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View Full Version : Prop Size / Gear Reduction / theory v actual


PendO
09-11-2005, 12:29 PM
Okay, so I have been trying to wrap my brain around the "theoretical" preformance differences caused by prop sizes. From the usawaterski.com website I can see what boats are AWSA approved http://www.usawaterski.org/graphics/downloads/2005AWSAApprovedTowboats.pdf and I found a good site to explain the basic prop terminology: http://www.acepropeller.com/clinic.asp , and a waterskimag.com online article about props and powerslot http://www.waterskimag.com/article.jsp?ID=12547

So ... is top speed reduced by powerslot/gear reduction in the M/C's and CC's, or is it the design of the hull that stands in the way of 50+mph? (Can I / Should I get a 13.7 X 17 for days when we are crusing, not skiing?)

Theory question: If you had two props with the same diameter, but one was a 13.7 X 15 and one was a 13.7 X 17 ... if they used the same gear reduction then the 13.7 X 17 would be faster at the same RPM's, right?

Looking at the engine/gear reduction combos for M/C I see that in 2005 the 15 is on the MCX 1.26:1 and the 17 is on the 1.56:1 ... when topped out does the LQ9 turn more RPM's than the MCX? Why not put the 1.26:1 in the LQ9? (did that change for 06?)

If someone wanted more mph in the 197 with MCX could they put on the 13.7 X 17 ... it would decrease the holeshot (increase the pitch), but once you got up to speed you would see a 13% increase (17/15) in the distance that the 17 should move in the same # of rotations as the 15?

Thanks for the input, time to watch some football;)

Editing - http://mymastercraft.com/Forum/view_thread.cfm?postid=2347&forum=2347&category=2 ... I like this thread on the mymc board, although I need to use a handheld gps to calibrate b/c our spedo never shows 48mph in the X-Star with the 14.5 X 21;(

PendO
09-11-2005, 10:32 PM
bump..........................................

whitedog
09-11-2005, 10:43 PM
What you are gussing is mostly correct. The two factors that you will not know forsure until you try differnt sizes is how your RPM will be affected. Often when goint to a higher pitched prop you will loose 200 - 400 RPM at WOT. This may or may not contribute to a faster top end. All depends how close to max RPM the boat runs now at WOT. Crusing economy should be better with a higher pitched prop but if you try to pull multiple skiers it will make a big difference in hole shot. Best way to find out is to demo several sizes and makes, they all perform differnt. Also a 4 blade vs a 3 blade vs a 3 blade CNC all can make big changes with in a given diameter and pitch.