Single hull vessels have a lot going for them. While a little slower than catamarans some say their motion isn’t as ‘stiff’.
The argument continues that this makes the ride easier on the stomachs of both crew and industrial personnel, whether turbine technicians or divers.
The issue is that they can roll, especially when stationary or moving slowly. In fact, every hull has an inherent response amplitude operator (RAO), a function used to determine the effect a sea state will have upon a vessel’s motion. A catamaran usually has a roll RAO value close to ‘1’, but this can be far greater for a single hull, creating challenges for support boats.
Dylan Froriep, Seakeeper applications engineer explained: “A monohull’s normal motion magnifies typical wave slopes of 3° to 5° between three and five times. So if you are onboard a vessel in a sea with a 5° wave slope, and the boat has a roll RAO value of three, the boat will have an average roll amplitude of 15°.”
However, a rather surprising demo at Seawork showed how a boat could be stabilised at the flick of a switch. In the tank was a model hull: put a hand on the edge, and it was all too easy to rock it up and down. But as soon as the gyro unit was engaged an invisible, almost vice-like grip made it almost impossible to continue the motion.
The effect is achieved by “turning the roll of the boat into ‘precession’... the fore and aft tilting of the flywheel-enclosed sphere”, said Froriep; the result is gyroscopic torque, perpendicular to the flywheel’s axis.
It’s certainly effective: “We aim for up to 95% roll reduction,” said the company’s Northern Europe sales representative, Andy Pearce. Each of Seakeeper’s models has a different level of angular momentum (gyroscopic ‘horsepower’), based on the weight, diameter and speed at which the flywheel spins, suitable for varying boat sizes. However, as each Seakeeper works independently, multiple units can be combined for additional stability: several have found a home on one single, larger vessel with very specific needs.
Moreover, the typical installation footprint suitable for a 24m boat is just 1m square, and it can be located pretty much anywhere onboard - not just centrally, as one might expect.
“There’s a lot of competition for that centreline,” pointed out Pearce. “But we don’t need it. Essentially the torque is exerted through the foundation and passed through the hull structure: even if the units are off-centre, it still creates the same moment, and the same mitigation.” He adds the positioning can vary vertically too: “While we have installed many in the engine room, we’ve also put a number outside, up on deck.”
There are some limitations: “For a high-speed vessel, we like to keep the units installed in the aft two-thirds of the vessel, away from the narrower, forward bow section,” he explained. “And while there’s no vertical installation limit, it does require a solid structural foundation underneath to transmit the stabilisation forces.”
The technology is gaining adherents. For example, Sima Charters has been utilising Seakeeper units to support efficient, hydrodynamic monohull design. The resulting vessels have reduced fuel costs but are still capable of working in 1.7m wave-heights: Sima now has five boats fitted with these gyro devices.
Further, when Subsea Partner went shopping for a light dive (daughter) craft for FPSO inspections on the Barents Sea, it specified a very steady platform as the returning divers would be facing extremely exposed, turbulent conditions without the luxury of a large vessel to absorb some of the impact. So designer Maritime Partner turned to Seakeeper to give the 13m long, 3m beam LDC ‘Trust’ the necessary stability.
With these vessels demonstrating resilience in extremely tough conditions, more gyro-stabilised monohulls should now begin to grab a broader share of the workboat market.
By Stevie Knight
