# Timeless 12-volt electrical axioms

In Jim Rogers’ recounting of an Atlantic crossing (Sailor is confirmed purist after Atlantic crossing Issue 130, May/June 2003), the attendant electrical problems more than amply illustrate the need to follow several age-old axioms of 12-volt electrical systems.

Axiom No. 1: To get full output of any alternator requires speed, usually 5,000 rpm. If the power takeoff on the engine and the sheave on the alternator are not properly sized, the alternator will not turn at 5,000 rpm to achieve full output.

Axiom No. 2: If Mr. Rogers did not replace the cables from the alternator to the battery bank, he may be trying to stuff too many amps down the cable, and just create a new source of heat in his engine compartment. There are many tables and formulas that help the boat owner in sizing the wire transmitting the generated electrical power from the alternator to the battery.

Axiom No. 3: Heat is a problem. Alternators are designed to achieve rated output at reasonable air temperatures. The warmer the air temperature in the engine compartment, the lower the output. And to top that off, in addition to generating 12-volt DC current, the alternator also generates heat, which must be dissipated somehow.

Axiom No. 4: Proper number and size of batteries is important. I don’t know what the total amperage draw is of all the electronic and electrical devices installed aboard this vessel, but Mr. Rogers should. The house bank of batteries should not be discharged below 50 percent before recharging. To do otherwise is hard on even deep-cycle batteries.

Axiom No. 5: Have a separate alternator and separate battery dedicated to starting the engine.

Axiom No. 6: Mr. Rogers made a comparison between his Honda Accord and his Swan 47. There is no comparison between the two. The alternator and battery in the Accord are properly sized to meet all the electrical needs of the automobile, and the engine compartment and the wiring of the Honda are designed to operate under normal and even abnormal conditions, and with the exception of a small clock, there is no electrical draw when the engine is not running. On the Swan 47, Mr. Rogers expects the battery bank to run a coffee maker, a watermaker, a microwave oven, the GPS, the VHF, the SSB, the radar, a laptop computer, wind instruments, an autopilot, a depth sounder, a stereo system, electric lighting, the refrigeration system and the solenoid for the stove, all while the engine is not running and charging batteries.

Axiom No. 7: V-belts should bear 180° (plus or minus) on the driving and driven sheaves. Introducing a water pump or other device creates greater possibility of slippage.

David M. Blakemore lives in Seattle and sails a 52-foot Ron Holland design, the electrical system of which he and his son, William Blakemore, an electrical engineer for Digeo Broadband in Kirkland, Wash., have upgraded extensively.