Short-scope Anchoring in Deep Water

The author observes his anchor in action in 40ft of water

The author observes his anchor in action in 40ft of water

Do you ever wonder how best to anchor when the water is a bit deeper than what you are used to? Do you wonder if you have enough rode to lay out a proper amount of scope? Are you shy about dropping the hook in busy anchorages where only deep-water spots are still available?

In deep anchorages that are either small or crowded, we occasionally need to anchor on shorter scope than might be considered ideal. In these situations we have used less than 4:1 scope in, say, 40-plus feet, where the weight of so much chain being laid out helps to keep the anchor rode lying close to the bottom. This is called the catenary effect, in which the rode’s weight holds it down and strongly dampens any horizontal loads.

The question then becomes, how effective is this really? Experience has taught us that this is a reasonable practice in deep water, but I wanted proof. So we decided to test our ground tackle when set with a scope of less than 4:1 in a deep anchorage and film the results. Our boat is a 49ft swing-keel Southerly, and our main anchor is a 73lb Rocna attached to 260ft of ⅜in chain. We often anchor with 4:1 scope and even less in deeper water. Although we have almost never dragged our anchor, even in stormy conditions, I was hoping to actually watch our anchor underwater in this situation and see if the weight of the chain made much difference.

Recommended Scope

My wife, Sheryl, and I have cruised extensively for 25 years throughout the Atlantic, the Caribbean, the Mediterranean and Northern Europe, and it turns out the answer to “what is the recommended scope?” is somewhat cultural. In Europe, for example, most boats have all-chain rode (if rodes are a mix of rope and chain, they usually have 30ft to 60ft of chain and the rest is rope) and the common practice is to anchor with a scope of 4:1, even in challenging crowded and tidal anchorages. In our 10 years of European cruising we rarely saw anyone anchoring with rope rode.

By contrast, in North American anchorages many cruisers use a short chain leader on a long rope rode. In this case there is no catenary effect. You need all-chain rode for catenary to work.

The anchor set with 3.7:1 scope...

The anchor set with 3.7:1 scope…

All-Chain Rode

Experts agree you need less rode if you have an all-chain rode. Some say 5:1 is good, some say you need 7:1 for real security, and others say 4:1 is fine. Britain’s Royal Yachting Association recommends that all-chain rodes be set with a minimum length of four times the water depth. (Presumably you are meant to count the height of the bow in the calculated vertical distance.) However, this simple scope calculation doesn’t take into account the effect of having all that weight of metal down when you have laid out a lot of chain in deep water. In reality, any ratio will be an oversimplification. In shallow water, for example, we always let out more rode than the rules suggest; in deep water, a little less.

But how is this effect felt at the anchor? Can we reduce scope to below 4:1 in deep water and still be secure? Again, I wanted to investigate and see for myself, and where better to do such tests than the warm clear waters of the British Virgin Islands? It’s an especially appropriate test area because there are a number of deepish anchorages where quite a few people try to fit in. In such situations it is better for all if we can anchor on a shorter scope.

Why Bother?

Why not just put out a healthy 6:1 scope every time you anchor? Well, it’s often a matter of courtesy. Anchoring with a 6:1 scope rather than 4:1 means there is room for fewer boats. Roughly twice as many boats can be accommodated in a given space if everyone sits on a 4:1 scope. If you find yourself sailing into a popular, small or deep anchorage, this might mean you take up quite a lot of room. Anyone anchoring in our test anchorage with 7:1 scope would have been forced to put out almost 400ft of rode—not the best way to be popular, since if everyone did that there would only be room for two or three boats. Even when we arrive first, we do not anchor with more scope than is necessary so that later arrivals will also have room.

...and with 3.2:1 scope

…and with 3.2:1 scope

Test Anchoring in Deep Water

In our busy, but not exactly crowded test anchorage, the wind was blowing a gusty 15 to 20 knots. It was deeper than normal—about 50ft with a nice sandy bottom. Laying out 165ft of chain at 3:1 scope meant we had 250lb of chain between our anchor and the bow of our boat. This would form a nice catenary curve, even when the wind is blowing strongly: or so we hoped.
Given the depth and the fact that it’s 5ft from the surface to the bow roller, the amount of chain we needed to achieve the different scope ratios was as follows:

5:1 Ratio: 55ft X 5 = 275ft
4:1 Ratio: 55ft X 4 = 220ft
3:1 Ratio: 55ft X 3 = 165ft

I calculated our 54hp Yanmar engine with its three-blade folding propeller would develop about 1,000lb of force at full power in reverse. This is roughly equal to the load on the anchor in a gale of 30 to 40 knots. So we would be simulating what we are likely to experience in normal conditions if it is blowing 20 knots and a rain squall comes through.

