View from the boatyard
Mike Pickles builds a shower base for a Nauticat 33 and explains how to seal it against the wet environment itÕll be facing in the heads
Mike Pickles constructs a new shower compartment base
We’re currently working on a Nauticat 33 which, having just been sold, has been brought in to remedy issues found on the pre-purchase survey – and to carry out some tLC for the new owner.
The priority job was to install a compression post as the mast was in danger of becoming keel-stepped instead of deck-stepped.
I covered installation of a compression post on a Seamaster 925 in PBO October 2017, and the solution for the Nauticat was very similar.
However, we did have to change the sole board layout, especially in the heads which also doubled as a shower.
The original was plywood with a GRP sheath and gelcoat finish on top. Sadly the underside was never treated and was merrily rotting away, so as we had to cut a new-shape board to accommodate the new stainless compression post, we took the opportunity to make a new shower base too.
The base was to be made so it could be easily removed. I don't like to make any areas of the hull inaccessible, so while the current vogue is to have a permanent, nicely-moulded shower tray, this is not compatible with easy access to the inside of the hull.
The base itself is a strange shape, so the first job was to create a template using 2in strips of hardboard and a glue gun. Using the template, the 19mm marine ply can then be cut out – and once a decision has been made on the position of the drain plug, work can be carried out to make the flat surface somewhat less flat.
We used a small orbital sander and 40-grit pads, and got creative. It's hard to get GRP to go round tight external turns such as sharp edges and corners, so as this base was to be sheathed, all the edges and corners had to be rounded off.
We were using polyester resin, and to ensure a good bond between the ply and the glass I tend to use chopped strand mat (CSM) for the first layer. In my experience, this has a far higher bond with a plywood surface than woven cloth.
So to start the sheathing process on the bottom face of the base we first coated the plywood with resin using a brush.
Go easy on the resin
Paint it on – don’t ladle or pour – and then spread it about. All you need to do is wet out the surface before laying on a layer of pre-cut 250gm CSM. Then, using a brush, add another light coat of resin before consolidating the layer with a roller.
If you have any dry areas, add a little more resin – but the trick is to use as little as possible. Resin has very little strength in itself, and using too much might speed up the wetting-out process, but the laminate will be resin-rich and as a result will be weaker, brittle and less likely to
bond well to the plywood.
Another reason for using just enough resin is cleanliness. Some folk seem to get GRP everywhere and end up with a huge mess, and much of this comes down to the uncontrolled sloshing of resin which, when you come to consolidation, will spray the stuff around in every direction.
Using polyester you have a great deal of control on cure times compared to using epoxy resin systems.
Adding a little more or less catalyst is a bit of an art, and I find it hard to explain how much to use at specific times as I seem to manage to add just enough.
You have no such flexibility with epoxy systems, but one sure-fire way to speed things up with polyester is to use a heat lamp which can allow you to get a workable cure in an hour or so.
Where epoxy wins is in bonding strength to a substrate such as plywood. However, to mitigate this using polyester resin, creating a dust-free and well-scored surface helps immensely.
When sheathing the top surface we followed up the layer of CSM with a layer of 300gm biaxial woven cloth. Again, a light coating of resin and a good roll will pull up any surplus resin from the lower layer. Use slightly more resin on the edges to help the cloth round the corners and stay stuck down during the cure.
After trimming and a quick sand using 100-grit paper round the edges, it's time to add 100mm-wide glass tape round the edges. By applying just the right amount of tension you can usually get the tape to lay nice and flat without too many creases.
The tape is thin, flexible and needs very little resin to wet out. In this case I was adding the edge tape on a Friday afternoon, and I added a little white pigment so that on my return the following week I would remember exactly which point I'd reached.
The whole base was then rough-sanded with 100-grit paper, washed and dried.
The sheathing was now essentially complete, so it was time to add the gelcoat. Surface coating using gelcoat has one big issue, and that is air bubbles: you add air during the stirring process and then again during application with a brush, roller or spray.
Thin layers
Gelcoat is thick, and it's very tempting and perfectly possible to add 2-3mm of gelcoat at one time. After an initial cure it’ll look pretty good... until, that is, you start to sand and then cut and polish the surface, after which you will see lots of tiny holes in the surface. Sanding or cutting further might remove these initial holes, but only for others to emerge underneath. The way round this is to apply multiple thin layers. I like to use four layers and then a top layer with added wax in styrene (also known as topcoat or flowcoat).
The wax comes to the surface, seals off the gelcoat from the surrounding air and allows it to cure, otherwise it will always remain tacky.
Once cured, the area can be sanded flat using 100-grit paper, which removes any major bumps and hollows and provides a key for the finish layers of gel.
The radiused edges needed to be squared off, so we put together a wood frame. A resin-proof tape was applied to the surfaces of the wood so nothing would stick to it, then further layers of gel were applied and the edges filled. Once cured, the frame was simply removed, leaving a solid, square edge.
The final process is marking, sanding, marking and more sanding until you’re happy with the surface fairness, followed by cutting with a suitable cutting compound.
Most fast-cut compounds will remove scratches made by 1,000-grit wet-and-dry paper, and you’ll get a pretty good surface finish. I like to go a couple of stages further with 1,500 and 2,000-grit paper before cutting with a fine-cut compound as the finish in my eyes is far superior and seems to stay cleaner for longer.