Sailing Instrumentation Systems
- Updated May 2009
Sailing information systems, wind speed and direction sensors have long
been common on sailboats and uncommon on powerboats. This situation
is changing, with increasing numbers of powerboats sporting dancing
wind vanes and spinning anemometer cups or the latest solid-state ultrasound wind speed and direction sensors in their superstructure. This
may be a consequence of the migration of some sailors to trawlers or
perhaps the result of the greater awareness of powerboat captains of
the need to take wind induced effects on piloting into account during
sailing information system consists of a few pieces of yarn or nylon
stocking tied to stays and attached to a sail. A small piece of wood
tied to a length of thread and a watch can provide a measure of the
boat's speed through the water. Once past these elgant measurement techniques
the sky is the limit. Electronic sensors can measure apparent wind direction,
wind speed, and boat speed. All of the data can be presented on any
one of a number of cockpit displays. The information can also be delivered
to integrated navigation systems such as chart plotters.
wind measurements on a vessel is quite unlike the problem posed by a
stationary structure. On a boat, especially a sailboat, the system must
cope with the fact that as the vessel operates it is likely to be rolling,
pitching, yawing, accelerating and dece erating. These motions can confuse
the wind vane, making it point closer to the bow of the boat when the
vessel is close hauled and accelerates down a wave front and point further
from the bow when decelerating on the same tack. These same motions
can cause the wind speed to vary considerably, just as the Genoa alternately
fills and sags as the boat moves in very light air on a choppy sea.
The heel and pitch angle of the vessel will cause the vane and the anemometer
to provide somewhat erroneous information since they are will no longer
be able to sense the wind velocity parallel to the surface of the water.
Further errors are introduced by the effects of mast twist and the up-wash
of air caused by the action of the sails and rigging. Many users will
choose to ignore these errors. For dedicated racers and others who want
the system to accurately compute not only apparent wind speed and direction
but also true wind and other related data these factors need to be accounted
for. The variation in wind velocity as a function of height above the
surface of the sea adds an another complication for the most dedicated
technical sailor. The more costly the wind system, the more completely
the system will correct for these second and third order effects. In
the most complete and complex instrumentation systems sensors for roll
(heel) and pitch angle provide additional inputs to the wind vector
calculations. Some of the latest ultrasonic wind sensors are equipped with rate gyros and accelerometers that make them usable at a sailboat's masthead. (Some of these devices also provide temperature, dew point and barometric pressure information.)
There is some disagreement
among manufacturers regarding what constitutes proper positioning of the
wind transducer assembly. While most specify that the sensing unit should
project forward of the mast truck while others suggest that the transducer
assembly project aft where, according to their installation manual, it
is better protected from damage from a foresail or its halyard and less
affected by updraft currents from the sails. The best procedure is to
follow the directions furnished by the manufacturer of the system being
installed. The most basic wind system will provide display of apparent
wind direction and wind speed. With inputs from other sensors, it is possible
to expand the wind information choices to include true wind direction,
true wind speed, true wind angle and leeway.
displays or between computer modules becomes necessary only when computation
and display of data such as velocity made good (VMG) is desired. In the
case of VMG computation it is necessary for the system to combine the
data from the wind sensor system with the hull speed information. The
exchange of data between instruments made by a single manufacturer is
likely to be well thought out and reliable. Exchange of data between instruments
from different makers is often difficult to extremely difficult. The fact
that manufacturers state that data available from the instrument is in
NMEA 0183 format does not always mean that interconnection and exchange
of data with other manufactures instruments using NMEA 0183 will function
properly, or at all. The NMEA format is imprecisely defined in many applications.
Unless the installation demands functions not available within any one
maker's product line it is advisable to choose a complete system from
one source. Even within a single makers product line there may be a requirement
for a translator module when interconnections are made. The interconnection challenge is significantly reduced in systems that employ the NMEA 2000 communication system.
instrumentation systems use dedicated computation / display modules, many
of today's multifunction display systems can integrate all of the sensor outputs to provide the helmsman with a choice of data displays. In such as system it
is possible to show data from any one or a number of sensors on a given
display. For example, a display can be used to selectively show hull speed,
depth or, when desired, both variables. The ability to selectively display
data can be of greatest value when more than one data display site is
needed, such as a remote data display at the chart table, plus full data
display at the helm station. Although the convenience of being able to
switch the data on a display may be attractive, there is a negative aspect.
In a moment of stress the viewer may not know what data is being displayed.
A compensating advantage offered by some of the multiple interconnected
systems is the ability to switch data from an inoperative display to another
unit, showing the data on a multiple line LCD. This fall back position
is only available where the data conversion from the transducer is done
within the transducer or a remote computer box separate from the display.
In systems where the computation is done in the display head failure of
the display often disables the computer portion of the system. Some of the latest chartplotter systems, for example the Furuno NavNet3D, running MaxSea software, can integrate sailboat performance data (polars) with weather information to provide racing sailors with a very powerful navigation toolset. This same technology can be very valuable on long offshore cruises and ocean crossings where taking optimum advantage of weather patterns can result in fast, efficient voyage times.
When considering a
wind sensor system for masthead mounting carefully examine the mounting
provisions and the design of the electrical connector. Remember, at some
time you or someone else will likely have to service the unit when it
is 10 to more than 100 feet in the air. A design that makes it easy to
connect or disconnect the cable and which can be readily demounted without
use of three different size wrenches will be particularly appreciated
when you are sitting in your bosun's chair at the masthead. Look for adequate
waterproofing of the electrical connector. When installing a masthead
sensor be sure the electrical cable is properly supported with a strain
relief to prevent damage from the unsupported weight of the cable down
the mast. If the sensor system is being installed on a sailboat mast be
sure to consider how you will prevent the connecting cable, the radio
coax and wiring to spreader and masthead lights from slapping against
the inside wall of the mast. Installing a suitable (not too large) length
of thin wall pvc pipe for use as a wiring conduit will pay handsome dividends
for anyone in the boat's cabin. Once the wiring is in place inside the
pvc pipe a length of strong line with pieces of closed cell foam plastic
tied at intervals can be pulled up the inside of the conduit, firmly clamping
the cables in place. The string and its pieces of foam can be pulled out
when the time comes to add more wires. Some instrument systems such as those offered by TackTick series use radio frequency links to transfer data from masthead sensors to data display modules and from hull data concentrator units to the data displays, In this system the masthead sensors and display modules are powered from integral solar cells that combined with integral rechargeable batteries will keep the devices operating during hours of darkness. The elimination of in-mast wiring and the autonomy of the display units can significantly reduce the time required for system installation.
systems also measure the boat's speed through the water and water temperature.
To some it may seem unnecessary to measure hull speed when the typical
GPS is already providing quite accurate speed data, however, the GPS can
only determine speed relative to the earth's surface, usually indicated
as SOG, speed over ground. Depending on the flow of the current, the speed
of the boat relative to the water may be quite different.
The most common hull speed measurement device uses a paddle wheel that projects from the bottom of the boat's hull (in some cases combined with a sonar depth sounder transducer and water temperature sensor). Other
types of speed sensors include ultrasonic sensors that measure speed relative
to the water by measuring the doppler shift in the frequency of the sound
energy reflected from the water back to the transducer. A number of other water speed sensing systems that eliminate the need for a protruding sensor have been used, however few have been successful in yacht service.