[Public-List] Prop Question

Mon Apr 18 14:03:58 PDT 2016

I already have the prop, but was interested to know that others had a 13-13 versus my 13-12.
The 2GM original was a 13 or 15 HP engine depending on your source. The 2GM20  was the same block with the liners removed thus more HP - and that is 18hp or 20.
 The folks I was talking to had Yanmar 2QM which I think is a 15 - thus they could carry a greater pitch.
Jonathan197

      From: Jonathan Bresler <262alberg30 at gmail.com>
 To: Mike Lehman <sail_505 at hotmail.com>; Alberg 30 Public List -- open to all <public-list at lists.alberg30.org> 
Cc: Michael (Mike) Meinhold <meinhold272 at gmail.com>; Jonathan Adams <laughing_gull at verizon.net>
 Sent: Monday, April 18, 2016 3:22 PM
 Subject: Re: [Public-List] Prop Question

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 theYanmar 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 sizedfor 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 BreslerS/V Constance Alberg 30 #262Annapolis/Eastport MD