jet-boat design considerations

Jet-Boat Design Considerations

© 2008 Ultra Dynamics of 13 Printed 5/17/08

A systemic approach for successful application of marine waterjet propulsion

Hulls

Engines

Transmissions & Driveshafts

Waterjet Selection

Jet Control Systems

Prepared by Ultra Dynamics Designer and manufacturer of UltraJet waterjet propulsion and control systems.



This document explores design considerations for the successful application of marine jet drives. This is a compilation and summary our experience, the experience of various boat builders and jet manufacturers, various technical papers, and other sources. It is offered as a guide only. We recommend you review the specific designs and needs of your boat with your boat builder, a professional naval architect and/or marine engineer.

"The Physics of Planing"

"Most maritime operations professionals spend quite a bit of time skimming along the water in some type of planing craft — whether it be a high-speed interceptor, rescue RIB or even a water jet propelled personal watercraft. I am sure, however, that few people give much thought to the actual workings of how their craft is able to rise up out of the water and move at speeds that, until recently, were unheard of.

The interaction of forces on a planing hull are actually very complex in all but a flat sea. At every instant in time, there is a different combination of buoyancy, dynamic lift, engine thrust, frictional resistance and gravity working on the boat. To complicate matters even more, the boat is moving along a boundary layer between water and air that just happens to be moving almost at random. All of this creates a dynamic system that is far more complex than even that of a fighter aircraft. In fact, even with today's computer technology, it is impossible to fully model this system in anything but a rudimentary manner."

J. L. Mitchell, COMBAT CRAFT magazine

Table Of Contents

Introduction ...............................................................................................................................................Page 3

Hulls

Monohedron .......................................................................................................................................Page 4

Warped ...............................................................................................................................................Page 4

Semi-Planing ......................................................................................................................................Page 4

Conventional Displacement ..............................................................................................................Page 5

High-Speed Displacement.................................................................................................................Page 5

Catamarans........................................................................................................................................Page 5

SWATH...............................................................................................................................................Page 5

Trimarans ...........................................................................................................................................Page 5

Flat-Bottom Hulls (Barges)................................................................................................................Page 5

Preparing The Hull....................................................................................................................................Page 6

Static Balance and Dynamic Trim ...........................................................................................................Page 7

Engine(s) ...................................................................................................................................................Page 8

Transmission(s) ........................................................................................................................................Page 9

Drive Shafts and Couplings .....................................................................................................................Page 10

The Jet Drive(s) ........................................................................................................................................Page 11

Control Systems .......................................................................................................................................Page 12

Ultra Dynamics Contact Details ...............................................................................................................Page 13



Introduction

A successful marine jet drive application involves four crucial elements, all of which need to operate in harmony, as a system, for the vessel to perform as desired. These elements are:

The Mission

The Hull

The Power Source

The Propulsor (Waterjet)

1. The Vessel's Mission

A full understanding of the vessel's mission is extremely important to the success of any propulsion system, including marine jet drives. This understanding begins with good communication between the naval architect, boat builder, and the boat owner when the boat is being specified and quoted. Simplified, the mission is the boat s primary task, where used, when used, and who will be operating the boat. Craft intended for offshore duty may have significantly different hull and propulsion compared to craft intended to operate in protected waters.

2. The Hull

There are many papers and opinions on the resistance and performance characteristics of various hull forms. The power developed by the power source and delivered as propulsive thrust by the propulsor is intended to overcome the resistance of these hulls and move the vessel up to a speed at which the propulsion thrust and hull resistance are in balance.

3. The Power Source

The power source is often a diesel engine in military, commercial revenue-producing, and larger pleasure craft. Modern diesel engines are complex machines with many components and sub systems. In most instances, these engines will reliably deliver the stated torque and speeds given the correct fuel quality and temperature, air temperature, turbo boost pressure, exhaust temperature, cooling system, alignment, and engine room temperature. Gasoline (petrol) engines and gas turbines may also be used with jet propulsion. It is important to consider the appropriate engine rating, and to decide if the optimum boat performance is required at full or at cruise throttle setting.

