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Can’t Fathom Depth in Metric?

Mar 3, 2017

Author: Fin

Fintan Hartnett is the Principal and Chief Instructor at Topmast Maritime Training and an RYA Yachtmaster Instructor as well as Sail Canada and ASA instructor.

Being back in North American waters has highlighted a rather unique feature of being a Canadian – units of depth. Canada falls somewhere between the metric system and the US (Imperial) system of measurement. We’re caught at that dangerous crossroads between modernity and antiquity. Guess which system is which. As a result we’re frequently pulling out charts for a given area which have numbers requiring further interpretation – failure to do so can have drastic consequences!

If, as an example, you happen to be nautically grounded in the metric system (as am I) and you find yourself running your fingers along the depth contours on a chart, comfortable the depth is enough to give good clearance under the keel, it can come as a shock to find oneself uncomfortably grounded on a sandbar – or worse. The mistake could be a simple failure to check the title block of the chart and find out the units of depth are in feet and not metres.

Our American friends, who sometimes appear hostile to the idea of the metric system, may make the same mistake – minus the consequences. When they accidentally interpret the number 3 on the chart as 3 feet they quickly move to avert disaster by heading for deeper water. The reality is they are perfectly safe in 3 metres of water (depending on their draught/draft). Where it may be bad for American sailors is if they mistake 20m of water for 20 ft and don’t put out enough chain when anchoring – c’est la vie.

Which leaves the last really disturbing standard of measuring depth, the use of fathoms. What is a fathom? I know plenty of Americans who don’t know the answer either, but the right answer is 6ft or 2m (more or less). When a chart displays the worrying annotation “soundings in fathom and feet” it’s time to break out the calculator and the aspirin.

Last Thought:
My last thought on this subject (for now) has to do with the depth sounder. On more than one occasion I’ve commissioned a brand spanking new boat and taken it out for a shakedown cruise only to find the depth sounder is reading in feet and not meters (stock standard for US products). Apart from the technological button pressing annoyance of switching the display to meters is making sure the depth sounder and the charts (paper and electronic) are using the same unit of depth, which may be a challenge here in North America.

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