[Public-List] Prop Question

[Jonathan Bresler via Public-List]

Might be worth getting The Propeller Handbook by David Gerr before choosing
a prop.
Just got a copy and have started reading it.  Plan to work through the
equations for the
Yanmar 2gm20f and the 3 blade 12rh14 installed on Constance.

When do you want to get a prop ?
Do you have horsepower, torque and fuel consumption curves for the engine ?
With those curves and the handbook, I am hoping to confirm that the prop is
well sized
for the boat and engine.   Will be trying to see what 2 blade would match
as a replacement.

Jonathan

On Mon, Apr 18, 2016 at 8:51 AM, via Public-List <
public-list at lists.alberg30.org> wrote:

>
>
> Prop pitch is measured in inches. So a prop with a 12 inch pitch would
> move 12 inches in one revolution.
>
>
> Mike Lehman
>
> ~~/)~/)~~
>
>
>
>
>
>
> On Sun, Apr 17, 2016 at 4:17 PM -0700, "Mike Meinhold via Public-List" <
> public-list at lists.alberg30.org> wrote:
>
>
>
>
>
> 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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-- 
Jonathan M Bresler
S/V Constance Alberg 30 #262
Annapolis/Eastport MD