[alberg30] Re: Props

[Robert E Johns]

From: Robert E Johns <bobjns at nais.com>

>From: A30240 at aol.com
>
>When I repowered Isa Lei, I used a 13x13.  This is attached to a Yanmar 2GM.
>With a clean bottom I get an honest 6+ at 2800, about 1/3 gph.  This reading
>is from a calibrated log.  At wide open it will burn a lot of fuel, throw a
>big bow wave and look like it is dragging the Bay behind me, just over 7.
>Needless to say since I'm hitting hull speed at 2800, this is where I cruise.
>The extra power is held in reserve in case someone wants to avoid "Keel
>Wheels".  As to shafts, if you have a good stainless shaft - use it.  If you
>still have the bronze one, you should get a new stainless one.  The two blade
>used with the Atomic 4 is a 13x7.
>
>Jim Davis

Dick and Jim,

I just dug out my old "Skene's Elements of Yacht design" by Francis S
Kinney. If you can find it it's got some good stuff on props, engines, and
shafts, although I wish some of the graphs covered more of the range that
fit our type of boat.

Jim, I assume that the RPMs you quoted were engine rpms and that the shaft
is geared to run at about half the engine rpms. Please let me know. One of
Skene's nomographs which I had used before shows shaft horsepower vs shaft
rpm vs prop diameter and if that 2800 rpm is shaft rpm then I am really
confused. I used this nomograph once to make a plot of horsepower that a
13" three-bladed prop could absorb at various rpms and then I plotted the
rpm vs horsepower of the Atomic 4 on the same sheet. The two curves crossed
at 1800 rpm. The following spring I took the boat out with a clean bottom
and no wind or waves, and opened the throttle wide. The tach went to 1800
exactly as predicted! The theory isn't supposed to work that well.

I'm not sure why you suggested changing from a bronze shaft to a stainless
one. In any case, Dick,  if you do use a bronze prop on a stainless shaft
you should install zincs. Our  A30, #397, was built in 1969 and we bought
it in 1976. It came without zincs and throughhulls were not bonded. I
elected to keep it that way, although the conventional wisdom at the time
said that you must  use zincs and bond throughhulls. When I had to replace
the shaft, I specified a bronze shaft to match the original. I have had no
problems with electrolysis. A side advantage, (which I didn't believe when
a friend suggested it) is that without a zinc I get almost  no barnacle
growth on shaft or prop. I know  this is controversial, and I am not
suggesting that anyone do the same thing. The only thing I am saying is
that it seems to work for me. But if the shaft and prop are not the same
material, there is no doubt in my mind that you must have zincs.

Jim, let me know if you have a reduction gear.

Regards,
	Bob Johns #397




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