Re: Cupping a Prop
Cupping refers to the turning of the trailing edge of the prop blade to varying degrees. It can be mild, mostly near the hub or moderate, extending all the way to the blade tip. Or, it can be radical, making the blades look like tulip petals.
However: We will just describe moderate cupping which is what most engines use.
The slight turning of the trailing edge remarkably improves the ability of the prop blade to "hold" the water. Because of this, a cupped prop can be run higher on the transom without cavitation.
Cupping does not increase the pitch of the prop, however, it will decrease the engine RPM some because of the added load it supplies. So: if you install a cupped prop with no other modifications to the boat or engine you may very well experience a decrease in top speed. If you play with the power trim and run it higher, or if you jack up the engine on the transom, there will be less resistance and you may see a slight increase in speed. If the boat was experiencing cavitation and/or slippage, then the cupping may very well increase the speed because of the better efficiency. All these factors vary with specific hull/engine combos.
Thus: A cupped 17 pitch prop may act like an uncupped 19 pitch prop in respect to engine RPM. General rule of thumb is that 2 inches increase in pitch will lower engine RPM by about 400 and cupping will lower RPM by about the same amount--Varying from engine to engine. Not a hard rule but a decent guesstimate for deciding which way to go with prop pitch.