Jet Drive

I am looking to buy a boat, but it has a jet drive. I have no knowledge about it and have a few questions.

?Can you give me a simple definition of what it is and how it works?

?How is the performance compared to other drives? (better, worse)

?Is there anything of interest about them I should know?

[Jeff]

Can you give me a simple definition of what it is and how it works?

A Jet Drive uses an impeller in a tube which sucks in water from the bottom and shoots it out a nozzel at the stern at a high rate of speed. As water is expelled backward, the boat moves forward (think of how a rocket works.)

Here is a fuller explanation from the Hamilton Jet Site:

http://www.hamjet.co.nz/index.cfm/Th...jet_Works.html

How a Waterjet Works

A waterjet generates propulsive thrust from the reaction created when water is forced in a rearward direction. It works in relation to Newton's Third Law of Motion - "every action has an equal and opposite reaction". A good example of this is the recoil felt on the shoulder when firing a rifle, or the thrust felt when holding a powerful fire hose.

Put simply, the discharge of a high velocity jet stream generates a reaction force in the opposite direction, which is transferred through the body of the jet unit to the craft's hull, propelling it forward (see diagram below).

In a boat hull the jet unit is mounted inboard in the aft section. Water enters the jet unit intake on the bottom of the boat, at boat speed, and is accelerated through the jet unit and discharged through the transom at a high velocity.

The picture opposite shows where water enters the jet unit via the Intake (A). The pumping unit, which includes the Impeller (B) and Stator (C), increases the pressure, or "head", of the flow. This high pressure flow is discharged at the nozzle (D) as a high velocity jet stream. The driveshaft attaches at the coupling (E) to turn the impeller.



Steering is achieved by changing the direction of the stream of water as it leaves the jet unit. Pointing the jet stream one way forces the stern of the boat in the opposite direction which puts the vessel into a turn.

Reverse is achieved by lowering an astern deflector into the jetstream after it leaves the nozzle. This reverses the direction of the force generated by the jet stream, forward and down, to keep the boat stationary or propel it in the astern direction.

http://www.hamjet.co.nz/index.cfm/Th...jet_Works.html

Or just http://www.hamjet.co.nz/

How is the performance compared to other drives? (better, worse)

Jet drives are most efficient for high speed planing craft and not so efficient for slower craft. Ultradynamics recommends on their site <http://www.ultradynamics.com/> that jet drives are more efficient over 20 knots, while propellers are more efficient under 20 knots.

In recreational usage, a jet drive is usually coupled to a higher horsepower motor than would be used in a prop driven application to arrive at a similar speed.

Jet drives do produce a fun ride. Unlike with a prop which requires time to go from stop to full power (except maybe for the case of a changing-pitch propeller), with a jet you can apply the full power of the engine no matter what the boat's speed.

Also Jet Drives have very little draft which is a big advantage for some applications. Another advantage could be safety if people are swimming or kids are involved in other watersports around the boat.

[Jeff]

Is there anything of interest about them I should know?

As I was looking for a graphic to illustrate my answer to your question, I discovered that Ultra Dynamics (Ultra Jet) has a really nice FAQ section on their site written by Graham Scott.

http://www.ultradynamics.com/section...sp?section=3-b

Here is a portion:

These design considerations are extracts from the manual "A Guide To The Application Of Marine Jet Drives" which is available from Ultra Dynamics.

5. Planing Monohull Configuration
The most efficient use of production jet drives is on planing vessels normally operating in the 20 to 40 knot range. 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 due to the absence of underwater propellers, shafts, rudders, struts, etc. Craft with jet drives do not have this appendage drag, therefore, the overall efficiency can be equal or greater than propellers are boat speeds greater than 20-25 knots. At slower boat speeds, the resistance of these appendages has less impact and propeller drives may be more efficient.

Monohedron Planing Hull Form
A typical monohedron hull form has the chine parallel to the keel line creating a constant deadrise angle from approximately mid-waterline to the transom. A monohedron hull suitable for jet drives will have a constant deadrise angle (from mid-waterline to the transom) in the range of 8 degrees to 25 degrees with most common being in the 12 degrees to 18 degrees range. However, deadrise angles less than 8 degrees can be used with jet drives in some applications. The hull resistance increases with larger deadrise angles, however, sea keeping and ride improves. Lower deadrise angles provide lower resistance than deep Vee hulls and the ability to plane at higher speeds but with a less comfortable ride on all but the most calm seas. Monohedron hull forms with moderate deadrise angles are generally suggested when cruising speeds greater than 30 knots are required. The location of the LCG is important to achieve optimum planing trim and best hull efficiency. Chine flats can also add lift and reduce resistance.
Warped Planing Hull Form
A typical warped hull form has a decreasing deadrise angle along the waterline length towards the transom. Warped hulls with jet propulsion offer better sea keeping and good load carrying capability compared to the monohedron hull form. Warped hulls offer less hump resistance so therefore will plane easier with less power. Warped hull forms are generally suggested for cruising speeds in the 20 to 30 knot range and when the ability to operate in a variety of sea conditions is required. Warped hulls tend towards a flatter trim, especially as boat speed increases. The flatter trim increases the wetted surface of the hull thereby increasing resistance and limiting boat speed. The location of the LCG is important to prevent the stem being driven into the water causing bow steering and the potential to broach. LCG is measured as a percentage of waterline length from mid-waterline or the transom at the static waterline. Chine flats can also add lift and reduce resistance.