4. The Propulsor (Waterjet)

The propulsion system converts power from the engine to the thrust that will propel the vessel. The propulsion thrust opposes the hull resistance that increases with boat speed. Maximum boat speed is achieved when thrust equals resistance. With waterjet propulsion, the conversion of power to thrust is performed by the least complex component in the propulsion system. A waterjet comprises an intake, an impeller housing, impeller and impeller shaft, nozzle, and the steering and reverse deflectors. The only moving parts involved in generating the thrust are the impeller shaft and impeller. Modern impeller technology can provide an accurate prediction of the power required at the drive shaft to produce a specified thrust and conversely, the jet rpm is an accurate indication of power available to the jet.



Hulls For Waterjet Propulsion The most common use of production jet drives is on planing vessels normally operating at cruise speeds of 20 to 45 knots (23 to 52 mph). Jet drives are a little lower in propulsive efficiency compared to propellers optimized for similar boat speed. However, this is offset by lower hull resistance of a jet boat due to the absence of drag from propellers, shafts, rudders, struts, etc. Therefore, the overall efficiency of waterjet propulsion can be equal or greater than propellers at boat speeds greater than 20-25 knots. At slower boat speeds, the appendage drag has less impact and the combination of hull and propellers may be more efficient. Hull Types

• With due consideration for unitque hull attributes, waterjet propulsion can be applied to most hull types including: • Monohedron planning monohulls/ • Warped planning monohulls • Semi-displacement monohulls • High-Speed displacement monohulls • Low-Speed displacement monohulls • Most catamaran hulls. • SWATH hulls • Trimaran hulls • Flat bottom hulls (barges).

• In all cases, consult your naval architect or other relevant professional for the most appropriate hull type for the craft stability, loading, mission, and anticipated operating and sea conditions.

• The paramount consideration for waterjet propulsion is to avoid ventilation and/or cavitation of the jet under all expected operating conditions.

Monohedron Planing Hulls

• Planing monohedron hulls are generally used with waterjet propulsion for high speed craft that cruise 25 to 40 knots with sprint speed in accord with the power to weight ratio.

• The typical monohedron hull has the chine parallel to the keel line creating a constant deadrise from approximately mid-waterline to the stern.

• Monohedron hulls suitable for jet drives will have a constant deadrise angle (from mid-waterline to the transom) in the range of 8 to 25 degrees with most common being in the range of 12 to 18 degrees.

• UltraJet units may be used in hulls with deadrise angles less than 8 degrees with adequate

immersion of the jets.

• Dee-V hulls with large deadrise angles have higher hull resistance but improved sea keeping and ride, compared to flatter hulls.

• Lower deadrise angles provide less hull resistance than deep-V hulls and the ability to plane at higher speeds. These hulls are well suited to bays and estuaries but may a less comfortable ride offshore.

• Location of the longitudinal center of gravity (LCG) is important to achieve optimum planing trim and best hull efficiency.

• Stepped hulls used in high-speed craft may not be suitable for waterjet propulsion if the air created by the steps could be drawn into the jet. This will cause ventilation of the jet and a loss of thrust and boat performance.

Warped Planing Hulls

• A typical warped hull has a decreasing deadrise angle along the waterline length towards the stern.

• Warped hulls with offer better sea keeping and good load carrying capability compared to the monohedron hull form.

• Warped hulls typically have less hump resistance so they plane easier with less power. Warped hull forms are generally suggested for cruising speeds in the 20 to 30 knot range when required to operate in a variety of sea conditions.

• Warped hulls tend towards a flatter boat trim, especially as boat speed increases. The flatter trim increases the wetted surface of the hull thereby increasing resistance and limiting boat speed.

• Location of the LCG is important to prevent the stem being driven into the water causing bow steering. LCG is measured as a percentage of waterline length from mid-waterline or the transom at the static waterline.

