Hydraulic cylinder and pump for autopilot

[exMaggieDrum]

I plan to install a Simrad autopilot system (AP24 display/control, AC42 computer, RFU, etc.). I am looking at installing an Accu-Steer hydraulic pump based on feedback from other cruisers and my research. I will probably get a SeaStar (Capilano/Hynautic) hydraulic cylinder.

The boat is a Goldenwave 42, displacement empty of 23,000 lbs, skeg hung rudder (I'm trying to get dimensions). It has a large wheel with chain drive to wire cable to the quadrant. I plan to leave the chain/cable system in and use it for manual steering. The hydraulic system will only be used for the autopilot, i.e. I will not have a hydraulic helm pump.

I can find several formulas and discussions on calculating the size of the cylinder for a specific helm pump (or v.v.). I have found very little on specifying/calculating the specific size (flow) of a pump to a specific size cylinder. I think I may know a way to do it but would really appreciate some feedback. One thing I do want is a beefy system for offshore work in heavy following seas conditions. I had a very stressful night trying ot avoid some rocks when the AP gave up, the wind pilot was out, and I had to hand steer (with a hydraulic helm pump - so all hydraulic). It was a beast.

I am also going to be contacting some vendors, e.g. Kobelt, Simrad, SeaStar, to get their recommendations. I did a long search on CF and did not find an answer to this specific issue.

So - pumps are rated in flow (cc or cu.in./sec). The higher rate pumps use more juice of course. Cylinders are specified in stroke (inches or cm) plus volume (cu.in. or cc). Plus they have different bar sizes but that is a matter of durability and overall strength/torque, not rate of turns). I want to be able to match any one pump to a suitable cylinder.

The goal is to have 7-9sec to go from hard port to hard starboard. That of course depends on the particular design of the rudder/quadrant but usually 70 degrees total is a good number. The force required is based on the length of the rudder arm for the cylinder plus the power of the cylinder.

A bigger cylinder (higher volume) will be capable of higher force and/or longer stroke. The pump must be able to pump enough fluid to take the cylinder from one side to the other. It will be a balanced cylinder and the pump will be a reversing pump fwiw.

An example of the problem is that Simrad sells three different size pumps. The RPU160 is a medium duty pump rated for 1.6 liters/minute, or 16.67cc/sec. If I want to go lock to lock in 7 seconds it can pump 116cc in that time. One Seastar cylinder that Simrad recommends for their pump has a volume of 310cc (with a 7" stroke) with a max torque (@ 1000psi for 35 degrees??). I can do the math for how long a rudder arm is needed for that but that is not the question. A longer arm has to travel farther and has more leverage so it takes less hydraulic power to turn the rudder and v.v. for a shorter arm.

So if the pump can pump 116cc in 7 sec to the cylinder and the cylinder is 310cc, does that mean it can push it lock to lock in 310/116 = 2.67sec. This seems like a bogus number (way to short), so either I must be reading it wrong or there are some inefficiencies that make it wrong.

How should this be calculated or I am looking at it the wrong way? I cannot find a rule of thumb ratio anywhere.

BTW - I believe this combination of pump and cylinder is way to small for what I want. If I got a Simrad pump it would be the RPU300 which is rated at 290 to 960 cc/sec. They recommend a SeaStar (Capilano BA 200-11) with a volume of 486cc. This pump has an 8A min and a 30A max draw. The RPU160 is 5-20A which is better for long distance in calm conditions.

Thanks for any feedback.

[Eigenvector]

So - pumps are rated in flow (cc or cu.in./sec). The higher rate pumps use more juice of course. Cylinders are specified in stroke (inches or cm) plus volume (cu.in. or cc). Plus they have different bar sizes but that is a matter of durability and overall strength/torque, not rate of turns). I want to be able to match any one pump to a suitable cylinder.

Just to clarify and make sure you don't get snagged.............
Pumps are rated by Volume/Time or Volume/Revolution and the key point at some given output pressure unless it is a positive displacement pump (which this 99.9% likely is)

Cylinders are rated on Bore Diameter, Stroke and Pressure. Pressure x Bore Area = Force. Bore Area x Stroke = Volume.

