More on navigating with logs & sounders
April 22nd 2007 21:42
The depth sounder and speed log. These two instruments are also much aligned as we think that a depth sounder is primarily there to make sure we don’t run on to the bottom and the speed log to prevent us getting a speeding ticket. Right on both counts but it is not the entire reason for having them on board.
Remember the wiggly lines I mentioned on the paper charts of the coast line? Well they are indicators of the depth of the sea. They start shallow on shore and get deeper as we move off shore. Marked in fathoms (six feet or almost two metres) or metres. The type is given on the chart, a lot of charts are so accurate that they have not yet transferred them from when they were surveyed with the old square riggers to now. These depth lines are running roughly parallel to the coast. With little more than our “eye” in the sea we can follow the 30 fathom line from Botany Bay to the Great Barrier reef where it starts just north o Fraser Island: What an easy course to do. Just set your depth finder ad you can use the alarm, which goes off if you are going too deep or shallow. Easy, not much thinking required and I can assure you lots of people do it. Not very professional but it works!
How do you know where you are exactly on wiggly lines that look the same on each part of the chart. Well you can use the dead reckoning method and judge your speed and distance by the speed log. This is just like the speedo on your car except it measures in nautical miles, which is what we use at sea. All distances travelled are in nautical miles and speed in knots. (1 knot equals 1 nautical mile of 1852 metres) So if you have travelled north for four hours at say six knots for ten hours following say the ninety metre line after leaving Sydney Harbour, you have travelled about sixty miles and should be off the coast at Newcastle.
Fishing trawlers use their sounders to effect when they are chasing fish. If they rely on simple gear like this to make a living then you should be prepared to rely on it as well. Sure the fishing boat can also have mega-bucks of sophisticated gear on board. With all the bells and whistles but as one fisho told me, “I took a greeny over the bow, it knocked out one of the bridge windows and a wall of water one metre by half a metre exploded in to the cabin. It put out nearly six hundred thousand dollars of electrics in on felled swoop. I used my old depth sounder, a paper chart and handheld compass to start fishing again when the storm subsided.”
It is no mere chance that the known world before Columbus was within known continental shelves. If sailors could not plumb the depths with their lead line it was too deep and they were at the edge of the world. In the land (or sea) of monsters, turn round and go back! The sailing directions for Portugal were similar to this. “Sail free of the channel and proceed west sounding as you go. Once you have grey mud and shell at forty fathoms turn south”. It was that simple, we are lead to believe. Of course they had rutters which were a pilots personal notes on what the land falls looked like so they could be recognised but with no more than string, lead and compass they could navigate huge stretches of the coast line. Out of site of land, despite what our history books tell us.
The KISS principle, keep it simple for simple sailors.
Remember the wiggly lines I mentioned on the paper charts of the coast line? Well they are indicators of the depth of the sea. They start shallow on shore and get deeper as we move off shore. Marked in fathoms (six feet or almost two metres) or metres. The type is given on the chart, a lot of charts are so accurate that they have not yet transferred them from when they were surveyed with the old square riggers to now. These depth lines are running roughly parallel to the coast. With little more than our “eye” in the sea we can follow the 30 fathom line from Botany Bay to the Great Barrier reef where it starts just north o Fraser Island: What an easy course to do. Just set your depth finder ad you can use the alarm, which goes off if you are going too deep or shallow. Easy, not much thinking required and I can assure you lots of people do it. Not very professional but it works!
How do you know where you are exactly on wiggly lines that look the same on each part of the chart. Well you can use the dead reckoning method and judge your speed and distance by the speed log. This is just like the speedo on your car except it measures in nautical miles, which is what we use at sea. All distances travelled are in nautical miles and speed in knots. (1 knot equals 1 nautical mile of 1852 metres) So if you have travelled north for four hours at say six knots for ten hours following say the ninety metre line after leaving Sydney Harbour, you have travelled about sixty miles and should be off the coast at Newcastle.
Fishing trawlers use their sounders to effect when they are chasing fish. If they rely on simple gear like this to make a living then you should be prepared to rely on it as well. Sure the fishing boat can also have mega-bucks of sophisticated gear on board. With all the bells and whistles but as one fisho told me, “I took a greeny over the bow, it knocked out one of the bridge windows and a wall of water one metre by half a metre exploded in to the cabin. It put out nearly six hundred thousand dollars of electrics in on felled swoop. I used my old depth sounder, a paper chart and handheld compass to start fishing again when the storm subsided.”
It is no mere chance that the known world before Columbus was within known continental shelves. If sailors could not plumb the depths with their lead line it was too deep and they were at the edge of the world. In the land (or sea) of monsters, turn round and go back! The sailing directions for Portugal were similar to this. “Sail free of the channel and proceed west sounding as you go. Once you have grey mud and shell at forty fathoms turn south”. It was that simple, we are lead to believe. Of course they had rutters which were a pilots personal notes on what the land falls looked like so they could be recognised but with no more than string, lead and compass they could navigate huge stretches of the coast line. Out of site of land, despite what our history books tell us.
The KISS principle, keep it simple for simple sailors.
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