Helwig F. Van Der Grinten
Is your anchor dragging?
When anchoring a large ship it is not as simple as it might seem to determine whether the anchor is holding. I have created four spreadsheets which makes this easy. One spreadsheet exists for each of the four quadrants of the globe: NE, NW, SE and SW.
First these spreadsheets calculate the radius of maximum swing by applying the distance between the antenna and the stern, antenna and the hawsepipe, and the amount of anchor chain let out. By applying simple trigonometry to both the distance between the GPS antenna and the hawsepipe and the heading of the ship to the GPS position at the moment the anchor is dropped, the drop point is determined. A circle can then be drawn on the chart around the drop point to indicate where the ship might be in all conditions of wind and current.
It is not enough to know that the antenna position remains well inside the radius of maximum swing because when a changing strong wind or a strong current occurs the antenna could be significantly closer to the drop point than the hawsepipe and a dragging anchor or slipping chain will not be immediately obvious.
Each spreadsheet applies the same trigonometry to subsequent position and heading checks to determine the position of the hawsepipe at any time. The distance between the hawsepipe and the drop point is calculated and compared to the amount of chain let out. If this ratio is greater than one, either the anchor is dragging or the windless is slipping.
I will provide to interested users four free spreadsheets with the proper dimensions for his or her ship.
Send me your:
Distance between the GPS antenna and the hawsepipe (m)
Distance between the GPS antenna and the stern of your ship (m)
see sample below
Email me at: email@example.com.
H. F. Van Der Grinten
October 6, 2007
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