09-17-2008, 09:17 AM #1
3+ mph gain with 4 blade vs 3 blade prop
I have been running a pitched-up 15/19R 3 blade on my RXP all summer (mods in signature, basically a full Stage 3 plus X Charger) and turning 8400 rpms with 83 mm (stock) nozzle. I finally got another GPS last weekend and was only running 78.2 mph at 8400 to 8450 rpm. I was disappointed at the top speed, but it pulls really hard and ran great all summer …
Well yesterday I swapped pumps and put in my other Skat-mod pump with a 15/20 4 blade. Carl at Island had trued this one up and pitched it up a bit as well; based on one test early in the summer I thought it would need to be pulled back a little, so I had pulled the trailing edge back about 1.5mm. Water and air ARE cooler now, about 65 degrees on both, but that was about the same as last time I ran the 3 blade too.
I threw the stock nozzle on, put the GPS in the top compartment and blasted off. Later I checked the GPS and max speed said 81.1 mph … I was surprised so I reset it and watched it … and clicked off an 81.5 mph! I was down to about ˝ tank of gas by then too. I was also ALL over the rev limiter on the Riva ECU, bouncing hard off 8600 anytime I pinned the throttle. I ran out of time but will swap to my 81.5mm nozzle this weekend to get the rpms down; I imagine I should easily be able to run a solid 82 mph if I can get the rpms down to 8500 – 8550 range.
I know my speeds are not amazing, many have done that and more with similar setups and cool air and water, but the difference in speed with the impeller swap really surprised me. With the right nozzle, I might actually be closer to a 4 mph gain by switching to the 4 blade.
No question part of the speed increase was due to the fact that my 3 blade is too steep, but it still turned 8400 rpms, so I thought the difference between the two would only be 1, maybe 2 mph. I was surprised and wanted to post this on the forum …
09-17-2008, 09:20 AM #2
That usually happens. 2mph seems to be the norm. I just got a 4 blade too. Did you loose holeshot a bit, because that is what happens....
09-17-2008, 09:25 AM #3
- Join Date
- Dec 2006
- Crest Hill, IL
With your setup, a 15/19 might not be enough prop...Have you tried the 15/22 3-blade? I have ridden a S3 boat with that prop, and it pulls as hard as the 15/19, but has the topend of the 15/20...
09-17-2008, 09:25 AM #4
great job vern!! How much did you loose on the holeshot? I have been thinking of useing a 15/19 for drag racing...
09-17-2008, 09:25 AM #5
Did you swap the other one back and get a verification??
09-17-2008, 09:36 AM #6
Vern, i have the same like you...In our GTI i tried all kinds of impellers and the best results ever was with Solas 15/20 4 blade (untouched) For example i did a test last weekend with skat 15.5/21.5 and at 8.500RPM i had 3Mph less than with 15/20 at same RPM. Really i don´t understan very well why of this but even with a solas 15/22R i have less top speed.
09-17-2008, 09:41 AM #7
I think Hydrojunkie has a pretty good idea, but I'll let him decide if he wants to expand on his theory.
09-17-2008, 09:46 AM #8
Hey Vern the same thing happens to me when I move to my 3 blade prop. I love my 15/22 prop. It does not pull quite as hard as the 15/19 but is pretty close. Plus the top end is easily 3 mph more.
09-17-2008, 09:49 AM #9
I honestly don't know if I lost holeshot ... if I did, it was very little, the ski still pulls very hard.
I have not tried a 15/22 3 blade ... this year the only props I have tried were the 15/19 3 blade and the 15/20 4 blade. The thing is that I had anticipated running more boost and had Carl at Island pitch both of them up. It seems that the 15/19 may not work as well when it is pitched up maybe? Not sure, but the 4 blade definitely has much better top end.
I did not swap back to verify ... I would have liked to, but I have such limited time to do this kind of thing. Not a true back to back test I know ... One other variable too, the 15/19 setup is a Skat triple bearing pump with SS liner and a full season+ on it, but clearance was still very good last time I checked it. The 15/20 setup I put in yesterday is a brand new Skat triple bearing pump, but just has a plastic liner in it now (brand new). Sooo, another variable, but the SS liners generally hold their clearance very well, so IMO that is not a big factor.
Now I am excited to put the 81.5mm nozzle in this weekend and get off the rev limiter and get even better numbers!
09-17-2008, 11:39 AM #10
nice work Vern, look forward to hearing the new numbers
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