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Setting Tandem Anchors

A Category IV hurricane was heading our way, and we did not want to leave our boat in a marina that would have no protection from the ravages of a storm of this magnitude. Instead, we chose a lesser evil, one in which our boat had a better chance of survival—we anchored it, using tandem anchors—

Tandem anchors can be a useful weapon in the cruiser’s arsenal—as long as they’re deployed and set properly

A Category IV hurricane was heading our way, and we did not want to leave our boat in a marina that would have no protection from the ravages of a storm of this magnitude. Instead, we chose a lesser evil, one in which our boat had a better chance of survival—we anchored it, using tandem anchors—one in front of the other.

By the time the storm hit it had been downgraded to Category III, but more than made up for it by stalling as it approached the coast. We were pummeled by hurricane-force winds for over 24 hours, with 12 hours of winds over 110 knots—and as far as the seas were concerned, well, let’s just say they made life aboard fairly miserable.

Many thought that anchoring out in a storm this strong was foolish, and staying onboard even more so. Maybe, maybe not, but with our hefty ground tackle, it worked. Even with over 10 miles of fetch, our boat did not drag, nor was any of our gear damaged. However, another boat that also had tandem anchors set did not fare so well, in spite of being in an anchorage that was better protected than ours. This boat broke loose, causing considerable damage to itself and other boats around the anchorage.

What made the difference? When the other boat’s ground tackle was recovered, it became clear that many of the protocols for using tandem anchors were not followed, demonstrating that the use of tandem anchors is not as straightforward as it might appear.

Holding Fast

One theory has it that the increased holding power of tandem anchors is the result of the primary anchor plowing a trough in the bottom as it sets, which allows the second, following anchor to set deeper once it reaches the softened bottom. However, it could also be said that tandem anchors hold better because they consistently carry the ground tackle load together, unlike two anchors deployed independently, in which case the load is seldom carried by both simultaneously.

How much tandem anchors increase holding power is dependent on many factors, but tests conducted by the U.S. Navy reveal that tandem anchors set in mud increase the holding power by 20 to 30 percent compared to the same two anchors deployed individually. The increase in holding power is less marked in harder bottoms, such as sand. (Note: we don’t know what type of anchors were used in this test.)

Some of the protocols for using tandem anchors are the same as those that apply to the use of any anchor, such as selecting appropriate anchors (in terms of weight and design) for any given situation and ensuring that there is an adequate amount of scope and room to swing. In addition to these elements, some considerations specific to tandem anchors also need to be addressed.

What is the Proper Anchor Rode?

Tandem anchors can be set on chain, rope or a combination of the two. In addition to connecting the anchors to the boat, the main rode’s other job is to cushion surge loads. If the rode itself can’t do this, a snubber or other mechanism must be used.

You will need shorter secondary rodes to attach the two anchors to the main rode, or alternatively to one another. The secondary rode need not cushion surge loads, but must resist chafe while minimizing stretch. Ideally, a secondary rode will be made up of chain of the requisite strength.

Polyester, Dyneema or Spectra ropes can also be used for secondary rodes, as long as they are strong enough. Nylon rope is an option, though to counter its stretchiness and low resistance to chafe, it should be of considerable diameter. (The larger a rope’s diameter, the greater its resistance to chafe and stretch.) When used as an anchor rode, the load carried by nylon rope should not exceed 20 percent of its tensile strength.

Which Anchor, Where?

Two anchors set in tandem need not be the same size or design, as long as both can set in the type of bottom in which they will be used. However, it is important that the second anchor can set with a zero rode-to-bottom angle, and each anchor should be of a design that can re-set on its own should it trip.

In my experience, tandem anchors set best with 15 to 20 feet between them. If they are too close together, the second anchor’s rode can interfere with the working of the primary anchor. If they’re too far apart, when the boat veers it takes longer for the second anchor to return to the furrow plowed by the primary anchor.

Some believe the larger of the two anchors should be set closer to the boat, as it will have greater holding power should the rode-to-bottom angle increase beyond a low angle. However, if enough rode is deployed for a minimum 10:1 scope, then it won’t matter which anchor is deployed first.

Why, When and How

There are two effective ways to deploy anchors in tandem: with a single secondary rode, or with a double secondary rode.

With a single secondary rode, the second anchor’s rode is attached directly to the primary anchor, which in turn is attached to the main rode. It is critical that the rode be attached in a way that doesn’t interfere with the primary anchor’s ability to set; the secondary rode should never be attached to the crown of the primary anchor or to its shackle.

Tangs or bolt holes provided for trip lines are seldom positioned correctly for attaching a tandem anchor, and as such they shouldn’t be used for this purpose unless the manufacturer indicates it’s OK to do so. The only anchor that has a dedicated attachment point for a tandem anchor is the Rocna. If you are tempted to add a tang or hole for attaching another anchor in tandem, consult the manufacturer first.

If you can’t arrange a satisfactory attachment for the second anchor’s rode, then you will have to use a double secondary rode configuration.

Here, each anchor’s secondary rode is attached independently to the end of the main rode. You can use shackles, splices or (not ideally) a knot for this job, usually attaching the secondary rodes to a large ring or shackle installed at the end of the main rode; this ring or shackle must be large enough to be able to easily accommodate both secondary rodes. Possibly, in place of a ring or shackle, the outboard end of the main rode could simply end in a loop of some sort, whether knotted in, lashed together or spliced.

