Kelly O

Lets say you have two sets of 5 blade SSM props, same diameter, same pitch, but one is 16 degree rake and the other is 18 degree.

What would the potential performance differences be on a stepped vee hull ?

UrbanDisturbance

From what I understand, the more Rake you have, the more bow lift. I may be wrong, but I think this would be an advantage for surface drives so you don't have to trim them up so much to get bow lift...I think.

tcelano

Comparing apples to apples, the prop with less rake should be faster, provided the boat's cg position was optimized for it.

The advantage of the higher raked prop is that you can leave the cg further forward, which makes for nicer midrange running, but still carry the bow as the center of pressure moves aft at higher speeds.

The reason I say the lower rake should theoretically be faster, comparing optimized setups, is that 1) in order to create bow lift, you are actually pressing down on the drive, which causes the hull to have to make extra lift to compensate, which equates to more induced drag, and 2) the blade angle is further from parallel to the thrust axis, causing more of the power to be used up making rooster tail.

Given no setup and balance optimization, there is a good chance you could throw the higher rake prop on and go faster due to the extra bow lift, at least on a non-stepped hull.

On the stepped hull, it's going to be less prone to these issues, and I'd expect the lower rake prop to be faster all-around.

Kelly O

Thanks for the replies thus far.

Any experience with stepped vees and different rake angles ?

bbladesprops

tcelano is basically right on. Bow lift is actually made via leveraging the transom though, not the drive.
In short, higher rake angles do hold the water to the blades longer creating more reaction to positive trim. Because the water is staying on the blade longer as positive trim is added, the higher rake angles do create more drag. Drag is speeds largest enemy. However, if the wetted hull area is decreased by more bow lift, the drag induced by the higher rake angle propeller will be insignificant as compared to the lessened drag of the hull. Therefore you go faster.

It is difficult to say any one rake angle will create the best or most efficient ride on any one true V or stepped hull. I've seen it work both ways. Lower rakes should be faster but only if the boat carrys itself correctly. Some stepped hulls will run faster if the front step is released (can be tough to do).

One other thing. Multi stepped bottoms create very aerated water. Sometime higher rakes will harness some of the slip this water induces.

Brett

I'CE

Brett, you are the expert, but all of your analysis is on "basically larger rakes prove to have larger surface areas" resulting in "more water surface grab per revolution" hull to hull V or cat each must be tested accordingly in order to optimize speed or cruse or combo. Racing enters another field. Carry on my friend. ps all dia/and/pitch being equal for comparison sake.

bbladesprops

Hi I'CE,

I didn't say higher rakes have more blade/surface area. I said as the rake angle increases the water stays attached to the blades longer as positive trim is added.

You are correct, each application takes on its own characteristics and needs to be aproached that way. Of course, experience can give you a little advantage. And indeed racing is a whole different ball game due to set up and CG.
I spent years with Race applications when I was at Mercury Racing. In those days we had quite the support team on wheels not to mention Lake X and the 2 movable boards of propellers we had on hand there at all times. Most of the recreational applications require a little different set of rules.

Brett