A bigger cylinder (higher volume) will be capable of higher force and/or longer stroke. The pump must be able to pump enough fluid to take the cylinder from one side to the other. It will be a balanced cylinder and the pump will be a reversing pump fwiw.

See above for corrections to this calculation. I assume that by the term "balanced" cylinder to mean a Double Ended Rod. This would be required as you have suggested due to the area difference on each side of the piston being different. On a single ended cylinder Area 1 = Piston Area / Area 2 = Piston Area - Rod Area.

A double ended cylinder generated an equal force in both directions and displaces the same volume in both directions. This is the only way you can get away without an oil reservoir.

So if the pump can pump 116cc in 7 sec to the cylinder and the cylinder is 310cc, does that mean it can push it lock to lock in 310/116 = 2.67sec. This seems like a bogus number (way to short), so either I must be reading it wrong or there are some inefficiencies that make it wrong.

Here's the error. You lost the 7 seconds. Should be
7 x 310 / 116 =18.7 sec.

Hope this helps. Like I tell my kids, I get paid to do math problems all day, every day.

[swansailor]

If you're contacting vendors, you might want to contact PYI, Inc.. They sell a linear drive from LeComble and Schmitt. I'm also putting in a Simrad system, boat with similar characteristics (Swan 43). I got the split unit because I have to mount the drive upside down under the cockpit sole.

[exMaggieDrum]

Originally Posted by swansailor

If you're contacting vendors, you might want to contact PYI, Inc.. They sell a linear drive from LeComble and Schmitt. I'm also putting in a Simrad system, boat with similar characteristics (Swan 43). I got the split unit because I have to mount the drive upside down under the cockpit sole.

Thanks for the tip. I am looking at the products they sell. They are not cheap though. I have heard good things about them. I will be visiting them at their booth at the Seattle Boat Show late next month.

[exMaggieDrum]

Originally Posted by Eigenvector

So - pumps are rated in flow (cc or cu.in./sec). The higher rate pumps use more juice of course. Cylinders are specified in stroke (inches or cm) plus volume (cu.in. or cc). Plus they have different bar sizes but that is a matter of durability and overall strength/torque, not rate of turns). I want to be able to match any one pump to a suitable cylinder.

Just to clarify and make sure you don't get snagged.............
Pumps are rated by Volume/Time or Volume/Revolution and the key point at some given output pressure unless it is a positive displacement pump (which this 99.9% likely is)

Cylinders are rated on Bore Diameter, Stroke and Pressure. Pressure x Bore Area = Force. Bore Area x Stroke = Volume.

A bigger cylinder (higher volume) will be capable of higher force and/or longer stroke. The pump must be able to pump enough fluid to take the cylinder from one side to the other. It will be a balanced cylinder and the pump will be a reversing pump fwiw.

See above for corrections to this calculation. I assume that by the term "balanced" cylinder to mean a Double Ended Rod. This would be required as you have suggested due to the area difference on each side of the piston being different. On a single ended cylinder Area 1 = Piston Area / Area 2 = Piston Area - Rod Area.

A double ended cylinder generated an equal force in both directions and displaces the same volume in both directions. This is the only way you can get away without an oil reservoir.

So if the pump can pump 116cc in 7 sec to the cylinder and the cylinder is 310cc, does that mean it can push it lock to lock in 310/116 = 2.67sec. This seems like a bogus number (way to short), so either I must be reading it wrong or there are some inefficiencies that make it wrong.

Here's the error. You lost the 7 seconds. Should be
7 x 310 / 116 =18.7 sec.

Hope this helps. Like I tell my kids, I get paid to do math problems all day, every day.

Eigenvector, thanks for your calculations. I made a reply to your note but I must not have submitted it properly or something. I finally just got down to the most straightforward calculation which is just:

Pump rate x time required for lock to lock = volume capable over that time

and then just compare that to the volumes of the cylinders to find one that matches that volume, or is greater, or less if you are willing to accept a slower turn rate. Pretty simple really. Or:

Cylinder volume / time to lock to lock = Pump rate necessary

Thanks again.
Joe