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?

22:17:

Having spent a lifetime of DIY jobs like decorating or garden organisation I use simple approximations all the time. When converting metric I reduce the actual conversion factor to a simple multiple that can be instantly done without pencil, paper, calculator or Google!

A Km is 5/8ths – ie between half and 2/3 of a mile So your 3 miles is about 5Km, 5 miles is 8 Km and 8miles in spitting distance of 13Km.

Metres are roughly yards+10%. 12 yards +10% is 36feet +3.6 feet. It’s not quite 10% and your boat is 3cm short of 12metres so I would say your boat is 39feet and 6 inches or an inch under that.

The yards +10% works for simple area too. A sq yard=9 sq feet, for metres add 10% for each dimension -so 9+ 10% +another 10% gives you 9 +.9 +.9 or 10.8 square feet to a square metre.

‘Close enough for folk music’ as they say 🙂

39.6 ft (11.97 multiplied by 39.7 divided by 12)

39.27 feet. Important because the CRT’s next band starts at 39’4″.

So my boat estimation (which I did honestly) was a couple of inches over. I’ll live with that as a rough rule of thumb estimate.

Of course not forgetting that a rough estimate should always be made as a cross-check to prevent gross errors in the exact calculation.

Exactly right and I’ve made a few of those – errors of scale.

What I was proud of was that in boat Mk 1, it was 5/8 of an inch out end to end. As for these mm, do you understand them? Make no sense to me. In wood, you need broader units and fractions of them.