Jason & I took the 10 HP jet to the water today and things went smoothly. The launch ramp at the lake has a gentle slope to it which enables us to do a lot of testing with the boat still tied to the trailer. This was very handy today since we have no steering on the boat.
First off, I found that with the center console, 5 gallons of gas and just me in the boat, the jet would not prime. As with all pumps, water must be up to the impeller center or above for the unit to start pumping water. I only had to shift my weight to the back of the boat and she caught prime and pump OK. It may be hard to see in the picture but there are slight indentions in the jet stream leaving the nozzle. These are caused by the stator vanes stopping just forward of the nozzle. By looking closely you can tell those indentions have a slight twist to them as they move away from the nozzle. This twist is going in the same direction as the impeller is turning. This means that the stator vanes need a larger discharge angle. Since the stator is still the ABS plastic part, we can change the solid model drawing and print another stator with a larger turning angle for test day #2.
One good thing I saw in looking down at the jet stream was that it was clear. I could see the red tail cone through the column of water. If the jet was not “happy” there would be tell-tell signs that the water was getting mixed up on its way through the jet. There would be varying degrees of white streaks in the jet stream. As I said, no white streaks here - at least not at fast idle.
OK, now for some thrust data to see if this little guy will do its thing. After seeing that the motor would run, that the drive line was going to run smooth and looking at the jet stream, we pulled the boat out of water and hooked up the thrust bridle. The bridle is two ropes with snap hooks that hook onto the tie-down eyes on the transom. The ropes are about 4 feet long and are tied into a common ring. We then hook in a 150 foot tow line to this ring. By adjusting the length of one rope we can get the boat to stay in one spot while thrust tests are being run. We launched the boat with the tow line attached to the ring and the other end tied to a 300 pound pull scale which in turn was attached to the tow truck.
The little digital tach had about a 2 second delay to get a steady reading so I just set the hand throttle and let the tach catch up and that’s where we took our readings. I recorded RPM and Jason recorded thrust at the truck.
Here are the readings:
RPM Thrust in pounds
1350 60
1620 90
1950 120
2040 140
I thought they were OK figures. I couldn’t hear any noise from the jet nor could I feel any excess vibration by placing my hand on the casing outside the impeller.
The only real surprise from this test was when I snuck back to take a peek at the jet stream at 2040 RPM……. CLEAR! I could still see the tail cone through the stream. That’s pretty rare!
I didn’t want to go any higher in RPM for fear of breaking the plastic stator vanes before we ran another thrust test with a smaller nozzle.
We came out of the water, took the 4.8” diameter nozzle off and installed a 4.425” diameter nozzle. To get some prospective of what we are up to here draw a 5 ¾ circle (the size of the impeller) and inside that draw a 4.8” circle (the size of the nozzle). Now you must admit, that’s a huge nozzle. Since it is so big I thought it best to try a smaller nozzle to find out where we were.
Here are the results with the small nozzle:
RPM Thrust in pounds
1350 55
1620 87
1950 112
2040 122
2300 150
2660 182
Well, looks like we need to try a larger nozzle rather than a smaller one.
The stator was still intact, so I unhooked the bridle and went for a ride. Our GPS decided to not find any satellites so no speed was recorded. Since this jet is designed for best performance around 10 MPH, any thing over that is gravy. We ran it up to 2600 PRM again and were above 10 MPH I believe. The Most impressive speed was about 3 to 4 MPH at only 1100 RPM.
I had to point the boat with the paddle and take a short run and paddle again, but got a good feel for the jet.
I got back to the trailer and picked up Jason, who’s a big boy. I’m around 215 and he is around 250. Couldn’t tell any difference in speed with the extra load!
For the first time out we had a good test with some good data. We’ll make a larger nozzle and test again. Using the best nozzle, we’ll measure the angle of the water coming off the impeller on the next time out also.
We will let you know.
--Len
TEST DAY #2
