Autopilot hydraulic pump control

[roland stockham]

Looking at a complete steering upgrade from mechanical to hydraulic. Old system has had it and in a center cockpit boat cable steering id difficult and expensive to fit.

I already have a near new raymarine wheelpilot but if I change to hydraulics it might be sensible to fit a below deck pump instead of the wheeldrive as the gearbox is recked. I am a bit worried that the small pump may not be powerful enough but the bigger ones may draw more current than the course computer can supply.

Anyone know if you can use solenoids between the computer and drive pump to reduce the output the computer has to supply? I other words is the computer output 'on/off' or proportional

[belizesailor]

If your boat was within displacement range for a Raymarine wheel pilot then you won't need much of a pump to steer it. Hydro auto is very energy efficient...just on/off as needed to steer.

Pump on my Raymarine ST-5000 system is compact. Including motor, its roughly the size of a quart jar, just a bit longer.

The pump does not draw a lot amps, but you can find the specs for your computer/pump combination on the Raymarine site to confirm.

[Neptune's Gear]

You are right to be concerned. The issue is not the average draw on the hydraulic pump, but the max. Think about the boat trying to broach down a wave at full speed. How much weight does it take on the helm to turn her? The pump will try to do that. I use a Simrad RPU-160. it CAN draw approx 42amps at stall. Of course it almost never does! But it can...
So, the required pump must be matched to the ram, otherwise the steering will be too slow - it's got to work quickly enough to steer properly. This can be calculated (volume of the pump (at working press) v volume of fluid the ram needs to put the helm hard over, v required time to hard over). Once you know that, and the stall current of the pump, you can see if the AP computer can supply it. In my experience it's best to go for the biggest pump that the AP computer can supply.
If the AP computer is too small, yes, you can absolutely use external switching. Don't use relays though, use FETs - MOSFETs to be more specific - they are like a solid state relay, and there are no contacts to wear out. Most relays are only good for a million or so switch cycles, which is not a lot in this application.
I did this with an older Simrad AP which suffered from the lack of AP computer power problem. Worked fine, but eventually the AP was replaced by a better one.
There is a commercially produced controller (used for robots) that can be bought online that will fill this function nicely. Let me know if you want details and I'll look it up - I've forgotten it's name, but I should be able to find it with a bit of google's help!
Good Luck
Matt

[belizesailor]

Wow, 42 amps. My little Raymarine Type I spec is 19 amps at max PSI. Rarely draws anywhere close to that though. What's your displacement?

OP: What boat is this on?

[Neptune's Gear]

Fully laden, 8000 kg. Large balanced spade rudder. The old AP computer only did 20 amps for a few secs. It was not enough in big seas from the quarter. The new one can do 40 amps and has never tripped.

Sent from my Nexus 7 using Cruisers Sailing Forum mobile app

[roland stockham]

Displacement is 7.2 ton. She is quite well behaved except in following seas when she tends to take of down the wave fronts. This can get into double fig speeds with enormous loads. On the last trip I blew the autopilot gearbox, broke the frame on the wind-pilot and eventually snapped the steering cables. So the whole steering needs upgrading. With a transom hung rudder the only ram made for the job is a vetus one which is on the size limit for the pump that matches the autopilot controller.
Currently thinking of keeping the cable steering but going to conduits to reduce friction and get a fairer run. Fitting separate hydraulic ram and pump for autopilot that is independent of wheel steering and repairing wind-vane (works through a tiller on aft deck) so three separate steering systems. Think matts suggestion of solid state relays sounds good

[Neptune's Gear]

It was a similar issue for me - top speed abt 18 knots surfing. I sorted it with one of these Robot Power Products - Simple-H & HV - H-bridge it worked perfectly for abt 20,000 miles. I then replaced the whole AP with a newer/better model. I can't recall what I did with the simple H now, or you could have had it!
Good luck with your mods!