To minimize any interference between the second anchor’s rode and the primary anchor, the primary anchor’s secondary rode should be about 1 to 2 feet long and the second anchor’s rode must be longer, say 15 to 20 feet.

Tandem anchors are a good weapon to have in your anchoring arsenal when you feel you may need some extra holding power. However, you shouldn’t regard this technique as a substitute for not having an adequately sized main anchor.

Finally, if you are going to use tandem anchors, make sure you have all the necessary gear, and practice deploying and retrieving them. A heavy blow, with the bow pitching in a seaway, is not when you should be deploying tandem anchors for the first time.

Retrieving Tandem Anchors

There is no simple or easy way to retrieve tandem anchors. You are strongly advised to wait until the wind and seas have moderated before attempting recovery. You may need to deploy another anchor to hold the boat in position while the tandem anchors are being retrieved.

In the simplest form of retrieval, the primary anchor is recovered and the secondary rode is then manhandled onboard, along with its anchor. This may be most easily achieved by fastening a length of line between the second anchor’s tripline hole and the main rode. This line should be some 10ft longer than the secondary rode. Floating line can be used to avoid entanglement with the rode or anchor. Once the primary anchor is at or in the bow roller, you can then use this line like a tripline to haul up the secondary anchor, followed by its rode—a lot easier than leaning over the pulpit and trying to get a grip on the secondary rode with a heavy anchor on its end.

Be prepared to think laterally; retrieval of heavy anchors may involve two or more people, a windlass or a winch, a davit or boom, a tackle, cushioning of some type to minimize the dings to the boat, or other contrivances—as well as some ingenuity.

If you are going to detach the secondary rode before its anchor is brought on board, bend a tether onto the rode and belay it to the boat to prevent the loss of this gear.

A buoyed tripline may aid the retrieval of the second anchor. It must be able to handle the strain that is associated with breaking out an anchor. This strain can be in the thousands of pounds for an anchor that can bury 10ft or more into a soft bottom. Half-inch rope is generally accepted as the smallest diameter line to use for handling comfort.

The Importance of Scope

The angle of the rode in relation to the bottom is the key to maximizing an anchor’s ability to hold. Those long-used ratios for scope—10:1 for rope rodes, 7:1 for rope and chain rodes and 5:1 for all chain rodes—allow for zero degree rode-to-bottom angles due to the catenary lays. It does not get any better than this. However, this applies only when anchoring in mild conditions.

As the wind picks up, the anchor rode is loaded and the rode-to-bottom angle increases. An anchor starts losing holding power as the rode-to-bottom angle increases beyond zero, but from a functional perspective, any increase up to 6 degrees is usually acceptable. As this angle increases, so does the risk of the anchor tripping. (For example, with its rode-to-bottom angle at 12 degrees an anchor has only half of the holding power as it would at 6 degrees.)

At a sustained wind speed of around 40 knots (less for an all-rope rode) an anchor rode begins to straighten out and its ability to decrease the rode-to-bottom angle through catenary effect disappears. The anchor’s holding power becomes dependent on the length of rode you have deployed, as this now determines the rode-to-bottom angle. This is why more rode is deployed as the wind increases.

Anchors can lose holding power at even lower wind speeds, thanks to waves and wind gusts. The depth of water increases as the boat crests a wave, while the length of the rode remains unchanged, thus increasing the rode-to-bottom angle. Wind gusts stretch the anchor rode, which also increases the rode-to-bottom angle. In some anchorages, gusts and waves together can decrease an anchor’s holding ability in wind speeds as low as 25 knots.

Calculating Rode-to-bottom Angle

Once your anchor rode is taut, a scope of 5:1 creates a rode-to-bottom angle of around 12 degrees; 10:1 produces an angle of around 6 degrees. To a certain extent, a larger anchor can compensate for less scope, but there are some factors to consider. Published holding-power figures for anchors are based on tests with long scopes, so when decreasing scope you can’t know at what point an anchor will trip. This tripping point can usually be found through experimentation, but this is a risky proposition. In severe weather, substituting a larger anchor for more scope may require an anchor so large it is impractical.

Even when you do everything right, an anchor can still drag. What then? First try letting out more scope. If the boat is still dragging and you are certain the scope is 10:1, it is likely that the anchor is too small, its design is wrong for the type of bottom, or both, in which case the anchor needs to be changed out to one that is more suitable for the conditions.

Many folks have been misled about the effectiveness of kellets. Once the wind picks up, kellets have very little effect on the catenary curve, though they may still remain effective for other purposes. As the wind picks up, letting out more rode has a much greater effect on the rode-to-bottom angle than does deploying a kellet.

Calculating Scope

The following elements are added together in order to calculate scope:

The depth of the water

The height of the bow roller above the water

Any additional depth of water contributed by the tide

But in gale force winds and above, the following factors also need to be included in the calculation:

The height of the storm surge

The height to the crest of the wavesTthe depth to which the anchor buries

Short the calculations on any one or more of these six elements, and you’ll likely not have deployed the correct amount of scope. If you can’t determine exact figures for any of these elements, err on the side of caution and use a scope greater than 10:1.

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