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Principle and Performance of Anchors

The anchor on your boat is like the parking brake on your car. When its on a hill or your boat's exposed to the wind, its essential that you have one; and a good one at that. Your boat, its contents and your life are worth more than a few dollars!

Don't skimp on the anchor and tackle. You can buy an excuse for an anchor for as little as ten or fifteen dollars but the first time it comes to needing it, you will realise you have simply wasted your money - and probably voided your insurance!

How do you know what type and size of anchor your boat really needs? There are numerous rules and regulations, some mandatory, some voluntary, all different. Recommendations are freely given by boating writers and experienced skippers but even two experts behind the counter of the ship chandlery where you buy your anchor, will argue on the details.

Boating writers and anchor manufacturers have listed sizes of anchors for varying boat sizes. The anchor sizes are always listed by weight and the boat sizes listed by overall length.

AnchorSizeBoatSize
2-4 kg4-9 lbs3 m9 feet
4-6 kg9-13 lbs5 m15 feet
6-8 kg13-17 lbs7 m21 feet
8-10 kg17-22 lbs9 m28 feet

Fig 1

An old English rule stipulates a pound of anchor for each foot of boat. The funny thing is that the weight of an anchor has little to do with its holding power and the length of a boat has little to do with its anchor loading.

The main reason the respective sizes of boats and their required anchors is expressed in length and weight, may be simply because everyone knows the length of their boat and the weight of an anchor. Very few boat owners know the frontal area or "windage" of their boat, nor the area and efficiency of their anchor flukes. Yet these may be the most important, if not the only criteria for judging what size anchor may hold what size boat, under a known set of conditions.

Making the same anchor twice as thick and twice as heavy would add very little to its holding power. It's the shape and the size of the anchor that matters.

Even the Regulations for Commercial Vessels pay little heed to these anchor holding facts. For example, the USL Code Table 1, Appendix H specifies a weight of X kgs for a vessel of Y length, for a vessel up to 75m in length. An acknowledgement for "high holding power" anchors is allowed to the extent of a 30% reduction in weight from the "normal" anchor. (A high holding power anchor is specified as one which holds more than 3 times its own weight. Modern anchors are designed to hold 30 times their own weight!)

So, this paper outlines;

  1. The "windage" of the boat (not its length)!.
  2. The expected wind velocity.
  3. The type and size of anchor rode.
  4. The SCOPE laid out.
  5. The type and size of anchor; its "efficiency index" (NOT necessarily its weight).
  6. The type of bottom ground.

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