Data acquisition is 'First Mate'Data acquisition is 'First Mate'

St. Lucia, Windward Islands-Our new boat, a Freedom 40/40, is named "Full Monty," because my wife, Karen, and I gave up our home and all connections to land in order to buy and live on it.

As a two-person crew, we need all the help we can get from reliable electronics to monitor the vessel, and data collection and control can play an instrumental role on a cruising sailboat. Although the boat's major systems, such as engine, batteries, inverter, GPS, autopilot, and radar, are commercial products, I created a number of secondary systems, developing first those that I needed most desperately. High on the list were enabling technologies, such as relays that could be used to allow a data acquisition module to control heavy electrical loads. I also gave high priority to power measurement and control, since there is no point using a computerized system if there is not enough power for a Dell Inspiron 7500 laptop.

Since data acquisition boards cannot be inserted into a laptop, I use Data Translation's DT9806 module, which offers a USB connection, to acquire raw electrical signals. The fact that the board is robust, small, and consumes very little power makes it ideal for a boat.

I needed a program that would collect, log, retrieve, display data, and control boat systems as required. I wrote my own program, called BoatMinder, in C++.

Batteries and nature. Full Monty has one battery for engine starting and a bank of four for the living quarters. I installed a Heart Interface Link 20 system for battery management. The Link senses the voltage and current draw and computes the state of charge. When we are connected to shore power, we can switch on an ac-powered battery charger. But as Karen and I enjoy the peace and quiet of sailing without the engine running when we're well away from shore, we have to rely on alternative ways of generating electricity.

First up, I installed a Ferris wind generator. To monitor wind generator power, as well as current, an analog input channel is dedicated to the battery voltage, sensed via a fused line at the battery terminal. We plan to use a Ferris tow-behind generator with a second regulator shared by solar panels.

Full Monty has on board eight solar panels of various sizes, with a total active area of 52 ft². Negative lines from the solar panels are brought all the way to the distribution center so I can monitor the solar power separate from other sources. The theoretical total current from the solar panels is 20.7A; here at latitude 14 degrees north, we have already seen close to that.

Oil and water. The Freedom 40/40 comes with a fuel gauge with a float. Since the gauge is located underneath the mattress in the aft cabin, it is a great advantage to have a remote readout.

For drinking water, float type fuel level sensors would not work in water-they would corrode and have incorrect buoyancy. So I surrounded a metal rod with a thin plastic sheath that could be inserted into the water through a watertight plate on top of the tank. The electrical capacitance between this rod and the water is then a measure of the water level in the tank.

While chlorine has been used to purify drinking water, there is a school of thought that oxidation by means of ozone is safer because it does not produce carcinogens. On board Full Monty a 12V pump forces air past an ultraviolet lamp inside a stainless steel housing. The radiation produces ozone. The system consumes about 30W, and intermittent operation, say four hours every day, is suitable. BoatMinder can preferentially turn the unit on when there is ample solar or wind power.

Some devices require that small digital output signals from the DT9806 be fed to a series of relays so that larger currents can be controlled. One is the refrigerator/freezer-the biggest energy consumer on board. BoatMinder ensures that it comes on only when ample power is coming in from sun and wind, or it is getting dangerously warm. Using power while it is being generated is more efficient because there is a loss when energy is stored in the batteries. BoatMinder also makes sure that the reefer does not accidentally stay on long enough to flatten the batteries.

Happy is the sailor who can see at a glance that he or she has plenty of diesel, wind, speed, pure water, electricity, water under the keel, and distance from dangerous shores. Happy is the sailor who can rely on his or her data acquisition systems to do the hard work.

For more information about data acquisition modules from Data Translation: Enter 537