Building A GRP Fly Bridge

Before you start you should know that since I have built mine, I have asked myself this question - would I do this again? I still have parts of the mould and I have learnt lots of tricks so the answer is YES but be warned it is a big job. If I didn’t have the time I would purchase something second-hand that was similar to what I wanted and then modify it. Having said that I am very pleased with the result here is a picture of my modified Shetland 570. I have to admit it is a picture that I am very proud of.

As I was a complete GRP novice I needed a book as a reference. Friends would give me tips and information that I would forget. The best book that I could find and would thoroughly recommend is (Blue Gee) Glassfibre and Epoxy Wood Book (2nd Edition) by David Benoy (Bar Code 5027215889010). I sourced mine from Aladdins Cave Chandlery at Bursledon Bridge, Hants. It cost me £20 but it had everything that I needed to know and much more.

Designing the mould

From start to finish the project took me a long time partly due to the unique shape that I wanted. I had to decide on a good position for it on the boat, design it, build it and then later (with a fishing season in between) completely line-out with formica to enable the component moulding to release. It actually took me a year! As it was to be positioned just forward of midships I felt I needed a longer steeper V shape at the front. This would make the overall look more acceptable too, matching existing lines of the bow and giving a similar overall look of the Raider Boats, which I like the look of but cost too much. This meant a bigger mould, more material and larger windows. I wanted to be able to look around the Fly Bridge while steering without obstruction. After taking a couple of side-on pictures and then crudely editing those pictures I came up with something close to what I thought would work.

Building the mould

I then proceeded to build the mould onto the boat to assume the lines and dimensions that I preferred. Using 3mm plywood sheets and batons I constructed the main mould, building the frame and sides first and then the two front panels, and the roof. To give the moulding inherent strength you need to add flanges with a radius to all edges. Due to the headroom I required at 6’3” I decided to invert the back edge for two reasons; I did not have to make the top 2” higher and any water should not run down my back while steering. I also added a larger angled face in the middle of that ‘lip’ to be able to fit my fluorescent strip light.

Once this was done I needed to make sure that I could lift off the mould and replace it without it changing shape and prove that it would still fit when replaced. I decided that the finished moulding should be bolted down through a flange along the front edges and then as per the original screen that I removed, bolt through the sides.

When I was happy that I had the mould shaped reasonably right (not perfect) I lined it with cheap white formica. I needed to take accurate templates of the sides first so I used Laminate Wood Flooring 2 mm Foam Underlay to make the templates, as it was large enough and very easy to cut and trim.

I used the templates to mark the shapes onto the formica and then used a Black & Decker ‘nibbler’ attachment to an old Black & Decker drill that a colleague gave me for cutting the formica to size. This could easily cut the shapes without the formica ‘splitting off’. Using a good Stanley knife is probably better on a straight cut. The formica is then stuck on with the proper contact adhesive. After this the all the internal angles and corners are then formed with a radius using plastercine. A radius tool gives the plasticine a smooth uniform finish. Any rough edges were rubber down and taped with the 2” packing tape. PLEASE DON’T DO THIS AND LEARN FROM MY MISTAKE. I had some bad marks created in the Gelcoat by using the Packing Tape ‘trick’ when building of the mould. Best avoid this and use fillers and rub down. There are no short cuts.


Ordering Materials

This was interesting as the chandlery would not sell me the quantities that I needed and the trade would not let me carry it in my car to my house. So I purchased it all ‘on-line’ from http://www.cfsnet.co.uk. It came on a lorry and was dropped off in my drive while I was at work. Please note that I bought the ‘good stuff’ there is the option of buying a lower standard product which for a budget flying Bridge may be sensible option. There were no questions asked and no problems. When I ordered a 5Kg can of Gelcoat or a 5Kg can Polyester Resin, 100g of Catalyst (Styrene) was sent free of charge (I suspect this worked around some of the rules).

I won’t give costs as you can find out very quickly on the above link but prices were about 25% to 30% of the normal cost when compared to any of the chandlers that I checked. For example 5kg of Clear Gelcoat was £22.40 plus VAT where 1Kg in the chandler’s shop was £17.99 including VAT!!!

