Anchor Rode – Calculating Capacity

by Jim Healy

In the United States, the American Boat and Yacht Council¹ (ABYC) publishes a table of rode sizing² that is suitable for use in coastal and intracoastal waters. This table makes rode selection safe and fairly easy for the vast majority of boaters. It also provides a criteria against which to evaluate installed ground tackle systems. I suggest the ABYC table be thought of as a “minimum standard.” For persons anticipating more demanding anchoring conditions, an understanding of rode forces is not just an interesting scientific curiosity; it can be a matter of life-and-death safety. Some cruisers will encounter – by the nature of their travels – more severe conditions than others. Coastal and intracoastal cruisers will not experience conditions that “blue water long-range cruisers” will encounter. Coastal and intracoastal cruisers will be able to seek and find protection that may not be available to blue water long-range cruisers.

In studying this subject, I have found it difficult to understand and integrate the huge volume of material written about it. After some years of trying to get my arms around it, I have arrived at this summary. For those wishing to verify the size of the ground tackle fit on their own boat, or those engaged in re-fit or equipment upgrade, I see the effort to manually size anchor rodes and snubbers as a multi-step process.

1. Understand the sources of energy that will have to be absorbed by the boat’s snubber, rode and anchor.
2. Determine the design capacity for the total load that the rode or snubber will be asked to withstand without damage.
3. Determine rope size required based on Average Breaking Strength.
4. Settle on a Safe Working Load (SWL), or a Working Load Limit (WLL), for the chosen rope size.
5. Determine the working length required to accommodate shock absorbing stretch.

Step 1: Understand the sources of energy that will have to be absorbed by the boat’s snubber, rode and anchor.

Anchor rode and/or rode snubbers experience loads that originate from three different sources:

1. wind-induced load consisting of nominal wind speed which is present more or less continuously, and gusts occurring in bursts;
2. surge-induced load which comes and goes with the rise and fall of passing waves; and,
3. loads cause by currents present in the water in which the boat is anchored.

These sources of load are independent of each other. Each source adds to the others within the rode. Wind loading tends to be predominantly static, with long-period variations caused by gusting. Heave/surge loads tends to be dynamic and variable with each passing wave. Current loading is a static, and a relatively minor contributor to total loading in more severe situations. Sizing anchor rode and/or snubber involves making many assumptions about the maximum potential load that the rope will experience from these three sources in combination.

The total load on the anchor rode must fall at or below the Safe Working Load (SWL), or Working Load Limit (WLL), of all of the manufactured and man-made components that make up the ground tackle and anchor rode “system.” This includes chain, rope, splices, pad eyes, shackles, swivels, toggles, thimbles, seizings, and whippings; i.e., all of the individual components that make up the mechanical connection between the anchor and the structural frame of the boat. In the ground tackle system, the weakest link is indeed the most likely cause of failure.

Step 2: Determine the design capacity for the total load that the rode or snubber will be asked to withstand without damage.

Wind-induced load is a major contributor to rode loading. Wind-induced loads increase as the square of wind speed. For Sanctuary, I calculated the wind loads that would be experienced under a range of wind conditions by a Monk 36 while at anchor. This analysis probably also applies to other classic trawlers with similar LOA and physical profiles; i.e., Albin, Grand Banks, CHB, Marine Trader, other Taiwanese, etc.

Once wind-induced load is applied to the rode, a portion of the total available SWL of the rope is “used up.” Additional load that is imposed by both seastate heave and surge forces, and water currents, must fall withing the limits of the remainder of rode capacity.

Some facts about our Sanctuary:

1. Sanctuary’s bow elevation presents a surface area, at zero degrees of yaw to the direction of the wind, of 165 ft² with the Flybridge enclosure in place, and 143 ft² without the flybridge enclosure. The Monk 36 (and other boats) tends to “sail,” or “horse,” or “veer” back-and-forth at anchor, and routinely reaches a yaw angle of +/- 30° to the wind, often more. The surface area elevation for Sanctuary, at a 30° yaw angle to the wind, is 214 ft² with the enclosure and 180 ft² without.
2. Whether referred to as “sail,” or “horse,” or “veer,” loads on the rode increase significantly when the angular travel reaches it limit, and the motion is slowed, stopped and direction-of-travel reversed.
3. With a yaw angle of 30° and a surface area of 214 ft², a wind of 50 knots will exert 1812.1 lbs of load.
4. In an open seaway, Earl Hinz³ suggests that heave/surge loads presented to the rode can equal wind loads. Most near coastal and coastal cruisers would seek the cover of a safe anchorage to ride out a blow, so the percentage of wind load would most probably be larger than the percentage of heave/surge load.
5. If we assume a moderately protected anchorage and allow 1160 lbs of wind force at a 30° yaw angle and 640 lbs for heave and surge, we reach our wind force limit at 40 knots.
6. In a highly protected anchorage with little heave/surge component, winds could range above 40 knots, but not higher than 50 knots.
7. This design assumption is marginal for summer thunderstorms, where wind forces at 70 knots could, all by themselves, drive periodic peak rode loads to 3500 lbs or more.

