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    Re: Welcome Aboard! New and returning...
    From: George Brandenburg
    Date: 2011 Feb 7, 13:26 -0800

    Not that it makes any difference in this case, but in freshman physics one learns that a floating object displaces an equivalent weight of water. Since ice is roughly 10% less dense than water, an ice sheet of uniform thickness will float with about 10% of its volume above the surface of the water. This means that the effective water level is about one foot below the surface of a ten foot thick sheet of ice.

    From: sjoquist.magnus---com
    Date: 7 Feb 2011 08:53

    Re horizon on sea surface covered with 4 feet of ice: On a cargo ship (where I have most of my navigational experience) where the height of eye may easily be 25 meters distance to horizon is around 10 nautical miles. At that distance 4 feet has no significance and can be neglected.
    If your height of eye is in the range where yachtsmen feel more at home (a couple of meters) you would have to be more careful, in particular watching out for ice ridges which could play a joke with you in the same manner as high swell. Why not checking with with an home made simple artificial horizon? Two (or more) observations with the horICEon and a couple with the artificial (i.e. a tin of water - double angle etc). Assuming you are not moving between the observations and that you take them more or less the same time, they should give you the same line of poition.

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