Well, I've PM'd Lurker and asked him the question. I'm gonna have egg on my face if I'm wrong!

I have got to say I may wind up in denial, because too many times I've painstakingly measured time over long distances and used a calculator to find say, a 7.5 knot speed and when entering this solution, hit exactly where I aimed. My experience just does not support this therory of whole number speeds.

I might add, that if you have played with the mission editor, you will find you can set boats speed at fine fractions, so why not in the campaign?
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RVAH-9 RA-5C Vigilante at NAS Sanford, Fl 1966
The coolest plane few have heard of!