Your sail number should match the boat number. the boat number can be determined from the serial number that is found on the transom of the boat. Here's an example of a Martin 16 serial number: CAZBL16138F505. The middle numbers after CAZBL16 is "138" which is the hull number and the serial number. When you order your new sails, In this case in the order you would specify to put "138" on the sails.