Home › Forum › Ask A Member › Prop Pitch vs Speed › Re: Prop Pitch vs Speed
More pitch does NOT always result in more speed. Your motor has a "happy spot" where it puts out its rated HP at a certain RPM. Typically on those old Johnrude 18s, that’s around 4500 RPM. If your motor were spinning 5000 RPM with the 9-inch prop, then putting the 10 on it might give you more speed as the higher pitch would bring the RPM down closer to the 4500 benchmark. However, if your motor were already pulling 4500 with the 9, putting the 10 on it would bring the RPM below that benchmark, thus not allowing it to reach its rated RPM/HP.
Think of it this way, you’re towing a trailer with a truck with a 6-speed transmission. You’re cruising along at 55mph in 5th gear on flat ground, akin to your 18hp revving at 4500 with prop A. You approach an uphill grade and suddenly your truck isn’t cruising along so smoothly anymore, so you downshift to 4th. This is akin to going to a power prop to tow a skier, using a prop with lower pitch. You have more load (uphill/skier) so you need to adjust the gear ratio/prop pitch to keep the motor happy. Same truck, but take away the trailer and you can now cruise along on flat ground in 6th gear, as you’ve taken away the load, akin to going UP in pitch. Hope that makes sense…
Bottom line is you need to borrow that tach and get a handle on where your RPM are. The 10 might only work well on a really light boat with minimal loading. You really want to avoid loading up the engine. Letting it rev a couple hundred over is better for it than lugging it a few hundred under.