The 12VDC powerpacks like Barnes and Fenner are not designed for
reversing. Will Hamm's system is simple and and an elegant use of off
the shelf parts. It looks like a fairly standard 2" bore steering
cylinder mounted at one end on a custom made swivel. The power unit is
a very small pump with a belt drive to what looks like a 1/4 HP DC
permanent magnet motor. The motor would have to be compatable with the
control unit but that means either a PM (2 wire) split field (3 wire) or
series wound (4 wire).
The thing is you have to balance the tiller arm length to the throw of
the cylinder to get 72º of movement then match the displacement of the
cylinder with the capacity of the power unit to get an acceptable hard
over time with suficient force. Then you do it backwards using
available parts to get the final tiller arm length. The thing is any
compromise reduces efficiency so components made for the purpose usually
work out better.
Here are some handy formulas. I did the calculations for Rutu and it
took 3 pages of scribbling and then decided to let a pro do it. :-)
Maximum pressure on the rudder = 1.6 x area of the rudder in sq. ft. x
boat speed in knots squared. If the hull is capable of surfing double it.
Torque = max pressure X distance from the cneter of force of the rudder
to the centerline of the rudder post. (The center of force on the rudder
is usually about 30 to 40% of the cord length. )
Cylinder force required = Torque/tiller arm length
Hydraulic pressure required (psi) = req'd force/net area of piston in
sq. in. Net area is the area of the cylinder less the area of the rod.
Hydraulic volume required (GPM) = net area of piston x throw length/231
Output of pump (GPM) = displacement x RPM/231
Horsepower required = GPM x PSI/1714 x .80 (efficiency of pump)
(multiply by 4 for a 15 second hard over time)
Steve wrote:
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.
I was kinda thinking the same thing. Infact, at first, I thought this unit
was a electro-hydraulic unit.
Since I'm not going to drive the hydraulics off the engine I really need to
find a compact efficient 12vdc power unit.
I don't think I need to get into a continious running pump since the boat
has a full keel and maintains it's course very well. 90% of the time, the AP
linear drive is just holding the tiller in position (static load). Something
that a hydrualic cylinder it very good at. The main reason the AH3000 linear
isn't satisfactory, is because in heavy weather, the rudder tiller exerts
enough force against the linear drive to force the motor to turn in reverse.
I'm afraid of what this is doing to the planetary gear reduction.
Thanks to your comments and that of Glenn, I will look farther for a 12vdc
(reversiable) PowerPack.. If anyone knows of something like this, please let
me know.. The pumps that Glenn referenced were just the pumps.. gotta be a
complete unit someplace, like truck power lifts, etc.
Steve
s/v Good Intentions
--
Glenn Ashmore
I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at:
http://www.rutuonline.com
Shameless Commercial Division:
http://www.spade-anchor-us.com