4 blade stern lift....why?
 

4 blade props give more stern lift. I have read here and always assumed that it was because of the extra lift an extra blade gives as its turning downward in its rotation. This doesnt seem correct however, as the same blade is pulling the hull down into the water on its "upswing". Two blades in their rotation across from each other should cancel each other out. It would seem that the up and down forces would be equal. I'm guessing that the answer is that the hull is easier to lift than to pull down into the water so the lifting blade wins the battle. But thats just a guess....anyone?

Only if the prop is way deep in the water, otherwise, the blades swinging up are throwing water in the air. The blades swinging down are trying to compress water. Same thing happens when you compare top and bottom of a prop.

Bulldog aka Ronnie

Outward rotation gives stern lift and inward rotation gives bow lift.

Lets make it a single engine with deep x dimension. I could see where a single engine surface piercing prop could be a handful. The prop would really want to walk the back end around.

That's why those egg beaters can only make right turns at speed!!

Hello Chris,

How is your recovery going?

I had a single engine boat similar to yours and used both three and four bladed props depending on the activity.

I primarily used the three blade for speed. It was 1-2 mph faster in smooth water. One less blade in the water. The x dimension was deep and there were no problems with planing etc.

I used the four blade 90% of the time. Planing was faster for skiing and watersports. Top end was a hair slower but unless I was staring at the GPS I never noticed. In rough water it hooked up better. Crusing speeds are usually slightly better at a given RPM.

In a single engine boat with a deep x dimension you are correct - the effects of lift/pull are equal and have no affect that you will notice.
However, the four blade - especially if high in pitch - will tend to twist the boat a little bit more than a three blade. If your forward prop rotation is clockwise (right hand) as viewed from behind the boat you will notice that the left side of the boat seems a little lower than the right side. You may find yourself using a little trim tab or the trim fin on the back of your drive to compensate. Or you can just move some heavy stuff (Battery, toolbox etc) to the right side of the boat.

Give me a call if you want to discuss further. See you at the boat show!

Steve

847-936-1334

The bow lift/stern lift also depends greatly on WHICH 4 blade prop you are running. I have run two different 4 blades on my boat, which doesn't have a particular deep x-dimension, but found that the two props acted differently, even though they are only 1" different in pitch. I run as an everyday prop a 23" Powertech 4 blade. Good bow lift, excellent handling, no outward lean. At the Shootout this year I borrowed Macdad260's 24" Bravo One. Got on plane just as well, but it handled differently. Less bow lift, a bit or lean, a little harder to steer, but it was about 3 mph faster. I'm told that a Hydromotive will again act differently than either of these two wheels.

my guess is that the blade on the
"downswing" is compressing as it travels
downward, (we all know how compressable
water is) the blade on the "upswing"will be
"scooping up" the water, and " throwing" it
towards the surface.
i have read that the lower half of the
prop is reponsible for the majority of
the thrust output.
with a blade coming into play every
90*, instead of every 120*,
you will develop more lift per propshaft
rotation.
does this make sense to any body else :eureka:
jt

Damn Foul, how much more bow lift do you want????:D :D :D

He needs more stern lift...its still in the water!