The three new surface effect ships (SES) recently delivered by Strategic Marine to Angola’s Energy Craft fleet are remarkable in more ways than one: sea trials demonstrated a top speed of 53knots but at a similar nautical-mile fuel consumption as far slower boats, writes Stevie Knight. The Crewliner 35 also delivers personnel without making them feel as if they’ve been travelling by cocktail shaker. However, the design’s inception was actually sparked by two dramatic crashes.
First, in 2014, came the sudden decline of the global oil and gas market. This meant day rates dropped like a stone for most vessels, says Eduard Ercegovic, technical director and co-founder of Aircat Vessels – who was then managing a fleet of chartered vessels for an offshore support company. The second was the 2016 Bond Offshore Helicopters Super Puma helicopter disaster in Norway, which claimed the lives of all 13 on board. This was followed by a sudden fall in helicopter availability.
Further, in the background was the ageing state of the long-range, 60-90-pax fast crew vessel (FCV) fleet – the vessel types that Ercegovic often chartered. The speed asked of FCVs means they can’t run forever, he explains: “They just get exhausted.” That left a niche in the market: what was needed was a more cost-effective alternative to helicopter transport and a more efficient, faster boat than a standard FCV.
So, Ercegovic and his colleague, Aircat Vessels managing director Jérôme Arnold, partnered with Norwegian naval architecture firm and SES specialist ESNA to create the Aircat 35 Crewliner. These vessels are basically a cross between a hovercraft and a catamaran; they generate an air cushion between the hulls to reduce resistance by lifting up to 80% of the boat’s weight out of the water. The effect is to reduce the vessel’s draught from 2.4m to a mere 0.8m.
This is achieved by a pair of large, 478kW fans, integrated into the forward half of the hulls. “These are not really custom-made – they’re actually the same blowers that you use for factory ventilation,” Ercegovic reveals. The dual fans push the air into the cushion that’s captured between two skirts; one fore, one aft of the boat’s high tunnel – but these have quite different characteristics. The forward skirt matches the bow angle and is made up of seven vertical, finger-like folds all nestled together, rather than a single sheet. If one of these fingers gets damaged, it will naturally deflate – but its sisters will automatically crowd in to take up the space, providing redundancy.
The rear skirt is very different and better described as a tiered structure of horizontal bags, maintained at just a little more pressure than the main cushion. These stern lobes, with the help of two vents, passively adapt to the waves by forming and reforming around the waves, to reduce pitching and a certain amount of roll – although that’s also minimised by the vessel’s 13.9m beam.
However, the main cushion is more actively modulated by four damper cassettes (vents) controlled by a computerised SES management system, which gathers data from multiple pressure sensors in the tunnel and from a motion reference unit (MRU). Since the electric actuators that open and close the dampers allow instant adjustment, the result is high-speed ride control.
“You can change the setting to maximise the lift and minimise the draught when you are going full speed in relatively calm seas,” says Ercegovic, adding that this leaves just enough draught for the propulsion and cooling to be effective. It’s also possible to dial it down since different, preset modes allow the crew to choose a ‘ride control sensitivity’. “There is some penalty to the speed if you increase the comfort, but it’s usually just a few knots,” Ercegovic says.
Read the full-length story and vessel technical particulars in the March 2025 issue of The Naval Architect
