How long will it take?

This one is for our non-boating friends.

We are often asked “how long does it take to get from (the place we are) to (this other place)?” and we hem and haw and say “well, it depends” and they look slightly annoyed and counter “just a ballpark” as if we are being purposefully vague and we make something up to be polite.

Why can’t we give a better estimate?

Let’s take the example of sailing from Vancouver to Silva Bay and let’s stick with the round number of 20 nautical miles as the approximate distance. We had a forecast of variable 5-15 knots.  FYI: A nautical mile is slightly longer than a statutory mile and knots are nautical miles per hour. Nautical miles are cooler than statutory miles because a nautical mile is one minute of latitude and so when looking at a chart you can use latitude to reckon distances.

Now first, what if we had 5 knots of wind? If that wind is coming from in front of us at about 60 degrees to our right or 60 degrees to our left, we might be able to sail at 4 knots (5 hours travel time). If that wind is behind us directly, we might be able to sail at 2 knots (10 hours travel time).

Readers will remember however, if the wind was directly ahead of us, we have to zig zag and so we might only make 1 knot toward our destination even if sailing faster through the water (20 hours).

If the wind was on the high end of the forecast and we have 15 knots of wind also at 60 degrees to one side, we might make 7 knots (3 hours of travel time) or if the wind was behind us we might make 5 knots (4 hours of travel time).
This means that in the simplest conditions we might take from 3 hours to 20 hours to make that trip.

Complications: A current against us would add time and a current with us would subtract time on either end. Waves slow us down if they are coming from ahead and speed us up if they are coming from behind. We might be the kind of boat that decides to motor if sailing less than a certain speed. As the distances get longer, the variability becomes greater. For example, a 40 mile trip in the same conditions could take 6 or 40 hours. Of course, I don’t know many people that would zig zag back and forth for 40 hours by choice. Finally, the length of the boat, the type of sails the boat has, its shape in and above the water, and the expertise of the crew all make a difference in how fast different boats go at the same wind speed.