GRP Fabrication

[jcmmarine]

I want to make a GRP hardtop for my cockpit, 11ft x 7 ft. I have made GRP repairs before and a locker cover, but never anything this extensive.

I am well aware of the importance of a good mould, but less sure about the materials and thickness I will need.

I do not need something that to walk on but it will need to support solar panels and a lowered radar mast.

Any advice would be welcome.

 

[Bajansailor]

I built a GRP 'bimini' hard top over our cockpit - it is made from single skin panels (CSM/WR/CSM) laid up on a sheet of Formica as a mould.
It supports a solar panel mounted transversely on the hard top - the panel is bolted to two GRP angle bars spanning the width of the bimini - these angles also help to stiffen up the bimini considerably.
This bimini has an upstand around its perimeter to further increase stiffness, and also to allow it to function as a rain collector.
If you want yours to support a lowered radar mast it would need to be a fair bit stronger - maybe use a 20 mm foam core with a CSM/WR/CSM each side, or a CSM and a Stitchmat.

I am trying to download a photo of it here using Photobucket, but the bucket seems to be on a go-slow tonight.... you can see a photo of it if you go to www.sail-trek.com, click on the 'Challengers', and then click on the first photo for Bajan Phoenix.

 

[William_H]

The doing of the job is fairly straight forward it is the design that is a bit tricky. A well designed mould will make it easier and do the job in polyester resin.Put a gel coat with pigment down first onto a well waxed surface. Make sure you roll the resin well into the glass as it is amazing how piholes can make it not water proof.
You might consider checking the price of carbon fibre to aid stiffness but it is probably too expensive compared to glass.
As Bajansailor says the solar panels should be an integral part of the design in that they will provide a lot of stiffness.
I imagine that the cnetre would be relatively flimsy while the outsides especially near the supports needs to be beefed up to take the loads.
Larger dimensions from bottom to top as in a turn up or turn down around the outside will aslo aid stiffness.
I would suggest perhaps one layer of woven rovings for the basic skin with a layer of light cloth both sides to make it look better. Then add additional layers around the outside. Perhaps an engineering student can help with identifying where the flex will most likely be and how to stiffen with added material (triangulation is the term).
certainly you don't want to make it too heavy (or too light)
Curves are stiffer than flat and can provide more headroom. Sections of foam can be imbedded to give greater thickness and so greater stiffness. good luck give it a go. olewill

 

[oldsaltoz]

G'day John,

Have you considered using foam? Can be made to any shape, thickness, has very good strength to weight, excellent insulation properties, easy to do even for a novice and no mould required.

I imagine you will need to leave a section out to provide access to the flybridge, perhaps even a hatch; a rounded top rolled edge will provide good stiffness without weight.

To get a near perfect curve on the edge of foam you need to cut the inside of the curve several times with a hand saw to within a few mm of cutting through, the closer the cuts, the sharper the curve.

When you have the required curve, just fill the saw cuts with resin and Micro-balloons then bent and clamp till cured.

Once you have the required shape just lay 2 x CSM and one roving's to finish, then sand just enough to remove high spots, apply more Micro-balloons and sand to required finish.

Now you only have to decide what colour you want it.

Things to think about when building a canopy / cockpit cover.
Cables can be built in for stern light, general lighting, cables also for speakers. Some cockpit covers may create an increase in exhaust fumes, so make some provision for forward facing hatches, also useful on warm days. Also build in any fitting you may need, Solar panels, boat hooks, fishing rods, life jacket overhead lockers etc.

Hope this helps.

Avagoodweeknd......

 

[jcmmarine]

Thanks Martin
PM sent

 

[jcmmarine]

Thanks All

Due to boat shape and wife insisting it match the rest of the boat, my size and design is fairly fixed. It is the construction thickness, strength, weight, number and size of stiffeners etc.
I am now thinking possibly along the lines of gelcoat/single skin/foam/ single skin.
Going to start making a mold tomorrow, that should keep me busy for a week whilst I ponder on what is to go in it.