• Some warped hulls have very little deadrise and minimum draft at the stern. This may require nacelles or other hull modifications to achieve sufficient immersion of the waterjet units.

Semi-Planing Hulls

• Semi-planing (sometimes referred to as semi-displacement) hulls are common on fishing, work, and pleasure boats.

• Semi-planing hulls typically cruise at 15 to 25 knots with the top speed being determined by



the power to weight ratio and hull form.

• Semi-planing hulls often have a full or partial keel, a deep forefoot, deep-V at the bow tapering to a relatively flat deadrise at the stern. These boats are also typically wide of beam at the stern.

• Some semi-planing hulls have little deadrise and minimum draft at the stern. This combination may require nacelles or other hull modifications to achieve sufficient immersion of the waterjet units.

• Hulls originally designed for propeller propulsion may have rocker at the stern. Hulls with rocker may require nacelles for successful conversion to waterjet propulsion.

Conventional Displacement Hulls

• Displacement vessels operate at speeds determined by waterline length and displacements. Hull shapes are typically for moving through the water (displacing water) rather than planing on the water surface.

• Waterjet propulsion may be used on displacement craft when the mission requires the unique attributes of waterjet propulsion, such as shallow water operation, avoidance of propeller repairs, excellent low speed maneuverability, low noise and vibration levels, safety for personnel and marine mammals in the water, low maintenance, simplicity, and durability.

• Displacement craft with missions requiring attributes of jet drives should use waterjets designed and optimized for low-speed applications. Most waterjets designed for high-speed applications are not suitable for displacement craft.

High-Speed Displacement Vessels (HSDV)

• Waterjet propulsion is being selected more often for HSDVs such as offshore patrol vessels and small to medium naval ships.

Catamarans

• Twin and quad engine/jet arrangements (two engines and jets per hull in larger catamarans) are a common form of propulsion for catamarans.

• Catamaran hulls may be planing or semi-planing depending on the hull form. Some catamarans use foils to provide lift and reduce hull resistance for higher speed.

• Catamaran hulls with hull forms similar to the

monohedron and warped hulls are common with jet drives. Symmetric hulls with deadrise in range of 10 degrees to 18 degrees are common.

• Some asymmetric hull shapes may require consideration to prevent ventilation of the jet from the tunnel side of the hull. Air from the tunnel may be drawn into the intake as the hull lifts with increasing boat speed.

• Some catamarans have relatively large deck areas in comparison to their water plane areas, and thus have space for large loads to be stowed. This may result in heavily laden craft that do not rise easily onto the plane and may have disappointing maximum speeds as a result of the high hull loading.

• Nacelles have been used with some catamaran hulls to ensure the jet intake is in non-aerated water.

• Catamarans with air delivered below the waterline for reduced hull resistance may not be suitable for waterjet propulsion due to risk of ventilation.

SWATH

• Waterjet propulsion may be used in specialty craft with submersible hulls. These operate as low-speed SWATH (Small Waterplane AreaTwin Hull) vessel when hulls are submersed and as higher-speed catamarans when operating without water ballast.

Trimarans

• Waterjet propulsion may be used in the center hull of smaller trimarans, especially passenger craft meeting low-wash regulations.

• Waterjet propulsion is also applicable to many larger trimaran craft.

Flat-Bottom Hulls (Barges)

• Waterjet propulsion is typically not recommended for flat bottom hulls without adaptation. Waterjets specifically designed for low speed craft may be used along with intake arrangements to avoid aerated water being drawn into the jet (ventilation).

Contact Ultra Dynamics if any of the points in this section are not clear or if additional information or illustrations are required. Please reference the paragraph number with your request.

Reference: “Taking the mystery out of speed predictions” by Alan Gilbert. See link to this paper at www.ultradynamics.com.



