T&T: Autopilot Technology

[Mike Maurice]

David Pike started my thinking off about autopilots and while the rest 
of you have been wallowing in hog heaven yesterday, I have been busy 
researching.

David says that his autopilot will not hold course except in calm 
conditions. This is most likely a compass problem, which may or may not 
be solved by installing a whole new autopilot.

The compass might be weak, but in fact his is most likely a flux gate in 
which case this does not apply.
The compass may be loose or there may be something magnetic nearby it 
that is loose.
The compass may be installed in a poor location that induces 
instability. In other words not near the boat's center of pitch and roll.
The boat is near 50 North and this may well be aggravating whatever 
problems the autopilot would otherwise have.

The compass is most likely not compensated. In short the installation 
may not have been completed originally and the instructions which should 
have been followed then and should be consulted now, have not been.

Have you read your instruction manual recently? Come to think of it, now 
why would you want to do something like that?

Next topic.

Most of the boats that we are concerned with use reversing DC motors to 
run the pumps or sprockets. Confining my remarks to the hydraulic pump 
kind. Most such pumps are in the range of about 1-3 liters per minute of 
capacity. Such motor systems only run when the pump is being engaged and 
may in fact be jiggled using a low voltage in order to keep the 
engagement low. In the case of larger systems the motor runs all the 
time and in one direction and there is a hydraulic valve that switches 
the direction of the flow in order to change the rudder direction.

It would appear that these reversing motors have to be relatively small 
for the simple fact is that the larger the motor the more mass that has 
to be accelerated and stopped with each change of direction. Small 
diameter motors seem to be the norm. Generally the amperage is on the 
order of 5-20 amps at 12 volts or about 250 watts. This is about 1/3 hp 
max, in the raw and in practice about 1/8 to 1/4 hp.

The control boxes that are generally available are designed to handle 
motors in this range. Any equipment that is larger than this is most 
likely targeted at commercial boats and the price sky rockets accordingly.

Scott Bulger and I have been discussing by phone the topic of the 
redundant autopilot and what makes a good choice for an installed backup 
system. You can assume we will start posting on this topic, but start 
another thread.

Regards,
Mike

_____________________________________
Capt. Mike Maurice
Beaverton Oregon(Near Portland)