Grasping the anatomy of a boat window

Updated: Nov 4

We see lots of variations, some which are a breeze to refurbish, others which are very time consuming and thus expensive to complete. Here I talk about what makes a window, how they can go together and why some are more expensive than others to process.


With some crossover....

Portlights are windows which normally open

Fixedlights or Fixed Portlights are windows that have a fixed pane

Hatches are hatches, except on some Beneteau models where they are used as portlights (!)

Sliders are portlights which have two channels, one with a fixed pane, the other which has a pane which slides along a channel fitted with a rubber/fabric liner known as 'flock'. Sliders will always leak, it's just a case of when and how much. They are the devils work but can be cured by replacement with a single fixed pane.

Flip-flop is an opening portlight where a section of the window is able to flip/flop about a horizontal pivot point. Often these sections are removeable. Most common on older motor boats like the Fairy Huntsman we're doing as I write this.

Hopper apparently exists but we've never seen one. Perhaps these are the opening portlights where a section can be moved vertically using winders. Probably from the same devil as sliders.

Windscreens, are what they say. Typically on a motor yacht, perhaps on a flybridge. If glass and curved we are not your port of call (try Seaglaze).

Frameless are typically acrylic panels which are screwed/bolted or fixed with adhesive. No frame involved.

Windows, the generic term is everything else, including larger cockpit windows


Ignoring frameless windows for the moment, most framed windows have an outer and an inner. Some only have an outer in which case the frame is more likely to be screwed than bolted. Where an inner and outer exist these clamp together to sandwich the panel in which the window is mounted. Generally the outer has the channel into which the glazing is fitted.

Note that when sending windows in for refurbishment we only need the part holding the glazing (so normally the outer).

Frames are generally anodised aluminium alloy, though plastic (eg Vetus), Stainless Steel (eg Bomar) or painted mild steel are alternatives.

Frames can have one or two channels, the latter may just be to accommodate a thicker panel, or be to accommodate sliders.

Frames are commonly manufactured in two or more sections which are connected by small alloy inserts known to us as fishplates. A frame with two or more fishplates is easier to disassemble to remove/replace glazing and facilitate cleaning/sealing. Frames can have mitre or radius corners, the latter common on newer windows. Some frames only have a single split (eg N C Bjerg as in HR Monsun 31) and these require the frame to be distorted to remove/replace the glazing which can be risky, but is possible. Fishplates often corrode so may need replacement as part of the refurbishment. They are drilled and tapped in situ so every pair is unique. As a consequence we only take out one end as otherwise we'd be left with a puzzle as to which fits where.

Frames can be fastened from the inside or outside with screws or machine screws (MS ie threaded bolts). If MS these can terminate in the other frame (inner/outer) or typically when screwed from outside in fixings which can be plain nuts or captive bushes known as interscrews (see later Blog post on Interscrews).

Screws and MS are generally Stainless Steel, mostly with raised countersunk heads. Older (<1980) boats, particularly Westerly, can have chromed brass fixings. Pan heads are an alternative to countersunk where the holes are not countersunk. Where the screw heads are on the outside in anodised frames some localised galvanic corrosion is almost inevitable on boats more than say 20 years old. This corrosion is rarely significant and is likely to clean up very well.

Frames have flanges through which the fastenings pass. The flanges facilitate the clamping mentioned above but also help hide the possibility that the cut out was done on a Friday afternoon with a Jigsaw after a visit to the Pub.

Anodised frames are generally very robust and rarely warrant replacement. Any corrosion is likely to be localised and probably looks worse than it actually is.

On older boats or where the manufacturer is no longer with us (eg Marsh Walters for Westerly) the option to replace will be both difficult to match and eye wateringly expensive as a bespoke undertaking so rarely viable.


I won't dwell. Glazing will be plastic or glass. Plastic means acrylic or polycarbonate. Glass means single (mono) sheet or laminate, but in both cases this must be toughened (so says ISO 12216). Glass is very stiff, very difficult to scratch, but will shatter into a million (ish) pieces, many of which will start a journey that ends in your bilge pumps. Glass is not as easy to source as plastic and will be more expensive, particularly in small quantities. Glass is expensive in curved form or in tints unless your wallet is bulging and probably not from us (try Seaglaze in Norwich).

Acrylic is hard, light, resists scratching well and available in tints as well as CLEAR. The most common/popular tint is GREY (9T21 Perspex code) but we have limited stock of BLUE/GREY (9T20, an older tint), BLACK (923 - actually a very dark grey), BRONZE (only in 6/8mm) and GREEN (as was common for Beneteau). To be honest if you have GREEN or BRONZE I would suggest a switch to GREY.

Polycarbonate has immense strength and impact resistance but is difficult to source in tints and more importantly is soft, so scratches easily. We do use a special abrasion resistant version (Makrolon AR) for the RNLI but this is more expensive than acrylic and should really only be considered if you happen to own/operate a lifeboat, or if you have hull fixedlights, ie windows in the topsides which will likely be immersed and could be hit by flotsam or a parking boat.


Most older boats will have either preformed rubber seals or channels filled with butyl based Marine Seal 033 sealant. We used to use and sell Marine Seal 033 as part of a DIY kit, but now concentrate on the far superior specialist silicone sealants. So no more Marine Seal and no more DIY kits. Sorry, but let me explain why...

Marine Seal is a two part butyl putty which when mixed takes two weeks to cure and can be messed with numerous times during this time. It's not unlike Linseed Oil putty used in your windows at your (older) home. So the plus point is that you have lots of time to turn your sows ear of a first pass into a swan. The negatives are that it takes two weeks to cure and in time will both harden, shrink and leak, just like yours is now. When removed it can look like rubber as it hardens to the channel shape.

In comparison silicone will last far longer, resisting hardening and shrinkage and retaining its flexibility and adhesion. So much much better. It also cures in 24-48 hours. The downside is that Silicone, not unlike the women in our lives are very picky about who they stick to. New silicone will not adhere to old, or in fact any sealant and the most common cause of sealant failure is inadequate surface cleaning. As an additional consequence we only get one chance for that perfect fillet, something which takes practice, experience and skill. This is why we consider application a workshop job, for us.

Silcone sealants come in low modulus (for windows) or high modulus (for hatches) and ours are sourced from ARBO. Typically BLACK, GREY and TRANSPARENT are the colours we use, though WHITE is available.

As for rubber, the case is closed. Rubber seals make manufacturing easier, quicker and thus less skilled and this is the primary reason why some use rubber to seal glazing. Rubber, like Marine Seal also hardens, shrinks and then leaks too. Also, in most cases replacements are no longer available so for all of these reasons we normally replace rubber with silicone sealant, something which gives a better long term solution.

Final thoughts

  • Frames plastered with kitchen silicone in a vain attempt to choke leaks require more cleaning which will add cost. Try and remove what you can before sending in.

  • Frames with very sharp points make it more difficult to achieve a good fillet.

  • Sliders are just painful from start to finish. Consider replacing with a fixed pane and if necessary maybe mounting a small opener within the pane (eg Lewmar New Std Size 0 portlight).

  • Our quotes do not cater for cleaning bedding materials from flanges; that's your job, but if necessary we will for an extra charge. If replacing the acrylic panes would it not be better to clean the flanges with the old panes in rather than risk damaging the new? I thought you'd agree.

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