Preparing The Hull

Some considerations when designing or preparing a hull for use with UltraJet propulsion:

• The forward and aft ends of the jet intakes must blend with the bottom of the hull and have no bumps, steps, ridges, gaps that might create turbulence ahead of the intake. There should be laminar flow over the intake opening when planing with the jet consuming only the water needed for propulsion.

• For single UltraJet installations, the traditional preparation of a V-hull form is to “flatten” the V for the jet intake. The forward end of the intake is blended forward into the apex of the V-hull (keel) with a triangular transition.

• A Nacelle is an alternative to flattening the V-

hull for the jet intake.

If interested in the nacelle concept, please

contact Ultra Dynamics for additional information.

• A small keel might be considered to aid directional stability in following or quartering seas. These have been successfully use by some boat builders. Consult your naval architect for suggestions regarding keels on your hull design.

• If a keel is used with a single jet, the aft end of the keel should be trimmed so that it is no closer than 5 feet (1.5 meters) from the forward end of the jet intake. The aft end of the keel should be faired similar to the trailing edge of an airplane wing so that there are no low-pressure areas ahead of the jet intake.

• Skegs or fins are sometimes added at the stern to aid in directional stability (tracking), especially for hulls with little deadrise at the stern.

• Ensure there are no through-hulls, water intakes, or other appendages any closer than 6.5 feet (2 meters) in front of the jet intakes.

• Underwater engine exhaust system should not be located near the front of the intakes due to the risk of exhaust gasses ventilating the impeller, resulting in a loss of thrust. Ideally, underwater exhaust should be placed beside the jet intake aft of the midpoint of the intake.

• Transom mounted engine exhaust should be above the waterline. Underwater exhaust in the transom may cause the exhaust gases to be carried under the hull and into the jet intake when in reverse. This will ventilate the impeller and result in a loss of thrust in reverse.

• The transom flange of most UltraJet units is at 90 degrees to the impeller shaft whereas many boat transoms are not vertical. The jet transom flange can be accommodated by insets to the transom, similar to the following photos

• All thrust generated by UltraJet units is directed from the integral thrust bearing through the intake housing to the hull. When the UltraJet unit is correctly installed, no thrust is applied to the transom.

Contact Ultra Dynamics if any of the above points are not clear or if additional information or illustrations are required.



Static Balance & Dynamic Trim

Consider the following points static balance to help achieve optimum dynamic trim when planing:

• The suggested starting point for the longitudinal center of gravity (LCG) for planning monohulls is approximately 35% to 40% of the water line length (LWL) from the transom at the waterline. The location of optimum LCG will vary by hull type and shape and center of buoyancy when planing. Consult the hull designer for optimum LCG location for the selected hull and the loads to be carried.

• Catamarans typically run with a flatter boat trim so the LCG may be 40 to 50% of waterline length from the transom. There is a wide range of catamaran hull shapes so contact the hull designer for optimum LCG location for the selected hull.

• If possible, the fuel tank(s) should be located close to the LCG so that the dynamic trim is not significantly changed as fuel is added or consumed.

• Engine and gen-set, if installed, should be located near the LCG. If possible, leave room to move the engine fore or aft along the stringers in case moving the engine(s) is required to optimize the balance and dynamic trim after sea trials.

• Allow for the additional weight of the jet drive plus the weight of the entrained water within the jet when computing the weights and moments. The weight of the entrained water will be carried in the boat when planing.

• Allow for the hull planing surface area lost to the jet intake when computing the hydrodynamic support of the hull when planing.

• Consider the differences in propulsive thrust vectors of the jet drives (horizontal near the waterline level) compared to the thrust vector of propellers (at an angle to the hull and further below the waterline).

• The need for trim tabs or interceptors should be closely evaluated. If trim tabs or interceptors are required to get quickly on plane it may indicate the weight and balance of the vessel may be less than optimum for jet drives. If trim tabs are to be used, ensure they are positioned out of the reversing thrust otherwise they may be inadvertently and suddenly removed.

