Hybrids may provide an answer for a section of patrol and pilot boats of the future, but the design demands care and a lot of discussion about the specifics.

An initial appraisal reveals energy storage systems could be a good fit. Patrol boats typically spend 60% of their time at below 6 knots said Andy Page of Chartwell Marine: electric running takes low loads off the engines and reduces emissions. For pilots, battery power is useful for loitering and harbour crossings on a down flow: there are other benefits including cutting both emissions and fuel bill.

But all-electric propulsion has limited application and can’t meet peak demand: pushing the water out of the way at higher speeds would sap the life out of batteries sized for more typical operations but scaling up would add to weight – and costs. Therefore, this element still requires combustion power. “For example when you need to punch through a 5 knot tide, then you need diesel engines”, said Page, “and pilots want to be absolutely sure they have the kick to pull away from the side of a ship.”

Likewise, Page pointed out that hybrids, unlike fully electric boats, are able to top up their own batteries during periods of loitering “so they’re not dependent on recharging infrastructure”.

But there are sizeable challenges.

“We can’t compromise on the service offered,” pointed out Page. For both patrol and pilot boats, redundancy is uppermost, therefore a parallel hybrid is, he believes, the most suitable design. This is the basis for the neat 9m Chasewell many of us spotted on the 2019 Seawork pontoon: two, independent 9.6kWh battery banks associated with a pair of 20kW motors yield up to four hours of 4kn to 6kn electric-only running. The hybrid transmission, supplied by Transfluid, is connected via a gearbox to a PTO-PTI so the integrity of the underlying propulsion is retained. Even an electrical blackout would still leave the twin 200hp Volvo D3 engines capable of operating the Hamilton 241 jets.

Therefore, he admitted there’s a lot of extra kit to fit onboard: “Taking all the hybrid equipment together, the electric and mechanical interfacing, and you are adding an extra 20% or so to the total volume. That space is harder to find on a smaller boat.”

Then there’s the weight: those 19.2kWh of lithium ion batteries weigh in at 260kg, “so we had to position them as far down as we could, close to the centreline”.

The hull itself needed to pull off a clever trick to be efficient at both ends of the operating profile, which required a complete redesign: “Rather than looking just at the bow and stern, the shape transitions with the demands of the speed all the way down its length... it was hard work although from a naval architect’s perspective, fun to do.”

The result is certainly flexible, and the Chasewell boat can get into shallows of less than 1m. But there’s also manoeuvrability: “Combine the jets with the immediate torque on the electric power, and you have a highly manoeuvrable system. When you drop the jet’s buckets, the boat immediately stops,” he said. “Pull one bucket up, and you can spin the boat around.”

However, Page underlined the issues in being on the cutting edge.

“Unlike a known OEM solution with a waterjet or conventional diesel engine, there have been technical questions you’d normally be able to source information on, but we and our suppliers and partners had to work it out for the first time.

He concluded: “Having met these issues for a boat of this size, and delivered a very suitable solution, we are confident we can do it for larger vessels... it’s very scalable, so we have started to roll out our larger Chasewell patrol/pilot boat designs up to 16m as well as other vessels including survey vessels and CTVs.

Further, new clients may soon be able take advantage of recent battery developments said Page: “The crossover from automotive (Formula E racing) may mean the electric-only range for these boats may well double in the next year or two.”

By Stevie Knight