Autopilots---size matters

We ran an article recently by marine electronics dealer John Barry that pointed out a problem which apparently is all too common—namely autopilots that don’t perform well because the hydraulic pumps were improperly sized by the installer.

Our goal in providing excerpts from that article is not to turn boaters into installers. Rather, we want boaters whose hydraulic pilots are under-performing to know that the culprit may be the wrong pump—and to talk to a dealer they trust about making the system work as advertised. At right is a hydraulic drive system from Lacomble & Schmitt.


By John Barry


Much has been written about autopilot sizing, and as technology evolves, new rules have emerged.  This discussion is meant to address some of the common issues associated with auto steering on hydraulically steered boats that are fitted with a reversible hydraulic pump as a drive system. 

Manufacturers have made pump selection easier by offering a “small-medium-large” choice for their products.  These pumps have different flow rates and different current draws, and drive selection is important to pilot performance.  Matching the pump to an electronics drive system is essential to prevent immediate smoke or improper steering, but that issue is not the thrust of this article. 

Easy steering vs a ‘pig’

Boats come in many sizes and shapes.  Autopilot performance starts with the naval architect who designed the hull and propulsion system.  The specified steering system added to the hull design determines whether the vessel is an easy steering, easy docking sweetheart or a squirrelly, difficult steering pig. 

I will resist the temptation to name brands, but suffice it to say that the steering characteristics of a boat are not in the control of the autopilot or the installer.  If you can make an autopilot drive the boat as well as an experienced operator can, you have accomplished the best you can do.  Remember, you cannot change the shape of the boat, nor her draft, windage or center of gravity, so don’t fight a losing battle.  The last resort is to change out the steering system or enlarge the rudders, an extreme that I have gone to in the past.

As installers, we want to get it right the first time.  Autopilots are one of the elusive products that sometimes need reconfiguration upon testing.  A complex device like this requires experience to choose the correct product every time.  So, how can we tell if the drive we have selected is the right one?  The choice is about flow rate, and flow rate is about hard-over-to-hard-over time (HOHO). Above is Garmin's GHP with Smart Pump.

The rule of thumb is that an autopilot pump should be capable of a 10 second HOHO time.  The gray area comes into play because boats have different full rudder positions and different steering characteristics.  One degree of rudder has a much different effect on steering depending on vessel speed, hull shape, rudder size, etc. At left is Simrad's AP70.

Pilots work within a range of steering parameters.  Algorithms written by smart guys over years have yielded an intelligence that allows today’s marine autopilots to work at various speeds and sea conditions.  Like many products, an improperly sized unit may appear to work, but yield substandard performance and/or high failure rates.

'Right-sizing' benefits

By sizing the hydraulic drive properly to the vessel, the electronics that control the drive can work comfortably within their capabilities, producing a lower duty cycle, less wear and tear on the steering and higher performance in adverse conditions.  Manufacturers publish guidelines for drive selection and these should be followed. Above at right is Furuno's NavPilot 700.

Typically you need the volume of the steering cylinder to specify a pump using this method.  When the volume is not known, an estimate can be made by measuring the cylinder diameter and throw using Volume = (Circular area x Throw).  An assumption for wall thickness of the cylinder for interior diameter is necessary also.  When in doubt, a variable flow rate pump can be installed. At right is a SeaStar pump.

Small steering corrections

One advantage of hydraulic steering using a reversible hydraulic pump is the ability to apply a small rudder correction to the vessel.  The voltage to the pump is ramped up slowly to move the pump just a little, flowing just a little fluid to the cylinder and applying less than 1 degree of rudder.  Although this type of tiny correction would be rarely called for on an inboard boat, having precise rudder movement allows the rudder to be placed in more accurate positions.  The ramping up of voltage to the pump effectively gives us a version of variable flow rate.  This allows an even wider selection of vessels that will work with a given drive.  Again, by sizing correctly the duty cycle is reduced and performance and reliability may be enhanced.  At left is Raymarine's Evolution pilot.             


About the author

John Barry owns Technical Marine Support, Inc. in Pleasant Prairie, WI. He is a NMEA Certified Marine Electronics Technician and instructs several NMEA technical courses, including Marine Electronics Installer and NMEA 2000 Network.


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Page 1 of 2 ( 6 comments)

hugo:(1/30/2016 2:00:32 AM) "Why is no integrated ais transceiver available? Only recivers.


Each AIS system consists of one VHF transmitter, two VHF TDMA receivers, one VHF DSC receiver, and standard marine electronic communications links (IEC 61162/NMEA 0183) to shipboard display and sensor systems (AIS Schematic). Position and timing information is normally derived from an integral or external global navigation satellite system (e.g. GPS) receiver, including a medium frequency differential GNSS receiver for precise position in coastal and inland waters. Other information broadcast by the AIS, if available, is electronically obtained from shipboard equipment through standard marine data connections. Heading information and course and speed over ground would normally be provided by all AIS-equipped ships. Other information, such as rate of turn, angle of heel, pitch and roll, and destination and ETA could also be provided. Check out:"
Islander Sailboat Info:(12/4/2015 9:49:32 AM) "Great post!! This is the missing introduction I've been looking for. Thank you for taking a complicated subject and making it very easy to understand." 11:16:16 PM) "Excellent posting! thanks a lot for sharing this information.
Lee:(7/27/2015 10:54:13 PM) "Just a note about the Sailor 6222 - it has no facility to output DSC messages, for example to a chart plotter. This was confirmed with the local Thrane rep."
Jack S/V Azure Te`:(5/5/2015 10:16:45 PM) "Thanks, helpful stuff. I'm reviewing the wiring on my sailboat. I found the power cord on the R/M RL80CRC+ has a drain(shield)wire , (+) and (-). The drain was connected to the same return as the (-), RayMarine states ,"if the vessel does not have a RF system connect the drain wire to the batts negative terminal" w/o further explanation, but I do have a RF grounding 2" copper strip attached to the SSB, Antenna tuner and external plate, so I believe the drain for the plotter should be connected to the copper strip - no ?"
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