Singlehanded sailors have one key problem: sleep.
While you’re asleep nobody is on watch so there is a high risk of collisions. To reduce these risks singlehanded sailor usually sleep in intervals of only 15 to 20 minutes.
But that sounds like an arbitrary number. Why not 40 minutes? Why not 10?
The biggest risk for us mini sailors is a collision with a fishing or cargo vessel. Since those ships often don’t (or can’t) alter course it is important for us to be able to still avoid a collision ourselves.
In order to do that we need to know how long it takes for another ship to cross our line of sight (where we cannot see it yet) until it arrives at our position.
Thanks to math we can calculate this!
The formula (simplified) is:
Distance of sight (nm) = 2.1 x ( sqrt(height of other ship in metres) + sqrt(height of my eyes))
Considering me being 1.75m in size I will assume 2m for the height of my eyes above sea level.
Now if I start seeing the toplight of a fishing vessel that’s 10m high then the ship will be about 9.6nm away.
If it’s a cargo ship which is 20m high I will be able to spot it from about 12.3 nautical miles away.
Now that we know the distance we can calculate the time if we know how fast the boats are approaching.
Assuming that the fishing vessel (10m high) does 8 knots while I do 5 knots and we’re approaching head on (i.e. our approaching speed is 13kn) then it will take 44 minutes until a theoretical collision. Still a fairly long amount of time.
But if we assume that our cargo vessel (20m high) does 30 knots while I am doing 10 knots downwind (i.e. we approach with 40kn) then it only takes 18 (!) minutes until both ships are in the same spot.
So it will be down to weather (=my own speed) and the type of vessels in the area (=the speed the others do) how long one sleeps in one interval.
I personally sleep in intervals of 15 to 20 minutes at a time.
I will explain how I make sure to wake up after one interval in a separate post at a later point in time.
This post is also available in: German