Thread: 15/20 vs 13/19
03-22-2009, 04:23 PM #1
15/20 vs 13/19
I see a lot of people running a 15/20 in stage 2 and 2+ skis. What are the differances and what other tuning is needed to use the 15/20?
All the advice i get is to use the 13/19 at stage 2 because it is a drop and go situation.
I woul like ot learn more about prop application and get the most wholeshot out of my bike.
03-22-2009, 07:48 PM #2
I had a 4 blade 14/17 pitched to a 14/21 and then went to a 4 blade 15/20 pitched to a 14/21 and WOW. Huge improvement in overall performance. Here is a thread with some testing I did and some good posts on there http://greenhulk.net/forums/showthre...light=impeller If you want more holeshot from what I hear a 3 blade is the way to go.
03-22-2009, 08:30 PM #3
thanks for the help... all the bending and pitching and.... i'm still confused. I know you want particular pitch to run ideal rpms based on what your motor is capable of, but i have no clue exactly where i should be and what tweaks to do to get there.
Is the stock tachometer accurate? If so i am running 8200 rpm in 55 degree water 60 degree air.
Finally got the motor figured out, but still have a lot to learn about props and tuning!
03-22-2009, 09:56 PM #4
03-23-2009, 08:17 AM #5
the one peice of info i need to understand is does raising the numbers on the pitch increase or decrese the actual pitch. Secondly, how does raising/ lowering pitch effect RPMs? does more pitch mean more RPM's? vise versa?
I understand that i have to establish the correct RPMs for my ski and mods and then find the correct prop to to make everything work. until i understand the effects of pitch on RPMs i can't dop that. I also don't understand why some would run a 13/19 on a stage 2 and some a 15/20. It seems like a drastic difference to me.
03-23-2009, 09:36 AM #6
You might want to read this thread. It hwlps alot:
Thr rule of thumb is 1mm (at the training edge) = 100 rpms. So in your case you would have to bend the trailing edge of your current impeller +1 mm to drop 100 rpms to 8100. If you are currently hitting the rev-limiter at 8200 rpms you may also be burning up the tip of your J-pipe.
Since you are adding an external IC and changing impellers, you will need to see what rpms the ski does after the changes and bend from there... Ron
03-23-2009, 10:41 AM #7
what do you mean by bruning the edge of the j pipe? I did the Rotax waterbox mod and it called for cutting some length off of the pipe. I imagine that the additional airflow may be causing additional RPM's ?
03-23-2009, 11:25 AM #8
If you are on the limited and continue to run it that way full throttle up on the limited he meant that you would burn the tip off the exhaust manifold and it doesn't take long either so if you rode it that way much you better look at your tip to see!
03-23-2009, 11:58 AM #9
A few things ...
- you cannot compare 3 blade to 4 blade impellers ... they are totally different and pitches are NOT comparable.
- Each mfr makes different styles of props, with different blade design, blade overlap, etc, etc, etc and you cannot compare advertised pitch numbers on props from different manufacturers.
- Each prop mfr (Solas or Skat primarily) will label and measure their props differently even if they happen to look similar.
- Different prop shops will tell you that the measured pitch rarely if ever matches the advertised pitch from the mfr.
03-23-2009, 12:44 PM #10
if that tip is burnt up what is involved with repairing/ replacing?
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