6. Other Hull Forms
Jet drives are suitable for use on semi-planing and displacement hulls when the benefits of jets (low draft, avoidance of propeller repairs, excellent low speed maneuverability, low noise and vibration levels, safety for personnel and aquatic mammals in the water, low maintenance, simplicity, and durability) are important, or essential, in the operation of the vessel.

Semi-Planing Hulls
This type of hull is common on fishing, work, and pleasure boats built in the Northeast, Atlantic States, and Florida. In the Northeast, this type of hull is known as a Downeast-style. Semi-planing (more commonly referred to as semi- displacement) hulls typically have a full or partial keel, a deep forefoot, a deep Vee entry at the bow tapering back towards amidships and a relatively flat deadrise at the stern. These boats are also typically wide of beam at the stern.
At first glance, these hulls may not be the most appropriate for jet drives, however, some jet drive manufacturers, including Ultra Dynamics, have introduced hull specific drives that suit the shallow immersion at the stern, and the overall arrangement of the hull. These hulls are typically cruise in the 15 to 25 knot range with the top speed being determined by the installed power.
Displacement hulls
These are usually larger, load carrying vessels that travel through the water no faster than hull speed which is limited by a relationship of the waterline length and displacement. Jet drives designed for planing craft are generally not the best solution for displacement craft requiring the benefits of jet propulsion as large diameter impellers and high water volume, rather than high water velocity, are required. Displacement craft with missions requiring some of the benefits of jet drives can use those specifically designed to provide high thrust at low boat speeds, such as some Ultra models or the tractor-style jet drives with large diameter, multi-blade (or vane) impellers.
Multi-hulls
These are catamarans and trimarans. Twin and quad (two engines and jets per hull in catamarans) arrangements are a very common form of propulsion. Multi- hulls tend to have a high length to beam ratio and, therefore, are sometimes difficult to model hull resistance with industry-standard prediction software.
Catamaran hulls with underwater profiles similar to the monohedron and warped hulls are preferred for jet drives. Symmetric hulls with deadrise in range of 10 degrees to 18 degrees are best for jet drives.
Be cautious about applying jet drives on unique hulls that use air, either as a cushion or lubricant to reduce friction between the hull and water. Aerated water at the intake will cause reduced thrust with a probably increase in engine speed, similar to the effect of a slipping clutch in an automobile. This will limit the ability to utilize the available power and will usually result in lower than expected boat performance.

7. Preparing The Hull
When preparing the hull for marine jet propulsion, the following points should be considered.

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 at the intake. There should be laminar flow over the intake opening when planing with the jet drawing in only the water needed for propulsion.
The intake will be on a flat surface fitted across the Vee and blended into the Vee hull with a triangular section.
A skeg keel might be considered to aid directional control in following or quartering seas. These have been successfully used by some boat builders. Consult the jet manufacturer and a naval architect for suggestions regarding keels on your hull design.
On single engine applications, the aft end of the skeg should be trimmed so that the aft end is no closer than 5 feet (1.5 meters) from the forward end of the jet intake. The aft end of the skeg 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.
There should be 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 anywhere in front of the intakes due to the risk of exhaust gasses being drawn into the intake and ventilating the impeller, with a resultant 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 jet drives can range from 90 to 102 degrees relative to the jet intake. If the transom is at another angle, rectangular or round insets sized to fit the jet flange will need to be welded or molded into the transom to accommodate the transom flange of the jet drive. Ultra Dynamics provides templates to prepare the openings in the transom for UltraJet® drives.
Some jet drives are mounted to the transom. These tend to have more of the jet outboard and transmit thrust to the transom. Others jet drives, including UltraJet® drives, transmit no thrust to the transom when correctly installed.

