On The Hook: Anchor Math and Management Part 2 - PassageMaker

On The Hook: Anchor Math and Management Part 2

Dive deeper into the science of properly setting you anchor. After all, when it comes to staying in place on the water, only one piece of gear is touching the earth. Make sure it stays that way.
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Most literature on anchoring seems to focus on the proper technique for setting an anchor, creating the illusion that this is all there is to it. No so. Difficulties setting the anchor, having the anchor drag or trip, or having the boat break free probably have more to do with inadequacy in one or more of the “big four” of anchoring—size, shape, scope and strength.

If each of these four factors meets or exceeds the minimum requirements that Mother Nature will demand of them, the odds are good the anchor will always set and not drag, and the boat will not break free. Scope was covered in Part 1 of the series, (“Anchor Math and Management,” PassageMaker July/August ’13 or click here) and some might say that was the easy stuff, so in this issue we will cover size, shape and strength.

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In addition to getting the big four to surpass the minimum requirements, there is a corollary to this rule, and one that many folks do not seem to understand. Each element of the big four performs specific functions—functions that differ from and are not interchangeable with the functions of the others. In some ways, substituting one of the big four to do the job of another may even increase the chances that an anchor may drag or trip, or something will break or deform.

Starting with anchor size, we find ourselves taking into account the anticipated bottom type and fluke angles of the various anchor designs.

Example showing the results of side loading on an anchor.

Example showing the results of side loading on an anchor.

Stiff, dense clay and firm, sandy bottoms are regarded as having the best holding power, with dense clay having 50 percent more holding power than firm sand. Then there are the softer bottoms, such as sticky clay, mud, loose, coarse sand and gravel, having two-thirds the holding power of sand, down to as little as one-third, depending on how soft the bottom is. So an anchor for use in soft bottoms, regardless of its design, needs to be bigger to have the same holding power as it would have in sand. 

An anchor’s fluke angle also plays a part in how well the anchor will not only set, but hold, too. To keep things simple, we break fluke angles down into two categories: narrow fluke angles, such as the pivoting fluke anchors, plow anchors, spade anchors and claw anchors, and wide fluke angles, such as the Navy Stockless design. The Fortress FX series and SuperMax anchors are adjustable for either narrow or wide fluke angles. 

For use in sand, a narrow fluke angle gives the anchor the best ability to set and the greatest holding power, whereas for mud and other softer bottoms, a wide fluke angle is best in its ability to set and hold. An anchor with a narrow fluke angle can be used in mud, but it will not set as easily and will have to be even bigger to have the same holding power as the wide-angle design.

However, beware of using a wide fluke-angle anchor in a bottom made of sand or dense clay. The odds are that the anchor will have trouble setting, and if it does set, it may hold poorly or not at all. That is why the Navy Stockless anchor, a design having a wide fluke angle (45 degrees), works poorly in sand. 

As the bottom becomes softer, the anchor needs to be bigger regardless of design, and if the fluke angle is ineffective, then the anchor needs to be bigger still.

SIZING THE ANCHOR

3 piece 100 lb. Luke anchor makes for great holding in a storm.

3 piece 100 lb. Luke anchor makes for great holding in a storm.

Since few manufacturers provide data on the holding power of their anchors or their tensile strength, mariners usually resort to the manufacturer’s sizing charts. The problem is found in the small print: “For use in sand, winds under 30 knots with moderate protection from seas, and the boat having the freedom to oscillate.”

Should conditions exceed even one of these parameters, gear needs to be bigger and stronger, even more so if more than one parameter is exceeded. What makes the choice of size so difficult is that these charts seldom explain how much bigger the ground tackle needs to be, leaving the mariner guessing when trying to size their ground tackle—guesswork that often results in gear that may fail to do the job.

Over the years through observations and experience, we have established rules of thumb for use with manufacturer’s sizing charts:

  • Go up at least one size for gale-force conditions and at least two sizes for storm-force conditions.
  • Add an additional one size for soft bottoms; two sizes if the bottom is very soft.
  • For an ineffective fluke angle, to all of the above, add an additional two sizes, three if the bottom is very soft.
  • Except for traditionally dimensioned fisherman-style anchors, wide fluke-angle anchors should not be used in sand, only in soft bottoms like mud.

Traditional fisherman anchors are sized differently from patent anchors. Generally speaking, for storm conditions, fisherman anchors with at least moderate-sized palms are sized at 2 pounds of anchor per foot of boat length; somewhat bigger for softer bottoms, and they can be somewhat smaller for lighter winds. 

We have yet to come across anyone who has had difficulties with dragging, tripping, deformation or breakage because their anchor was too big. We think their anchor was big enough.

IMPORTANCE OF SHAPE

The shape of the anchor, or in other words, its design, incorporates many different factors, with size and fluke angle being just two of them. The entire goal of an anchor’s design is to allow the anchor to set as easily as is possible in a specific type of bottom and then, to hold as best as its size and shape will allow for in that particular type of bottom.

3 piece Luke anchor- disassembled.

3 piece Luke anchor- disassembled.

