for all purposes. Sikaflex 295-UV is the premium product, and this is formulated to do both sticking and sealing; most other products are not.

The ability of adhesives to accommodate movement is in direct proportion to their thickness. So a thin layer of adhesive, say 1mm thick, being pulled 2mm sideways is being elongated by about 200%. Using bolts to pull a window down hard, squeezing out the adhesive until it is very thin, almost guarantees a broken seal later on. Better practice is to place nylon spacing washers to ensure that an even thickness of bonding agent remains. 3mm thick is about right.

(4) UV degradation causes many problems, and ultimately destroys the plastic of the window. Some adhesives may not resist UV very well, and they are still exposed to it even under the acrylic or polycarbonate. Many boats are now using a painted perimeter band on the top surface of the windows, to prevent this degradation, and this measure is recommended by Sikaflex even for use with their Sikaflex 295-UV, which has good resistance. The paint also helps to hide any air bubbles or dodgy bits!

(5) Removal. If the acrylic is only going to last 10 years it needs to be replaceable, so it has to be possible to get it off. But if the adhesive is doing its job, this will be less than straightforward. Hacking away with chisels, smashing the window to get it off, is hard work and no fun, and may risk damage to the hull side as well.

…and the solutions

(1) Adhesive and bolts. In this version the bolts are there just as a back-up for the adhesive. They may therefore be quite widely spaced. But if they are tightened too much they will in time distort the plastic and can break the seal. Therefore most of the work is being done by the adhesive. Nylon washers will help maintain the proper thickness of gunge. Sikaflex recommend that the bead of 295-UV is not less than 3mm thick, on windows below about 1.1m. One cartridge of 295-UV will do about 2 metres of 40mm x 3mm, so with my 40mm overlap, assuming the perimeter is about 3m, I would require 1 ½ cartridges per window. Plus the primer to aid adhesion. It works out at around £40 per window. (Sika have comprehensive instructions and some very informative stuff on their website about making triangular beads so air is not trapped.)

(2) One solution suggested to me was to use a non-setting permanently flexible mastic, such as Arboseal. Butyl tape is used in domestic glazing for the same purpose. The marine window manufacturer who told me this said that the RNLI always specify this sort of installation. Except that most of their windows seem to be in frames, which makes life a lot easier. I would be concerned at the possibility that the flexible mastic would gradually squeeze out over time, even if one used nylon washers. But it would be reasonably easy to remove and replace. Not convinced by this one.

(3) Yet another solution, used by Aussie builders and some production boatbuilders, is, wait for it, double-sided tape! 3M make some Very High Bond (VHB) tape, and their thickest version, VHB 1991, is just over 2mm thick. This sticks the windows in place, and silicone is squirted into the gaps around the edges to make a seal. But, going back to our 1 metre window, if the expansion reaches 2mm, the tape will be sheared to 45 degrees. Much more and it will surely come apart. I would only use this on smaller windows and inset ones. Low modulus neutral-cure silicone is available from the building trade, used for sealing around door frames, windows and all areas where there is a lot of movement. Dow Corning 791 comes recommended.

But what if we go in the opposite direction, and rely on our bolts, which we need anyway for security, so that the compound we put between the window and the hull only has to do one job? It is then really just a gasket. I am indebted to Rick Palmer for this solution, which is the one I am adopting.

(4) Rick first used foam rubber for his windows about 35 years ago. They never leaked, and best of all, they were easily removeable for replacement. For his latest cat he is using a one-piece gasket of foam rubber, adhesive-backed for positioning, with just a squirt of silicone around the edges. The bolts are pushed through the material, and apparently seal adequately without silicone. Most significantly, the rubber and/or window can slide across each other, so there should not be any risk of that permanent breakdown of the seal that is so often seen. I would think that it is important not to do the bolts up too tightly – it should be allowed to move. The bolts can always be tightened a little later on. After all, the silicone should stop any water ingress anyway.

My research took me to Portmere Rubber, who supply many types of rubber, solid and foamed, in a myriad of thicknesses. Their recommendation to me was to use black EPDM rubber, which is very resistant to UV. Many people have used neoprene in the past, and it is somewhat less resistant. They offer it in 2mm and 3mm, adhesive-backed (although the adhesive is not waterproof), and can cut their 2m x 1m sheets to any width. (20 metres of 38mm wide and 2m of 600mm wide x 3mm thick cost me £63 delivered.)

My cunning plan for watertight windows: The holes around the edges of the windows are at 70mm intervals, drilled oversize to 7mm. The overlap between the cabinroof and the window is 40mm, so the 3mm foamed EPDM will be 38mm wide. I shall cut the corners from 600mm wide EPDM. A little silicone will seal any gaps between the pieces. (This is cheaper than using a single-piece gasket, and should be as effective.) Bolts will be 6mm flange button head, which gives a wide flat head for even pressure. I shall stick the rubber to the cabinroof (Rick prefers the window side) and bolt the window down carefully. Then finally I shall run a small bead of silicone round the edges, where there should be a little 2mm deep gap to be filled.

When it comes time to replace the windows, I can remove them easily and use them as templates for the new ones. If they should leak, I can tighten the bolts a little. I’ll let you know how I get on.


Portmere Rubber, Southampton.
Tel: 023 8022 3628

How to install windows using conventional method

Installing polycarbonate windows with double-sided tape

Detailed Sikaflex instructions