8. Static Balance and Dynamic Trim.
Some items to consider regarding the static balance of the boat which help have a good starting point for optimum dynamic trim when planing:

A suggested starting point for the longitudinal center of gravity (LCG) to be 40% of the water line length (LWL) from the transom at the waterline, or at Station 6 on lines drawing. The location of optimum LCG will vary by hull type.
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.
The 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 water within the jet when computing the weights and moments.
Allow for the 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 a propellers (at an angle to the hull and further below the waterline). The jet drives may have a lower tendency to lift the bow resulting in a flatter trim and lower boat speed if the LCG is not moved aft to compensate.
The use of trim tabs is discouraged. If trim tabs are required to get quickly on plane it may indicate the weight and balance of the vessel is less than optimum for jet drives.
If additional lift should be required aft to get "on step" planing surface extensions of the hull under the jet drives have been utilized on some planing craft. The extensions add lift without the drag associated with trim tabs. Consult the jet manufacturer and your naval architect for advice.
A detailed weight and moment calculation is imperative as the successful planing hull cannot have a little ballast added here or there as an afterthought. Also consider are the probable weight and location of the personal gear to be brought on board by the owner.

This is from http://www.ultradynamics.com/section...sp?section=3-b and of course their main web site is http://www.ultradynamics.com/

Also following this thread http://boatdesign.net/forums/showthr...p?threadid=392 Ultra Jet provides DXF Drawings on their website which is nice, though you have to register for a password with them.

In any case, they have a great web site, and it is worth al look at their FAQ, also including http://www.ultradynamics.com/section...sp?section=1-b as you consider your first jet drive.

[diver]

We have just taken delivery of a second hand mob fast response RIB. It is powered by a mermaid 4cyl Turbo charged Diesel giving 140hp with a P.P 90 water jet. Max speed quoted as 26knots. During recent runs we have been only able to achive 15knots.
We belive there is a problem with the jet unit, engine RPM and Turbo are as design.
Not been able to find a site for a manual or info.
Anyone have ideas or links i could try?
Any help would be welcome.
Richard

[yipster]

let me thank you for the links above Jeff !
(and keep myself posted for more on jets

[Pazz]

With regards to your pp 90, The most probable cause would be the impeller to housing clearance, this is a critical part of getting the power out of a jet unit. I have a pp 65 on a V8 rover engine with 155 hp, this pushes my 16.5 ft baot at over 35 knotts, I also have the setup for adjusting the clearance on a pp 65 which may be the same as yours.
I hope the above is of some help.

just out of interest, there is a guy near us selling a 18ft speedboat with a V12 Jag E type engine driving a Castoldi jet, I've seen it going (and heard it!!!) he wants £1500 for it..

[diver]

[quote=Pazz]With regards to your pp 90, The most probable cause would be the impeller to housing clearance, this is a critical part of getting the power out of a jet unit. I have a pp 65 on a V8 rover engine with 155 hp, this pushes my 16.5 ft baot at over 35 knotts, I also have the setup for adjusting the clearance on a pp 65 which may be the same as yours.
I hope the above is of some help.

do you have the procedure for setting up the clearance?
We stripped the Jet down yesterday to check for damage and found the following.
blade clearance 18thou
small dents/damage to impeller.
Stator vanes have some small indentations and bumps.
Main cause for concern are the following. drive shaft has a large amount of up/down play.
Direction cone is not conical it looks as if its has had some damage to the bottom (6.0 clock position) It has been flattened. comparing to a spare cone we have its out by 1.5" from a circle.
richard

[Pete7]

Originally Posted by diver

We have just taken delivery of a second hand mob fast response RIB. It is powered by a mermaid 4cyl Turbo charged Diesel giving 140hp with a P.P 90 water jet. Max speed quoted as 26knots. During recent runs we have been only able to achive 15knots. We belive there is a problem with the jet unit, engine RPM and Turbo are as design. Richard

Richard is this a Pacific 22 Rib ?

Pete

[diver]

its simlar to a pacific rib. inboard mermaid 140hp diesel unit powering the jet

[Pete7]

The reason I ask is because I had a drive of this one two years ago.

http://www.boatsandoutboards.co.uk/view/EML252/

Absolute shambles, we only got 15 knots and the thing went sideways down the estuary horrendous we took it home after two miles and gave it back to the owner explaining we didn't want anything to do with it. It needed an awful lot of sorting.

Much prefer the stern drive version:

www.polar-ribs.co.uk

Pete

[phil959]

can anyone tel me at the moment i have 165 hp mercruiser at 3600 rpm with the impeller that i have now i only get 15 knots . but engine will only rev to 2500 rpm ,if i buy a lesser pitch to suit my new engine and alow it to rev to its max do you think i will get better proformence.hope someone can help me
phillip

[user1636]

they are fun but you must remember they only turn when thrust is being created. If you panic and let off the gas the boat will not turn.

[egon]

hey can anyone tell me where i can buy a new inner ring for the impeller in a p p waterjet

[Pete7]

Try Vosper Thornycroft:

http://www.vtplc.com

Pete

[ed fitz]

I recently returned from Ca. looking at several rib jet boats the gov was selling.My question is the ribs had Hamilton #273 and looked different from the pictures of Hamilton 274s.The appeared to have an open clean out and seemed to be much heaver than the 274s yet the impellers seemed similar. Does any body have any knowledge about this.