Water generators offer the best way to produce renewable electricity aboard a cruising yacht whilst under sail on an extended passage.

Water generators extract energy from the boat’s motion through the water – essentially, this is still wind energy in that the water generator recovers a small percentage of the energy captured by the yacht’s sail plan.

The inertia in the yacht’s hull evens out the variations in wind velocity and direction, and the result is a very steady and constant power output from the water generator, in contrast to a dedicated wind generator, where the output will tend to fluctuate continually.

There are three main types of water generator. The earliest designs use a heavy impeller or spinner attached to a long line. These work well but can be awkward to deploy and recover. Also, there is no satisfactory method to control the impeller depth, which will rise through the water as boat speed increases. The impeller has a tendency to surface at higher boat speeds (i.e. 6 – 7 knots) and generation stops. As these systems use a long tow line, the parasite drag and subsequent loss of boat speed is higher than other systems, and typically, maximum power outputs are around 100 watts.

More recently, the ‘outboard leg’ type of water generator has been introduced. These are attached to the yacht with a system resembling a dinghy rudder fitting. As the generator is fixed at the transom, any vertical movement of the yacht’s stern is transmitted to the unit, so wave action and heel angle directly affect the depth of the water impeller. Also, the units rely on a mechanical ‘fuse’ in the hold down lines (which keep the impeller submerged) to protect the submerged leg in the event of impact with flotsam.

The ‘outboard leg’ type machines can offer impressive performance but utilises an internal gearbox to step up the impeller RPM to match the small three-phase alternator. They also require a specific electronic converter unit to rectify the AC to DC and step the high output voltage from the alternator down to the level of the yacht’s system.

Depending on make and model, ‘outboard leg’ type generators weigh between 8 and 18 kg, excluding mounting.

The Sail-Gen system is different, sharing the same operating principle as the DuoGen’s water mode, which is well proven over hundreds of thousands of sea miles.

The Sail-Gen is direct-drive, i.e. 1:1 ratio impeller to alternator. There is no gearbox to wear and add friction.
The Sail-Gen’s alternator is not submerged, and is therefore less liable to failure through water ingress.
The Sail-Gen’s running depth is controlled by a ‘hydro foil’ we refer to as a ‘dive plane’. This means that the water impeller flies through the wave action at a constant depth and is not directly affected by the pitching movement of the yacht, as occurs with the ‘outboard leg’ pattern.
The impeller running depth can be optimised by a simple adjustment of the dive plane angle.
The dive plane prevents the water impeller from surfacing in all but the most extreme situations.
The Sail-Gen dive plane is active, and moves to a positive angle as tension is applied to the recovery lanyard. This makes recovery of the water generator simple and easy, regardless of boat speed.
An impact wire can be fitted to move the dive plane positive in the event of a collision with floating debris. This allows the Sail-Gen to ride over the obstructions, minimising any damage.
The Sail-Gen is built using high quality, corrosion-resistant materials, including cast aluminium impeller, carbon fibre driveshaft and a welded aluminium frame. The aluminium is hard anodised, followed by a polyester powder coat.

The slow speed, permanent magnet alternator is robust, highly efficient and well proven. Featuring stainless steel shaft and bearings, powerful ferrite magnets and heavy copper windings, this unit is capable of sustained outputs in excess of 500 watts, and can be directly connected to a battery if required.

The Sail-Gen is supplied complete with an alternator yoke and ‘c’ bracket. Together, these form the gimballed mount for the Sail-Gen and allow the unit to pivot both vertically and horizontally when deployed. These gimbals resolve and dissipate the bending forces produced by wave and steering action.

In its ‘parked’ position when not in use, the Sail-Gen is held in a vertical position by a clamp and strap integral to the supplied mount hardware.

The standard Sail-Gen features a 280mm diameter impeller for yachts with passage speeds up to 9 knots. A hi-speed 240mm diameter impeller is available for boat speeds up to 15 knots.

To prevent battery damage through overcharging, we recommend that a charge regulator forms part of the installation. This should be a ‘dump’ type regulator rated at a minimum of 40 amps at 12 volts.

Drag and the resulting speed loss from water generators are often over-stated. Drag from the Sail-Gen is less than 15 kg at 6 knots, and typical boat speed losses at that speed are in the order of a tenth of a knot or less.

The Sail-Gen weighs 16.7 kg excluding mounting.