Windows on M

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john.pompei
Windows on M

I want to replace the two windows in the coach roof by the mast as they have crazed and leak. I think the leak is due to them being trod on and as they seem to be quite thin they bow under the weight thus causing the seal to part.

My first thought was to replace them with thicker acrylic which would bow less if stood upon but it would be necessary to have the edges chamfered so that there was not an edge to trip over. I guess this would be quite expensive.

Another idea might be to get replacements made in toughened glass , I have no idea what this would cost though.

I know that the windows are not designed to be stood on, but there location make them difficult to always avoid.

On a similar topic I have a small leak in one of the side windows despite tightening the retaining screws. BTW it seems as if the screws are self tappers as some won’t tighten?

Any advice on these topics would be welcome.

Regards,

John

ASHAITI 2008M

rick.jones
rick.jones's picture
Hi John

Hi John

You'd probably want to get acrylic professionally cut, so adding chamfering should not add too much to the cost. It's all too easy to crack acrylic if you try to cut it with the wrong tools.

I wouldn't go for glass, too heavy, and even toughened would be prone to cracking under stress. I'm sure a Mac hull flexes in a heavy sea!

I had a problem with window leaks in my X, and it proved to be just the screw holes that were leaking, there was nothing wrong with the window sealant itself. I just took all the screws out then replaced them with black Sikaflex in the holes. I'm not sure if the M windows are fitted the same way though.

Rick Jones (Treasurer) 1994 Mac 19 "White Lightning", Isle of Wight

mike.clarke
Hi John,

Hi John,

We too have a slight leak from our port side deck window so am interested in repair / replacement. Our windows are in quite good condition so I was thinking of removing and replacing with new sealant. I have identified an appropriate black sealant /adhesive as Sikaflex 291i which is a PU adhesive specifically for marine use - don't use normal silicone sealant as it won't bond to fibreglass or the window material properly. The Sikaflex is more expensive than most sealants but is the gold standard for marine use and the cost is small compared to the work needed to use it. I'm still not sure if the window material is acrylic or more probably polycarbonate. Although polycarbonate is more prone to UV yellowing which doesn't appear to be case with our windows. I don't know how to tell the difference without damaging the window though. I think I will try to replace it with new sealant in the first instance. If that fails I guess we will have to get a polycarbonate replacement made. I don't think they are acrylic as it is soft so easily marked and can break under impact unless its been properly stress relieved. I m not sure you can replace them with glass as this is unlikely to seal given the flexing of the deck that occurs when standing on the deck area. If you want to use glass then I think you would need to have a proper frame made up to hold it to have any chance of a proper seal given the necessary flexibility of the surrounding area - this would be expensive. As you point out the windows are held in with self tapping screws - I can see these needing to be replaced with small stainless nut and bolts if the window is completely replaced.

Has anybody else replaced their windows or knows what they are made of?

Mike C - Tarka 26M

mike.clarke
PS Sorry -  but I just

PS Sorry - but I just realised I specified the wrong Sikaflex number in my note - the correct sealant is Sikaflex 296 which is specifically for sealing marine windows. The Sikaflex 291i I mentioned in the note above is an excellent marine grade adhesive sealant but is NOT suitable for use with acrylic or polycarbonate according to their tech data sheet as it can make acrylic craze (but its excellent at bonding fibreglass to just about anything else!).

Mike C - Tarka 26M

rick.jones
rick.jones's picture
i'm pretty sure the windows

i'm pretty sure the windows are acrylic, assuming they used the same material in the M as in earlier boats. The X came with a blank panel that you could build your own electricals into, and obviously made of the same material as the windows. I drilled and cut quite a few holes in this, and having worked with acrylic panels in the past, it never occurred to me that it was anything other.

In modifying what the previous owner had done, I eventually pepper-potted it so much I had to start again (!), and bought a new piece of dark-smoked acrylic, which was just the same as the original.

