03-17-2008, 09:26 PM #1
- Join Date
- Nov 2006
What do these pitch numbers really mean?
Ok, starting from the basics; a Solas prop gives you a prop spec'd a 13/19, they also have a 13/20, and to keep it simple a 14/20. Some people change the pitch on the trailing edge, others change the leading edge.
My question is how to the 2 numbers (leading vs. trailing) correlate between each other?? What's the difference between a prop with a higher difference between leading and trailing numbers and one with a lower value?
Could this have something to do with the ability of the prop to accelerate the water through the pump??
Then to confuse things a bit, NuJet measures with one #. (6.5, 5.5, 5.0, etc....)
03-18-2008, 07:53 AM #2
Variable pitch props were designed to get the best of both worlds. Leading and trailing are releated to a point but usually the leading dictates the amount of water pulled in from the pump or "bite" and the trailing edge dictates thrust pressure. Variable pitch props are not the most efficient but allow a little bit of a transmission act for skis giving good acceleration along with top speed. Billy (RX951) could explain it a lot better.
03-18-2008, 07:55 AM #3
Yes, and all the numbers, leading and trailing is all dependent upon engine RPM and other hydrodynamic factors such as exit nozzle and wear ring to impeller clearance in order to process water efficiently. other factors can include; intake grate opening, engine power, stator vane size, impeller hub and root angle design ect..
If you place a Solas Concorde 13/19 next to a Solas Concorde 14/20, there will be some size on the hub and shape differences.
The Solas Dynafly is also a totally different impeller. Due to its hub design, when it is pitched up or down, depending upon how much, the OD of the impeller can change due to the shape of the blades.
Some GPR's that can produce alot of power are able to run a higher trailing edge to take advantage of more top end speed. Only if they are able to produce the extra rpm/power to take on the extra "load" on the engine. In some applications, depending upon the impeller, the training edge will get depitched liek the older concorde impellers in order to help "unload" the engine and allow it to spin a higher RPM and put the motor back in its powerband.
Some 3 seater applications can have a lower leading and trailing edge on the impeller pitch in order to allow the engine to operate at the proper RPM to compenssate for the extra "load" the motor has to work pushing the extra weight of the craft. ex. 12/18 or a 13/19.
Now, if extra power to be added such as high compression heads, VF3 reeds and some pump shoe or hull work to lessen drag, then a 14/20 can/will be applied since modifications have been added to produce more RPM & power.
Every engine has a powerband that is needs to operate in to prevent detonation and to maximize engine efficiency. When it comes to jet pumps, which are not that efficient, it takes higher RPM's to allow the motor to build its most power.
Now for PWC that run closed course, they want maximum "bite" as Duke mentioned and they will typically run a pump/grate/handling combination that will allow the craft to stay hooked up all the time. Their impeller pitch numbers will also differ compared to a top end PWC.
Last edited by RX951; 03-18-2008 at 08:19 AM.
03-18-2008, 04:22 PM #4
- Join Date
- Nov 2006
Thank you gentlemen, that makes things quite clear.
Ps. how do you compare a NuJet which has a different pitch value; in this case a 6.5?
03-18-2008, 05:05 PM #5
03-18-2008, 05:12 PM #6
6.5 means it is not a variable pitch impeller. The pitch or angle carries through out the impeller at the same angle. I think 90 % sure.
03-18-2008, 06:31 PM #7
03-18-2008, 07:04 PM #8
- Join Date
- Nov 2006
Thanks for your post. One thing I'm a little puzzled about is the reduction in leading (bite) edge pitch. Some take a 14/20 and change it to a 13/20; would this mean you still need the extra hp to run the trailing edge? Why is the leading edge descreased; would you not want the most bite as possible (hp permitting)?
03-18-2008, 08:45 PM #9
To low, and the motor will burnt he prop out of the hole. Similar to spinning your tires from the starting line in a car. You want to be able to load up the motor with your final mods just enough to bite hard and give your ski a good holeshot with minimal prop spin and pull through the powerband without any flat spots or dull midrange.
If the motor does not have enough mods to allow it to rev quick, depitching the leading edge will help it spin up quicker.
also, I've seen where the training edge is too steep say a 22, and they will depitch it to a 21 or a 20 to take some load off of the engine at top speed. the motor may be lugged down say in the 6950 rpm range and they need to bring it up to 7100+. so depitching the trailing edge can allow the impeller to to "unload" some necessary rpms.
Watch those RPM's in the cooler months vs. the hotter months. One can typically see anywhere from a 80-150 rpm drop in rpms going from cooler months to the summer hot days where the air is thinner. In the winter the air is dense [more molecules of O2 available] and motors will turn more rpms. Each motor varies as will the from gain/loss.
so, when you are tuning an impeller to your motor, keep track of the modifications you have, time of year, weather, rpms ect....
I also highly advise a good digital tachometer when tuning your engine.
Good experienced tuners have the know-how to tweak that prop down to the +/- in a 50 or less rpm window. Not knowing what your doing pitching your own prop can possibly lead to engine damage and/or dissatisfied results.
Its common for some people to have a winter prop and summer prop.
Depending upon the application, rpms can be altered by changing the reduction nozzle also.
Some of the 4-tec owners [RXP/RXT] have adjustable reduction nozzle rings to adjust/tune their rpms
Last edited by RX951; 03-18-2008 at 08:50 PM.
03-18-2008, 09:04 PM #10
I don't know what pitch Nu-Jet 6.5's start out with but I had one pitched to a 13/19 by SpeedFreeks and set back. That impeller worked excellent.
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