I am responding to Quentin Kinderman’s recent letter in which he stated that dual alternator belts are warranted for a 150-amp alternator setup to solve the issues of slippage and failure (Fan belts first Issue 131, July/Aug. 2003). I respectfully disagree with Kinderman’s findings. I have a 150-amp Powerline alternator on my Yanmar 27-hp in my Bristol 30. It uses a single belt to drive it, and I have had absolutely no issues with belt failure or slippage.
The problem with dual-belt setups is that it is impossible to adjust each belt independently, so one invariably runs tighter and “hotter” than the other. Adding dual pulleys to a single-pulley water-pump shaft will only overstress the shaft bearings and lead to early failure. In my experience, dual belts are not the solution. The solution is the type of belt used. I use Gates Rubber Co. Green Stripe XL belts. These are extreme-duty cycle belts. I am still using the original belt after more than 500 hours of running. It has never slipped, and the bearing surfaces look as good as new. You can get these belts from any auto-parts store.
Obviously, the other consideration in belt slippage and failure is its proper tension. Over-tensioning the belt(s), whether single or dual, will only lead to failure of the water pump and/or alternator bearings. The belt needs the correct amount of slack, based on the engine specifications. I have found with the Green Stripe belt that once adjusted correctly, no more adjustment is needed.
David Browne sails a Bristol 30, which he has completely rewired and repowered, from St. Petersburg, Fla.
Roger Hellyar-Brook, manager of the systems program at the Landing School in Kennebunk, Maine, and the author of Ocean Navigator’s Engines & Systems email newsletter, responds:
A single multiple-horsepower belt adjusted correctly is the better answer to the problem of driving large alternators, especially if it replaces the standard alternator, as the factory circulation pump is nearly always a single belt drive. The belt recommended is often specified for engine-driven refrigeration compressors. If twin belts are used, they must be purchased as a pair and replaced as a pair. If the alternator setup is an additional one and its drag on the engine is approaching 3 hp, twin belts may be the answer to belt slippage.
