More on estimation
February 21st 2007 03:23
More on estimating:
In the last couple of blogs I have said how you can accurately judge distances and how you can estimate them quite accurately. Here are a few more tips.
Speed through the water: To work out your speed without a log and without using a string log dropped off the stern with marks on it that are timed as they run through your fingers try this. You know your vessels length. Now I know that it grows longer when you are standing at the bar talking to your mates and grows shorter when you are booking for a haul out or paying for a marina berth. Use the actual length. From the bow time as you drop an object in the water until it passes the stern. Lets say 12 metres of length in 5 seconds. We now have distance and time and from that we can work out speed. The equation is 12 metres times 1.852 (to get a nautical mile answer) but lets just multiply by two as we are estimating. So 24 divided by five equals four point eight knots and if you called it five you would be close enough.
Height over the water: Not long ago, I had to take a large (fifty-five foot) ketch up the swamp behind the islands off Brisbane to avoid some cyclone affected water at sea. We knew our depth and could estimate the depth of the swamp under out keel but suddenly we had power lines across our path. Not something you come across at sea so we had to work out if we could fit under the live wires, fry or turn back.
I put the question to the crew and they came up with many novel ideas but the natural and most accurate was to run a plumb line up the mast on a spare halyard and measure its length, add some metres for the mast top gear. Then plumb the depth between the water and the top of the bottom of the mast. We had about three metres to spare. Alongside we could have held a stick out at arms length. One end of the stick at the top of the past and our thumb moved down the stick to the water line. Now role the stick on its side allowing for the water line and measure to the tip along the wharf.
From your position at the helm of the average yacht the horizon is three nautical miles. If a ship is heading toward you and you can just make out its white bow wave on the horizon, it is likely to be three miles away, (or even much less). If you are travelling at six knots and he is travelling at twenty-four knots your joining speed is thirty knots. Three miles at thirty knots is six minutes. You had better get out of the road as quick as you can!
Some large ships have a blind spot as far as out as one and a half miles so very soon you will could be hidden from view and become shark bait. If you are in a GRP or fibreglass boat it is unlikely he has picked you up on radar or if you are quite small he may not see you in the sea clutter on his radar. It gets scary out there!
In the last couple of blogs I have said how you can accurately judge distances and how you can estimate them quite accurately. Here are a few more tips.
Speed through the water: To work out your speed without a log and without using a string log dropped off the stern with marks on it that are timed as they run through your fingers try this. You know your vessels length. Now I know that it grows longer when you are standing at the bar talking to your mates and grows shorter when you are booking for a haul out or paying for a marina berth. Use the actual length. From the bow time as you drop an object in the water until it passes the stern. Lets say 12 metres of length in 5 seconds. We now have distance and time and from that we can work out speed. The equation is 12 metres times 1.852 (to get a nautical mile answer) but lets just multiply by two as we are estimating. So 24 divided by five equals four point eight knots and if you called it five you would be close enough.
Height over the water: Not long ago, I had to take a large (fifty-five foot) ketch up the swamp behind the islands off Brisbane to avoid some cyclone affected water at sea. We knew our depth and could estimate the depth of the swamp under out keel but suddenly we had power lines across our path. Not something you come across at sea so we had to work out if we could fit under the live wires, fry or turn back.
I put the question to the crew and they came up with many novel ideas but the natural and most accurate was to run a plumb line up the mast on a spare halyard and measure its length, add some metres for the mast top gear. Then plumb the depth between the water and the top of the bottom of the mast. We had about three metres to spare. Alongside we could have held a stick out at arms length. One end of the stick at the top of the past and our thumb moved down the stick to the water line. Now role the stick on its side allowing for the water line and measure to the tip along the wharf.
From your position at the helm of the average yacht the horizon is three nautical miles. If a ship is heading toward you and you can just make out its white bow wave on the horizon, it is likely to be three miles away, (or even much less). If you are travelling at six knots and he is travelling at twenty-four knots your joining speed is thirty knots. Three miles at thirty knots is six minutes. You had better get out of the road as quick as you can!
Some large ships have a blind spot as far as out as one and a half miles so very soon you will could be hidden from view and become shark bait. If you are in a GRP or fibreglass boat it is unlikely he has picked you up on radar or if you are quite small he may not see you in the sea clutter on his radar. It gets scary out there!
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