• If additional lift aft is required aft to get "on step" planning-surface extensions under the waterjet(s) have been successfully applied by

some boat builders. The extensions add lift without the drag associated with trim tabs. Planing extensions may be fixed or have variable deployment by means of hydraulic or electric actuators. Consult your naval architect for advice and request separate UltraJet document on trim tabs and planing extensions.

• Define the displacement units to be used from the beginning of the project be they in pounds, kilograms, tons, tonnes, etc. and be consistent throughout the project. Even the term “ton” can lead to displacement errors of up to 10%. US short ton = 2000 lb. (907 kg) UK long ton = 2240 lb. (1016 kg) Metric ton = 2204 lb. (1000 kg) Tonne = 2204 lb. (1000 kg)

• A detailed weight and moment calculation is recommended for the successful planing hull. It is not practical to add a little ballast here or there as an afterthought. Also consider the probable weight and location of the personal gear to be brought on board by the owner. A weight and moment calculation worksheet (Excel) is available from Ultra Dynamics.




The Engine(s)

• UltraJet marine jet drives may be powered by

diesel or gasoline (petrol) engines in the range of 150 to 1,800 bhp (112 to 1350 kW) each.

• Diesel engines from most manufacturers are compatible with UltraJet propulsion.

For links to most diesel engine web sites go to

www.ultradynamics.com and click on the menu item “Helpful Links.”

• UltraJet impeller power absorption is rated in kilowatts (kW) and brake horsepower (bhp). Some diesel engine manufacturers rate in metric horsepower (mhp) for recreational boats while the commercial version of the same engine are rated in brake horsepower (bhp). Be consistent with the power units. Request or download separate Ultra document titled “Waterjet impeller Nomenclature (Impellers Demystified)”

• Diesel engine manufacturers rate their products differently, some with maximum power with fuel and air temperature at 25°C (77°F) and others at 40°C (104°F). Select a power rating commensurate with the anticipated operating conditions as there can be a significant difference in power. For example, a boat that meets performance specifications when commissioned in the spring in Canada may perform poorly in the hot and humid environment of Panama.

• Many engine manufacturers also list shaft horsepower (shp) being the power at the transmission drive flange. Shaft hp is generally defined as rated engine power less 3% to 4% to allow for losses within the transmission and driveline.

• Some engine manufacturers may require engine rpm at full throttle be greater than the

rated rpm published in their technical literature. This may be 50+ rpm higher than published for waterjets and 100+ rpm higher than published for propellers. Depending on the power and torque curves of the specific engines, this action may result in less than the published power being available at the rated rpm. Check with the engine supplier if higher than rated rpm is required and if so, request a statement from the engine supplier on the power available at the higher rpm.

• When estimating the power at the waterjet dive flange, take into account the parasitic power demand for hydraulic pumps, water pumps, larger alternators, air conditioning compressor, and engine room ventilation fans, if any are installed. One or more of these devices will consume marginal power that is then not available to the waterjet(s) for boat propulsion.

• Waterjet propulsion thrust is transmitted to the integral thrust bearing and then through the intake case to the hull. Thrust is not transmitted via the drive shaft to the engine, or marine transmission, if installed. Thus, the engine (and transmission) may be mounted on vibration absorbing mounts.

• The catalog power ratings or dynamometer test report power rating may not always be realized when the engine is installed in the vessel. For more information, click on menu item “Jet Boat

Design & Operation” at www.ultradynamics.com to see Ultra Document “UltraJet Commissioning

Guide.”

• It is suggested that the engine mounting system be configured to permit the engine to be moved fore or aft throughout a distance limited by bulkheads, fuel tanks, etc. This will order to adjust the dynamic trim, if required, in the first of a boat type. See preceding comments on LCG and trim.

Links to the web sites of most shaft and coupling manufacturers at the UltraJet web site www.ultradynamics.com and click menu item “Helpful Links.”




