Hi Dr SUP
I surf my 14' Search all the time with the same leash as all my boards
9' Da Kine
the rail saver, leash string and leash plug distance from the back of the board probably add up to at least another foot, so 10' of leash.
on my back foot so my front foot gets near the nose without issue.... not tip riding like the most awesome Ryan Burch, but having fun up front.
I would bet a 10' leash would do the trick if you are not tip riding, but simply having fun on a long board.
I have been walking around on longboards , using an ankle leash for close to 50 years without issue... you learn to dance around it.
It is easy to do spinners etc with an ankle leash, but that would be near impossible with a waist leash.
Pic might not look like much... but it's pretty far up front for a 14 ft board