These electric curved fin stabilisers from Sleipner could be a total game changer

Curved fin stabilisers matching the performance of hydraulic ones, with easier fitting and functionality, could be a game-changer. Hugo Andreae reports…

After years of resisting calls to develop an electric version of its popular hydraulic Vector stabilising fins, Sleipner has finally bowed to pressure from boatbuilders and revealed a brand new range of all-electric fins.

Over four years in development and featuring a number of new innovations to maximise the efficiency, quietness and safety of both the actuators and the fins, Sleipner is confident that its latest offering will not only match the performance of its existing hydraulically driven fins but also outperform its electrically driven competitors.

Speaking to Motor Boat & Yachting, Sleipner CEO, Ronny Skauen, said that it had taken this long to develop the new system because of his determination to ensure the same high levels of safety and reliability inherent in the company’s hydraulically activated fins.

“Hydraulics are very powerful, very quiet and very reliable and can cope with shock overloads,” said Skauen. “Even if they do fail you can always release the pressure to let the fins return to their neutral position.

“Electro-mechanical systems tend to be noisier and there’s a greater risk that the motor or gearbox could seize, leaving them stuck in the up or down position. It took us a long time to find effective solutions to these issues.”

Safety first

One of those solutions is a very unusual gearbox design called a Harmonic Drive Strain Wave gear that uses a unique elliptical cog bearing that can withstand extreme loadings while delivering exceptional reliability and very high gearing ratios. It can even slip without destroying its interlocking teeth.

Skauen admits it is a costlier and marginally more power hungry solution than traditional planetary gears but points out that there’s a reason the Mars Rover space vehicle and Lockheed F35 fighter jet also use the same type of gearing.

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Resolving the noise issue also took some lateral thinking as the problem isn’t just the sound of the motor and gears accelerating and decelerating causing a distinctive (and more noticeable) rise and fall in pitch than the near silence of a hydraulic piston or background thrum of a generator, but also the actuator’s tendency to reverberate through the hull.

Skauen says that the hull moulding can act like a loudspeaker membrane with certain resonant frequencies causing it to vibrate and create noises in areas of the boat some distance from the actuator itself.

However, unlike an engine that can be isolated from the hull structure on rubber mounts, the fin mounts have to be absolutely rigid to cope with the enormous rotational torque forces. The solution was a new damping material and arrangement that retained the necessary torsional rigidity while still isolating the actuator from the hull.


Elongated cutaway shape improves efficiency

Arguably the biggest change of all is in the shape of the fins themselves. Sleipner has been championing its patented curved Vector fins for over a decade now due to the extra stability at rest and fuel efficiency underway that they offer over traditional straight fins.

This is because Vector fins splay outwards from the hull creating more vertical stabilising force and less horizontal force at anchor than a straight fin with its forces acting at a constant 90 degrees to the deadrise angle of the hull. The Vector fins also generate more lift underway at speed to help offset the drag created by any external fin.

Sleipner’s latest design, developed specifically for its new electric actuators, is its most extreme yet, boasting even more significant gains in stabilisation at rest and drag reduction at speed as well as a decrease in unwanted side-effects such as sway, yaw and anchor walk (when the fins paddle the boat forward over its own anchor).

Stability gain

They still curve upwards like the original Vector fins, only more so, but they are also considerably longer and shallower than previous designs, with a cutaway shape that keeps the faster-moving tips further away from the hull than the slower-moving leading edges.

This helps prevent an unwanted interaction between the fin and hull that was reducing the size of the efficiency gains. The end result is an unprecedented gain in stabilisation efficiency as well as increased lift underway. Sleipner’s own measurements show that these new fins are now twice as effective at anchor as flat fins of the same size and speed.

The other big advantage of the new electric fins is that they allow a larger rotational swing than hydraulic arms, which are typically limited to 35 degrees either side of neutral. This gives greater flexibility in how fast, how far and how long the fins are rotated in order to achieve the best stabilising effect.


Why curved fins generate more of the desirable vertical stabilising force and less of the undesirable horizontal one

It also enables more sophisticated algorithm controls to counter the effect of anchor walk by adding an occasional backwards paddle rather than constantly paddling one way or the other as some rival electric fins do.

The end result is Sleipner’s most efficient fin shape yet and one which will, in time, be adapted for use on its hydraulic range too. Skauen falls short of saying that Sleipner’s new electric fin systems are superior to its hydraulic ones but does admit that they have a number of advantages both for builders, who no longer need to allow for hydraulic pipe runs and electro-hydraulic pumps, and users who can enjoy all the benefits of Vector fins without some of the less desirable side effects.

Although full details and prices of Sleipner’s new electric fin stabiliser range haven’t yet been confirmed, there are likely to be five different packages with fin sizes from 0.6m² to 2.5m² covering boats from 55ft up to 130ft with fully installed prices starting from around €90,000 inc VAT.

First published in the June 2023 issue of MBY.

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