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
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
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.
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