[Public-List] Prop Question

[via Public-List]

So we have been talking about the one constant; our boats, and four variables; diameter, pitch, number of blades, and usable HP.  Not to make this even more complicated, but shouldn't we also include transmission reduction ratios.  They can range from the Atomic 4 with no reduction to most diesels in the mid 2x range.  Quest has a little one lung BMW diesel with a 2.7/1 reduction gear.  That provides a reasonable amount of torque but a relatively slow shaft speed, thus a three bladed 13x14 prop.  To achieve maximum torque in the Atomic 4 the prop is spinning as much as three times faster and has about half the pitch.  

And more importantly, I need Mike to tell me what prop to use as soon as I figure out how to hide a new Beta 16 from my wife.  

Richard Hazlegrove
Quest 433

 

When discussing prop options for our boats, shouldn't we include the transmission gear reduction into the mix?  I believe that the Atomic 4 has a 1/1,  other diesels 2.3/1  My boat is powered with a single lung BMW diesel with a 2.7/1 reduction ratio.  It spins a 13x14 three bladed prop.  

-----Original Message-----
From: Mike Meinhold via Public-List [mailto:public-list at lists.alberg30.org] 
Sent: Sunday, April 17, 2016 7:17 PM
To: Jonathan Adams; Alberg 30 Public List -- open to all
Subject: Re: [Public-List] Prop Question

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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