On Powerboats - Waterjets: the other marine propulsion

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Shallow draft, maneuverability and high cruising speeds are among the benefits of jet-driven boats

Shallow draft, maneuverability and high cruising speeds are among the benefits of jet-driven boats

The popularity of waterjet power in the recreational boating industry was once driven at least in part by the sheer novelty, but this type of propulsion has since gained acceptance based on actual merit. With their unique capabilities, waterjets make a lot of sense for a number of applications — from very small boats, such as personal watercraft, to larger vessels for which shallow draft, immunity from floating objects like lobster pot buoys, high cruising speed, swimmer safety and maneuverability are primary considerations.

How they work

Waterjets draw water in through an inlet grate in the hull bottom, accelerate it by means of an impeller, and shoot it out the transom in a small, high-velocity stream through a steerable nozzle. The nozzle is fitted with a deflector (sometimes called a reversing bucket) that drops down and deflects the water stream forward to stop or reverse the boat. This backing capability allows a jet-driven boat to stop quickly, since the engine need not be slowed and shifted into reverse — just drop the bucket and voila.

Jet-driven boats can be handled easily by just setting the throttles and then using the steering and buckets to maneuver, providing essentially infinite thrust adjustment — unlike an inboard, where you have a choice of either in gear or out of gear. Triple- or quadruple-engine waterjets usually eliminate the steering nozzles and reversing buckets on the inboard engines.

Because water is drawn up and into the waterjet housing and the thrust is high relative to the vessel’s center of gravity, the hull tends to run at a lower trim angle than with a conventional propeller. This should be factored into the hull’s design and weight distribution. Other design elements to consider: The waterjet takes up a lot of room, with space needed for both the engine and the waterjet, and the waterjet also holds a lot of entrained water, which adds weight to the boat.

A waterjet is unique in that the impeller pulls less power from the engine than a propeller at midrange throttle settings. For example, an UltraJet 376 rated at 700 hp at 2,200 rpm would need to run at 1,920 rpm to absorb the same horsepower (470) as an inboard running at 1,850 rpm. It will get the same efficiency (miles per gallon) as the slower-turning inboard, but it will need to run 50 to 75 rpm faster than an inboard to produce the same thrust and hull speed. While an impeller’s discharge velocity can be twice the vessel’s speed through the water, a conventional inboard propeller moves the water just 10 to 20 percent faster than the hull is moving (this difference equates to the propeller’s slip), but it moves a much larger column of water, and that’s how it gets its midrange thrust or “traction.”

While the diameter of both the waterjet impeller and the propeller are based on vessel displacement, pitch selection is different. Impeller pitch is matched solely to engine power, while a propeller’s pitch is selected according to hull speed in order to control slip and to allow the engine to make its rated rpm.

One reason waterjets are so efficient in their design speed range is that a propeller has to work harder due to its interaction with surrounding water flow, which is at a different angle than the propeller shaft, subjecting the individual propeller blades to variable loading each time they swing though a revolution. Since the waterjet impeller is enclosed in a pipe-like tunnel and receives an even flow of water, it works in a more benign environment. And since all of its discharge is contained and directed aft rather than partially outward, it picks up more efficiencies. Though a waterjet’s intake grate creates drag, it’s nothing like the inboard, which is dragging its running gear — struts, shafts, props and rudders — through the water, especially as speed exceeds 30 knots.

Matching waterjet to mission

Waterjets took a bum rap early on, through no fault of the technology, when they were sold by vendors anxious to make a sale with the least-expensive unit. This resulted in applications that performed poorly, especially at lower power settings. What everyone learned is how important it is to match the waterjet to the application. Whether it’s a high-speed rescue boat or a heavier workboat, if the supplier knows how the vessel will be used, an appropriate unit can be selected that’s optimized for the vessel’s power-to-weight ratio, speed range, sea conditions, displacement variation and overall mission.

There are waterjet models for a wide range of applications. For instance, waterjets with low-volume, high-velocity impellers work well in high-speed, light-displacement vessels but are relatively inefficient below 25 knots. On the other hand, waterjets that produce greater water volume — that’s what produces low-end thrust — and lower velocity work well for heavier, slower boats when operating at as low as 70 to 80 percent power but are less efficient at high speeds.

If you’re choosing between two waterjet models, the bigger of the two may well be the way to go simply because of the midrange thrust issue. The biggest challenge for any propulsion system is getting the boat up on plane. That’s because there is a lot of resistance (hull drag) while the boat is climbing over the hump and the engine has yet to develop its full power. In fact, if the propeller demand for an inboard exceeds the power the engine can develop while trying to get over the hump, it won’t be able to plane. Making rpm isn’t a problem for the engine driving a waterjet impeller, but since the resistance for any boat while getting up on plane remains high, it’s easy to see why the waterjet’s ability to achieve sufficient thrust in the midrange is so important.

Another issue is the cavitation produced by a hull that’s moving too slowly for a too-small, high-turning impeller, and this in turn can cause erosion in the aluminum impeller housing and also in the impeller itself. In other words, the water moving through the bottom grate and toward the impeller, largely a function of hull speed, has to be moving fast enough in relation to the water being discharged from the impeller to prevent cavitation. And since many waterjets operate most efficiently at speeds exceeding 25 knots, a boat must have enough continuous horsepower to continuously cruise at these speeds with a full load, painted bottom, canvas up and when running into the wind. The point is to choose a waterjet model that matches the power, hull and mission. And make sure to sea trial a waterjet boat with a full load to make sure it performs well at all speeds.

