Saturday, 13 December 2008

Fathom this...

A fathom used to be the most common measurement of depth at sea (it has now been largely superseded by the meter). It could also be used to measure distance although this was less common.

I am fascinated by the history or Etymology of words. I understand that the word Fathom is derived from the old English word Faedm meaning to embrace, this in turn was derived from the Danish word Favn which literally means to have ones arms outstretched. The Swedes have a similar word Famm which also means to embrace.

The natural (and quickest) way for a sailor manning the lead on a sailing ship to measure the depth was for him to measure it in armspans as he pulled it in. Thus the Danish word favn passed into common usage and came to mean the distance between the tips of the middle fingers when a man's arms are outstretched sideways to their fullest extent. Fortunately this distance is roughly 6 feet so this has become accepted as the true distance of a Fathom. In a rare moment of poetic beauty an early act of parliament defined it thus: "the length of a man's arms around the object of his affections."

Of course in an age of echo-sounders and GPS plotters it is no longer acceptable to use a hug as a unit of measurement and so the Fathom has become almost obsolete. Most charts are now metric and give depths in metres. However, we still occasionally use Fathoms on-board the lifeboat. Why? I'm not sure really as it does cause a certain amount of confusion. As an example, our echo-sounder will give a reading in metres, feet or fathoms. We generally use metres or feet as we understand them. However, our anchor cable is marked in Fathoms (every 15 Fathoms in fact). And so to decide how much anchor cable to let out you must measure the depth in feet and then as a rough rule of thumb multiply this by 3 to give the required length of cable for normal conditions. You then need to divide this by 6 to give the number of fathoms required! So, with a depth of 30ft, multiply by 3 gives 90ft of cable needed, divide by 6 equals 15 fathoms. Luckily in this case our cable is marked every 15 fathoms.

To add slightly to the confusion we take a fathom to equal 2 metres whereas it is actually only 1.83 meters. Where the Fathom is used to measure distance it is multiplied by 100 to give a cable. A cable is often taken to be 200 meters whereas it is actually only 183 metres. A cable is also normally accepted as being one tenth of a nautical mile, again, this is not actually the case and 10 cables are less than one nautical mile. Confused yet?

But, if you think about it, why worry too much. These measurements were used in a time when there was no accurate way to measure anything on a sailing ship. If you can't measure accurately there is no point getting too stressed about what units you are using!

(thanks to: 'the Oxford companion to Ships and the Sea', Lena (mother-in-law) and Lisa (Danish mate))


Joe Crow said...

Excellent post John, I do enjoy reading your blog.

lifeboatjohn said...

Cheers'd always nice to get feedback, especially when it is positive!

Hope things are going well with you guys?


Anonymous said...

15 fathoms is a shackle! derived from when chain was supplied in such units and the shackle was used to identify the amount of chain out.

lifeboatjohn said...

Curiously we never use the expression 'shackle' in the RNLI though the term is still in common usage in RN.

Unfortunately though things are never quite as simple as you might expect. A shackle of cable was originally (Pre 1949)supplied to the Navy as a length of 12.5 Fathoms or 75ft. Later this was changed and became 15 Fathoms or 90ft! The orginal measurement meant that there were 8 shackles to a cable (assuming you take a cable to be 600ft rather than the more accurate measurement of 608ft), unfortunately the more modern length of 90ft is not a simple divisor of a cable there being 6.66 modern shackle to the cable.

I'm beginning to appreciate the metric system!


derrick said...

nice post. Interesting stuff too. :)