Hull Design & Seaworthiness


Wild South steaming in Foveaux Strait, New Zealand

Seaworthiness Considerations

The key design objectives for the project were seaworthiness, comfort and efficiency, i.e. low fuel consumption

Seaworthiness encompasses good stability and flawless seakeeping. On a motor yacht, this relates to hull shape and – to a lesser extent – propulsion and steering arrangements. There are two primary contributors to vessel stability: hull shape and vertical position of the centre of gravity.

In the context of a cruising motor yacht, artificially lowering the centre of gravity would involve carrying ballast low down. This increases displacement and equates to burning fuel while pointlessly lugging around the ocean a large amount of unnecessary weight. Stability in a motor yacht is best obtained through adequate hull shape.

The motion of a boat at sea is also directly coupled to hull shape and mass distribution. Mass distribution determines centre of gravity and inertia, hull shape defines how buoyancy forces impact the hull in waves.

Stability and motion at sea are coupled and should be considered together. Unlike in a sailing yacht, where stability is needed to develop driving power, a motor launch only requires stability to be safe. A vessel can easily be too stable: accidents were recorded on early ore carriers as people were thrown into partitions as the ship righted herself violently in a seaway. Suspended cradles were subsequently built to hold the heavy cargo away from the bottom of the ship and reduce stability. Anyone who tried steaming a fixed-keel sailing yacht without its mast in a sea will attest how incredibly violent its motion can be. It is often impossible to stand on deck. Both the centre of gravity and inertia are too low.

In the case of the Wild South design, which was going to be light and quite beamy to offer good living quarters, full attention was required to create a stable boat that would still offer a gentle motion at sea. Stable is important in order to limit roll angle, so we needed a hull that would stiffen up quite quickly, but not excessively, and gradually. In the same time, we wanted a very large range of positive stability; this is what brings the boat back if it is ever rolled.

The hull sections were drawn with these goals in mind, with a combination of a chine and a curved bottom. The curvature in the bottom controls how the righting moment of the hull increases with heel and the chine above the waterline is instrumental in producing high righting moments at moderate heel angles to limit roll without creating undue drag.

Light boats tend to move more quickly, especially in terms of roll, so increasing transverse inertia through carefully planned weight distribution was relevant. Wild South has a relatively short roll period, but it doesn’t move far and doesn’t stop abruptly. It makes all the difference between being able to stand up in the middle of the cabin or afterdeck, and having to hang on not to be thrown around. The boat is even steadier under way at sea, due to the naturally stabilising effects of the keel and rudder at speed. It is a very comfortable vessel.

Building volume, or more exactly waterplane area, in the stern sections was essential in the design. As a wave approaches aft, the sooner it starts lifting the stern, the sooner the hull can accelerate and keep up with the crest, before the slope of the wave becomes significant. Not only it dramatically reduces power requirements, but it keeps the boat away from the unstable water found in the crest in bad weather.

Stern sections also need to allow a clean exit for the flow leaving the hull. This prompted for a low transom above the waterline and limited curvature in the buttocks lines.

In order to preserve course stability and good control at the helm, the build-up of pressure at the bow as speed increases must be minimised. The forward sections with the fine entry and reduced forefoot operate in conjunction with the lateral keel area aft and rudder to provide outstanding tracking and control in following seas.

Unlike sailing hulls, motor yachts don’t travel through the water heeled, but minimising the coupling between heel and trim is still critical. In quartering seas, the hull tends to roll and any tendency to dip the bow down leads to an increase in resistance forward and course stability issues.

Punching into head seas, the hull largely follows the contour of the waves without burying the bow provided the sea is long enough in relation to the hull length. In short steep seas, the best tactic has been heading straight into them and control speed. The hull resistance is too low and the power installed high enough to launch the boat off the crests if too much throttle is used.

Steering, course stability and seakeeping have proved flawless in all conditions. A basic electric wheel pilot easily holds the helm in all weather, only performing very small corrections.

The weight distribution combined with the volume of the cabin makes the boat fully self-righting. Natural watertight subdivisions in the hull between the crash bulkhead, forepeak, machinery space and aft compartment lead to a vessel extremely resilient to external damage.

Performance and Economy

The hull shape was designed to minimise wave-making resistance.  The objective was creating a boat easy to push so only a minimal amount of power would be required to cruise. The first comment professional seamen make when they witness the boat steaming is: “There is nothing happening”. There is very little bow wave and, unless driven very hard, it leaves a near-flat wake behind.

Wake at 8.7kts

Wake at 8.7kts

Bow wave at 8.7kts

Bow wave steaming at 8.7 knots over a calm sea

Flow at transom at 8.7kts

Water leaving the transom at 8 knots

Only minimal turbulence is observed behind the centre of the transom at 8 knots (the exhaust slot underneath the hull also discharges there). A little faster and the flow leaving the hull clears completely.

This leads to excellent fuel economy. Figures below the 6 litre/hour mark at 8.5 knots, or 1.4 nautical mile/litre make for very low running costs and very long range. Top speed can exceed 10 knots in flat water and regularly climbs in the double-digits in beam or following seas without using any great amounts of engine power.

I personally wanted to make the hull longer by some 1.5 to 2 metres for the beam prescribed, increasing slenderness, but this was opposed categorically by the owner, who wanted to keep the boat compact for berthing and manoeuvring in close quarters. Those who keep the vessel in marinas would also find cost advantages associated with a shorter hull.

He had a valid point for the intended use and this does not preclude developing a longer version. A Wild South 42 for example would retain all the stability and motion characteristics of the Wild South 37 with higher speeds, better economy at speed and the advantage of increased length and slenderness for punching in head seas. A slightly larger model again, possibly with twin screws, would easily undertake ocean passages.