Hard over time

B

BillH

New member
Hi, I’m putting together a AP system for my 40’ heavy displacement cutter. Around 30,000#, full keel with barn door rudder.
I had a naval architect calculate the hydraulic cylinder- 339 cc- +- 21cu/inch. He recommended a 2.0 liter pump which is around 10 sec / 70 degrees hard over. I emailed furuno support the verify this is appropriate and compatible with the 711c AP. The tech support recommended the 1.6l pump- around 12.7 seconds. I’m curious why the discrepancy?
Thanks, Bill
 
I suspect it will make no difference in practice which pump you choose. Many settings affect the autopilot response besides the nominal maximum time to hard over. The autopilot is generally making a large number of smaller course changes, not swinging from hard to port to hard to starboard. The difference in response time to make a 3 or 4 degree change in rudder angle would be fractions of a second between the two pump sizes and not enough to affect autopilot steering performance in all likelihood.

My guess is that the tech was trying to save you some money because the smaller 1.6l pump would be cheaper.
 
Quitsa said:
I suspect it will make no difference in practice which pump you choose. Many settings affect the autopilot response besides the nominal maximum time to hard over. The autopilot is generally making a large number of smaller course changes, not swinging from hard to port to hard to starboard. The difference in response time to make a 3 or 4 degree change in rudder angle would be fractions of a second between the two pump sizes and not enough to affect autopilot steering performance in all likelihood.

My guess is that the tech was trying to save you some money because the smaller 1.6l pump would be cheaper.
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Quitsa, interesting to break it down that way. I would guess the difference in speed may be less noticeable on my boat vs. a more performance type hull?
 
BillH said:
Quitsa, interesting to break it down that way. I would guess the difference in speed may be less noticeable on my boat vs. a more performance type hull?
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That's quite likely although it would be a function of hull design and the relative size of the rudder. Usually fast planing boats have smaller rudders than displacement hull boats. If you have an SCX-20 or other device than can measure rate of turn, it would give you an idea of how rapidly the boat changes course based on rudder position.

While operating the boat, having too slow a rudder response time would risk the autopilot overcorrecting and steering a bit of an S curve around the course line. That could only happen if the steering algorithm in the "self-learning" feature of the autopilot couldn't adapt because the boat's behavior was outside its design parameters. This just seems very unlikely to me, which is why I think the Furuno tech was taking the view that you should use the smallest pump that works with the hydraulic steering ram and helm pump on the boat.
 
Quitsa- thanks again for your comments. The SCX-20 is on my list. I have purchased the Hy Pro PR 20 RU pump- 2L. The 1.5L version was the same price. This is a new model from Hy Pro that has the solenoid bypass/ reservoir built into the pump unit.
 
A small hint for once you have the system operational -- if you push in the knob on the 711C control head, it will give you the option of adjusting the response setting. Experiment with high and lower settings until you see the boat tracking the straightest course.
 
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