I did a partnering deal like that many years ago. A guy in the UK had his boat in the US but he couldn't use it very often. In exchange for use of the boat, I covered all the operating expenses (except insurance) and over the two years I used it I made many "improvements" in terms of equipment / sailing gear (e.g. new batteries, loran, a radar).
The deal worked out well for both of us -- he got his boat back in better shape than he left it and I learned that you should never calculate the cost / weekend or cost/sailing hour. I also learned that the only way you can make sense of owning a sailboat is to live on it, preferably in a warm climate where rum costs no more than $5 a fifth.
PS -- Twenty years later we're still very good friends and have sailed many miles together.