View Full Version : physics of prop dynamics???

Ole Miss Rebels
11-01-2007, 12:10 PM
i put a new prop on my 2005 xstar and it drove entirely differently(better). i went from a 13.7 x 17 stock to what eric at oj suggested (14.25 x 14). wow, what a difference. it had tremendously better hole shot, smoother cruising at wakeboard speed, better lock-in on perfect pass, slightly higher rpm, from 3100 up to 3300 rpm at 21.5 mph. and modest gas consumption increase. i know the first number is a measurement of diameter of the prop and the second number is the number of inches the boat moves forward with one revolution of the prop.

my technical question is? what change does diameter effect on the performance of the boat and how do you adjust the pitch when you adjust the diameter? secondly, what does the "cup" mean and what does it do? thanks in advance, i just would like a basic understanding of how props work.

88 PS190
11-01-2007, 12:22 PM
With the Prop you are balancing different aspects. Diameter adds surface area, steeper pitch results in more forwards motion per turn than a shallower pitch prop. Cup is almost like a kicker at the end of the blade as if it got steeper all of a sudden and changes the way a prop reacts.

When you select a prop you are selecting for a prop that isn't so steep that you are overloaded and the engine cannot respond by getting into the power band, yet is steep enough that you get the speed you need in the RPM zone, and that you don't run too many rpm to cruise. Your slightly larger new prop, though not as steep will allow the engine to spin the prop easier, and it should slip less. As a result you gain holeshot. Once you are up to speed you notice the change in RPM, bigger prop would lower rpm, less pitch means more rpm so you ended up at 3300 rpm at speed. Note this means your top speed dropped.

For alot of reasons props are selected specifically to application. If you spend alot of time with loaded ballast, full fuel tanks, and alot of people and board as a primary purpose then it makes sense to move the direction you choose. The guys at OJ and ACME always steer people correctly.

People who have direct drive and slalom alot would choose differently.

Either way these new props work alot better than the old cast props.