# Jet Drive

Discussion in 'Jet Drives' started by Guest, Mar 12, 2002.

1. ### GuestGuest

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?

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### JeffModerator

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/The_Hamilton_Waterjet/How_a_Waterjet_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/The_Hamilton_Waterjet/How_a_Waterjet_Works.html

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

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.

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### JeffModerator

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.

Here is a portion:

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

Also following this thread http://boatdesign.net/forums/showthread.php?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/sections/ultrajet/next.asp?section=1-b as you consider your first jet drive.

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### diverJunior Member

help with P P 90 water jet unit

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

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### yipsterdesigner

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

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### PazzNew Member

pp 90

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..

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### diverJunior Member

p 65 setup

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### Pete7Junior Member

Richard is this a Pacific 22 Rib ?

Pete

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### diverJunior Member

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

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### Pete7Junior Member

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

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### phil959Junior Member

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

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### user1636New Member

driving jet boats

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.

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### egonNew Member

p p waterjet

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

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### Pete7Junior Member

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### ed fitzJunior Member

jet drives

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.

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