How long will it take?
We are all familiar with the equation “speed equals distance over time” but when you are whizzing around in a small boat it is inordinately difficult to get your brain to do the simplest sums. We could use the calculator on our phone, or a clever slide rule device, but it is probably bumpy and wet… and we should be looking out! However, by carefully selecting the speed at which we plan our navigation exercises we can reduce the amount of grey matter required and make navigation easy and fun. How do we do that ?
Well, 60kts is equivalent to 60 nautical miles an hour, and as there are 60 minutes in an hour, we can say that 60kts equates to 1nm every minute. Well, we’d be very lucky or rich to own a boat that can regularly do 60kts… but using sub multiples of this fact enables us to work out speed, distance, time calculations very easily.
First off, if 60 knots means it takes a minute to travel a mile, then at 30knots (half of 60kts) it should take twice as long… i.e. 2mins to travel a mile. At 20kts (a third of 60kts) it will take 3 minutes. And so on. If you plan your navigation at these easy multiples then ETAs/Time of travel are easy! For most small powerboats, 20kts is a good speed for general navigation.
Going slower, say for the pilotage phase at the end of a trip, 6kts is a good speed. It is a tenth of 60kts, so it takes 10 mins to travel a mile… or… more easily for pilotage planning… 1 minute to travel 0.1nm. We can thus measure the distance between buoys, or turning points, and easily convert it to time to travel. For example, 0.4 miles will take us 4 minutes.
In Summary:
30kts is equivalent to 2 mins for 1 nm
20kts is equivalent to 3 mins for 1 nm
10kts is equivalent to 6mins for 1 nm
…and for pilotage…
6kts is equivalent to 1 min for every 0.1 nm
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