You could start like this if you wished:
The ratio of consecutive Fibonacci numbers tends to the golden ratio approx = 1.618 as n increases and so on but rules of thumb are still useful.
Take 3 miles, 5 miles and 8 miles for example – how many km do they come out to? Roughly?
There are all sorts of RofT – for example, a kg is 2.2 lbs, so 15 kg comes to about 32 or 33 lbs, the weight of a sandbag of ballast in the boat. I weigh around 92.5 kg at this moment.
Knots are interesting too – a knot is 1.15078 mph – I tend to think of 20 knots as 23 mph.
A square metre is 10.76 sq ft. For metres, divide feet by 3.28.
My boat is 11.97 metres now – reduced length, in order to fit the rudder in under 42 ft, nothing else overhanging bow and stern. What length is my boat in feet?