I signed up from the Torsk internet site. If I start to talk to a tourist and they seem super interested I often ask if they would like to volunteer. Ultimately, your question was how to keep volunteers.
1. Make it fun.
2. Make each and every volunteer feel priceless no matter what they do on the boat.
3. Each week, for the Torsk folks, Wednesday a general email is sent out asking who will be on board. Nice reminder that the boat is still there and needs your help
4. Plan on lunch for the volunteers on board. Lunch meaning hot dogs/chips/soda. Really, what will hot dogs and rolls set you back?
5. Work together, it is a social event as well as a restoration event. If it is a social event as well as a restoration event, it becomes fun to attend and not just another job.
6. Remember it is VOLUNTEER and your members can make it when they can. It is not a job. We all have those in one form or another. The hatch is always open no matter what Saturday you can make it. Again, it is supposed to be a fun, no pressure day restoring something cool for generations to come.