Thread: Prop/rpm question
10-29-2014, 08:54 PM #1
So ran my ski all summer with a 15/21 prop and 83mm nozzle, it would gps 77.5 mph with a full tank at 8380rpm. Ski has a riva ecu, fizzle he cooler, and et137 etc. I sucked up a rock and bent the prop back in September. Bought a 15/20 prop trying to get my rpms up and tested it yesterday. Ski ran 75.4 with a full tank and turned 8620 rpms. How come I lost 2 mph with more rpm? Also what's the rev limiter on a riva ecu? Thought I should be hitting it. I was hoping with the cold water and 15/20 I would see 80mph.
10-29-2014, 09:00 PM #2
Oh yeah this is my 07 rxp
10-29-2014, 09:08 PM #3
I'm still trying to figure out this whole prop thing on ski's. On a boat the pitch of a prop is rated by how far it should travel on a spin in other word a 17 degree prop should move 17 inches per spin a 21 degree prop should move 21 inches. On a boat a higher degree prop has a higher top speed with less hole shot a lower degree prop has more hole shot and a lower top speed not sure the same applies on these a tleast it wouldn't seem to be not the consensus I have seen on here interested to see the responses
10-29-2014, 09:12 PM #4
Here is the explanation on boat props once again not sure if it applies here
When it's time to buy a new propeller, diameter and pitch are the watchwords. To be able to select or upgrade to a new propeller, you must understand these terms and how they translate to real-world performance.
Diameter is the distance across the circle made by the blade tips. In general, "Diameter is determined primarily by the rpm at which the propeller will be turning and the amount of power that will be delivered to the propeller," according to Mercury Marine's manual, Everything You Need to Know About Propellers. Diameter "usually increases for propellers used on slower boats and decreases for faster boats," the manual continues. Further, "If all other variables remain constant, diameter will increase as power increases; diameter will increase as propeller rpm decreases."
Pitch is defined as "the distance a propeller would move in one revolution if it were moving through a soft solid, like a screw through wood." For example, a 21-pitch propeller would move forward 21 inches in one revolution.
Think of a propeller as you would a car's axle ratio. The lower the ratio, the more pulling power from a standstill. The same is true with a prop. The lower the pitch, the better your hole-shot. However, this comes at a price: top speed. The lower pitch makes the engine reach maximum rpm at slower speeds.
Conversely, a higher pitch will deliver greater top speeds, but slower acceleration. Be aware that lower-horsepower engines can bog down if fitted with a propeller with too high a pitch and diameter, and that can wear heavily on internal engine parts.
If you're changing pitch on a recreational boat, remember that each inch of pitch is worth about 200 rpm. Lowering the pitch will increase rpm and vice versa. For example, going from a 23 pitch to a 21 pitch will increase engine rpm by about 400 revolutions.
10-29-2014, 09:41 PM #5
As i understand it mate its the actual angle of the prop blades measured in degrees. For example a 15/20 prop would have an angle of 15 degrees to start with and then transition to 20 degrees at its trailing edge.
10-29-2014, 09:45 PM #6
Original post........Was the wear ring replaced?
10-29-2014, 09:56 PM #7
Boats are completely different from my experience. I played with props on both my maxum and baja. It was much easier than the skis. Skis are a whole different breed. So far I've ran 5 different props in the rxp with some crappy results in the past. I had a 14/20 before the 15/21 and the ski ran 72-73mph, it was sad. Speedfreak told me to try the 15/21 and boom the ski was night an day faster. Probably gonna call him in the spring and have him repair and repitch the 15/21.
10-29-2014, 09:57 PM #8
I did not replace the wear ring this time as it was still in spec.
10-29-2014, 10:09 PM #9
Yes I'm not 100% sure but so far your results would show that the same standard was true you reduced the pitch and the rpm's went up but the top speed went down which is the same effect this would have in a boat but once again this is a true guess not a ski prop pro by any stretch of the means just had a little experience doing it with my boat last year and didnt like the results. The rpm's went up top speed went down. I plan to do the same thing here soon on my ski I'm going to buy a lower and higher pitched prop and make a comparison just to figure out what works best.
10-29-2014, 10:17 PM #10
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