Does the prop pitch effect RPM/speed?

[Sea Mitch]

Took my prospective boat for sea trial. The 1993 yamaha 250 on a grady 232 gulfstream (4025lb) maxed out at 4900 RPMs and about 31 kph. This seemed low. The guy said that the prop had a "19" pitch and the std prop is a "17" pitch. This is why that happens. What is he talking about! HELP!

 

[Dhadley]

Re: Does the prop pitch effect RPM/speed?

You bet it effects it! So does prop type, blade configuration, blade area, cup, type of pitch (progressive, ect.) set up of engine on the boat (X dimension) and center of gravity (set back - if any).<br /><br />Going to a smaller pitch of the same type prop would help but it also sounds like an evaluation of the set up is in order to maximize performance of your rig.<br /><br />If you want to, try raising the motor up on the transom (increase x dimension) and record the RPM and speed increases.<br /><br />Good rig! Let us know how you make out.

 

[bsrigley]

Re: Does the prop pitch effect RPM/speed?

first boat,<br />Yes, the correct prop. pitch and diameter is critical. putting the wrong prop on a boat is probably the easiest and most common way to make a boat UNDERperfom. <br /><br />the "pitch" is a measurement of how far a propeller would move through the water in one complete revolution with 0 resistance. thus a 17 pitch prop. would theoreticly travel forward 17 inches if it were turned one complete time with no resistance. a 19 pitch would travel 19 inches, etc.<br />All motors have an optimal WOT (Wide Open Throtle)RPM. I don't know what your's should be, but I'm sure someone on this BBS does. You may also try contacting Yamaha.<br /><br />In theory, a 19 pitch prop. would make the boat travel faster than a 17 pitch prop. (assuming that both props are the same diameter)BUT, ONLY if your engine has enough power to turn the "faster" prop. If it does not then the 19 pitch prop. would actualy be counterproductive and make the boat slower because the motor will not be able to get up to a high enough RPM to produce the torque needed to turn the larger prop. fast enough to get the boat up on plane.<br /><br />Also, a prop. with too low of a pitch can cause problems. With a lower pitch, say a 15, the boat may get up on plane quickly, but have a lower top speed, and the motors RPMs can go too high.<br /><br />Finally, the diameter of the prop. is also critical. Larger diameter=slower take off buter higher top speed.(again, assuming no limitations on engine power) Smaller diameter= possible faster take off but lower top speed.<br /><br />One more tip: you may try contacting propeller manufacturers. they can often tell you pretty accurately what prop would work for your rig.<br /><br />sorry for the long-winded explanation. hope it is of some help.<br /><br />Good luck