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Sea-trialing an M-hull - Soundings Online

Sea-trialing an M-hull

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From the top of Arjuna’s tuna tower, the boats on San DiegoBay look like miniatures as twin 460-hp Cummins diesels rev to get the M 40 Sportfisher up to speed. The strange-looking hull and the angular shape of the deckhouse give the vessel a Darth Vader quality that startles other boaters.

“Let’s give our guests a heads-up for a turn,” Arjuna’s owner and skipper, John Grimstad, suggested.

Read the other story in this package:   M-hull: new thinking in boat design

This courtesy seemed a bit extraneous, but a few seconds later it was clear why it was necessary. Without throttling back, Grimstad spun the boat in a tight 180-degree turn. As if on rails, it carved a perfect semicircle, running through the maneuver with no perceptible heel. The G-force was mighty, and all on board had to brace and hold on to avoid being thrust against the cockpit coaming like a sock in the spin cycle.

Exceptional cornering ability and a smoother ride are two attributes of the M-hull, which also suppresses wake at slow speeds and produces lift at high velocity, translating to less drag and better performance. Now on a westerly heading, we headed straight into the sea breeze, but the GPS indicated that Arjuna went faster with the same throttle setting, despite her towering superstructure and a slack tide.

“This boat is a so-called surface-effect craft that achieves better speed because the energy of the wind and the bow wake are captured inside the planing tunnels of the M-hull,” explained Bill Burns, co-founder of M Ship Co., the company that designed Arjuna. “This shape produces a combination of aero- and hydrodynamic lift that literally picks up the boat, making it go faster even though it has to overcome increased windage.”

In layman’s terms it works similar to an airplane that takes off into the wind because of better lift. Burns says that a smaller M-hull prototype achieved up to 10 knots more speed going straight into the wind.

A test drive was all it took to convince Grimstad about the M-hull’s qualities. “I’ve been around boats all my life, so I saw the potential of this idea,” he says. To him it was an ideal platform for entertaining friends and family, either on weekend trips to Catalina or chasing Albacore and billfish along the Baja coast. That’s why the boat was fitted with an extra 200-gallon fuel tank, a tower, rod holders, outriggers, a bait well, complete cockpit galley and a marine barbecue.

The boat was built by Knight & Carver in San Diego (www.knightandcarver.com ) using E-glass, vinylester resin, and Core-Cell foam. But for Grimstad, comfort and amenities were more important than pinching pounds, so Arjuna carries more than her design weight. And there’s another reason: M-hulls can pack a lot, courtesy of the greater beam and the wider bow section. Consequently, stowage under the cabin sole is generous, and both engines plus the relevant on-board systems are easily accessed.

Arjuna’s master stateroom is fitted with an angled double berth and a fold-down flat-screen monitor for watching DVDs and checking vital vessel data. The galley includes a ceramic cooktop, stainless-steel sink, and fridge with freezer box under the cabin sole. There’s an enclosed head/shower and a dinette to starboard, which converts to a double berth. The Furuno NavNet monitor at the helm has a repeater screen on the flybridge, so the skipper can check the “heartbeat” of the boat.

Grimstad demonstrates how the system allows him to adjust performance based on desired range and available fuel supply. Cruising at 18 knots and 1,880 rpm, Arjuna burns about 23 gallons of diesel per hour; at 12 knots and 1,400 rpm fuel consumption decreases to 11 gph. “If I want to go really far and have time, I put her in idle, which is still good for 5.7 knots. That burns only 1 gallon per hour, so 600 gallons are good for nearly 3,000 miles.”

Meanwhile, Arjuna had the Point Loma Lighthouse abeam. Idling along, the wave tops smacked the underside of the planing tunnels, but as soon as Grimstad opened the throttle, the hull rose perceptibly, and the ride became calm and smooth. Going downwind it was impossible to ignore the M-hull’s directional stability and lack of slamming.

“It’s very easy to oversteer this boat when you’re used to conventional hull shapes that roll and yaw in a following sea,” Grimstad says with a grin, casually one-handing Arjuna’s helm and content to be an early adopter of M-hull technology.