Quantities for a job similar to mine:

bullet Gelcoat - About 3Kg of Clear Polyester Gelcoat for 2 coats (The text books say 1M squared area = 500g of Gelcoat, which worked for me).
bullet Gelcoat Colour Pigment - I needed about 200g so I had to buy a 500g can of pigment. To add colour to 500g of Clear Gelcoat use 5% to 10% of the pigment. I always used just over 5%. Therefore for 500g I used 30g (30cc) of pigment.
bullet Polyester Resin - 12.5Kg
bullet 300g M5 Chopped Strand Mat = 10Kg. I probably used about 7Kg of the 10Kg roll. (450g CSM may be a better option but I don’t know).
bullet Fluffy rollers or 2” paint brushes.
bullet 3 or 4 Mixing Sticks (mixing Gelcoat will make them soft, mixing resin does not).
bullet 3 Clear Graduated 1.2 ltr Buckets.
bullet 2 Clear Calibrated Measuring ‘Cups’ up to 30cc - for adding pigment and catalyst.
bullet Acetone – 1 ltr for cleaning equipment.
bullet Plenty of rags.
bullet Mould release wax.
bullet Consolidating roller tool.
bullet Disposable or Marigold Gloves.
bullet Eye protectors.
bullet Strips of Wood.

** Try to obtain a colour pigment that is close to what you want or choose a colour contrast. You can aim to get an exact match but that will be your choice.  If you go for an exact match you must read the book referenced above to learn how to do this. I went for a similar (near match) colour ‘Light Cream’.

Note after the first coat of Gel I noted the 2” Packing Tape had started to ‘pucker’- boy was I pi**ed off. Hopefully as warned above you did not do a similar thing. See pics below. I applied a thick second coating over these areas as I knew I would be rubbing it down to get a better finish. This tidying of the moulding took a lot of time.

Preparation Prevents PPPP Poor Performance!!

If like me, you are a complete novice, I would do exactly what the reference book advises. Simply follow the book and build 2 small projects. It gives you an idea of just how good and easy it will be and gives some confidence that what you are doing will actually work!

Warning: Work ideally in 15 -20 degrees C.  Work in a well ventilated area. Use eye protectors if working in any awkward positions. Read all material label directions first.

Gelcoating: This part took me 6 hours from start to finish.

bullet Clean the mould by wiping down with Acetone.
bullet Polish the mould well with at least 2 good coats of mould release wax onto formica, you need more layers on other surfaces.
bullet Pour 500g of Clear Gelcoat into a graduated pot.
bullet Mix in 30g of pigment (just over 5%) and stir in well, particularly into the corners of the pot.
bullet Mix in 10g of Styrene/Catalyst (use 2% catalyst for Gelcoat) and stir in well; particularly into the corners of the pot, it starts to cure from now!
bullet Paint on thick to all surfaces of the mould, slap it on!
bullet ONLY MIX ENOUGH THAT YOU CAN USE IN 10 MINUTES! Hence 500g at a time.
bullet After a complete coat, leave for 2 hours to ‘Gel’.
bullet Allow to set for 12 hours.


‘Laying Up’ Preparation

For 300g CSM (1oz) you will need 6 layers to make a 5mm thick moulding (6oz lay-up)!!! For each Kg of CSM you lay you will need about 2Kg of Polyester Resin!!

‘Crack out’ the mixing pots that were used for mixing the Gelcoat it is thick stuff. Be careful not to crack the mixing pot itself. They are now ready to use for mixing the resin.

Warning this work can be a messy job if you are not prepared before you begin. I did the lay-up part in sessions as it took much longer than I anticipated.

Cut all CSM pieces to approximate size before starting to mix resin.

After using the resin at the end of the session I wiped the pots clean with Acetone.

Just get on with it ……. ‘Laying Up’

bullet Mix 500g of Polyester Resin with 5g of Styrene/catalyst and mix well. This is much thinner than Gelcoat.
bullet Paint a good coat onto the Gelcoat.
bullet Lay on the cut pieces of CSM and roll flat with the consolidator roller.
bullet Stipple more resin with a brush into the white areas and keep going until the surface appears translucent with no white areas (indicating dry of resin or trapped air).
bullet Leave for 1 hour between layers, by the time I had finished on the first layer and had coffee it was time to start the next.
bullet Lay up to 4 layers at anyone time (if you can) and then leave for 24 hours to cure.
bullet It is wise to put strips of wood into larger areas to ‘stiffen’. It is the resultant ‘top hat’ profile of the mat laid across the strips of wood that actually give the strength.
bullet I added the wood strips after the first 2 layers of mat.
bullet I also added 2 pieces of conduit for cables to be fed down through from the mast, this also added strength.
bullet I completed 6-7 layers to all sides with overlaps on all the corners.

