Ultra Dynamics | Ultra Jet - Frequently Asked Questions about Hyrojets & Waterjet Propulsion

1. Who is UltraJet and Ultra Dynamics?
Ultra Dynamics, and the predecessor company, Dowty, have manufactured marine waterjet propulsion since the 1950's. Ultra (and Dowty) were primarily involved with military applications for 30 years until the late 1980s, in commercial vessels in the early 90's and in military, government, homeland security, fishing, workboats, passenger vessels, and recreational boats since the early 1990s. See Recently Launched for examples of boats for these applications.

2. How do the UltraJet drives compare with other jet drives?
While we are proud of our products, the more important opinions come from our customers, especially those production builders who have switched from other suppliers. They tell us the quality is better, prices are lower, installation is simple, boat performance is better, controls integration is simpler, and parts - when rarely required - are less expensive.

3. Are jet boats faster than propeller driven boats?
Some are, however, the maximum speed of planing boats is determined by the power, weight (displacement) of the boat, the hull shape, and trim. As a general rule, maximum boat speed is determined by the power to weight ratio, regardless if propelled by jet or propeller.

4. How do jet drives compare with underwater propellers for efficiency?
Boats with waterjet propulsion are generally more efficient above 20 knots in hulls optimized for jets. Propellers are generally more efficient than jets in boats designed to operate at less than 20 knots. However, in some applications the attributes of waterjets, such as bollard pull, maneuverability, shallow-draft, and safety considerations override efficiency, especially if when little time is spent a the lower speeds.

5. I have heard that jet boats are less efficient and have poor performance?
It is true that there have been unsuccessful jet boats. The poor performance of these boats can usually be identified as:

The jet was selected on price rather than selecting a model with the thrust needed to carry weight of the boat, full fuel, full passenger load, owner's gear, and equipment.
There was a misunderstanding of the power actually available to the jet(s) when considering mission variables and transmission losses.
The selection was initially correct for the specified displacement but the boat was built heavier than specified and/or the boat was not well balanced
The jets were fitted to hulls not suitable for jet drives.
Jet drives with relatively low efficiency and quality were used on some boats, i.e. the incorrect jets for the application and mission.

6. How well does a boat fitted with UltraJets accelerate?
When selected appropriate to the power to weight ratio, UltraJet units permit full power acceleration from a standing start and utilization of full power, if required, at any boat speed.

7. Why hasn�t waterjet propulsion been used on more pleasure boats?
Often fear of the unknown by some boat builders resulting in higher prices for the jet option to cover their first time costs. These high prices probably discouraged potential buyers. Forward-thinking boat builders who offer waterjet propulsion comment that simpler installation of jets offsets the slightly higher component cost.

8. Will the jet pick up seaweed?
They can, however, the risk is lower than for an underwater propeller. Keep the power low when at low boat speed over areas of seaweed. The risk is very low at cruise speed. Small and floating sea grass will go through the jet without concern.

9. How will I know if the intake is blocked with seaweed or other debris?
The engine rpm will increase for the effected jet and boat speed will drop slightly. This indicates the jet is not pumping as much water due to a partial blockage of the intake.

10. How is seaweed or other debris cleared from the jet intake?
There are several methods with the first being to Backflush the jets. Backflushing is running the jet backwards for a short period by engaging transmission reverse. This pumps water out of the intake to blow off debris caught on the intake grill. Rare blockages that are not removed by backflushing can be cleared from inside the boat via the access cover. Other options are available for mission-specific applications.

11. Can the jets handle sand and gravel?
Yes, the occasional gulp of sand or gravel is not detrimental to the jet drive. In fact, waterjets are much less susceptible to damage than an underwater propeller in similar conditions. Continuous operation in sandy or silted water will accelerate wear of the impeller. Remember to keep the power low when crossing sandy shoals so that there is reduced tendency to draw sand or gravel into the jet.

12. What if the boat runs aground?
Some jet boats have a small keel or skeg but otherwise most jet boats have no underwater appendages. Therefore, if grounded at low speed on a shoal, the high reversing thrust of the UltraJet drives will usually allow self-recovery by reversing the boat out of trouble.

13. What about lobster and crab pots?
Not a problem. Drive right over the floats as there is no underwater machinery to foul the float lines.

14. Are vessels fitted with UltraJets Manatee-safe?
Yes, manatees will not be injured or cut by boats fitted with UltraJet propulsion when operating in No-Wake or Manatee Zones.

15. Can I beach a jet boat for a picnic?
Absolutely. Drop a bow anchor and reverse into the beach. Place a stern anchor on shore. You can then step off the stern to the beach. (Be aware of tidal depth variations during the time at the beach).

16. Can I expect improved service life from the diesel engine and marine transmission?
Yes, absolutely! Jet drives are universally recognized for being much easier on engines and transmissions. With jet drives, cyclic loads such as powering into waves are not transmitted to the transmissions or engines. Overloading the boat has little effect on the transmissions or engines, only resulting in lower boat speed. Also, the transmissions are engaged and disengaged far less frequently compared to underwater propeller drives, especially advantageous for sport fishing boats.

17. If I do not engage and disengage the transmissions, how can I maneuver the boat?
Maneuvering a vessel with single or twin UltraJet drives is accomplished by vectoring thrust using the reversing and steering deflectors on the jets. You operate the reversing levers at the helm in a similar manner to transmission shift levers, however, control of the thrust is proportional and directional. The engine throttle can be set to provide the thrust required for the maneuver in the prevailing conditions.

18. What is the key to maneuvering a jet boat?
Keep the transmissions (if installed) engaged in forward, set the engine rpm at 600-1200 rpm, depending on prevailing wind and current, then make small adjustments to the reversing deflectors and steering.

19. How often will I need to change the impellers?
Owners of pleasure boats should expect five or more years of normal use from the stainless steel impellers. Minor damage to the blades caused by large stones can typically be repaired by a propeller shop experienced with stainless steel propellers.

20. What routine service is required for the jet drives in vessels equipped with UltraJet drives?
Jet drives are probably the lowest maintenance item on the boat. Routine maintenance can be summarized as:

Before each use:

Check oil level in hydraulic fluid reservoir.
With marine transmissions in neutral and engine not running, check that the jet drive shaft rotates freely.

Once a month:

Check condition of the impeller by feel through the inspection cover (Again with the engine not running).
Inspect the condition of the anodes (zincs) on jet and hull.
Check security of electrical connections in the bonding system.
Check electrical connections to hydraulic valves when electrohydraulic or joystick control is installed.

Annually or when hauling the boat for bottom paint or other maintenance, inspect the intake grills and general condition of the underwater components of the jet drive.
Apply a suitable antifouling coating to the underwater components of the jet drive and inside the intake.

Author: Graham Scott
Copyright � 2007 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