1. ## Impeller vs Propeller

There is a common misinterpretation about how an impeller’s degrees of pitch are directly related to inches of travel the boat will produce. This is not true. A propeller’s hydrodynamic characteristics have been described as moving almost “like a screw through wood” by marine propulsionists. . Attached is a graph from Mercury Marine. .

However, a jet boat/pwc will not move until pressure is discharged from the pump nozzle. The degrees of pitch on the label of a pwc impeller are simply degrees of angle at the OD and are in no way an estimate of travel per revolution. The pitch “label” on a pwc impeller is best to be considered as just that. A “Label” because it really only represents two elements (of many) that determine an impellers profile. In addition, each mfg labels their impellers differently. They use different elements to best describe the form of the function and its general nature.

As if to say, y in f(x) = yx, where y determines only the slope of the line described by f(x).

Root angle, blade length, blade size, overlap and hub are also very crucial components to consider in determining an impellers profile. Nevertheless, moving the OD at any point of the impeller, at any part of the blade is changing the pitch and depending on where you altar the pitch will change the characteristics of your jets propulsion. The leading edge affects volume, pressure and hookup. The trailing edge affects RPM, bottom end acceleration and top speed. The pitch of your impeller is only what represents the measurement at the OD and nothing more. You should consider all of the above mentioned elements before choosing your impeller.

Hope this helps,

G
.

2. my propeller graph didn't come out..

see if this works.

3. Originally Posted by g-fly
However, a jet boat/pwc will not move until pressure is discharged from the pump nozzle.
There are two variables which determine the thrust provided by the jet pump, the mass of the water flowing through the pump, and the change in velocity of the water relative the speed of the boat. Pressure is only a byproduct of the pump, and cannot determine the thrust. In fact, after the water exits the pump nozzle, there is no pressure, only mass and velocity.

The equation for jet thrust is:

F = M (V2-V1)

where

F=thrust
M=mass flow per unit of time
V2 = exit velocity
V1 = inlet velocity

If you can increase M or V2, then you will increase our thrust, and consequently the acceleration and top speed.

Michael

4. Thanks Michael.
Gerrit was trying to give an example in terms most people can relate to, in order to answer a lot of very basic questions that we get comparing the two types of drives. The point in this thread being to give an understanding of how very different jet pumps and a boat propeller are.

Pressure may very well be a byproduct of the pump, but velocity won't increase without the nozzle creating the pressure. Considering a pump is up inside the hull behind the intake duct, there are also a whole lot of variables that affect the "mass" of water getting to it. Real world results with many variables is a little different from well known equations on paper.
V1 changes with various intake grates, duct sizes, etc. You can't increase V2 without the nozzle- take it off your ski and see how much "F" you get. This is the point in comparing a propeller and impeller on the original thread.

Point is, our goal is to try to help the majority of questions with the benefit of a forum like this, alocating a reasonable amount of time to it. This type of post helps reduce lengthy phone calls and time taken that we simply can't afford to take and have a profitable business. We certainly can't cover all circumstances, or hold a class lesson, but we try to help as best we can.

Regards,
David