Prop "Stern Lift"

[kaydano]

I've heard and read several times in the last few days about some propellers providing more "stern lift" than others. What that is exactly has been bugging me... I get that it "lifts the stern". How it does this is what has been bugging me.

It dawned on me just a while ago that while a normal prop pushes water parallel to the prop shaft (pushing the boat forward), a prop probably also pushes some water in an outward direction, perpendicular to the shaft (and in the same plane as the prop). If a prop was deep, deep under water, these forces would cancel each other out. But if the prop was near the surface, you might get more push downward against solid water (no Derrick, not ice) than you would near the surface (where some of this outward force simply pushes water up into the air), resulting in a "lift" to the stern of the boat? Is that it? So is "stern lift" then the ability of the prop to actually waste some energy pushing water out perpendicular to the shaft instead of along the shaft (which normally pushes the boat forward)? Seems like it's wasting some energy that could have been used to push the boat forward, but if it lifts the stern enough, it might help certain boats (certain hull shapes, like a pontoon) get a better planing angle, which in some cases must net out to being a benefit greater than the amount of wasted energy...

 

[BigKahuna]

http://continuouswave.com/whaler/reference/prop3.html Check this out, Kaydano...............

 

[Ben & Suzy]

Good find BK. That should make things a bit more clear. And that only touches on the different styles of props. There are just so many elements to blade design that effect the way a prop performs.

 

[kaydano]

Wow, and I thought I typed long posts...

Thanks for finding that!

Some of it seemed hard to swallow, like the the stream of white water in the center of a boat's wake 20-30 feet behind the boat having any effect on a prop that's since been long gone, but I think he does sum it all up nicely in just one sentence:

"The blade entering the water on the downward rotation side generates more lift than the blade entering the air on the upward side, with a net result that the stern is lifted."

Abe Lincoln once said "If I had had more time, this letter would be shorter". Not sure I would have ever boiled it down that well, but I think that's the main point in the least words.

Thanks again for posting that Daril.