Rick Jones (Treasurer) 1994 Mac 19 "White Lightning", Isle of Wight

john.pompei
Hi Mike

Hi Mike

I worked in the technical service dept of ICI plastics division on sheet materials in my youth and although that’s a long time ago now the material basic properties will still be much the same.

Im pretty sure the windows on the Mac are acrylic especially as mine have crazed so much. Cast acrylic such as Perspex is harder than Polycarbonate, more UV resistant but less impact resistant. When cutting, drilling or machining most sheet plastics it’s important not to allow too much heat to build up and it was always recommended to”back off” drill bits.

Fixing any plastic sheet to a dissimilar material needs to be done using oversize holes as the coefficient of expansion is large and changes in temperature will cause cracking or distortion. Crazing is caused by stress and aggressive chemicals such as spirit. For items such as boat windows cast not extruded acrylic is best as it starts off with less built in stress and will therefore last longer.

In the case of the Mac windows as they are fixed to fibreglass I guess both materials will expand by similar amounts so the fixings holes need not be so oversized.

Regarding the sealants, I seem to remember someone in the association recommending a form of adhesive tape rather than liquid sealant, it may have been Roly. Anyway, when I return from my holiday in a couple of weeks I will lookout some names of companies specialising in acrylic fabrication and make some enquiries. I’ll of course let you know my findings.

Regards,

John ASHANTI 2008M

ps

When we met in Tollesbury I seem to recall that you had a new main made by someone in West Mersey. I am in the market for a new one so perhaps you could pass on the details.

mike.clarke
Hi John,

Hi John,

Thanks for the info on the windows - I was coming to the conclusion that they were cast acrylic because as you say that have not yellowed like polycarbonate although ours don't show any significant crazing. The Sikaflex marine sealants are the recommended fixing method and I've been checking their data sheets and find there is a specific one for acrylic windows onto fibreglass - its Sikaflex 295UV - I have some on order from Amazon which appears to be coming from Germany for £17.57. If the windows are glass then the correct sealant is Sikaflex 296UV. Some people use Dow neutral cure silicone but even Dow don't recommend this as silicone doesn't stick well to acrylic (or gelcoat). The Sikaflex marine adhesives are polyurethane based and are moisture setting and designed for the job. I think the windows leak because of the self tapping screws used - as you said the recommendation to use oversize holes and bolts to allow some movement although the bolts are actually unnecessary if they windows are bonded properly (like car windscreens). I suspect tight self tapping screws are part of the reason for both the crazing and the leaks.

The sailmakers we used was was Nick Mussett on Mersea island - the sails he made for us are excellent quality and set much better and have lasted longer than the original Doyles. We had the jib made up with a UV protector as it lives on the furler. If you use a bolt rope on your mast you need to make sure its the right size as the original Doyles was not metric rope. Nick had to make ours a size larger than originally fitted to make sure it ran and stayed in the mast slot. His quote was also significantly better than the big name lofts and the sails every bit as good in my opinion.

N & J Mussett

UNIT 26, WALDEGRAVES LANE

CO5 8SE West Mersea

Essex England

Phone: 01206 385658

Hope this helps.

Mike C - Tarka 26M

john.pompei
Hi Mike,

Hi Mike,

Thanks for the info on the sailmaker.

Regards

John

simon.armitage
I think the tape was Butyl

I think the tape was Butyl which is widely available and in different sizes, even on Amazon. I remember Mike Floutier waxing lyrical about it before I had even heard of it. He bought his from the USA.

Simon Armitage

Sowenna 26M

john.pompei
Hi Simon,

Hi Simon,

I’ve been online and it does seem tha Butyl tape is reccomended for bedding all sorts marine fittings including windows.

It seems quite cheap and not messy, so I may well try.

Thanks & Regards

John

ASHANTI. 2008M

john.pompei
I’ve just noticed an article

I’ve just noticed an article in November’s PBO on sealing deck & windows. It recommends Sikaflex Polyurethene products and butyl rubber tape for windows.

It also strongly advises good cleaning especially removing any silicone .

it looks like we are on the right track

Regards

John

ASHANTI 2008M

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