Slipping clutches also heat transmission oil. Over time, this can be seen right on the dipstick. "Instead of being that nice amber color, [the oil] will start to look like molasses," says David Richardson, a sales representative for Twin Disc (www.twindisc.com). "When the clutches start to really slip, it could turn black." One sign of clutch problems is a propeller shaft that turns when the engine is in neutral and the boat is stationary. "As they heat up and cool, instead of being flat, the clutches become distorted," Richardson explains. The result is a transmission that never completely disengages. "When you put the boat in neutral, you'll see the propeller shaft still turning slowly," he says. "There should be no movement of the propeller in neutral unless it is being affected [by current or the boat's forward motion]."