Thanks Aaron.
That pretty well confirms what I thought. Chain is effective at a bit longer range than grape, and did not have the accuracy of ball. When using it at point blank which would be about short range on the ruler it did not need to be too accurate as the sails, masts and rigging gave a huge spread of target. I was talking to an Army gunnery Officer who did some cannon tests for Brigadier Sir Peter Young when he was setting up the Sealed Knot. They tried chain from a cannon on his estate and it shredded a Copse of trees at the end of the meadow.
Having that as a criteria, I am now happy to institute an unofficial rule for my games to use chain at short range, and test to see if it unbalances the play in any way. At least that is a way to get it into use more often.

Rob.