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From: Arild Jensen (no email)
Date: Tue Dec 30 2003 - 21:13:53 EST
Bill Kimley wrote:
I installed a #2 pump on Diesel Duck 44 that had a 13" cylinder (BA-175)
and it failed on the first four day cruise. The locking pin broke on the motor
shaft. Installed the smaller #1 pump and it has worked fine for a 9000
mile cruise from Hong Kong around the Pacific finally landing in San Francisco.
According to a company rep (AutoNav) the high volume of the #2 pump
instantly moves the small piston so far that the computer causes the pump
to operate back and forth in multiple very short spurts. This can stress the
shaft. Note: AutoNav, like me, uses the same computer for #1 and #2.
Bill Kimley
REPLY
Looking at the Diesel Duck 44 and a Hatteras LRC 42 there seem to be a
considerable difference in rudder size not to mention big differences in prop
size and speed. This also means that the same amount of piston movement will
produce considerably different rate of turn in the two vessels.
In addition, the autopilot computer brains uses an algorithm to arrive at the
best response for given situations.
Some autopilot brains have a more complex program than others.
For example the Raymarine/Autohelm pilots require the installer to calibrate
the rudder movement and adjust the stop points by way of the rudder angle
sensor. Other autopilots use physical limit switches to detect when the rudder
is hard over.
Some autopilots use the compass as the main sensor to detect rate of turn and
the point when the hull actually begins to swing.
As was mentioned before, the actual voltage reaching the pump motor will
determine how fast the pump moves the oil.
A vessel which is sensitive to the helm can usually get away with a smaller
pump than a vessel that requires a lot of rudder movement to produce a given
amount and rate of turn. This is the art of autopilot installing as opposed to
the science.
Knowing how a particular brand of autopilot responds to a given boat hull is not
easy to predict ahead of time without having a past use database.
Going back to the original question; if Skooch is fairly responsive to the helm
then a type 1 pump will likely work.
If Skooch requires a fair amount of wheel to get the boat turning at the normal
cruising speed then upgrading to the type 2 pump may be required.
If a Type 1 is too slow and a type 2 is to fast flow rate adjusters can be
inserted into the pump lines going to the rudder ram.
I had to do that to one boat I installed an autopilot on. to get the correct
rate of turn.
cheers
Arild
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