It seems that tipping is always a problem with yacht charters. There those people who maintain that an owner/Captain should not be tipped,'after all he owns the yacht and the charter fee goes in his pocket'!
There are those who over tip....yes that happens.. and where it's nice to have happen, it's also embrassing too !!
For the first group of guests, I have to say this.. In most cases they are comfortable tipping a 'Rent a Captain' a few bucks for driving them around, but when it comes to tipping a professional owner/captain there hands get stuck in there pockets. Perhaps it's the fact that the owner has this large yacht that they would also like to own,

or perhaps its an overall state of confusion...

... what ever the reason, tip the owner if he has meet your expectations of showing you a good time. Just like the 'rent a captian' the owner/captain also lives off tips too!!
So how much ?
15% tip of the charter fee is extremely nice, but way over board. If you had the best time in the world, the service was just incredible, the wind was perfect and the food was to die for then perhaps 10% would be justified. But that would be for the whole boat and given to the captain for distrubution around to the crew.
A much more diplomatic way is to tip each crew something according to there post, when you finish you will most likely have tipped about 5% of the charter fee, a much more resonable amount..
Good luck and have fun, crewed yacht charter is amongst the best ways of having a successful sailing holiday