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The Sea Trials – Yanmar 315/Ultrajet

Yanmar 315/Ultrajet

Hunt likes the Yanmar 315 for its high power-to-weight ratio (dry, the engine weighs just 944 pounds) and chose the Ultrajet 251 propulsion unit in part because it is available with joystick control, which makes dockside handling a lot easier.

Like any waterjet in this class, the impeller cannot absorb much horsepower until the engine is turning close to its rated rpm. That means the boat starts to get on plane and seriously move at 3,300 rpm, just 600 rpm off the top. That’s no fault of the boat or the drivetrain; it’s just the physics of water flow through a small-diameter pump. But once the boat is up to 3,500 rpm, which the Yanmar can cruise at comfortably all day long, the Hunt 25 is making about 25 knots, and top end with 60 percent fuel, a clean bottom and two people aboard is about 31 knots.


This boat was headed to the Mediterranean for life as a tender aboard a megayacht, and jet power was chosen for its shoal draft, swimmer safety and maneuverability. (The boat was hauled for delivery before I had a chance to record time to plane, time to 20 knots, and 30-knot-turn data.) This is not the boat to buy if you want to cruise at 20 knots, go faster than anyone else, or are looking to squeeze every last mile out of a tank of fuel. However, it does what it does very well.

“Our jet customers find this propulsion to be very liberating,” says Willard. “They don’t have to worry so much about lobster pots or shallow water and, with a boat of this size, our owners take them right up to the beach for a picnic.”

Of course, this is something you can actually do with a 25-footer drawing just 18 inches (hull draft), so it’s a practical, useful capability in a boat of this size. Beaching a million-dollar 40-foot jet on a regular basis isn’t something an owner is likely to do, so shoal draft opens up a new dimension operationally in a small boat like this. The impeller will ventilate more easily in rough water than a prop, so keep that in mind if you run far offshore often.

One thing quickly became apparent when running this boat down sea in the 2- to 3-foot Narragansett Bay chop — how well it tracks running with the waves. If you’re in the market for a waterjet, the hull form will make the boat sink or swim handling-wise. The Hunt has enough deadrise and the proper center of gravity to give it just the right amount of directional stability. If it’s too flat aft, the boat will fishtail so much you’ll never be able to take your hands off the wheel. And it will spin out in a turn, which is dangerous for the occupants. Too much deadrise and the boat won’t be able to get out of its own way, struggling to get up on plane.

The Hunt design also includes a fairly fine bow with high chines for a smooth head-sea ride, but not so deep or fine that it bow-steers running down sea. The result is a hydrodynamically well-balanced hull. (A few other popular specialty jetboats I’ve run are anything but.)

Our jetboat’s steering had a very quick single turn from lock-to-lock, so that takes some getting used to. Absent a rudder or lower unit, the waterjet-powered boat tends to wander, so the tendency is to oversteer until you get the hang of it. By the time I was done driving the boat for a few hours, the steering ratio seemed about right.

Dockside, the Ultrajet joystick worked like a charm. In normal maneuvering mode around the dock, just push the stick to the side to steer in that direction. There’s also a “backing” mode, which is best used when you’re facing astern. When you push the stick to port, so goes the stern. When backing down in normal “ahead” mode, the stern would go in the opposite direction, because of the way water is deflected off the reversing bucket.

With any jet, you can also leave the engine in gear — turning the impeller — and drop the bucket from any speed to deflect thrust forward and stop the boat on a dime. Of course, this is also a safety advantage if you see something ahead at the last moment.