[Public-List] Prop Question

Wed Apr 20 05:13:02 PDT 2016

Jonathan,

The uncertainties in some of the information needed  for prop selection
make it difficult to use equations to check the suitability of a particular
prop for the A30 / with various engines..

This list is all the data we need
- EHP -  power required to tow the boat at speed -  can be estimated from
known performance on other engine/prop combinations - probably a 10-20 %
uncertainty
- Engine peformance - torque or power vs RPM - well-known for new engines
- Propeller "open water" performance - thrust and torque vs advance
coefficient - hard to obtain, particularly for 2-bladed props
- estimated shaft losses - may be fairly large
- wake fraction (how much the boat slows the flow to the propeller) -
 unknown, but quite large for our big keel in front of the prop
   (This can be a detailed wake map if we are also concerned about
vibration)
- thrust deduction (how much the propeller adds to the drag on the ship) -
very small effect

Mike

On Mon, Apr 18, 2016 at 3:22 PM, Jonathan Bresler <262alberg30 at gmail.com>
wrote:

> 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
>