03-25-2008, 05:11 PM #1
Measuring thrust and pump pressure
Check out this link: http://marinetoys.ca/mtoys/product_i...5ddd9da703eda0
This digital gauge measures prop slip on the fly to help ensure your hull, drive and propeller are dialed in for your particular boat/engine/drive combination. The prop slip formula is available free all over the net and many performance boaters have used it to determine optimal setup on their boats; the advantage of the gauge is it does it on the fly and at varying speeds rather than the very manual process with just the formula. But the gauge ain't cheap, and it don't work on jet pumps ...
What I have been wondering about is the jet pump equivalent to this though. Theoretically the more thrust we generate the faster we go, if everything else remains the same. So can we measure thrust ON our skis? Can we measure pressure inside the pump venturi? Can we measure water flow in gph? What can we measure and what would give us some indication of the impact of different impellers, nozzles, etc?
I did find that you can buy manual water pressure gauges that will measure water pressure from 0 psi all the way up to 3,000 psi for less than $50. Most use a 1/4 npt fitting and some have a 'lazy hand' that records max pressure attained. Other more sophisticated electronic ones are available too for more money but since our 'environment' is a bit wet I focused more on the manual gauges.
Just curious if anyone has attempted to measure thrust, gph, pressure in psi, etc for our pumps. Anybody know the approximate gph or water pressure a typical RXP achieves in the pump?
Arguably if we had two identically prepared RXPs that are 2 mph different, and we measured the pump pressure and found it identical, we could then conclude it is a hull/hydrodynamics issue and focus on that. If we found the slower one had less pump pressure, then we could focus on pump/impeller/nozzle (and/or engine) issues. In other words if we KNEW pump pressure on similar skis, we could then pursue secondary speed improvements in the hull/grate, etc. By knowing and comparing pump pressure we could potentially come up with our own modified 'slip' formula, which actually measures hydrodynamic efficiency of the hull.
03-25-2008, 07:23 PM #2
- Join Date
- Aug 2006
- Gonzales, La
There you go again. . . trying to bring out hobby up to a higher level.
In short, I think that you are absolutely correct. Getting more data on the pump will tell us much more about prop selection, diffuser selection, grate design, etc. . . than speed testing.
I think it would be relatively easy to measure pressure upstream and downstream of the impeller. Downstream pressure is proportional some kinda way to thrust.
With all the prop testing I've been doing and will continue to do, it would have been great to put this data together for all that.
03-25-2008, 07:35 PM #3
03-25-2008, 07:38 PM #4
- Join Date
- Nov 2007
- Odessa, TX
03-25-2008, 08:21 PM #5
you could also measure upstream pressure before the impeller in the pump tunnel, to determine max load before overstuffing.
03-25-2008, 10:11 PM #6
Mike, the problems you ran into are what made me sit back and think 'there has to be a better way to determine low speed/speed loss' ... hopefully we can figure out a way to do this a little more scientifically. I know speed changes do prove out the same thing, but there are so many variables it would be nice to have a somewhat standard way to measure pump pressure.
The pitot tube concept is good, but not sure how/where to rig that and unsure if it would hold up? I was thinking more along the lines of 1/4" npt threaded hole ... and there is a place that looks tailor-made on the upper left top of the nozzle ... nice big flat thick boss just waiting to be drilled and tapped. My pump is not in the boat now, so I am not sure how much room there is above the pump ... might need to drill, tap, then run hard line or SS flex hose out to a good place to mount a gauge.
Measuring and relieving pressure ahead of the impeller in the pump tunnel would be tougher ... unlike the GPRs with the 2" pump extension our RXPS just don't have a good spot to drill and mount relief valves.
03-25-2008, 10:27 PM #7
This is a really interesting concept. Definately makes sense to measure the high pressure side of the pump, but you are not accounting for volume of water being discharged. I would think that you would need to represent your findings in PSI@GPM or something like that.
On the rxp, you guys could drill and tap the pump for guages or relief valves into the wear ring just ahead of the prop. there is plenty of room there. On the downstream side, you just need to settle on an exact spot in the venturi if you want to compare notes. I know that on the yammi, the visibility spout nipple would be perfect for that
03-26-2008, 07:39 AM #8
- Join Date
- May 2005
You have to be careful on the discharge side - that physics thing and bernoulli or some other guy - A perpendicular port may caause suction and not pressure. I think you need a pitot tube facing the force - The polaris pwc''s had a pitot tube right under the driveshaft of the intake side that was very good at measureing speed. In theory - rear discharge pressure should relate to speed - except that the hull or the intake bubble could be causing drag. Like why filled ride plate holes increase speed - pump pressure should be the same ? .02
03-26-2008, 10:07 AM #9
Addicted, you are right, there is room to drill and tap into the wear ring area ahead of the impeller to measure (and maybe relieve) pump tunnel pressure ... I am not sure if that would benefit our RXPs since we don't seem to have the 'buck' to worry about, and that seems to be the primary reason to relieve pressure there.
On the thrust measuring, probably the 'simplest' is by positioning the ski in a pool and tying it off and measuring its 'push' or thrust against a stationary surface. But that does us no good, too expensive and complicated.
Tim, good point, you are right that a perpendicular hole is not great for measuring water pressure when its going by so fast ... and that made me wonder about a different way to measure. The Yami and S-D stock bilge pumps use a tube that creates suction due to water rushing past ... I like the Yami single tube setup far better, it has almost zero variability, and what I wondered is if we put a very sensitive VACUUM gauge on that, would more thrust and GPH register as additional vaccum? If so, that would be by far the simplest way to measure water volume and speed to a degree ... this is similar to what many racers use to evacuate crankcase pressure too, they mount a venturi tube just downstream of the collector in the header and the vacuum created results in PCV.
03-26-2008, 11:03 AM #10
Isn't this exactly what JD1 experienced as he got his RXP to astronomical speeds and isn't this exactly what is happening with the enlarged inlets on the X models as people are increasing their speed as well?
Seems the more and more power we keep throwing at these things the more likely it is to become a prevalent problem...
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