howtobuild

Some basic tips, how to build a model on a Fibreglass hull.

Building a model using a moulded GRP hull is really quite easy, and many people will have already learned much of what follows through, perhaps from assembling a kit or two, or by reading articles on the subject in the model magazines. I’m not going to cover every single aspect in detail here, but I’ll just try to give you some idea of the work involved in fairly general terms, and also provide answers to some of the questions that customers raise.

After you’ve made a rigid stand to support the hull, the first job is to fit inwales, deck beams and bulkheads inside the hull. Inwales are strips of wood that are glued to the hull sides just below deck level. They strengthen the hull, provide a firm location for the deck, and also reinforce the hull/deck joint. Firm balsa wood ¼” (6mm) square is fine for this, though harder woods like spruce may be used if preferred. Thin (3mm) plywood is strong enough for most bulkheads. These aren’t usually needed for strength reasons, but it’s a good idea to divide the interior of the hull into separate watertight compartments.

It’s quite simple to bond wood securely to GRP. The first step is to lightly sand all areas where adhesive will be applied, brush and/or vacuum to remove dust and loose fibres, and finally wipe clean with a little acetone or cellulose thinners on a rag, before applying adhesive. The most suitable adhesives are polyester (fibreglass) resin, or alternatively a 'slow' epoxy adhesive such as Devcon 2-Ton. Most of the 'fast' or '5-minute' epoxies are not quite as good for bonding to GRP. If using polyester resin, I often find it helpful to thicken it first by stirring in a small amount of P-38 or Plastic Padding filler paste. The extra viscosity stops the resin running downhill from wherever you apply it, and the improved gap-filling properties mean that your joints don’t have to be made quite so accurately. Whatever adhesive you choose, use large numbers of spring type clothes pegs to clamp everything until the glue has set.

After first deciding positions for motors etc, deck cross-beams and bulkheads can be fitted, make templates from thin card to find the correct shape. One or two bulkheads are useful to separate and locate motors, batteries and radio gear, but they are really only needed to add strength in heavier displacement hulls like tugs that will have to carry a lot of weight or ballast. When all these wood parts have been fitted and the hull is quite rigid, components like motors, propellers and rudder shafts can be installed. Tack them in place with filler paste or epoxy, then bond more securely once you are quite certain that everything is perfectly aligned, as mistakes will be hard to correct later. It’s just common sense that all motors and control linkages should be thoroughly tested before decking is glued in place. You should also carry out flotation tests as construction progresses to check what ballast if any, will have to be added to the finished model.

Bonding the inwales to the sides of an Island class OPV hull, plenty of peg clamps.

Short lengths of inwale being glued in place to support a raised foredeck.The inside surfaces of the hull that will be visible on the finished model have been filled with P-38 and sanded smooth.

Two more typical hulls showing the kind of finished structure of inwales and deck cross beams that is needed. The hull at the top is a scale 36R racing yacht, and the lower hull is an electric-powered small steam yacht about 900mm long. No bulkheads were needed for the yacht, but a single one was fitted in the steam yacht before the decking was glued in place.