If you want to go faster than 45 or 50 knots, leaving outboards and sterndrives out of the discussion, surface-piercing propellers become the power of choice when it comes to pure efficiency. Of course, some waterjet applications will run well at slower than 25 knots, mostly in boats with low resistance and light-bottom loading. But then again, many won’t. Some waterjets will also go faster, but with a big drop in efficiency. Just make sure to sea trial any waterjet boat you’re looking at and make a note of rpm vs. speed and fuel flow.

Advantages

One of the greatest advantages of waterjets is draft. Since they don’t have propellers or rudders that project below the bottom of the hull, the boat’s draft is as shallow as it can possibly be. Vibration and noise levels typically are much lower than conventional inboards, too, with no propellers moving in choppy water flow. Plus, the engines don’t absorb the impeller thrust — the thrust is transmitted through the intake housing to the hull — so they can be soft-mounted and allowed to “dance” around freely.

The lack of props (or anything else) on the outside of the hull makes waterjets the safest possible drivetrain, which is a big deal if your boat spends a lot of time with people swimming and diving from it. That’s one of the reasons jet-powered PWC are so popular. Short of a direct hit — which, incidentally, seems to happen often enough and isn’t surprising, considering that kids and more than a few idiots are driving these 60- to 70-mph, 800-pound missiles unsupervised — it’s pretty hard to hurt someone with one.

If a salesperson is taking you for a demo ride in a jet-driven boat, count on them showing off a little, doing donuts or maybe taking a spin through the nearest string of lobster pots. In fact, that’s what the venerable Ted Hood used to do when sashaying around off Newport, R.I., in one of his gorgeous Little Harbor Whisperjets on a demo ride, as I seem to recall. The fact is, once you get the hang of it, it’s hard not to show off a little when driving a twin waterjet boat. They’re a kick to drive and, in the right hands, are much more maneuverable than a conventional twin-screw inboard. When they’re integrated with a bow thruster and controllable by a single joystick, you can go in any direction you want maneuvering around the dock. And while you can slide the boat around dockside in any direction you please with twin waterjets, a bow thruster and joystick control, so can a good stiff breeze, and for the same reason — there’s so little underwater resistance.

Waterjet requirements

So why isn’t every boat waterjet propelled? There are a number of reasons, including initial cost, hull form requirements, midrange performance and rough-water capability. Waterjets are more expensive than inboards since relatively few are built each year, resulting in less automation and more labor during manufacture.

A waterjet needs a fairly deep-vee hull, too, for a number of reasons. While an inboard has rudders with plenty of bite to control its heading, a waterjet relies a lot on the shape and weight distribution of the hull to run straight. It needs a buoyant entry — one that’s not too deep and fine — to prevent the bow from digging in and steering the boat down sea. It also needs plenty of deadrise aft — at least 16 degrees, 20 or 22 degrees is better — to keep it running straight. (A deep-vee creates more resistance to turning, acting a little like one long rudder, which is what makes it such a great down-sea boat.)

Waterjets are also susceptible to fouling with weeds and other debris. Sometimes back-flushing works; if not, cleanout ports are provided in the waterjet housing by the impeller. On the other hand, you might have to go over the side to clear a line fouling a prop that a waterjet would go right over, so each system has its advantages.

Some waterjet boatbuilders, like Hinckley, install hull fins aft to help keep the stern from sliding out like a ski boat in a high-speed turn, which can aerate the impeller, causing it to stall, like slipping a clutch. But turning in your own length at 35 knots in a 40-footer will really give your guests something to remember you by. It’s also important to keep air from getting to the impellers, since air makes an impeller stall and it’s tough to control a boat absent thrust. This is another area where ample deadrise comes in, since a deep-vee hull will encourage bow-generated bubbles to flow outward to the chines before they make it all the way back to the intakes. Hull strakes also have to be situated so they don’t channel air to the intakes.

The waterjets must be submerged deeply enough — at least to the centerline of the impeller shaft and preferably to the top of the tunnel, though not always possible with twin waterjets in a deep-vee hull — to keep them primed and to ensure a good, air-free bite on solid water in a chop. That’s another reason a vee-bottom planing hull with plenty of depth under the cockpit is a good way to go. The hull’s buoyancy aft also must suffice to accommodate the added weight of waterjets (full of water) in the stern. In some hulls this means shifting the engines farther forward to keep the boat from trimming too far by the stern and running bow high. All these requirements make some hulls poorly suited to waterjet propulsion. Low-deadrise boats and traditional round-bilge semidisplacement hulls such as Maine-style lobster boats require modification to be successful, at the least.

Sea state limitations

Even with a good, deep-deadrise hull design, a waterjet has speed limitations in rough water, largely because of the constant-water flow-to-the-impeller issue. It seems there’s just no substitute for having propellers a couple of feet below the keel, where there’s (nearly always) plenty of solid water for steady prop-generated thrust. However, waterjets are successfully used on pilot boats that, of course, routinely operate in rough water, so a combination of appropriate waterjets, hull form and not pushing the speed envelope can produce good results. And remember that even with a good hull design and proper longitudinal center of gravity, a waterjet can be a lot of work to steer offshore running down sea for hours on end.

While no system is perfect, the waterjet will be the closest thing to perfection for owners — and applications — who place a high premium on its clear strengths. Shoal draft, inherent safety, maneuverability, smooth and quiet running, and the sheer fun of driving a jet-driven boat make this power well worth considering.

Eric Sorensen was the founding director of the J.D. Power and Associates marine practice and is the author of “Sorensen’s Guide to Powerboats: How to Evaluate Design, Construction and Performance.” A longtime licensed captain, he can be reached, and his book purchased, at eric@sorensensguide.com .

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