Articles
Ground Tackle Considerations
The selection of ground tackle for a cruising boat involves the careful consideration and balancing of a number of elements, including not only the anchor but also the rode, shackles, bow rollers, windlass, and snubber. While a detailed treatment is beyond the scope of this article, some factors to consider are presented.
Boat Size, Wind and Wave Loading
A good place to start in considering a ground tackle system is to estimate the loads experienced under various wind conditions. Table 1 is adapted from an American Boat and Yacht Council (ABYC) publication, and provides estimated loading from the combination of wind and wave action, for various sized boats and at four wind strengths. This table is conservative in that the loads indicated are nearly 4x those which would be experienced due to wind alone. This safety factor is intended to account for pitching, surging, yawing, and other dynamic effects which can considerably increase loading.
Table 1. Loads for Ground Tackle Systems (adapted from ABYC) |
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Boat Length (LOA) - feet |
Beam (Bmax) - feet |
Load on Ground Tackle - pounds |
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Sail |
Power |
15 knots |
30 knots |
42 knots |
60 knots |
|
25 |
8 |
9 |
125 |
490 |
980 |
1,960 |
30 |
9 |
11 |
175 |
700 |
1,400 |
2,800 |
35 |
10 |
13 |
225 |
900 |
1,800 |
3,600 |
40 |
11 |
14 |
300 |
1,200 |
2,400 |
4,800 |
50 |
13 |
16 |
400 |
1,600 |
3,200 |
6,400 |
60 |
15 |
18 |
500 |
2,000 |
4,000 |
8,000 |
In using this table, select either the appropriate length or beam, whichever gives the greater loading. A coastal cruiser should design their ground tackle system to handle 42 knots of wind, which represents typical gale conditions on the East or West coast of North America. A cruiser who ventures offshore should consider the 42 knot column as the minimum, and would more conservatively design their ground tackle based on the 60 knot figures. In selecting rode and shackles, the Working Load Limit (WLL) should meet or exceed the expected loading.
Rode Selection
For offshore cruising, there is really no substitute for an all-chain rode. While the weight is a burden, both for the crew and the sailing performance, this is more than compensated by the resistance to chafe and the ability to reduce scope compared to an all-rope or combination rope-chain rode. For some of the really deep anchorages, it is reasonable to carry a length of heavy nylon rode, sized according to the chart above, to extend the rode.
When using a chain rode, a nylon snubber should be used to minimize shock loading on the attachments points in case wind and waves pick up, removing catenary from the chain.
Anchor Considerations for Differing Bottom Types
The characteristics of the bottom have one of the largest impacts on an anchor’s ability to set.
- Soft Mud or Soft Sand. In a soft bottom almost any anchor will set, but holding power is directly related to the projected fluke area, which is determined both by the anchor’s “angle of attack” and the size of its fluke(s). Look for an anchor which maximizes this projected area.
- Hard Mud or Hard Sand. While hard mud or sand provide excellent holding once an anchor is set, they can provide more of a challenge for an anchor to penetrate. Look for an anchor designed to position itself with significant weight on the tip to aid in penetration. In this situation, an aluminum anchor is at a disadvantage, due to its lower overall weight for a given size.
- Weeds or Grass. Similar to hard mud or sand, in that an anchor must have sufficient weight on its tip to penetrate, but with the additional requirement of a “sharper” tip to slice through the vegetation.
- Rocky Bottom. No anchor will hold well with a flat rock bottom, but an anchor which can “hook” rocks on the bottom can make the best of a difficult situation…often a fisherman or grapnel type anchor is recommended in these situations, but the better choice is to find another anchorage if possible. Beware of the possibility that a “claw” type anchor can scoop up a rock and thereafter fail to set properly.
- Mixed Sand and Coral. Frequently found in South Pacific anchorages, the trick is to drop the hook into a sandy patch large enough to prevent snagging the rode (or the boat!) on coral. Once done, this has the same considerations as soft or hard sand.
Set The Anchor
All too frequently a cruiser drops the hook and assumes that all is well, only to find out when the wind picks up in the middle of the night that the anchor is not holding. The time to find that out is when you drop the hook, so be sure to back down on the anchor at around 2/3 power, and check for dragging both by monitoring the rode for vibration and by sighting a range ashore.
Veering Loads
When winds shift or tides change, the loads on an anchor can veer by up to 180 degrees. Many anchors will pop out of the bottom under these conditions, and have to re-set. Look for an anchor which is designed to remain buried under veering loads – sometimes called “roll-stability”.
With careful consideration, it is possible to assemble a ground tackle system which will provide safe and reliable anchoring across a wide range of conditions. Make the effort to do it right, and you will be rewarded with a safer journey, and a better nights sleep.
While an article such as this will necessarily provide only an overview of the process involved in selecting a complete anchoring system, the interested reader may refer to the publications cited below for a more detailed treatment. Rocna has also created an extensive wikipedia style resource on anchoring, found at www.rocna.com/kb. Refer to this site as well for information on rode selection, anchoring tips, accessories, and a wealth of other useful information.
References
Hinz, Earl R. The Complete Book of Anchoring and Mooring, 2nd Edition, Cornell Maritime Press, 1994
Calder, Nigel. Cruisers Handbook: A Compendium for Coastal and Offshore Sailors, International Marine/McGraw-Hill, 2001