[Public-List] Prop Question

[Mike Meinhold via Public-List]

Pitch is the angle of the blades at 3/4 of the diameter.  Higher pitch
develops more thrust at the same RPM, or the same thrust at lower RPM,
while demanding higher torque from the engine. Pitch is chosen so you
operate the engine at the most efficient RPM and output power when the boat
is moving at a desired speed.

There is only so much you can get out of pitch before you are developing
too low a pressure on the back side of the blade, and caveating. Then you
have to add blade area.

Say you are swinging a 13 x 12 (13 " diameter, 12 " pitch) and you are
hitting optimum RPM and not quite getting to hull speed. Then you may
change to a 13" pitch and get closer to the optimum.

Conversely, if your 13 x 12 was getting to hull speed before it go to
optimum engine RPM, you would pitch down

 The boats with Atomic 4's have 13 x 7 2-blades .

 I am trying to decide now what prop I will put on the other side of my new
Beta 16!  I am decided on a 2-blade for sure.

Mike
Rinn Duin #272



On Sun, Apr 17, 2016 at 6:55 PM, Jonathan Adams via Public-List <
public-list at lists.alberg30.org> wrote:

> ok so my prop is a 13 - 12, and talking with a few others recently they
> have a 13 - 13
> The boats in question have Yanmar 2GMs or 2QMs in them (~15 hp).
> I believe the second number is for the pitch - does this make a
> difference?
> Jonathan197
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