@FSH 210 Sport &
@jdkaeser
Both of your numbers match up with some of the calculations I have done after a day of boating using similar online calculators for engine performance variance. In essence, using them to compare issues with cavitation and power loss/performance relative to air temp changes throughout the day. However, I can state unequivocally that I gained several hundred RPM when switching to a higher elevation pitch on the impeller - which means I am getting further into the powerband of the motor. The motor provides plenty of power for our activities. Performing a dead water start for a slalom skiing run highlights my issue with launch cavitation - an issue of too much power for ambient conditions and my boats impeller/cone setup. Towing a 3-person tube during the hottest part of the day being when the boat struggles the most for at speed performance - when I lack power.
It is a balance that I am trying to find - my current impeller is better for towing the tube, but cavitation makes my dead water slalom starts more difficult. It is for this reason I am hesitant to tune the impeller for a lower elevation. Once I am above about 17 MPH, I cannot create cavitation while towing for any watersport.
Maybe it is easier to describe a typical summer day on the lake at high elevation and the issues experienced during each activity:
6:00 AM - On the water for slalom skiing - air temp in the low-60's and water temps in the low-70's. Cavitation likely if too much throttle applied during my launch. Once up and on plane - no issues and I can ski up to about 35 MPH (which is too fast for my ski level and ski setup)
* I wish my setup was able to put available power down better so that I could have a better launch experience. Things are great once I get the ski up on a plane.
7:00 AM - Wake Boarders go out and air temps around 70. Wakeboards launch great (more surface area on wakeboards results in much less human anchor from me on my ski). However, I can still cavitate if I don't apply gradual throttle input.
* I am happy with the performance of the boat during these conditions.
8:00 AM - Air temps in the mid-70's - if conditions permit, I will try to water ski again. Cavitation slightly less of an issue as available engine power drops, but still likely with aggressive throttle input.
* Functionally no change from my morning set other than a slightly lower probability of cavitation from throttle input - chalk this up to slightly degraded power.
Noon-5:00 PM - depending on water surface conditions and passenger inclination, we may tube. At this point, air temps are in the 90+/- range. Water temps near the surface tend to be a little higher here (maybe mid to high 70's) and I think I start to have issues with heat sinking also limiting performance. The boat really just cannot throw a tube around and drops in speed on turns and speed recovery coming out of turns is just too poor to have a really aggressive tubing experience. Some of this is due to performance related to air/water temps, but it is also largely due to pump efficiency in sharp turns and drag from the tube with hundreds of pounds of people on it.
If water conditions permit, and I try to ski during peak heat my issues change. If I have more than about 600 lbs of people in the boat and try to do a deep water slalom ski start I have been dragged as long as a full minute before the ski comes up on plane - I can tell you my forearms looks like they are stuffed with cube steak after one or two launches - basically, if I fall, I am lucky if my arms have the strength for one more launch.
*This is where I wish I could adjust my setup the most - gaining some low-end torque to get me out of the water during high temp days.
So there is the conundrum - I cannot raise low-end torque for hot weather launches OR improve mid-day tube towing without exacerbating the cavitation during low-temp morning launches. Too much power for the impeller setup in the morning and too little power to overcome mid-day heat.
I *THINK* if I go to a lower elevation impeller pitch it will improve both of these issues - to a degree.
Reasoning: Tuning the impeller for a lower elevation, in my understanding, brings the leading edge of the impeller closer to the edge of the pump housing - increasing pump efficiency and lowering the probability of cavitation. The lower-elevation pitch also acts similar to a taller gear - I lose some low-end grunt but I gain top-end. This improves my early morning starts when excess power is available and I don't need the extra low-end grunt of the higher-elevation current tune on the impeller. The improvement of pump efficiency from the lower elevation tune will also improve the delivery of power during hot temps, but I think this will be offset by the loss of low-end grunt. Maybe this is where the ribbon delete and air-filter come into play - pitch for a lower elevation and improve available HP? Or am I really just spinning my wheels - changing things just to end up functionally right where I already am?
I haven't thought about this much since my boating season ended in early October, so I am a little rusty and I welcome any more knowledgable people to cut me to pieces.
For everyone following along and providing insight, thanks for all your help over the past year. Merry Christmas!
