I have no instrument for that. No GPS, or else.
But I beleive, once in a narrow channel on Lake Toro called, France Island Passage, F5, shore was 100 ft from canoe and going rather faster....to me 10 wasn't exagerate... but make that 9 knots. 2 reefs on mainsail, full to mizzen, man that was amazing, my buddy enjoyed a lot.
I will buy a GPS for summer, not too expensive, then I'll be able to have an accurate reading.
Here's a map section to give you an idea about location.
Wind was from West, on the run (backwind), passage 1 mile approx. from x to y.
Lake Toro is in Quebec, about 100 miles north from Montreal, a beautiful lake. Many motor boats, but that's ok. Get that on Google map.
We also sailed a gentle day on lake Sawin where few motors, 10HP max are allowed, it's also nice for sailing.