Pump Volume vs. Peak RPM

[Pro-pulsion]

I was thinking about this through the course of the day and wondered about any potential data floating around. When under full throttle off of idle, even with a heavily modified big bore, can the engine ever reach max RPM and achieve a properly loaded pump? My thoughts are no and only because my theory is that the pump needs to spin up the load, the impeller needs to draw in the volume and under no real driven intake of water volume. Does this sound about right? So what I mean is, the intake grate is not loading the pump to full volume since the ski is barely moving. Therefore engine may spin up to max RPM, but would be underloaded until the ski was moving fast enough to help drive the water volume into the pump channel. Make sense? Is there any data, flow charts or like that that have shown any sort of volume measurements vs. RPM? It was an intriguing thought...figured it was worth asking too

 

[kevbo]

When measuring thrust and comparing it to the inlet/outlet area, i find find that the column of incoming water has to be moving over 20MPH. A heavy couch can feed a pump very well but a light standup will always be boiling water over the blades reducing its thrust capacity. This is why a massive inlet really helps, look a the Japanese hulls as an extreme example.

 

[Balduccimoto]

This is a good question and I have thought of this before. It would be interesting to see real world data as a measure of efficiency vs thrust vs inlet speed from various hull speeds. It's possible to do a little analysis with a CFD program to get an idea of what kind of efficiency losses there are. I tried doing a quick search and I don't think there is any information regarding this scenario because many pump analysis is with a static inlet.

I have a CFD program and there is a generic jet pump model on www.grabcad.com .. If I can miraculously find the time, I could come up with a graph like this:

This graph isn't really applicable because specific speed is the speed after the impeller.

 

[kevbo]

Install a static port in the heel of the inlet and a pitot behind the nozzle. I think you will find that for a given rpm, thrust will follow inlet pressure.

 

[Pro-pulsion]

I know the principals of aerodynamics are exactly the same as hydrodynamics. That being said, the impeller works very similarly to the airplane wing creating an area of low pressure to draw the water in. The point of the couch vs. the standup is excellent. My buddy always wondered why my SuperJet having a larger engine has a hard time passing his old beater couch. I never thought of the immediate boil effect the lighter hull has to deal with. I always figured it was just due to a larger pump channel on the couch and until now never gave it any further thought.

 

[kevbo]

It is a massive infrastructure change and a tough sell but freestyle skis would benefit running larger pumps at lower pressures. We are beginning to see the idea take hold. There's just so much existing equipment in this country. Maybe the Japs can lead the way since their stuff is produced in lower volume.

 

[D-Roc]

I love my big pump. Best part on my ski.


Dirty River Rider