So, this 15" wide stripe runs the length of the hull from the bow to the stern just under the hull/deck joint, correct?
You do realize that this stripe will almost certainly have to be distorted as it follows the hull shape from the bow to the stern and that it will not have an even appearance after it is applied?
The only real way to do something like this that is so wide is to manually place a line of tape at where the top of your stripe would be and another where the bottom of the strip would be -- this one you will have to stand back a distance and adjust as necessary to account for the changing angle and curvature of the hull and so it looks correct to the eye.
then apply an oversize chunk of vinyl or several pieces as necessary and VERY CAREFULLY score the surface of the vinyl with an X-Acto along where it meets the underlying tape -- then remove the excess and tape. This will take a fairly long time.
Boot Stripe = stripe just above and parallel to the waterline
Cove Stripe = stripe just under the hull deck joint or along the top of the topsides (the section of hull that is above water)