Most factory engines are equipped with a 35 or 60 amp alternator with an internal regulator.  This factory combination will take 3 to 5 hours engine running time to put 100 Amp Hours (AH) back into your batteries.  These alternators are fine on automobiles, but do not belong on a sailboat.  With only the factory alternator charging the battery, at 0.7 gallons per hour and diesel fuel at $1.50 per gallon, the cost just for fuel will be about $150 to $300 per year for the vacationing sailor and $1,500 per year for the full time cruiser.  Add oil and maintenance; consider the cost of rebuilding a diesel engine, and you can see why cutting engine running time to an absolute minimum is essential.  By replacing the factory alternator with a high output one designed for marine service and is an exact replacement, will cut your engine time in half.  Adding a smart regulator will cut engine running time another 50%.  What took 3 to 5 hours will now take one hour.

High output alternators are available from 115 to 135 amps, 65 amps at 24 volts, in a small case, which is what 95% of the boats need.  Large case alternator output is from 160 to 200 amps, and 135 amps at 24 volts.  A new, important recent development has made the small case alternator much better.  We are now offering our own FW series of alternators available with an isolated or floating ground.  Floating ground alternators stop high output current from flowing through the engine block and are our standard.  Although grounded small case alternators are still available, we don’t recommend them.  Small case alternator design in the past has always used a grounded case which allowed some output current to flow through the case and the engine block before reaching the battery.  With a floating ground alternator this can’t happen.

The above outputs are SAE cold ratings of 80 F. After operating 5 minutes at 100 amps they become hot or about 200 F and the output is reduced about 15%.  The 105 to 125 amp alternators are sufficient for most boats.  Single output alternators are recommended, but dual output are available.  All of our alternators have electronic tachometer outputs.  Alternators using external smart 3-step regulators are the best way to go as they will cut charging time in half while doing the best possible charging job on expensive batteries.  There are many good smart voltage regulators on the market today.  See alternator voltage regulators for description.  Two types of mounts are available, standard American with a 1” or 2” single foot and the Japanese dual foot “saddle” mount which measures 3 1/8” between brackets.