Hi Alan. The actuator assembly sounds pretty typical. As for the speed of
stroke, they are pretty slow for sure. Oil compensating should be fairly easy
and definitely needed if more than 15 feet or so of depth is anticipated. My
thought on this is......water is heavier than oil, and oil is heavier than
air. By making reservoirs part of the envelope, natural separation of the
three should be possible using just gravity.
If a small bubble is still inside the actuator, it should eventually work
it's way to the upper reservoir. If a little water enters the envelope, it
should go to the low spot.
So......if we take the actuator, encase it in a stout fiberglass casing,
bond into the casing some threaded PVC female couplings, one on top and one on
bottom. Screw into these PVC fittings some hose barb adapters, attach some
clear flexible tubing, and plug the ends with threaded PVC fittings and a
plug.
This creates the oil reservoirs, above and below the actuator. Now, as
the actuator rod extends, a vacuum is created in the FRP casing, and the oil
in the reservoirs will fill that space as the flexible tubes compress. The
reservoirs would need to be large enough and flexible enough to squeeze
whether there was additional outside pressure or not. I'm thinking maybe 1
inch diameter and 6 inches long for each.
With the actuator movement pushing the oil back and forth, any small air
bubbles should work their way to the top of the upper reservoir in short
order. Like wise, if a small amount of water got in there, it would eventually
settle at the bottom of the lower reservoir. Hopefully before it did any
damage. With the clear tubing reservoirs, we can see if there's air bubbles or
water at a glance, and drain or top off as required.
This was my original thoughts on the Minnkota compensation too. I'm just
not sure we need to take these devices apart. Air bubbles will find their way
to the top if we give them a path, a little agitation, and time.
Gotta make a method to "top it off" that's easy so maintenance is
minimal.
Now, I'll admit, I may be full of S#&* here as I've never
actually done it yet, so any criticism is welcome and appreciated. Frank
D.