The more common types of bottoms that will be encountered by a recreational boater, which must be considered when choosing an anchor’s design, are sand, mud, marl, clay, gravel, rocks, and growing in any of these bottoms, weeds. If the crew has to struggle to get their anchor to set in one of these bottoms, more than likely they are using the wrong design for that bottom. 

When it comes to anchor designs, the most common mistakes that we see being made are owners attempting to use anchors that are designed and sized for use in a sandy bottom used in softer bottoms, and attempting to use anchor designs in weeds that do not reliably set in this type of bottom.

Since it is what the anchor cannot do that more than likely will create problems, our rule of thumb for choosing an anchor design is to not just look at the anchor’s assets but, more importantly, consider what it cannot do or will do only with difficulty and then choose designs that minimize or eliminate these liabilities.

THE FOURTH FACTOR

Strength is the fourth factor that needs to be considered when choosing ground tackle. Each and every component in the ground tackle system should be picked so that its working load limit (WLL) is equal to or greater than the maximum load calculated for the ground tackle. No item should be sized to its proof load (PL), its minimum break load (MBL) or to its tensile strength. Only the WLL of an item should be used when sizing gear for use in ground tackle.

When sizing ground tackle for strength, there are a few other aspects to consider:

  • Should one piece of gear, having the WLL that it needs, be too large for the adjacent pieces to mate with it, upsize these adjacent pieces, do not downsize. A great example of fitting an intermediate piece is having a local welder install oversized links in the end links of chain, allowing the use of commonly available shackles that have a WLL that matches that of the chain.
  • WLLs are based on straight-line pull. Bending loads (side loads) can cause an item to deform or break with up to 50 percent less load than what the item can resist in straight-line pull. Anticipate a bending load by ensuring that the item has double the WLL than specified for straight-line loads.
  • Unlike most elements of ground tackle, rope is not assigned a WLL. Rope only comes rated for breaking strength (also called tensile strength). To calculate what size rope is needed, multiply the maximum load that was calculated on the ground tackle by 6.6 to obtain the breaking strength that the rope will need. Once this breaking strength is known, go to the rope manufacturer’s sizing chart and match it with the size of rope that provides this breaking strength. Sized in this manner, three-strand nylon rope will also resist chafe better than a thinner diameter rope and still have plenty of stretch to cushion surge loads.

MULTIPLE ANCHORS

To avoid the need of handling gear on a daily basis that is larger and heavier than that which may be needed only occasionally, have two sets of ground tackle. One set is sized for more typical loads and is often called the main bower. For those who may find themselves away from a slip when bad weather hits, the second set of gear should be sized for short-duration, 60-knot winds in the softest bottom that will be encountered.

Common attachment point for multiple anchors.

Common attachment point for multiple anchors.

Mariners should also consider carrying a third set of gear, storm gear, and not just for storms but for when the adequacy of the main bower might be called into question. This gear should be sized for long-duration conditions, the heaviest that could occur in the softest bottom that will be encountered—and don’t forget to include wind gusts in the calculations.

Timeliness is an important part of anchoring that should not be overlooked. What is a surprise to many folks is that timeliness is something that is applicable even before the boat is put in the water. Our rule of thumb for shopping for ground tackle is to shop early, months before it will be needed. Once all of the bits and pieces are on hand, trial-fit everything together, making certain that all of the components mate easily with their adjacent partners, and if they do not, make it so. Then, get it all on board, even before the boat is launched. Then go out and practice deploying and retrieving this gear. Work out any bugs early, altering, adapting or replacing things that need it, including the arrangements on the bow of the boat.

WRONG APPROACH

Some folks choose anchors based on the anchor’s ease of being stowed on the bow roller, but we think this is the wrong approach. Instead, we think it is better to obtain anchors that will be successful in the worst conditions and the type of bottom that will be encountered, and then, adapt or alter the boat as is necessary to fit this gear. This forethought also applies to the size, strength and locations of the cleats, samson posts or bitts where rodes, snubbers and bridles will be belayed, all sized for the worst weather, not the best.

One important point to remember is that recommendations for sizing ground tackle are minimum recommendations, starting points—not stopping points. With Mother Nature always seemingly wanting an opportunity to chuckle at mere mortals’ tendency to underestimate her power, when it comes to anchoring, bigger is always better than smaller.

When should you get the storm anchor out? We think the answer to that question is the same as to this question: When is the best time to reef? When you first wonder if you should.

What should you take away from this? If you have been anchoring fine for years in varied conditions, you are doing just fine. But if your anchor has dragged or tripped or a part of it has broken or become deformed or chafed, then your ground tackle needs tweaking.

Anchor platform setup for 2 anchors, and snubber arch.

Anchor platform setup for 2 anchors, and snubber arch.

First, using the principles articulated in Part 1, calculate the load on the ground tackle, taking into account boat size, wind speed, the boat’s exposure to seas and waves, wind gusts, duration, the boat’s ability to freely oscillate and currents. Then match the bottom to be anchored in with an anchor design that will not fail in that bottom, size the anchor to hold in that bottom and make certain that each piece of gear in the ground tackle has a WLL that will meet or exceed the load on the ground tackle (factoring any bending loads). 

Then follow the rules of thumb for scope. Remember, too, that anchoring is not a race, nor is it for the lazy.

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