The only thing I ever partnered in was a 24' sail boat with 2 other friends.
I worked out nicely only because we understood the rules.
1. Whoever screws it up must fix it ASAP. Cant afford to fix it? Fix it anyway.
2. When general maintanance work needs to be done on it, EVERYONE must do their best to meet with the other partners at the same time and do the work together. That way no one say's, "Oh yea', I put 5 hours into this or that" when it might look like only one hour.
3. Regular running costs are to be divided equally. Period. If you barely use it tough poo poo.
4. Keep a calendar at a persons house who will maintain usage schedule.
We discovered that we all were pretty busy doing our own stuff and scheduling was rarely an issue. In fact we ended up using it together most of the time. After work and long weekends at Angel Island BBQing and comsuming large quantities of Kamikaze's.
BTW, that was the name of the boat... Imagine....