Powerful DC engine-mounted alternators require large doses of horsepower to produce their electricity. This is normally transmitted via V-belts, from the power take off (PTO) pulley on the front of the engine crankshaft to a pulley on the alternator. Traditional V-belts have a hard time dealing with really big alternators, and require careful alignment, heavy duty tensioning hardware, and early replacement (for a lot more on this subject, see Offshore Cruising Encyclopedia).

The John Deere diesels which are aboard Wind Horse use a new (to us) type of drive belt off their PTOs. These are called Poly-V belts, and are ribbon-like in appearance. They are reported to be more forgiving than traditional V-belt designs, but we took a wait-and-see attitude.

The alternator installation on Wind Horse is much the same as we’ve used on our other larger designs. Electrodyne large frame alternators with external diode and cooling fans are mounted on each engine. These units will run at a constant 165 to 180 amps at 28 volts (depending on cruise RPM) for hours on end. That’s over 4.5kW of power per alternator. They’re very efficient, but still each require 10 horsepower to drive them at full output.

This boat requires more DC power than in the past. She is set up to run all of her accessories, from washer and drier to air conditioning, while under way from big inverters. There is also a much larger battery bank to charge. These alternators spend more hours at higher loads than we have seen before.

Which brings us to some real world experience with the Poly V-belts. We left New Zealand with four pairs of belts on board as spares. If we’d been using a pair of traditional B-sized V-belts we would have changed them every 250 to 500 hours. In September we passed the 1400-hour mark on Wind Horse‘s engines still with the original Poly-V belts installed. So we decided to pull them for inspection and then change to new belts.

These belts are tensioned with an eccentric pulley which has an internal spring. It has worked well, much to our relief (we did not know what to think when we first saw this). The belt is removed by compressing the pulley/spring affair using a socket wrench as in the above photo.

The belts are inspected by bending them backwards (shown above). This is the belt from the starboard engine. If you count in to the third rib from the right you will see a small crack. This is a sign that it’s time for a change. There were eight of these small cracks on the starboard belt. The port belt was fine. Both are now replaced.

One thing which needs to be kept in mind with big alternator loads is whether any other accessories – such as a fresh water cooling pump – share the drive belt. The bearings on these will see an increased load. On our John Deere diesels we carry spare idler pulleys, spare spring tensioners, and fresh water pumps, along with the high strength bolts that hold them in place. We remove and inspect the belts before long passages, or every 300 hours. With the belts off it is easy to check for bearing play in the pumps, idlers, and tensioning pulleys.