Who cited this http://www.shaftlok.com/FreewheelStoryShaftLok.htm as the
indisputable response to argument about free-wheeling a prop? I looked at
the article and am confused. I thought the person who brought it up was
arguing against free-wheeling as a means to reduce drag compared to locking
the shaft. But the author argues both sides of the issue. Here is an
excerpt:
"Because it may help sailors understand the forces and the conditions, let's
take a look at twin screw motor cruisers. Consider the case of a faiths twin
screw motorboat. Suppose she is running at 12 knots on one engine. (her top
speed on two engines may be 20 knots or so.) Her powerful engines will
require a large blade area within the confines of the propeller diameter in
order to absorb the power without cavitating. The blades will be wide. The
pitch will be about as large as the diameter coarse pitched. The speed of
flow into the propeller will be around 10 knots. The drag of such a
propeller is shown in Figure 3 (15 x 15 propeller). Note that the actual
drag involved in this case are hundreds of pounds. Also, a shaft friction of
1 pound - foot only slows the shaft minutely, so that in this case it is
clearly better to allow the idle shaft to revolve."
What am I missing here? Wish I could find the original post.
Anyway, I usually trail a shaft when trolling slowly. I have never made it
a practice to cruise with a single prop pushing. From what I have seen here
on this list, a twin screw trawler saves little if any fuel in single-engine
mod. OTOH, I am glad to know that if reduced to one engine through a
casualty, I can run the snot out of the other one while trailing the one
shaft to get some place to fix the dead one.
Since I view maintenance on two engines NOT to be twice the effort for one,
my argument to those who think they are "saving hours" on a diesel engine is
to point out that I fully expect these engines to either outlive me or still
be running fine when/if I sell the boat. I simply cannot run them enough.
Rich Gano
CALYPSO (GB-42-295)
Southport, FL
Who cited this http://www.shaftlok.com/FreewheelStoryShaftLok.htm as the
indisputable response to argument about free-wheeling a prop? I looked at
the article and am confused. I thought the person who brought it up was
arguing against free-wheeling as a means to reduce drag compared to locking
the shaft. But the author argues both sides of the issue. Here is an
excerpt:
"Because it may help sailors understand the forces and the conditions, let's
take a look at twin screw motor cruisers. Consider the case of a faiths twin
screw motorboat. Suppose she is running at 12 knots on one engine. (her top
speed on two engines may be 20 knots or so.) Her powerful engines will
require a large blade area within the confines of the propeller diameter in
order to absorb the power without cavitating. The blades will be wide. The
pitch will be about as large as the diameter coarse pitched. The speed of
flow into the propeller will be around 10 knots. The drag of such a
propeller is shown in Figure 3 (15 x 15 propeller). Note that the actual
drag involved in this case are hundreds of pounds. Also, a shaft friction of
1 pound - foot only slows the shaft minutely, so that in this case it is
clearly better to allow the idle shaft to revolve."
What am I missing here? Wish I could find the original post.
Anyway, I usually trail a shaft when trolling slowly. I have never made it
a practice to cruise with a single prop pushing. From what I have seen here
on this list, a twin screw trawler saves little if any fuel in single-engine
mod. OTOH, I am glad to know that if reduced to one engine through a
casualty, I can run the snot out of the other one while trailing the one
shaft to get some place to fix the dead one.
Since I view maintenance on two engines NOT to be twice the effort for one,
my argument to those who think they are "saving hours" on a diesel engine is
to point out that I fully expect these engines to either outlive me or still
be running fine when/if I sell the boat. I simply cannot run them enough.
Rich Gano
CALYPSO (GB-42-295)
Southport, FL
