Posted by: Joe | 24 July, 2013

Replacing Porthole Glasses

After 30 years our glass has misted up due to water ingress into the interlayer film that is sandwiched between the  two layers of glass. Most of the portholes has a slight de-lamination around the edges but we had one in the aft cabin you couldn’t even see though! Some of the glass in the round ports are also cracked so I will be doing these as well.


My plan is to get the glass out and get a template made up and some new ones cut. Then they have to be re bedded into the frames using a sealent and while I am at it I might as well change the rubber seals as well!

Removing the glass

By far the easiest way to get them out was to smash them! But as a liveaboard with a small one this makes quite a dangerous mess.

Round Portholes

I spent a lot of time scratching my head over these as they have a screw in collar to hold the glass in. I did make my own tool:

(I will get a picture of this but it is just a bit of wood with some bolts at the right width)

Even though I tried to free it up it just didn’t budge. I found this was the only way to get the round ones out. As these are below the deck you can knock most of the glass outside and using a rag inside over the top you don’t get too much inside. Once smashed I picked out all the little bits around the edge with a screwdriver the collar just unscrewed!


Rectangular Portholes

For these I was able to get them out without making too much of a mess. First you need to take out frame collar. Remove the small screws under the rubber seal, then I used a bit of petrol on a brush around the edges that then dissolved the sikaflex type sealant that sealing the glass in the frame.  Then slide a sharp knife down between the glass and the frame, then gently bit by bit you can get the inset frame out:


Then I set to work with the petrol and a knife and got the glass out in one bit, minus a few chips.

Making a Template & Find Supplier

All the rectangular ports are slightly different. But I used a good piece of cardboard and got one just about right with enough clearance to stay in the frame. You might want to test your template in a few of the ports to be happy it will work for all of them. The round ones are a bit easier I found them to be 162mm diameter, this has a bit of clearance so you can get it into the frame but also enough overlap.

Finding a local supplier was a bit difficult as not many glass suppliers like the small hand cut jobs or jobs for boats! You could go down the expensive laser or sand cutting route but that would set you back a lot.

I had to put the bulk order in to get it through Glass Solutions in Cardiff, UK. We did have to get a couple remade as on the first one the put a stamp on it (required in the building trade) and some of the edges in the bulk order came out quite chipped and the tolerances where quite big. I suppose that’s what you get for going with hand cut glass though!?

They used two sheets of UK standard 6.4mm with a STADIP Bronze film for the tinting interlayer. 

Finding a Sealent
Now this took a bit of research as I wanted something I could remove again in future if I needed to. Currently a polyurethane sealent like sikaflex 291 was used but this would be tough to get out again.

Quotes from Boat US:
Think of polyurethane as an adhesive rather than a sealant.. Polyurethane is the best sealant for the hull-to-deck joint. It is also a good choice for through-hull fittings and for rubrails and toerails.
Sika 295 – Their solution to fixing glazing. But as with all skia products it is quite expensive! Plus this was Polyurethane based.
American industry leader in marine and building supplies suggest a Polysulfide caulk on this page. BoatUS suggest that Pulysulfide should not be used on Polycarbonate.

I found quite good info in this page about sealant suggestions for particular applications:

In the end I decided on 3m 4200. It is polyurethane based but medium strength allows for disassembly.. the real truth I suppose would be for me to take one apart after sealing it 😉

Refitting/Sealing Glass

This was quite straight forward. I cleaned up the frame with a wire brush then put the sealant around the frame and bed the glass in. Then you need to put the small screws back in the rectangular frames and clamp down the porthole. With the round I just put the sealant in the frame put the glass in then a bit more sealant and screw the collar back in (I used my handy tool I created to get them nice and tight).
Putting the sealent on

Putting the sealent on


With the glass fitted I then set to trying to source some seals to keep the opening porthole watertight when shut.

Now the question comes over what material to use to seal the Bronze portholes!?

Reading forums and speaking to Seals Direct a natural rubber recovers after compression much better then a synthetic rubber like EPDM. The natural rubber that Seals Direct sell is a emi soft natural rubber (45°sh) is flexible and semi compressible which but does not come in quite the right sizes so we had to get some cut which was costly at £5 per meter!

The spaces we have to fill are:

  • Rectangular Portholes = 7mm square. Seals  Direct were happy to cut the 6mm natural rubber sheet into 7mm strips which should work ok as these ports have a nice lip on the porthole body to seal against.
  • Round Portholes = 4mm square. As the natural rubber is so hard to cut they would not cut the 4mm sheet so they suggested a solid 5mm neoprene cord. Not as compressable but the next apparently the next best thing.

Glueing them in we used a waterproof contact adhesive.

To test these properly under pressure you need to heel the boat right over. Which we will be doing in a couple of weeks so will let update as to how well these seals are working.

Other ideas I had:

In future I might have to try the EPDM rubber and see how compressible it is. I did get some commercial rubber samples from a local company but it was not compressible at all. The hardest thing I have found is getting the size we need and finding time for all the calling around. The next UK supplier I might try in future would be Nufox as they do a 4.5mm square and 7mm square by the meter online.

Another solution would be silicone as Polymax do a silicon rubber square section that us available by the meter. The only problem with silicone is I don’t think it seals that well on an application where you are opening and closing the porthole. We tried making our own silicone seals by filling the space with fresh silicone then using a circle cake tin bigger then the porthole (with lots of washing up liquid on it) we were able to flatten off the top leave it to dry and cut some neat edges. Healing the boat over to test these though they did leak 😦 So as I said I don’t think silicone would work. 


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