# 100 hp centrifugal jet pump design help

Discussion in 'Jet Drives' started by beef supreme, Apr 15, 2013.

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### beef supremeNew Member

I am building a jet boat as a project this year. I am going construct the centrifugal impeller myself but don't know what dimensions I need to make it to get the most out of the engines without over loading them. Here's what I have to work with:

The power source is going to be approximately 90 hp at 3600 rpm with about 130 ft-lb of torque (actually 2 45ish hp, 724cc v-twins 180 degrees out of phase run off a single high performance carburetor off a 1500cc motorcycle). That power is going to be transmitted by two B section v-belts and pulleys. What I'm trying to accomplish is to get as close to 90 hp out of a water jet as possible. I can do about 50 square inches at the input and output of the impeller and a nozzle of about 12.5 square inches at the rear of the boat. My question is what diameter do I want to make the impeller assuming the circumference times the height of the opening at the edge is equal to the intake (50 square inches) and what rpm?

If anyone has a 100 or so horsepower centrifugal jet boat a few specs would be so awesome. inlet area, nozzle size, impeller dimensions and impeller rpms

Any insight on the matter would be greatly appreciated. Thanks

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### BarrySenior Member

Hi
Jet thrust 101
The thrust that a jet pump delivers is due to the force that is developed from accelerating a mass, the water , and changing the velocity of the mass, acceleration
F=mass times acceleration (again simplified as the force is continuous then the mass is really mass flow rate)

So you can get the force by changing two variables. The amount of mass (water) that you move or change the acceleration from inlet to outlet.

There is a chance that you do not understand the term Centrifugal impeller.
Basically consider 3 major pumps ignore the stator issue at this time

1) Axial flow, where for simplicities sake is one or two or three propeller shaped impellers contained in a housing that moves a lot of water without a lot of acceleration,
or Force = a lot of water times a smaller acceleration

( an aside, one way to develop a higher outlet velocity is to build pressure in the pump housing just ahead of the nozzle and Axial flow propellers, impellers due not build pressure efficiently as the flow is more directed axially down the length of the axis of the pump shaft. This is due to the hydrodynamics of the foil crossection of a blade of the impellor)
You can look at the old Hamilton 773 or Kodiak two or three stage pumps to see an example of an axial pump)
I will call this a high volume low pressure pump

2) Mixed Flow Pump
In this pump, the water is not only moved down the axis but also is pushed radially to the outside edges of the pump body which creates more pressure so that this pressure can push the water out through a nozzle at a higher velocity. So typically the mass flow rate is less than an axial pump but the pressure in the pump body is higher which increases the water speed
Force = less mass flow rate than the axial but more outlet velocity which means more acceleration
Look at a Berkley pump impellor
We would call this a medium volume medium pressure pump

3) Centrifugal pump
Generally with this type of pump normally the water would enter the middle of the impellor at its axis and all of the water is pushed centrifugally outward to the edges of the pump case which increases the pressure significantly. This higher pressure is able to push the water out the nozzle at a higher velocity than the other pumps. Unfortunately, this high pressure comes at extremely high efficiency costs, ie higher fuel burn to the force generated compared to the other pumps above.
That is why you do not see centrifugal pumps in jet boats.

Some history, John Hamilton, recognized as the first person to put a pump into a boat used a centrifugal pump and it did not work well.

I do not know of a single company who builds a jet pump for boats. There are too many issues that such a pump would present.
The fuel cost issue, debris issues as a centrifugal pump that builds high pressure will be subject to cavitation in the presence of debris in the pump, (even mixed flow pumps exhibit this event), ventilation problems, if you suck in air entrained in rapids or air launching the boat, it will be hard for the pump to reprime when it comes back in contact with green water, again mixed flow pumps have this problem.

So you should reconsider a centrifugal pump as your thrust source.

Your design should also consider a stator design to gain efficiency especially if you are putting in more than one stage.

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### BarrySenior Member

a typing error
When I wrote that " I do not know of a single company who builds a jet pump for boats, I meant who builds a centrifugal jet pump
line 5 up from the bottom

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### baeckmoHydrodynamics

The Gills thrusters are entirely axial impeller units, not radial ("centrifugal").

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Yes according to the web site now.
They used to have ones from around 100hp that had a centrifugal pump mounted vertically and suction was is the centre and a radial exhaust that was directional so they could produce thrust 360
I have a photo somewhere

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### BarrySenior Member

Please post the picture of the centrifugal pump if you can

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after opening my mouth , this could well have an axial flow pump above it, it was sent to me saying it was centrifugal? Might have to eat my words?

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### baeckmoHydrodynamics

Naah, but I think Voith had something such. Anyway, the point is that the characteristics of centrifugals are unsuitable for propulsion since they produce low flow for a given power, which results in low thrust per engine kilowatt. This has been shown in other threads under the "Jet" theme. I suggest Barry should check there first!

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### BarrySenior Member

Baeckmo
I did not catch your point when you said that I should check another thread first
The originator suggested that he was going to use a centrifugal pump
My response was was that of the three pump types, the centrifugal is the poorest choice, with poor efficiency, and other problems
I suggested that the originator should reconsider using the centrifugal pump that he wanted to use
We have built a large number of jet powered boats and prefer the axial flow over mixed and found them, when properly matched ( impeller to horsepower) that they are more fuel efficient than mixed and cause less issues when injecting debris

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### baeckmoHydrodynamics

Sorry Barry, I was adressing the "beef supreme" regarding his/her choice of pump type. I fully agree with you regarding the axial pumps, and have explained the technical reasons for this in various threads about jets.

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### tom kaneSenior Member

Hamilton jet and others tried centrifugal pumps and some had twin pumps side by side a European model .

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if you need to change the flow direction by 90degrees does a centrifugal ever make sense?

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### philSweetSenior Member

There are some engineering considerations as well. In an axial pump, the thrust is transferred to the shafting. In a centrifugal pump, it isn't. Sometimes, that is important to the selection process. It certainly needs to be considered when designing stuff.

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### BarrySenior Member

Tom Kane
Thanks for the pict
Years ago I had an old black and white VHS film of Hamilton running the grand canyon of the Colorado. Then I lent it out and it never came home. On that tape was a documentary of Hamilton's development of the jet boat and the prototypes that he had built to get things right including the centrifugal pump
Phil
In an axial pump the thrust is transferred to the shafting
Also in a mixed
But in both axial and mixed there is also thrust transferred to the pump housing as the water flow is ran into the stators.
The stators which are fixed and part of the pump, change the direction of the flow.
When you change the direction, you are changing the velocity of the water, and acceleration is a change in velocity. When you have a mass flow (rate) and accelerate it, you get a corresponding force on the stators

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