just can''''t help it.
Here''s a thought about how to keep the grandparents reassured about the safety and well-being of their grandchild during this grand adventure -- Communications! Take a cell-phone or satellite phone and keep in touch with them-- Every Day! (at least at first) E-mail pictures to them all the time. Then maybe they too can become involved in the adventure.
As to how to ease them into it before hand, the RV analogy is a very good one. Also, communicate with the grandparents during the re-fitting of the boat and the preparations for the journey -- keep them filled in on exactly what you are doing to prepare and why, so that they can have confidence in just how well-founded the boat and its crew are.