I think you misunderstand the typical hydraulic auto-pilot for
mid-size boats where the manual steering is not hydraulic. They
typically have a dedicated reversible hydraulic pump driving (as Glenn
points out) a balanced cylinder. The AP electronics turns the electric
motor driving the pump on and off in the required direction.

Larger vessels, including Fintry, have a setup like the one you
describe below, where an hydraulic pump runs continuously with
hydraulic servos (fluid operated relays) supplying the fluid to the
ram(s) as required. This is similar to automotive power steering.

I think I'd rather have an hydraulic cylinder and hoses back there
rather than an electrically driven screw about the same size. The
hoses would be bigger than the wires, but the Surplus Center drive is
not designed for your environment.

Finally, think harder about the broader design issues here. In
particular, what happens when a big wave pushes the rudder harder than
the AP can push back? This is the touchy area for screw drive APs, as
the drive has to stall safely, and then go on pushing as soon as it's
able. An hydraulic unit does this easily, but a direct electric drive
has problems -- electric motors don't like to stop and you can't just
un-clutch the thing, as it has to stop, not backdrive.

Also think about duty cycle and design life. An AP is working all the
time, back and forth. Very few applications require this kind of
activity, so it may be an area where adapting something from elsewhere
is difficult.

As a complete aside, I wonder about the need for a balanced cylinder
in a hydraulic drive. As Glenn says, with an ordinary single ended
cylinder, you have different thrust in the two directions. But,
modern autopilots are pretty sophisticated and adjusting for that in
software might be cheaper than the extra cost and maintenance (two
seals rather than one) of a balanced cylinder.

Jim Woodward
www.mvFintry.com

"Steve" wrote in message ...
I had considered hydraulic and if I already had hydraulic on the engine, I
probably would reconsider it.

Originally I had planned on hydraulic for the anchor windlass but found the
Lewmar 1000 vertical and I'm now happy with that.

Also any hydraulic cylinder and hoses would be right in the cockpit since my
rudder is 'outboard' and no provision for an inboard cylinder.

Additionally, I would have to find or build some kinda electrical to
hydraulic control valve for the AP control head to operate.

After reading Glenns comments regarding the operating speed of the screw
drive, I went to the boat and did a test to determine the rate of travel of
the OEM (AH3000) linear drive. It is exactly 1 inch per min. with no load.
The unit I'm considering is 3/4 inch per min.. 25% slower, however since the
this unit has 3 times the thrust I could move the tiller attachment pin aft
an appropriate amount to increase the rudder stop to stop travel time and
still have the advantage of the increased dynamic and static load.

I'm going to contact the fellows at Surplus Center and see if I can get some
more spec. info on this unit. They have been very help full in the past.

I am however taking Glenns warnings into consideration and thanks for the
heads up.

Steve
s/v Good Intentions