Sailing Instrumentation Systems
by Chuck Husick

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 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 a passage.

The simplest 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.

Making accurate 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.

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.

Interconnection between 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.

While traditional instrumentation systems use dedicated computation / display modules, many of the newer offerings employ multifunction display modules in conjunction with built-in or remote data processing modules. 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.

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.

Many instrumentation 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 types of hull speed measurement devices are pitot tube probes and paddle wheels. The former operate by sensing the pressure created by the water flowing past the boat's hull on the end of a sensing tube. The latter operate electrically, counting the rate at which a small paddle wheel projecting slightly below the hull is rotated by the passing flow of water. 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. Water temperature sensors are often incorporated in hull speed transducers. Most of the paddle wheel type sensors can supply speed information to chart plotters and similar equipment. Some chart plotters can combine hull speed, SOG, magnetic heading and course over ground (COG) information to compute the set and drift of the current.