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Anchor Nomogram

So when you ask what length boat will a 10kg anchor hold you may as well ask "How long is a piece of string?" As I have shown the answer depends on a combination of six main parameters:

  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.

To make the effects of these parameters more understandable I have combined the main graphs in the form of a NOMOGRAM - a sort of anchor computer. (Fig 21)

From this you determine the load your anchor should hold under various conditions or conversely what size and type of anchor you will need for your boat in different circumstances.

This paper has been prepared for the benefit of the leisure boating industry - principally cruising yachtsmen. The same principles expounded apply to the anchoring of floating marinas. They have not been mentioned for two reasons:

The parameters in my nomogram still apply but, for interest the following special circumstances must be considered.

Wind loading is not calculated so much on the marina itself, as on the vessels that can be berthed and tied to it, broadside to the wind. Wind loading is therefore greater than that of a vessel riding at anchor.

Anchors normally used are:

A. Concrete blocks with an efficiency index of 0.5

B. Mushroom anchors with an efficiency index of 2.0

C. Ships anchors (Stockless or Dreadnought) with an efficiency index of 3.0

D. Ships anchors Admiralty Pattern with an efficiency index of 5.0.

Rodes of heavy ground chain and lighter top chain contribute greatly to anchoring capacity.

The anchors can be buried in the seabed with mechanical equipment and divers thereby improving the efficiency of holding power by a factor of up to 10 times.

I would like to take this opportunity now to introduce a new type of anchor I have developed. It is called the Flook and is a Flying Anchor capable of self deployment. That is, when launched into the water, it glides out at an angle of 5:1 under the water and lays itself out away from the vessel 50ft for every 10ft of depth.

When it lands out on the seabed, a backward jerk on the rode opens its flukes and digs them into the bottom. The geometry is such that rode angle of about 25 degrees will dig the flukes in (the shank/fluke angle is some 30 degrees) and this can be achieved without any chain catenary at a scope of 2.5:1.

Because of the large Delta Wing a very high fluke area to weight ratio is obtained, greater than most lightweight sand and mud burying anchors. Hence the Flook, in addition to its "Flying" self deployment, has an extraordinarily high holding power and efficiency index of at least 40:1 in sand or mud.

The burying capability of the Flook is achieved not only by the Fluke-Shank angle but by the fact that the Fluke is displaced vertically below the shank in a manner similar to the Bruce and the CQR anchors. This is done with a unique Pantograph linkage.

The use of this Flook anchor will revolutionise anchoring techniques of the past. No longer will it be necessary to know the depth of water and to measure out a rode of appropriate length. This happens automatically. No longer is it necessary to drop anchor upwind, or upstream to back down on your final anchoring point. No longer is it necessary to row out an anchor in the dinghy to set a stern anchor or a kedge. No longer is it difficult to lay a Bahamian Moor or a Hammerlock to prevent swinging.

Full details of the Flook anchor and its unique features follow and the very interesting underwater video available.



nomogramme is best seen with eyes open


How to Use The Anchor Computer

1. To determine what forces will be imposed on your vessel under different storm conditions:

a. Start on Graph A on the vertical scale left side at the wind speed (V) expected in knots.

b. Draw a horizontal line across until it intersects the shape coefficient curves of your particular vessel.

c. From this intersection, draw a line vertically down or up, until it meets the oblique line for the appropriate cross sectional area of your vessel.

d. From this point, move horizontally to the right hand vertical scale, where the wind load is kg force, including surge and veer factors, can be read off.

2. To determine what holding power your anchor will have under certain conditions:

I. a. Start on Graph B from the left side vertical line (kgs left side - lbs right side).

b. Draw a horizontal line until it intersects the type of anchor expressed by its Efficiency Index in sand (e.g. 6 for Admiralty pattern, 40 for Flook).

c. Suspend a vertical line to the bottom line, which gives the maximum holding power of the anchor (in kgs force) under ideal conditions). (Example shown - 10 kg(22 lbs) anchor with E.I.= 30.

II. a. Continue to extend this vertical line onto Graph C, until it intersects the modifying line for type of rode used.

b. From this point, show a line horizontally.

III. a. Continue this horizontal line to the Graph D, which again modifies the holding power for the effect of cope, until it intersects the appropriate line for scope.

b. From this point, draw a line vertically to the top.

IV. a. Continue this vertical line upwards into Graph E, until it intersects the appropriate line for bottom type; e.g. sand, weed, etc.

b. From this point, draw a line horizontally to the left side vertical scale and read off the final holding power modified for the conditions applied.

3. If the holding power of your anchor, under the conditions applicable, is equal to or greater than the loads predicted from the wind and surge pressures, you should be safe in your anchorage.

4. If the holding power is less than the predicted forces:

a. Continue from the right hand vertical scale of Graph A, where the wind load was determined (combined Wind Load/Anchor Holding Strength Scale).

b. Move horizontally to the "bottom condition" curve in Graph E.

c. Drop vertically from this intersection, until you intersect the appropriate scope of rode line on Graph D.

d. From this point, continue horizontally from right to left until you meet the applicable "Type of Rode" line on Graph C.

e. From this point, extend a line vertically upwards, where it will intersect a number of radial efficiency lines in Graph B.

f. From the various points of intersection, draw lines horizontally to the left or right, and read off (in kgs on the left, lbs on the right) the weights of each type of anchor, which will hold the load required.

5. The effect of altering any of the parameters on a particular situation, can readily and graphically be seen.

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