Find your vent line that goes to the thru-hull vent fitting, measure across the fitting or hose. If you can measure across the actual fitting the OD of fitting will be either 5/8" - 3/4" or 1" When your line size is 5/8" ID you should have a hose that is over 3/4" - 7/8" outside diameter depending on the thickness of the hose material used.