H77
You didn't mention if this looseness was evident when you were rigged to sail. That is, are you finding this hull slop (10 inches) when your jib is up and you've hauled down on your jib halyard to tighted the rig. I suspect it will be much less than 10 inches when tight and what difference remains you'd have to consider whether it affects the boats performance.
While the looseness is inherent to the H16, it usually isn't great and doesn't really affect the performance for the averge to above average sailor. Probably, like most of us H16 sailors, it just kinda bugs you 'cause it shouldn't move like that. But in reality, the H16 does fine with what little movement you have between the hulls when rigged and tight.
With the jib down, you always have much more movement between the hulls
Cheers
Ron
H16 '82