Hello Group:
From time to time there are queries on the forum regarding
feathering props. I recently installed a PYI MaxProp (3-blade
Classic) on our BCC and thought I’d share the process/results to date.
After considerable hand wringing and research I finally opted
to install the 15" version of the prop a couple of months ago. It
almost fit in the aperture w/o cutting the shaft but when the blades
were feathered there was the possibility of the rudder coming in
contact with the folded blades when the rudder was hard over; plus it
sat too far back anyway. (See MaxProp Initial Fit.jpg) I had the
shaft cut/re-keyed so that the prop fits just about in the center of
the aperture. With a shaft donut zinc installed there is one inch of
clearance from the cutlass bearing housing to the donut zinc. (See
MaxProp Final Fit.jpg) I set the prop with 20 degrees of blade angle
(10.3" pitch) (X=K and Y=E).
As others have noted, the most dramatic difference is in
reverse (which was really dramatic in our case as we originally had a
fixed two blade). A lot more power and one can actually spin it up
in reverse quickly, get sternway, then throttle down and actually get
the boat to turn the way one wants. (Had a hard time doing that with
the two blade). Going from ahead to reverse, or vice versa, is
seamless, i.e., there is no discernable lag from one direction to the
other.
Going ahead improved significantly as well since we went from two to
three blades. I was able to get some reasonably accurate data on
speed using GPS (w/ WAAS/Differential functioning) during a run
through the ICW canal east of Choctawachee Bay w/ no wind and no
discernable current over a period of 10-15 minutes. Bottom of the
boat was clean and fair. Results were (engine is the ubiquitous
Yanmar 3GM30F):
2250 rpm: 6.0 - 6.1 knots
2500 RPM: 6.6 Knots
Since I was in brown water at the time I couldn’t bring
myself to run the engine up to 3400 RPM (visions of silt impacting my
impeller more furiously) to see what would happen. After a hundred
plus hours of motoring in varied conditions (just completed a trip
from the Gulf of Mexico to Jacksonville FL) I suspect the prop is
somewhat underpitched. The engine will easily go all the way to 3600
RPM (in fact I have to be careful not to over rev it) with little
change in boat speed from around 2800. Apparently the prop could use
more “bite” to take better advantage of the engine’s power at higher
RPM’s but I confess to not having any kind of appreciation for fluid
dynamics of propellers.
As far as the effects of the feathering feature, Quis scit?
During the recent sail across the Gulf I was on a very deep reach in
light air for a number of hours and was making 4.5 - 5.0 knots which
seemed reasonable so I guess the prop helped? but I couldn’t prove it.
On a separate note; we also installed a Free Hand Wind Vane
during the last haulout. Neat gadget.
Pete
S/V TwoLoose
p.s. I could not figure out how to attach pictures to this email so
I ended up putting pics on the photos section of this forum in
the “TwoLoose” folder