top of page

Grasping the anatomy of a boat window

Updated: Dec 30, 2020

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.

To save time you can jump to specific sections using the bullet links below. You might want to get a cup of tea.

Ok, let's dive in....


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 may 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 my 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. Flanges can be flat or may show waves or mini-channels on the underside which allow bedding compound to be retained even when a fitted flange appears flush.

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.

Frame section matching

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.

Why is this? When I started looking at this, like you I naively assumed one could just order some section from a wholesaler or extruder and employ someone to bend the frames in the same way as a plumber would buy and bend pipes to suit. Sadly no.

Most frame builders ‘own’ one or more section designs for which they alone have tooling. This tooling is used by whoever they employ to create the extruded lengths (which are then anodized by a third party). Some of the larger frame manufacturers (eg Seaglaze, Trend) may have several sections, while most of the smaller frame builders have just one. When builders disappear (eg Marsh Walters) so does their tooling. But frame builders are reluctant to consider 'copying' a section (even if obsolete) for fear of being sued for breaching design copywrite. This all adds up to a 'probably impossible' scenario.

So if you have a boat of an age where frames merit replacement, the chances are the section will not be available and more annoyingly something close probably won’t be either. This is one reason why frame replacement can be a nightmare.

Take for example the case of the Moody Eclipse, the ones with the deckhouse in 33 and 38 foot versions. Built by Marine Projects in the 1980's (who built Princess Motor Yachts in the same period) several at some point went rogue with windows sealed by a Sikaflex type adhesive (the devils work). Now, up for refurbishment it's a no as the time taken trying to extract and clean the black hell from the Moody frames is such that buying a new boat becomes more viable. Can we find out who made the frames and might, just lead us to the section? That will also be a no, so far and believe me we have asked everyone we can. A few years back a group of E33/38 owners had some new frames made by Seaglaze, but their closest section is viewed as 'heavy' so we're trying to find an alternative for another group of owners. This is how I now know how naive I was to think sourcing frame section would be easy!


You guessed it, there is no frame! Most older frameless windows are made from plastic, though the reader will have spotted that new yachts (both powered and sail) are now often covered with flush glazing panels, more often bonded to the hull and quite possibly made from glass as well as plastic. Let's deal with the trad versions first.

Trad : typically acrylic, fixed with bolts or screws at 75-100mm spacing around (but say 15mm inboard from) the perimeter. In the old days bedding compound would have been applied between the mating surfaces but this can be very messy (one client said it brought back memories of a 1980's fondue party), particularly inside around the cut-out. Today we recommend (but don't supply, eBay does that) a single sided tape from UK company SCAPA, named SCAPA 3507 in grey. This gives a much neater and less fondue result.

We normally use the old glazing as a template and put a bevel on the edge to make the pane edge softer. We recommend pan head fixings as countersunk fixings have a habit of splitting the acrylic in time.

More Modern : Note that we are not yet at 'Current' but have entered a dangerous period where acrylic panes are 'bonded' rather than attached with mechanical fixings. Beneteau is a culprit amongst others here. So, on the face of it you use an adhesive (think Sikaflex style) to glue the pane to your hull. No horrible fixings poking their heads from your lovely tinted glazing. Sounds too good to be true?

So Beneteau (I can't remember which model) added a rebate to the acrylic to accommodate the adhesive and allow the pane to fit flush within a recessed opening. Sleek beyond words, the punters lapped them up. 20+ years on, the acrylic is now crazed. If this was a trad install, then no problem, undo the fixings, release the tape and remove for replacement. But this is not Trad. The first issue is that being flush you can't get a tool of any shape in to lever the pane away. But that pails into insignificance because having STUCK the pane with a well know adhesive it ain't coming off anytime soon. Hair pulling, bigger heavier tools, some hours and finally desperation now have the bloody thing off. It's in pieces (desperation became frustration became a large hammer and chisel) which makes using it as a template tricky, but more importantly it still has most of your Gelcoat attached. Yes friends, your Gelcoat proved to be the weakest link and replacing the glazing has morphed into a hull regel job.... Not so keen on 'More Modern' now are we?

Current : Now I suspect you think I'm going to say this is worse, but perhaps not. The reason 'More Modern' fails is because the glazing (acrylic) fails. If it didn't craze, it could stay attached for ever and in many respects this is why GLASS is an ideal material for bonded frameless windows. Difficult to scratch and with superior UV resistance it's pretty ideal (but please don't break it). Not so good for our business but I'll let that go for Happy Sailors.

One last thought about the adhesive route. Adhesives suffer from UV and as a result suppliers will insist on the use of a UV protection paint/primer. This is normally black and is applied around the edge of the pane (glass or plastic) beyond the area where the adhesive will be (as UV can reflect from a boundary into the adhesive). This is why most (if not all) bonded frameless windows have a dark border. Now, having finished your tea is a good time to pop down the Club Bar and show your fellow sailors what an expert on Frameless Windows you've become...


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 should really be toughened (though strangely the latest ISO 12216:2019 does not make this mandatory). 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 or shape as plastic and will be more expensive, particularly in small quantities. Tints are available in glass but these may not be exact matches for those in plastic.

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.

Plastic can be curved or formed; Glass generally is flat. If a tight curve is required (eg 90 degree on some deckhouse window corners) then holding a bent piece of plastic in a frame is unlikely to work (unless very thin eg 3mm). In such cases the panel will have to be 'Drape Formed', which is something we can't do. Briefly, if you have a large oven and can heat a sheet up to the required temperature (~180C), the plastic becomes 'limp' and can be 'draped' over a male mould prior to cooling. When cool the sheet will have adopted the new shape without any need for additional bending. If you need this I suggest you google 'drape forming', or if local speak to Project Plastics in Colchester. If you need curved glass try Seaglaze or Trend.


Framed windows on 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 (period) 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 (or a plumber). This is why we consider application a workshop job, for us.

Silicone 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). Please don't take offence if we decline your sliders...

  • 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.

  • For frameless windows, take care with the edges as machining will replicate any imperfections, unless we are forced down the more expensive route of building a CAD model for the CNC.


Recent Posts

See All

SCAPA tape - sourcing

There are numerous sources online, but we've found the people at HADLEIGH ENTERPRISES LTD both knowledgeable and helpful. No kickbacks for us, but I believe good service should be rewarded. They can

Lewmar Low/Medium Profile hatch - release hinges

There are various versions of this hatch, but here we focus on the older variant with Type B hinges. For more details on the various types see here. Whether you have the hatch removed or still in pla

Lewmar Low/Medium Profile hatch - split lid frame

If your lid is still attached to the base you will need to remove the hinges first as the lid cannot be split when still attached to the base. For this read the sister article 'Lewmar Low/Medium Prof


bottom of page