Cracking out

Before I did this I trimmed the mould with a small angle grinder with a disk fitted meant for stone blocks and floor tiles. Due to the amount of dust generated I asked my daughter to Vacuum alongside the cutting edge to catch some of the dust (we both wore gloves, face mask and goggles).

I then ‘cracked out’ the moulding by working around the edges after removing most of the batons used to stiffen the mould. I attempted to preserve the mould but failed it was pretty much destroyed! I have kept the sides as they will come in useful for something else yet I am sure. If not re-build into another 570 Fly Bridge Mould.

At this point I did a test fit on the boat and then removed it to finish off.

Finishing the Fly Bridge

Cut access holes to the ends of the built-in cable conduit.

Trim all sharp corners of the flanges to give a good ‘seat’ for Titanfast trim.

Rubdown any ‘spiky’ ends of glass and all the cut edges.

Make a template for a window and when you are happy with size and position, mark and cut out the holes for the windows. Note I did this after I had already purchased the 5mm Clear Perspex/Acrylic off cut. This worked well as they did not have the exact size that I needed but had an off cut very close to what I wanted, so I compromised. It therefore became much cheaper to purchase than a larger sheet (and with no waste).

If necessary fill any poor areas of Gelcoat, I had to do this due to the Packing Tape that I used ‘puckering up’. Repairs should be fine as the mix is constant so the colour should be identical (assume using same pigment), after a rubdown and polish of the repaired area the repair should be undetectable.

Next use an orbital sander to finish any other ‘poor’ areas of Gelcoat with 150 wet and dry but finishing with 240, likely areas will be where plastercine was used to give an internal radius to the mould.

Turn over and paint the inside.

Polish the Gelcoat (easier to work on and would save some climbing around on the boat while it was on the trailer).

Fitting the windows

I sourced the Acrylic/Perspex sheet 5mm from ALDA PLASTICS, Unit G1, Liners Industrial Estate, Pitt Rd, Freemantle, Southampton 023 8023 5423.

I then used a rubber seal (part number BRS1123), buying 6M at £1.80/M, to fit the Acrylic/Perspex windows sourced  from Alexander Comrie & Son Ltd, Second Avenue, Millbrook  Road, Southampton 023 80702911.

There is a knack with this. That is get the correct rubber i.e. one with the correct gap for the material that fits into both slots. I struggled as I could not source the correct rubber as one groove was correct at 4.5mm the other grove just 1.5mm a bit tight but it still worked. Also the gap between the hole and the Acrylic window has to be correct, in my c\ase about 5mm. It actually took three of us to fit the first window. Then when we got the hang of it just me and my wife Alison managed top fit the second window.


As I tried to show here the filler needs to be fed into the grove and then pushed in using a special ‘tool’. A Tool is available but I made mine from a wire boom after a friend described it to me.  One trick here was to push the ‘tool’ rather than pull the ‘tool’.

Fitting stainless A Frame and mast
I cut down a fitting that I previously purchased from Beaulieu Boat Jumble I then had a stainless foot welded accros the bottom of both ends by Hythe Marine Services who made a top job of the welding. It made sense to fit all this while it was on the garage floor stillas this saved me some effort climbing around the boat. I fitted the mast and fed the cables for the VHF Antenna, Anchor light and Nav Light through the conduit.

Fitting to Boat
I finished all edges with Black ‘Titanfast’ trim, buying10M at £1.95/M also sourced  from Alexander Comrie & Son Ltd, Second Avenue, Millbrook  Road, Southampton 023 80702911. Fitting was a three man job. Two had to hold it inplace, as a certain amount of force enabled a better fit, while the other drilled the holes. We bolted down through the front edge lip and down through the cabin roof and screwed through bottom edge of the sides ignoring original holes left by the original screen. I filled gaps with silicon and bolted down tight. Last thing was to cut all the stainless bolt ends off with an angle grinder. By bolting and keeping the old screen I still have the option of swapping them back if I choose to sell her later.



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