The Transmission(s)

• Due to the inherent forward, reverse, and neutral thrust capability of waterjet propulsion, a marine transmission is not mandatory. Many jet powered commercial vessels, especially fishing boats, operate without transmissions to save weight, initial cost, and maintenance.

• The selection of a marine transmission with a waterjet propulsion is often guided by a combination of the mission, skill level of operators, and type of engine.

• A speed reduction ratio may be required to match the waterjet to some high-speed diesel engines and gasoline engines.

• A speed reduction ratio may be required when a larger jet than normal for the power is selected due to higher displacement and/or higher thrust required for the mission.

• The engine to jet speed reduction ratios for diesel engines are typically in the range of 1:1 to 1.4:1. Gasoline (petrol) engines may require engine to jet speed reduction ratios in the range 1.4:1 to 2.0:1.

• Even when a diesel engines can operate with a 1:1 ratio, a small reduction ratio may be desirable for lower thrust at engine idle. This can help keep the boat speed at reasonable levels in no-wake zones. This is also useful on sport fishing boats to maintain a lower trolling speed (without trolling valve on transmission) without sacrificing maximum and cruise boat speeds.

• A speed-up transmission ratio may be required to match the waterjet to some low-speed diesel engines.

• A transmission with a neutral position (or a clutch) will permit operation of the engine for tuning, servicing, power generation, battery charging, etc. without residual disturbance of the water at the dock.

• For waterjet-propelled craft consider transmission controls separate from the engine throttles. A separate push/pull knob located near the helm is suggested so that the transmission is not unintentionally engaged or disengaged. The transmission(s) should be engaged at the beginning of the trip and disengaged just prior to engine shut-down with all maneuvering accomplished with the jet controls only.

• Most modern commercial and recreational engines have electronic fuel management (throttle). That often leads to an electric shift on the transmission. The helm controls are commonly dual function [shift and throttle] with limited options for single function control heads. The electric shift logic in these controls includes, neutral safety [engine start in transmission neutral only, shift delay, shift protection for high speed [rpm] shift and other features.

• A transmission permits the jet drive to be "back-flushed" by engaging transmission reverse. Back-flushing of the jet enables clearing larger obstructions from the jet intake grill when operating in contaminated waters, such as rivers and estuaries, or where there may be large amounts of seaweed, plastic sheeting, cardboard, etc. NOTE: The static waterline should be at least to the centerline of the impeller shaft for back flushing to work. Back flushing works best when the jet is immersed to the top of the impeller.

• If back flushing is a mission requirement, ensure the jet-driven hydraulic pump can be run in reverse at speed for short periods.

• In some applications, the UltraJet control hydraulic pump is driven by the transmission PTO which always rotates when the engine is running and always the same direction of rotation.

For links to the web sites of most marine transmission manufacturers go to the UltraJet web site at www.ultradynamics.com and click menu item “Helpful Links.”




Drive Shafts and Couplings

The design and selection of the drive shaft between the engine and jet drive (or marine transmission and jet drive if a transmission is used) is extremely important for long term trouble-free operation. An inappropriately sized or out-of-balance drive shaft may cause vibration and noise culminating in premature failure of the universal or constant velocity joints. Collateral damage may occur to the hull, engine, transmission, jet drive, and other equipment the propulsion system. Some recommendations and suggestions are:

• Propulsive thrust is transmitted from the waterjet thrust bearings through the jet intake to the hull rather than via the drive shaft to the engine (or marine transmission if installed), the drive shafts may be canted at slight angles to facilitate installation of the drive shaft between the engine, or marine transmission, and the waterjet drive flange.

• As the waterjet is fixed to the hull, universal joints, constant velocity (CV) joints, or flexible couplings will accommodate movement of the engine on the engine mounts.

