Larry,
If it's like every other one that I've seen, it's simple. First, I'm surprised that there's only one belt. With engines and alternators of your size, they usually run twin belts. Check the pulleys and see if there are twin grooves on the engine and alternator pulleys. If there are, use twin belts. If you have the original setup, you can call Sam's Marine (this website) or Hatteras (links) and get the right belts. If not, maybe your old belt or starboard belt still has the legible numbers. So get the right belts, always replacing both belts of a twin belt drive, and then:
1. Turn the power off, just to be sure. This can be done with power on, but you can't touch any wire terminals with metal tools.
2. Remove the belt covers, if any, by finding the bolts that hold them on and removing.
3. Look at the alternator to determine which bolt (usually just one) is the clamping bolt for adjusting. Most alternators have a big long bolt at one end that goes through a frame or sleeve and holds the alternator onto the engine. This long bolt is a rotation point to allow the alternator to swing in and out from the engine pulley, which adjusts tension on the belts. The other bolt is a short one that clamps the alternator to a slotted arm that lets the alternator move when the bolt is loose, but holds it tight when the bolt is tight.
4. Loosen, but don't remove the nuts on both bolts. Move the alternator towards the engine pulley so you can slip the new belt(s) over both the engine and alternator pulleys. Pull down with your hand to tighten the belts against the pulleys, making sure they're aligned in the pulley grooves.
5. Tension the belts. Frequently you can put a long, heavy thing like a hammer handle or a length of pipe between the engine and the alternator body to get good leverage to tighten the belts. While there is tension on the belts, tighten the clamping bolt to hold the tension. Then tighten the other bolts holding the alternator. Don't overtighten and break these bolts. You probably can't overtighten them with a 5 inch handle wrench or socket wrench, but you can with an 8 inch handle.
6. Check the tension. The belts should "give" about 1/2 inch or so when pushed or pulled like a bowstring. Twin belts should have the same tension on each one (they will if you replace them in pairs.
7. Replace the cover.
8. Turn the power back on and get a reading from your helm battery voltmeters with the engines off. Then run the engines and go watch the belts. They should run true. Check that you're getting more volts on your helm voltmeters after starting the engines than you had when the engines were off.
9. After a few hours running time. Check belt tensions again and adjust if necessary.
Doug Shuman
