[Public-List] Prop Question

Mon Apr 18 16:33:41 PDT 2016

A while back I compared specs of my QM15 withthe GM18. The GM is lighter
with no sleeves, (fresh water cooling is a big  bonus)but stroke, cylinder
size (displacement) are the same. They get the HP increase by bumping up
rated rpm.
I don't have a tach,  but guessing at  2/3 rpm moves Checkmate at about 6
knots, full ahead 7 1/2 with clean bottom. Full ahead on a dead push (as in
when aground) loads the engine excessively (black smoke exhaust). That's
the 13 X 12 wheel.
All in all, I think that's about right with the 2.14:1 reduction.
I normally set engine rpms to run at 5 1/2 knots. Little less than 1/2
gallon per hr. Fuel consumption.
Because of the aperture limitation, I see no advantage of big, heavy
horsepower. Maybe more pitch and 25 hp. Will improve making port with gale
headwinds and nasty seas. Those are conditions that call for a larger
diameter wheel. Heck, it calls for a bigger boat lol.
Michael Grosh
#220
On Apr 18, 2016 5:07 PM, "Jonathan Adams via Public-List" <
public-list at lists.alberg30.org> wrote:

> 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
>
>
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