T&T: Autopilot Pumps

[Dave Cooper]

<Mike wrote: There is a practical limit to the size of motor that can be
reversed. It is around 1/4 hp, I suspect.>

Power consumption and the sailboat crowd with their minimal power
requirements seem to drive the major recreational autopilot market
manufactures. Seldom do we see robust commercial rudder drives on our
trawlers.

Swan Song is not a minimalist trawler and as such we have the power to use
to run a commercial sized rudder control system. We nearly went with a
constant running dual DC pumps with dual direction manifold system which
unfortunately don't have the MTBF of the reversing motors especially in
brush life. They also add complexity and component count to the system which
decreases its reliability, IMHO. Our reversing DC motors draw 25 amps at a
nominal 24 volts each. This is about 600 watts or about 3/4 horsepower if
they were 100% efficient. We have two of them. They are rated 180 cu/in per
min. They aren't cheap...~ Over $1500 each but have performed without issue.

Our rams are 15.5 cu in each, full stroke (2 1/2" X 11") and we use about
75% of that stroke for +/- 40 degrees of rudder. and the hard over hard over
time is 4 secs with both pumps working so we should have 23.25 cu in divided
by the two pumps divided by the time (4 sec) or 2.90 cu in per sec. Rounding
that back up = ~174 cu in per minute. So it checks both ways.......I think
;-)

I did know the ft-pounds of force but can't seem to find that data right
now. My feeling at the time I designed it was that the rudder itself is the
weak link not the quadrant nor the rams nor the ram support structure nor
the pumps. Our pressure bypass is set at ~1200 lbs....perhaps a bit high
but.....

Simrad, Raymarine and a few other manufactures I had contacted recommended
pumps in the order of 90 to 120 cu in/min. They seem to consider hard over
hard over times of 8-12 seconds good. This is 1/2 to 1/3 less than we have
now and which we are very, very satisfied with.

I haven't found another boat with this response time nor one that tracks
dead on under virtually any sea state. I have to chock it up to rudder size,
the autopilot and the rudder response time.

As always YMMV....

Dave & Nancy
Swan Song
Roughwater 58