I own a 36' sailboat with a 60HP marinized VW diesel engine. A single belt drives the water pump from the crankshaft pulley. The only adjustment is achieved by adding and removing spacers between the two halves of the water pump pulley. A strange but ingenious method that requires a specific size and length belt to accommodate this. The existing belt was too long and sloppy. I talked to a V-Belt Guys tech who was very knowledgeable and told me how to measure and choose the right belt for me. That turned out to be a 15245 belt and I placed an order. The belt arrived promptly at my door. I installed the belt and it worked well and was within the specific tolerances needed for this adjustable pulley.