Model Boat Moulds for Fibreglass and Plastic Hulls

A mould lets you make one hull shape many times. It’s how most commercial RC hulls are produced, and it’s also a good approach for a modeller who wants several hulls for a club class, or a tough, waterproof hull for a working model. This guide covers the plug-and-mould process for fibreglass hulls and the simpler route of vacuum-forming plastic.

Plugs, moulds and hulls: the terms

• Plug (or buck): a master copy of the hull’s outside shape. It’s usually made of wood, foam or filler and finished to a high gloss.
• Female mould: a rigid shell taken off the plug. Hulls are laid up inside it, so their outer surface copies the mould’s inner surface.
• Male mould: a form that material is shaped over. Vacuum-formed plastic hulls are often made over a male buck.
• Release agent: a wax and/or film coating that stops resin bonding to the plug or mould.

Every flaw in the plug shows up in the mould, and every flaw in the mould shows up in every hull. In female moulds this works in reverse: a bump on the mould surface makes a dent in the hull, and a dent makes a bump. The plug and mould surfaces are where most of the work goes.

Buy a hull, buy a mould, or make your own

• Buying a ready-made hull is quickest. Many scale and sport hull shapes are sold by specialist suppliers.
• Buying or borrowing a mould is sometimes possible through clubs, or when a small maker retires.
• Making your own makes sense if the shape isn’t available, or you need more than one hull. Expect it to take longer than building a single wooden hull, because you’re making a plug and a mould before the first hull.

Making the plug

Plug construction methods

Any of the wooden hull methods can produce a plug, and it doesn’t need to be watertight or strong. Common choices:

• A bread-and-butter stack carved to shape. See building a bread-and-butter model boat hull.
• A planked or sheeted hull over frames, filled and faired.
• Foam shaped over or between profile templates, then coated with filler or a thin layer of glass and resin.

The general setup for frames and shadows is covered in making a model boat hull.

Surface finish and release

The plug needs a hard, smooth, non-porous surface:

1. Fair the shape, checking it with templates and battens.
2. Seal the surface, for example with sanding sealer, primer or a thin resin coat.
3. Fill and sand through progressively finer grades.
4. Finish with a high-gloss paint or primer that’s compatible with the resin you’ll use.
5. Apply release agent, typically several coats of a mould release wax, buffed between coats, sometimes followed by a film-forming release such as PVA release agent.

Always follow the release and resin manufacturers’ instructions. A release failure can bond the mould to the plug and ruin both.

Undercuts, tumblehome and split moulds

A hull must be able to come straight out of the mould. If the sides tuck in above the widest point (tumblehome), or there are other undercuts, a one-piece mould will lock the hull in. Solutions:

• Split moulds: make the mould in two halves along the centreline, with flanges that bolt together. The halves come apart to release the hull.
• Mould to the widest point: mould the hull only up to the line of maximum beam, then add the topsides or bulwarks separately.

Even without undercuts, slight “draft” (sides that open outward) makes release easier.

Making a fibreglass female mould

1. If needed, fit a temporary flange (a dam of card, ply or plastic) around the plug’s edge to form a mould lip.
2. Apply a tooling or mould gelcoat if using polyester, and let it reach the stage the manufacturer specifies.
3. Lay up glass in layers, working out air bubbles with a brush or roller.
4. Build up enough thickness for the mould to stay rigid. For a larger model mould, add stiffening ribs or a frame on the outside.
5. Let it cure fully, then ease the mould off the plug. Wedges or plastic spatulas can help start the separation.
6. Clean and polish the mould surface, then apply release agent before the first hull.

Laying up a fibreglass hull in the mould

Polyester or epoxy

Many model hulls are made with polyester and gelcoat, especially commercially. Epoxy is often preferred where the hull will be bonded to wooden parts, or where fumes are a concern in a home workshop.

Don’t mix systems carelessly. Polyester won’t reliably cure against epoxy, and some release agents work better with one than the other.

Cloth and mat weights for models

Model hulls use much lighter reinforcement than full-size boats. Typical lay-ups combine:

• A surface coat (gelcoat, or a thin resin coat)
• A light glass cloth or tissue first, to pick up detail and stop the weave printing through
• Heavier cloth or chopped strand mat behind it for stiffness
• Local reinforcement at the keel, transom, shaft log and other stress points

Build up gradually and keep each layer well wetted out but not swimming in resin. Excess resin adds weight without strength.

Vacuum-formed plastic hulls

For smaller hulls, especially static models, waterline models and lightweight RC models, vacuum forming is a practical alternative:

• A male buck (often solid wood or resin) is placed on a vacuum table.
• A sheet of styrene, ABS or similar plastic is heated until it sags, then pulled down over the buck by vacuum.
• The formed shell is trimmed, then braced inside with bulkheads or stringers.

Thin vacuum-formed hulls are light but flexible, so they usually need internal stiffening. Joints and openings must be sealed for working models. Home-built vacuum boxes using a household vacuum cleaner and an oven are popular for small parts, but heating plastic sheet calls for care with temperature and ventilation.

Safety: resins, fumes and dust

• Work in a well-ventilated area. Polyester resin and gelcoat give off styrene vapour, and a suitable respirator is widely recommended.
• Wear nitrile gloves; epoxy can cause skin sensitisation with repeated contact.
• Glass fibre dust from trimming and sanding irritates skin and lungs. Wear a dust mask, eye protection and long sleeves.
• Mixed resin can heat up as it cures, especially in a deep cup. Mix only what you can use and don’t leave a large mixed batch sitting.
• Follow each product’s safety data sheet.

Common faults and fixes

If you’d prefer a wooden hull that’s still tough and watertight, glass-sheathing a planked hull is an alternative. See how to waterproof a wooden model boat hull with epoxy and glass cloth.