I just purchased a used boat, 1997 Glastron SE 170 bowrider with a 1997 Johnson 130 outboard and it came with 2 different, 3 bladed aluminum props. 14.25x19 and 14.0x21. Which one is geared lower for more pulling power vs top end speed ? I assume the 19 pitch but not sure how the bigger diameter fits into the picture. I haven't had a chance to try them yet so don't know what the difference in RPM might be. Thanks
The difference between the area of a circle of 14" vs 14.25 isn't much: In percentage is 3.5% . The difference between 19 and 21 is 10% without any regard for the actual physics of the effect of each on prop thrust in water which really is what interests you. Real answer to your question is how does each affect the performance of the prop.
Usually pusher props are larger in diameter and smaller in pitch to get the desired thrust......pushing heavy loads. Speed props are usually smaller in diameter and higher in pitch, made for pushing light loads fast. Then comes things like blade area (regardless of D and P dimensions), and "Rake"....bending the blades back from the root to accomplish desired "tuning" results in maybe bow or stern lift, and then cupping to hold better in shallow mounting installations, or hard turns at high speeds and all that engineering stuff prop designers take into consideration, like achris mentioned.
Props are gears with some slippage due to the medium...water vs wood or metal among other things. For which does what, less pitch, less distance travelled per revolution, less power required per revolution, less speed but more power available to keep the prop turning (developing thrust) under increased load....like water skiing.
