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Should a Jet Drive Be Considered For Your Next Boat?
Marine Jet Drives -- The Wave of the Future
Why consider jet propulsion for your boat? Just as the technology of cell phones, televisions, and clock radios are undergoing a transition from analog to digital, marine propulsion technology appears to be moving quickly to offer alternatives to the venerable propeller. Just as today's digital technology is expanding the potential of the entire electronics scene, jet propulsion is providing many new advantages in the marine industry, from pleasure craft to offshore work boats.
What is marine jet propulsion? Basically, it is a function of differences in the mass-flow of water. Water enters and exits a jet drive at differing velocities, power is converted to thrust via an impeller, and the thrust propels the boat. Steering right and left is accomplished by directing the system's exit flow in one direction or the other, much like directing the thrust of a submerged propeller-driven system. Forward and reverse motion is infinitely controllable through a reverse deflector that simply diverts the thrust fore or aft.
Today's jet-drive technology provides recreational and commercial boaters with many practical benefits that are not present with propeller-driven units. What are some of these benefits?
Safety - Water skiers, swimmers, recreational and commercial divers are safe from the threat of injury (or worse) from turning propellers. There is minimal potential for damage to jet-driven boats from hitting floating debris, and there are no propellers to foul on stray ropes or fishing lines. Jet-propelled boats simply ride over floats and lines attached to lobster and crab pots. Jet driven boats have expanded access to shallower waters, particularly in tidal areas. The potential for major drive damage from running aground, as with submerged propellers, is eliminated.
Maneuverability - Jet-driven boats have excellent maneuverability, even at low speeds, because of vectored (directed) thrust. Whatever a situation requires, the needed amount of thrust can be directed fore or aft, port or starboard. Thrust is applied smoothly by control of the reverse deflector, an integral component of the unit. This vectored thrust affords jet-driven boats very responsive steering, ideal for control in narrow waterways and avoiding other vessels. There is infinite control of fore and aft movement, even in severe wind and sea conditions.
Economy - Jet drives are easy on engines and transmissions. A jet drive, well matched to an engine and hull, allows the engine to operate at optimum cruise throttle settings, thus providing better fuel economy. In start-and -stop situations, (for example, water taxis, lobster and crab fishing boats), the engine is set at optimum throttle, the transmission is left in forward, and all stopping, holding and accelerating is smoothly controlled with the reserving deflector. Basic maintenance consists of monitoring bearing oil levels and checking for anode depletion. If service is required, many external parts can be owner-serviced in the water, without hauling the boat (such work can often be done on a beach or rack which is exposed at low tide). This is especially beneficial if inspection or service is needed miles from a full service marina. If operated in relatively clean water, many years of service may be expected from the encased impeller. End result: lower operating costs.
Comfort - Compared to propeller-driven systems, jet drives greatly reduce on-board noise and vibration levels. Jet drives rotate at higher speeds than propellers, and the impellers are encased in precision-crafted housings. Thus, vibration levels are much lower than with propeller-driven units. Considering that thrust is developed within the jet itself and is transmitted directly to the hull, engines may be fitted with flexible engine mounts to further reduce engine-generated.
Environment - Vessels with jet drives have much lower underwater noise signatures than vessels with propellers. In addition to being a relevant consideration for military applications, this in an especially significant factor where marine life and other environmental matters are concerned. Propeller-driven boats are restricted in several areas of Florida because of potential harm to fragile coral. Along with being safer for swimmers, the jet drive is also safer for marine life. The endangered West Indian manatee is a good example. Since 1976, more that 43% of manatee deaths were attributed to humans, most of them in boating related incidents, primarily from impact with underwater propellers. Manatees, if struck by jet-driven boats, are much less likely to be injured.
Other Applications - Jet driven vessels are ideal for use in towing or recovery operations because maximum thrust for any throttle setting is available, at even the lowest boat speed. Jet-driven boats are often able to get closer to the a grounded vessel to assist in the recovery. Jet drives have no equal in their ability to stop a planing boat. Approximately 50% of forward thrust may be utilized when the reversing deflector is deployed (it is essential to warn and prepare passengers and crew before performing an emergency stop).
While jet-driven watercraft are definitely the "wave" of the future, they may not be suitable for all applications. Propeller drives still have their place!
Jet drives should best be considered as alternatives to propellers, not necessarily as replacements. Jet drives have unique advantages for vessels operating in certain roles. Propeller-driven vessels have other attributes. Matching their attributes to the particular vessel's configuration and mission is advised.
Indeed, much is happening in the marine propulsion arena these days. The trend toward jets is real, primarily because of the various benefits over propeller drives as noted in this article. So, to summarize....Jet driven boats give you more maneuverability, tend to be more efficient, more economical, quieter, safer and more environmentally sound. You'll be seeing more Jet drives on the waves in the future!
Author: Garry Cornell
Copyright © 1999 Ultra Dynamics, Inc.
The above document may be copied or downloaded for personal use. For permission to reproduce this document in quantity or for use in other media, please contact gscott @ ultradynamics.com |
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