Cupping a Prop

Hello all, when a cupped prop is sold by its manufacturer, is its pitch as listed the true pitch, or the theoretical pitch?

Also, if was to buy a "regular" stainless 4 blade prop, and take it to get cupped, and I need it to be 17 pitch, what size should I buy? For example, would a regular 19 pitch cupped then become a theoretical 17 pitch, or would cupping a 15 pitch then make it 17?

Thanks guys

Re: Cupping a Prop

I am not a prop expert, but I do have some experience with cupped props. So here goes: The propeller goes by it's original pitch. In other words a 17" cupped prop is a 17" pitch prop with an extra 'cup' or roll in the trailing edge of the blades. This cup effectively increases the pitch somewhat. It will bring your rpm"s down some and may bite the water better, reducing cavitation.

In my experience cupping the prop will not increase the pitch as much as you are expecting. My cupped props seemed to be less than 1" pitch increase.

I hope this helps, and look to hear the experiences and opinions of other boaters.

Re: Cupping a Prop

what I have experianced is that cupping will give the equivelent of about 1/2 degree of pitch, the big difference is bite, you will get a much better hole shot, plane at lower speed and reduce slippage

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.