• IMPORTANT: A torsional coupling, or equivalent, is required at the engine flywheel when a marine transmission is not used. A torsional vibration analysis (TVA) is mandatory without a transmission between engine and jet. See the Designer s Manual for the selected UltraJet model. The UltraJet web site www.ultradynamics.com has additional information on torsional couplings and torsional vibration analysis. Click on the links “Helpful Hints” and “Additional Resources.”

• The angles formed at the forward and aft universal joints must be equal to avoid vibration, noise, and premature failure of the universal joints and other components. Other installation requirements may be mandated by the universal joint supplier.

• Shafts with constant velocity (CV) joints may have unequal angles, however, the CV joint with the greater angle may have lower service life.

• The centerlines of the engine crankshaft and impeller shaft should be parallel. If a marine

transmission is used, the centerline of the transmission output shaft should be parallel with the centerline of the waterjet impeller shaft.

• Automotive or truck universals are not suitable for waterjets driven by diesel engines as the torque load increases exponentially (cubed) with higher rpm. On-highway trucks are near the lowest torque with highest rpm (high gear on flat road). Industrial-grade universal or CV joints should be considered.

• Maximum angles for universal joints are relative to drive shaft torque and rotational speed (rpm). The maximum angles range from approximately 3 degrees at 5,000 rpm to approximately 8 degrees at 2,000 rpm. Consult the shaft supplier for recommended maximum angles for the intended torque and rpm.

• For twin-waterjets with 5-degree down angle, the compound angles formed by the inclined impeller shafts AND the hull deadrise angle require additional consideration. Discuss the requirements with the waterjet and driveline suppliers.

• If the engine is well forward in the boat, the driveline may require two sections and a center bearing support. Please consult the drive shaft supplier for recommendations.

• Contact the driveshaft supplier when the driveshaft must pass through a watertight bulkhead.

• Consult the drive shaft manufacturer for limitations in torque capacity, alignment, balance, etc. Also request a torsional vibration analysis (TVA) to include mass moment of inertia data from the engine, transmission, coupling(s), driveshaft, and impeller and impeller shaft. The torsional coupling and/or engine supplier typically prepares the TVA.

• Additional guidance on the selection and application of drivelines with universal and constant velocity joints may be obtained from the Society of Automotive Engineers (SAE) Recommended Practice, Document SAE J901 MAR95.

For links to the web sites of most shaft and coupling manufacturers go to the UltraJet web site at www.ultradynamics.com and click menu item “Helpful Links.”




The Jet Drives

• Selection of the UltraJet model is primarily

based on the loaded and normal operating displacements of the craft. Hull shape, planing trim, the power to the jet after calculating transmission losses and parasitic power demands, and the intended use (mission) are also to be considered. These factors also apply to other methods of marine propulsion.

• Maximum boat speed in calm water is primarily based on the power to weight ratio of the craft, regardless of propulsion method.

• Ensure the selected jet is capable of absorbing the available power, without cavitation, at the boat speed determined by the power to weight ratio. The maximum power listed in waterjet literature may only be possible at higher boat speeds.

• Additional guidance on the selection and specification of waterjet propulsion is available a the UltraJet web site www.ultradynamics.com and click menu item “Jet Specification & Selection.”

• Jet drives benefit from the movement through the water to help "prime" the jet intake. Therefore, the maximum power absorption (without cavitation) of all waterjets increases with boat speed. Conversely, waterjets may not be able to absorb all of the available engine power at the lower boat speeds. This applies to all waterjets, regardless of manufacturer.

• Boats often become heavier than originally specified. Allow for potential overweight condition in boat speed estimates and waterjet selection.

• To select the appropriate jet drive for a particular application, first determine the anticipated boat speed using industry-standard methods based on power to weight ratios or thrust versus resistance calculations. Then select a jet drive that is capable of absorbing full engine power without cavitation at that boat speed. If a marine transmission is to be used, be sure to use the shaft horsepower (shp) at the transmission output flange to determine the

power available to the jet drive.

