knots versus mph
A statute mile is 5,280 feet and a nautical mile is 6,076 feet. Since mph refers to statute miles and knots refers to nautical miles, just take the ratio of 1.15 (close enough but not quite exact). So if a boat is traveling at 10 knots, it is going 11.5 mph, or 15% more in that unit of measurement.
BTW, I couldn''t recall the exact number of feet for the nautical mile, so I did a quick web search with the words "nautical" "statute" and "knot" and up popped many useful sites.