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

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

Steve,

You might contact WH. The power pack I have is a simple 1/4 HP
DC motor the drive a hydraulic pump via a belt. Pretty compact.
I suspect they would sell you a unit for a reasonable price. Motor
only runs when moving the runner. But that is a function of the
controller rather than the powerpack itself.

Doug
s/v Callista

"Steve" wrote in message
...

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

Doug,

Several metions of WH and I thought I saw it referred to as 'something'
Harris but I'm not familiar with WH. I assume he designs and provides
hydraulic packages.

Need a little help here..

Steve
s/v Good Intentions

WH autopilots. Will Ham makes an un-sexy, but elegant and bulletproof
autopilot system in a little shop out on Banbridge Island. Extremely
popular with serious cruisers and small commercial boats. The Dashews
love them if that means anything. It does not have some of the "smart"
and "learning" features of some fancier brands but you can set just
about everything for the best performance.

I think in a PS owner survey it was the only brand that reported no
failures and had the lowest failure rate in the SSCA survey.

Steve wrote:

Doug,

Several metions of WH and I thought I saw it referred to as 'something'
Harris but I'm not familiar with WH. I assume he designs and provides
hydraulic packages.

Need a little help here..

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

You can find them at www.whautopilots.com. One of the things
I really liked about it is the fact that the major controls come out
as knobs. Gain, Yaw, and Counter Rudder can be easily adjusted.
In all sorts of conditions I have rarely adjusted anything except
the Gain, and then not by much.

Doug
s/v Callista

"Glenn Ashmore" wrote in message
news:YZUlb.90635$sp2.28254@lakeread04...
WH autopilots. Will Ham makes an un-sexy, but elegant and bulletproof
autopilot system in a little shop out on Banbridge Island. Extremely
popular with serious cruisers and small commercial boats. The Dashews
love them if that means anything. It does not have some of the "smart"
and "learning" features of some fancier brands but you can set just
about everything for the best performance.

I think in a PS owner survey it was the only brand that reported no
failures and had the lowest failure rate in the SSCA survey.

Steve wrote:

Doug,

Several metions of WH and I thought I saw it referred to as 'something'
Harris but I'm not familiar with WH. I assume he designs and provides
hydraulic packages.

Need a little help here..

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

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

Thanks for the formulas Glenn.

I ran the numbers to get some idea of what I might be looking for.

Seems I should be using about a 2"X20"X1.5" cylinder, 318psi @ .186gpm.

Using a slightly oversize pump at a lower RPM, I would need a 1/4hp motor to
achieve the 15 sec. hard over time.

Seems do-able on paper. I just don't know what the max current draw might be
for this motor. I'm sure someone has a formula for this as well.

Steve
s/v Good Intentions

Sorry, another typo..

"Steve" wrote in message
...

Seems I should be using about a 2"X20"X1.5" cylinder, 318psi @ .186gpm.


Should read .... 2"X20"X 1 1/8" cylinder, .......

Steve
s/v Good Intentions

Your power requirement is pretty close to what I got for Rutu. Now you
will find that there are so many alternatives that your brain will start
fogging over.

You have a starting point but the cylinder is going to be pretty big and
heavy and the tiller arm will have to be 18 to 19" long. You have
plenty of room to work with the pressure. With 700 lb. on a 12" throw
you can get by with a much smaller rod diameter. Look for a 1.5" to 2"
cylinder with a 5/8 to 3/4" rod and 12 to 14" of throw. It will be a
lot lighter and you can use a shorter tiller. The displacement will be
less and the pressure will go up about the same so the required HP will
be about the same.

Also, small 12VDC motors develop HP with RPM more than torque and the
lower you gear it the less efficient it gets. Try to stay under 3:1.

For example a 2x12x.625 cylinder will take about 34 cu. in. for a full
throw or about 136 CI/minute for a 15 second hard over. Now say we
choose a 3600 RPM PM motor (Surplus Center #10-1779 would be close but a
little over powered) and start with a belt ratio of 2:1 so the pump is
running 1800 RPM. 136/1800 is .075 so we need a pump with about that
displacement. The Surplus Center #9-5567 is a little small so the belt
ratio would have to be more like 1.8:1

Admitedly that is not an ideal setup but it will be very hard to get the
perfect match with surplus parts. The thing is, when you add up the
cost of all the parts including the mounts, fittings, hoses etc, you
will come out spending $500-600 and many hours of thinking and labor and
still not be quite sure that it will work the way you want. That makes
the $1,200 price of a complete professionally designed unit seem a lot
more attractive. You would have to be a compulsive DIYer like me to
rationalize it. :-)

OTOH, if you go through the exercise of designing one you have a lot
better understanding of how they work and know more about how to select
the correct one for your boat.

Steve wrote:

Sorry, another typo..

"Steve" wrote in message
...

Seems I should be using about a 2"X20"X1.5" cylinder, 318psi @ .186gpm.



Should read .... 2"X20"X 1 1/8" cylinder, .......

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