• Most UltraJet units mount to an aluminum intake frame supplied by Ultra and installed in the hull by the boatbuilder. Stainless steel studs, washers, and nuts will clamp the jet to the intake block. All forward and reverse propulsive thrust will be taken through the jet intake, to the intake block, and then distributed through the hull and stringers. Contact Ultra Dynamics for drawings of intake frames for aluminum, composite, wood, and steel hulls.

• Waterjet units should be installed so that the static waterline is at least to the centerline of the impeller. This immersion is required for initial prime when the jet is engaged. See previous section “Preparing The Hull.”

• Contact Ultra Dynamics if any of the above

points are not clear or if additional information or illustrations are required AND refer to the Designer s Manual for the selected UltraJet model.



Jet Controls

The foregoing sections deal with installation for a harmonious relationship between the mission, hull, power, and waterjet. When a system is selected for optimum thrust and boat performance, the ability to smoothly control the thrust is now paramount, especially when maneuvering in close quarters.

Ultra Dynamics offers a full range of control systems to match mission, performance, and experience level of the operators. Available are simple controls actuated by push/pull cable through to the JetMaster® joystick system with Programmable Logic Controller (PLC).

Intuitive, smooth and controlled vectoring the jet thrust is the essential final link to achieve successful waterjet-propelled craft.

• Hydromechanical (HM) Control The Hydromechanical control provides proportional operation of the reversing deflector when actuated by a push/pull cable from the helm. The HM control converts low efforts at the helm to hydraulic actuation of the reverse deflector, even at full throttle. The HM control is available on the UltraJet models 251, 305, 305HT, 340, 377, and 410.

• Electrohydraulic Jog (EHJ) Control The Electrohydraulic Jog control uses a joystick or toggle at the helm to electrically actuate a solenoid-operated hydraulic valve on the jet. Hydraulic flow from this valve actuates the reversing deflector. The EHJ control is available on all UltraJet models but used mostly on the 251, 305, 305HT, and 340

• Electrohydraulic Proportional (EHK) Control The Electrohydraulic Proportional control operates the reversing deflector proportional to the movement of the helm lever movement. The system is available with single or twin lever helm control for single or twin-jets respectively. The EHP control is available on all UltraJet models.

• JetMaster-Pro Control The JetMaster-Pro is a competitively priced full-featured control system based on a Programmable Logic Controller (PLC) and proprietary software developed by Ultra

Dynamics for waterjet propulsion. The system offers many features previously only available on more expensive systems.

• JetMaster PLC Joystick Control Ultra Dynamics, and predecessor Company Ultra Hydraulics, have been supplying joystick controls for vessels under 75 feet since 1995. These Ultra-developed joystick systems are used to accurately control forward, reverse and neutral positioning of the boat using vectored thrust from the jet drive(s). The JetMaster joystick controls are available on all UltraJet models.

• Customized Control Solutions Many UltraJet customers have custom control systems tailored for their unique mission requirements. While custom versions and enhancements have been developed, the wide range of standard features and options typically meet the requirements of most applications.

Visit the “Jet Control Systems” page at www.ultradynamics.com for more information on each of the control options.




Contact Ultra Dynamics.

Ultra Dynamics Limited Ultra Dynamics Marine, LLC. Cheltenham, GL51 9NY England Columbus, Ohio 43230 USA Tel: 44-1242-707900 Tel: 1-614-759-9000 Fax: 44-1242-707901 Fax: 1-614-759-9046 E-Mail: [email protected] E-mail: [email protected] Web: www.ultrajet.co.uk Web: www.ultradynamics.com

Last updated: May 16, 2008

N.B. The considerations and concepts discussed in this document are presented only as a guide for UltraJet customers. Ultra Dynamics does not attest to the suitability of any or all of the concepts in this document for use with every UltraJet application. Please discuss and verify the selection and use of these concepts with Ultra Dynamics and with the appropriate marine design professionals.

The latest version of this guide is available from the UltraJet web site at www.ultradynamics.com. This document may be updated without prior notice as new information becomes available.

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