TEST DAY #2

TEST DAY #2

Things have been busy at NAMJ, so we are working the testing in when we can.

Today’s test will be a two-fold project:

            1. Test a larger nozzle

            2. Measure angle of water coming off the impeller and entering the stator

The first nozzle we tested had a 4.8” diameter. The second nozzle was 4.425” diameter (.375” smaller).  The test showed a decrease in bollard pull with the smaller nozzle. Yes, I know bollard pull or static thrust doesn’t tell the whole story, but that’s all the information I have for now.  I need to know the horse power going into the jet at each of these data points to get a true picture of what’s really going on.  The #2 nozzle may give 1% less thrust at a given RPM but at 2% less horsepower, thus it could be a better choice. We can measure HP when we convert over to a 38 volt DC motor.

Back to the testing; since I went .375” smaller with the #2 nozzle, I’ll go .375” larger with the #3 nozzle. We printed off a 5.125” diameter nozzle in the 3D printer in about 4 hours. It came out ready to test, no machining required.  Off to the lake for testing.

First, we performed the angle test with the pitot tube set up (reference -  back to introduction for some explanation of this). I chose 3 RPMs for this test.  At each RPM I took readings at 3 different radiuses by moving the pitot tube in or out of the mounting tube.  O-degrees would indicate that the water was flowing straight back toward the nozzle.

Here’s the data I took with the boat in the water up to the top of the nozzle while still on the trailer:

            RPM    Angle@H/PSI              Angle@M/PSI             Angle@T/PSI

            1600                30/4                             30/4                             4.25/5

            2000                30/6                             30/7                             42.5/7.5

            2400                30/9.5                          30/9.75                        42.5/10.75

H = .25” out from the impeller hub. [1.375” r]  M = about mid-stream, [2.05”r] and T = 025” inside outer housing [2.65” r].

Several things of interest show up here. First, the angles at each radius do not change with RPM. In most jets the angle increases with RPM.  I’m not sure but I believe the fact that there is no change indicates a very stable impeller hydraulically. That’s good.

Next the pressure, (this is only dynamic pressure) increases as you go outward from the  hub and also with RPM. How about a little figuring to see how we are doing. With pumps most calculations are done using head rather than PSI.  Head = PSI x 2.31/sp.gr. Specific  gravity for water is 1. So just multiply PSI by 2.31 to get head. Head is measured in feet. This gives us 11.55 ft @ 1600 RPM,  17.33 ft @ 2000 RPM & 24.83 ft @ 24000 RPM. Since head varies as the square of the RPM, we have new (RPM/old RPM) sq x old head = new head. Using 1600 as the base you get the following:

            RPM                        Measured H              Calculated H

1600                                                        11.55 ft

2000                            17.33 ft                        18.05 ft

2400                            24.83 ft                        25.99 ft

This shows the impeller is basically following the affinity laws.  If you put a pressure tap flush with the outside housing you would see an increase in static pressure which would be the difference between measured & calculated figures. Dynamic H + Static H = Total H.

The last and most interesting thing the data shows is the angles are flip-flopped from what you would expect. The angle of the impeller blade at the hub is always greater than at the tip which is a function of tangential velocity. Therefore, the water stream coming off the impeller base should be greater than that off the tip. As you can see the base is 30 & the tip is 42.5. Can’t say I know what is causing that. For now, I’ll just make another stator that will match these angles and retest.  We’ll try to figure that one out later.

Now, for the longer nozzle #3 test, the wind was blowing 15-20 kts, 90 degrees to the launch ramp so we launched the boat and went into a cove and tied the spring scale to a tree.

Here’s the data:

            RPM                Thrust in Pounds

1240                                                         55

1620                            96

1990                            144

2210                            180

2470                            220

2620                            245

2750 MAX                  255

The jet stopped the engine at 2750 RPM. With different RPM’s it is hard to compare to the #1 nozzle. By using the scatter chart option in Excel you can plot the data and easily see how it compares. Looks like the #3 nozzle is the winner.

Jason and I went for a short ride again but with the wind we couldn’t tell much about speed. One thing that gets talked about by some “experts” is how a jet absorbs power. When Jason & I ran the boat we were going over 10 knots and the maximum RPM was 2750 --- the same as our maximum RPM at static.

How this can be, you may ask.  Well, basically if a jet impeller is not cavitating it will absorb horsepower at the cube of RPM.  Head varies as the square of RPM and flow or Q varies directly with the RPM.  The reason the impeller requires horsepower is to increase the head across the impeller. We measured 24.83 Ft H at 2600 RPM at 0 boat speed. If I had measured in the same place at the same RPM with the boat going say 10 knots the head would be around 26 ft or so.  Some will say that would show up in a different RPM at speed.  Not so, because the head rise across the impeller would still be 24.82 ft.  Thus, requiring the same HP input thus resulting in the same maximum RPM. The difference in head would be the intake head going from negative H at static to near 0 at 10 knots.