Re: Why? RPM Question.
Look at it this way...
If you're driving a pickup truck, towing a large, heavy trailer, going up hill, you would down shift to bring your RPM up and relieve the stress on the engine. If you didn't, that stress would result in pinging and possibly burning up the engine. Once you've crested the hill you would upshift once more to your regular crusing gear and bring the RPM down. It takes less power to go on a flat surface or downhill than it does uphill. Well, in a boat you are ALWAYS going up hill, so you need to leave it in a "downshift position", ie. a prop that allows the RPM to come up to a point where the motor isn't working as hard to maintain the top speed. It may sound counterintuitive, but a motor running at 5500 RPM and 40MPH is not working as hard as the same motor running at 4500 RPM and 40MPH (with a higher pitch prop). The 4500 RPM motor is lugging, .ie running uphill in high gear. The 5500 RPM motor is not lugging, .ie has been downshifted. It makes less heat and runs more efficiently. Less heat also means less coking (buildup of carbon on the piston dome and ring lands).
Not sure if that helps, but I hope it does...
- Scott
