Got a question for you on props.

I have a two SS props - same pitch, same diameter. One is a four
blade and one is a three blade. Same manufacturer.

The four blade has a cup or more of a twist in the blades than the
three blade.

Is there a reason for that?

Short Wave Sportfishing wrote:
Got a question for you on props.

I have a two SS props - same pitch, same diameter. One is a four
blade and one is a three blade. Same manufacturer.

The four blade has a cup or more of a twist in the blades than the
three blade.

Is there a reason for that?

There is a purpose and reason for everything, grasshopper.

Short Wave Sportfishing wrote:
Got a question for you on props.

I have a two SS props - same pitch, same diameter. One is a four
blade and one is a three blade. Same manufacturer.

The four blade has a cup or more of a twist in the blades than the
three blade.

Is there a reason for that?

It is part of the design to reach the intended pitch.

If you look at both props, you should be able to see that the 3 blade
probably has a greater rake angle where the blades meet the root or hub,
and the trailing edge is noticeably cupped. This design is to reduce
ventilation or blowout at higher trim angles or jack plate settings, and
as a bow lift enhancement at speed.

The 4 blade is cupped along the rake to enhance the bow-lifting feature
desired from that design. The 4 blade may have a slightly thinner blade
cross-section too, for a given application. Each blade is only loaded
w/ 75% of the power compared to a 3 blade situation.

Rob

On Fri, 15 Jun 2007 08:08:39 -0400, trainfan1
wrote:

Short Wave Sportfishing wrote:
Got a question for you on props.

I have a two SS props - same pitch, same diameter. One is a four
blade and one is a three blade. Same manufacturer.

The four blade has a cup or more of a twist in the blades than the
three blade.

Is there a reason for that?

It is part of the design to reach the intended pitch.

If you look at both props, you should be able to see that the 3 blade
probably has a greater rake angle where the blades meet the root or hub,
and the trailing edge is noticeably cupped. This design is to reduce
ventilation or blowout at higher trim angles or jack plate settings, and
as a bow lift enhancement at speed.

The 4 blade is cupped along the rake to enhance the bow-lifting feature
desired from that design. The 4 blade may have a slightly thinner blade
cross-section too, for a given application. Each blade is only loaded
w/ 75% of the power compared to a 3 blade situation.

Interesting.

I need to learn a lot more about this.

Thanks.

On Fri, 15 Jun 2007 10:57:42 GMT, Short Wave Sportfishing
wrote:

The four blade has a cup or more of a twist in the blades than the
three blade.

Is there a reason for that?

Cupping is a way of increasing the effective pitch. If you have
enough torque to spin them, the cupped props should give you a bit
more speed at the same RPM.