Construction

 

 

The design was always intended to be built in PVC foam core/E-glass sandwich laminate. It is a construction method that is light, stiff and strong and leads to very low maintenance as paint systems benefit greatly from the inert fibreglass substrate.

Unit construction is also easy to undertake for small yards or amateur boat builders by building a disposable male mould and no specialised equipment is needed. Furthermore, all flat panels can be manufactured on a large table from fully dimensioned drawings and fitted to the boat with minimal effort, only requiring bonding and fibreglassing in place at the connecting edges.

The boat can be built very economically using E-glass and polyester resin, with an epoxy resin barrier coat underwater to eliminate any possibility of osmosis. Glass/epoxy laminate is always an option, more expensive, but friendlier in the workshop. There would be no real advantage in using epoxy with exotic fibres and this would lead to a very significant cost increase.

In terms of weight, this type of sandwich construction compares readily with aluminium when the glass skins are built-up to offer the impact resistance that is desirable in a cruising vessel, rather than designing to a pure tensile strength criteria to obtain a hull as light as possible.

Hull, Structure and Subdivision

The hull is largely supported longitudinally between transverse bulkheads and partial bulkheads by natural stiffeners such as engine bearers and tank boundaries.

The chine, gunnel and stem areas are solid GRP for impact resistance. The keel girder is a heavily glassed hollow section.

The curved bottom sections lend the hull the mechanical properties of an eggshell-shaped membrane, hugely strong in resisting external pressure forces when compared to the commonly found developable panels. The shape is easily obtained due to the natural elasticity available in PVC foam panels and their limited size.

Starting from the bow, a vertical full-height crash bulkhead is found at 300mm behind the forward waterline. The chain and anchor locker occupies the upper half of the volume.

The machinery space underneath the main cabin floor is contained between two watertight bulkheads and the lazarette is sealed as well forward of the propeller. Hull damage with water ingress from either end would hardly affect buoyancy or trim.

Flooding of the engine room would not compromise overall buoyancy either, but would raise the waterline.

Superstructure

The deck and cabin are built out of foam sandwich as well. Window panes are typically tempered glass, urethane-bonded. This was a main driver for keeping the cabin sides flat. Glass is the most durable and scratch resistant material for windows. In terms of strength, it compares to acrylic: both break around the same pressure, but the glass doesn’t flex as far.

The forward cabin windows are curved and must be built from acrylic or polycarbonate.

Stiffening of the panels is achieved by fibreglassing in longitudinally split PVC piping to form hollow girders.

The main cabin and forward cabin roofs can be manufactured on the floor, using simple cambered moulds.

Interior

The interior structure is preferably built out of composite panels as well and tied into the hull and superstructure as relevant. Hardwood trims or veneers can be used as desired for visual effect.

However, considerable flexibility exists with layouts and construction methods, within the weight budget of the design. While the interior construction can provide additional support to the shell, strictly speaking, only a few panels are structurally important.

Interior Layout

Steering

The preferred steering system should be of manual hydraulic type, with a rotary pump behind the wheel and a double-acting cylinder connected to an arm in the lazarette. The balanced rudder blade does not warrant using any form of power-steering. This is one less dependency and common cause of issues.

The rudder blade itself is built out of profiled PVC foam and E-Glass on a stainless steel stock. The bottom bearing shoe is removable, which allows dropping the whole rudder out by turning it at 90 degrees and lowering it. The vertical clearance required below the keel horn to remove the rudder is less than 200mm (8”).

The rudder stock bearing inside the hull is well above the waterline at rest.

Propulsion

Most marine diesels in the 40-75HP range, with a little further upside possible as long as the intent is engine life and propulsion efficiency rather than boat speed, are valid options.
Preferred installation should include a wet exhaust system and waterlock muffler.

Suitable 3-bladed propellers for cruising should be found close to 19′ diameter for 17′ pitch with a target shaft speed of about 1000RPM near the top end, but subject to specification on a case-by-case.

Comments or suggestions? Go to the Wild South discussion.