Test 1—Steady pull in reverse

First we drove the engine astern at 2,500 rpm, simulating winds over 30 knots. Throughout, the chain entered the water at a steep angle and continued to hang in a catenary curve, lying at a steep angle to the bow and parallel to the bottom near the anchor—just the way it was supposed to.

Test 2—The Crash Test

Next we decided to push things a little. In this case we anchored in 40ft of water for a total of 45ft from bow-to-bottom. We laid out 145ft of chain—a scope of 3.2:1 with a weight of 220lb.

As the wind and current push the boat back, the anchor chain, which hangs straight down to the bottom when the boat is at rest, forms a catenary curve. This keeps the rode parallel to the bottom

As the wind and current push the boat back, the anchor chain, which hangs straight down to the bottom when the boat is at rest, forms a catenary curve. This keeps the rode parallel to the bottom

Our goal this time was to see what happens in stronger gusts and lulls that allow a boat to build up momentum and jerk back harder. We did this by jerking back on the anchor from a position somewhat ahead of it, then putting the engine hard astern. This was not a precise test, since the speed we attained in reverse varied with each test. But it applied much greater loads than just running slowly up to full reverse throttle the way we had in the first test. While she was at the helm working the throttle, Sheryl also tried to estimate whether the forces she was applying were equivalent to what we have felt on the boat in past squalls.

We were at rest with chain hanging straight down when Sheryl put the engine in full astern, and she was able to develop some good sternway before the chain came taut, bouncing our 36,000lb boat back forward again. In fact, the force on the boat was quite impressive. As Sheryl described it: “The bow immediately snapped around to line up with the anchor, and then the bow got pulled down from the force on the chain due to the anchor holding so well. It felt like when gusts of 40 to 50 knots whip the bow around.”

Underwater I watched the chain lift up off the bottom until it was coming up at an angle that actually raised the shank a little. As it did so, the anchor moved about 12in, in the process digging itself in even deeper. I wondered what would have happened with another style of anchor. It might have pulled out and dragged.

Next Sheryl let out 170ft of chain—a scope of 3.7:1. We then ran another test with a similar full-power fast reverse that caused the bow to snap back around as the road came taut. From below I could see that the extra scope made a big difference, as the shank never even came up off the bottom. The anchor barely moved this time.

The Rocna anchor set with 3:1 scope—note the chain is nearly parallel to the bottom and we are driving full throttle in reverse

The Rocna anchor set with 3:1 scope—note the chain is nearly parallel to the bottom and we are driving full throttle in reverse

Conclusions

In normal conditions (not storm force) an all-chain rode will allow reduced scope in deep anchorages. We found 3:1 scope to be OK for anchoring in 50ft of water. Increasing scope to 4:1 is better and is a good idea if strong squalls are forecast. Even an increase to 3.5:1 will make the anchor more effective.

Remember: do not try this unless you are anchoring on an all-chain rode.

Anchoring Hints

Go for a heavy anchor—in any given anchorage a heavier anchor will perform better than a lighter one. If you are outfitting a boat for cruising or looking to upgrade, why not increase the size your anchor at the same time?

It can be difficult judging the distance to your neighbors in deep anchorages, since more rode means you need to be farther away from nearby boats. Try a range-finder app or even a laser rangefinder.

Use binoculars with a range finder (such as our Fujinon 7X50 binos) to judge neighborly distance.

Use GPS and Radar to plot the positions of nearby boats.

Judge neighborly distances in boatlengths. If your boat is 33ft, you can guess the distance to a nearby boat by counting how many boatlengths away you are.

Ask neighbors how much rode they have out to make sure you will not hit each other.

Paul Shard has been cruising for 25 years and is the host of the Distant Shores TV series at distantshores.ca

 

Photography by Paul Shard

October 2015

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