Step 3: Determine rope size required based on Average Breaking Strength.

Different manufacturers of 3-strand nylon rope have different specifications for the “tensile strength,” or Maximum Breaking Strength, of their products. Using 5/8”, 3-strand nylon rope as an example, there is a considerable range of manufacturer-published tensile strengths (Average Breaking Strength). Some examples:

Step 4: Settle on a Safe Working Load (SWL), or a Working Load Limit (WLL), for the chosen rope size.

The industry standards group for rope manufacturers, The Cordage Institute, specifies that the Safe Working Load (SWL), or Working Load Limit (WLL), of a rope “shall be” determined by dividing the Tensile Strength by a Safety Factor. Safety factors increase from about 5 for

• non-critical applications,
• used under normal service conditions,
• where rope is in good condition,
• with appropriate splices,

to 20 or more for lifting applications and to 25 for life-line applications. The Cordage Institute guidance is, SWLs should be reduced where life, limb, or valuable property are involved, or in exceptional service conditions, such as shock loads, sustained loads, etc.

In the example of our 5/8″ average Breaking Strength rode/snubber rope, here are three data points for safety factor:

10500 / 5 = 2100 lbs
10500 / 6 = 1750 lbs
10500 / 8 = 1320 lbs

Note: There is also a further useful data point for consideration in this discussion. The ABYC Standard, H-40, shows an SWL for 5/8”, 3-strand, nylon line of 1114 lbs. This specification appears to have been de-rated by a further 200 lbs. A table note states: “Working loads for nylon rope are based on factors of safety, line strength loss due to knots and splices and additional factors including abrasion and aging. The point is, consistent with the criteria of the Cordage Institute, aging and condition of materials does affect SWL.

It is clear that SWL is exceeded in operation earlier, and more often, than most boaters think.

Step 5: Determine the working length required to accommodate shock absorbing stretch.

The first 4 steps, above, deal with slow, steady loading of the rode, and slow, steady energy release; that is, winds and seas at largely steady-state velocity. But that is not natural! Under more variable conditions, instantaneous loads within the rode can rise very dramatically. I highlighted the Cordage Institute suggestion, above, that SWLs should be reduced where life, limb, or valuable property are involved, or in exceptional service conditions, such as shock loads, sustained loads, etc. Note that reduction is reflected in the ABYC recommended number of 1114 lbs for 5/8” rope.

Very high instantaneous loads can cause deck hardware to fail and can cause the inelastic and relatively brittle metal components of rode to break. That is why all-chain is not a good choice of material for anchor rodes that will see severe or extreme conditions. To decrease very high instantaneous loads, elasticity in the rode is absolutely essential. The elastic component of the road absorbs high instantaneous shock loads and disperses the energy that would otherwise be transmitted to deck hardware and anchors.

Elasticity and shock-absorption capability is found in the ability of Nylon fibers to stretch and return to their original length, without damage up to a point. In the process of stretching, energy is absorbed and released. The following figures are from New England Rope. The numbers vary somewhat among different rope manufacturers, and they vary among the various types of rope made by the same manufacturer, but they remain proportionally similar. For three-strand nylon rope:

at 7.5% loading, three-strand nylon rope stretches 3%;
at 10% loading, three-strand nylon rope stretches 5%;
at 15% loading, three-strand nylon rope stretches 8%;
at 20% loading, three-strand nylon rope stretches 10.4%; and,
at 30% loading, three-strand nylon rope stretches 13%.

Rope manufactures and most experts recommend using 3-strand nylon rope at an SWL of 15% of maximum loading. This is to minimize potential damage to the internal fibers and microfibers of the rope and maximize the useful life of the asset.

Example 1:

Using the generic Average Breaking Strength of 10500 lbs, as derived in Step 3, above, the choice of a maximum of 15% loading for a Safe Working Load limit would allow 1575 lbs, or a safety factor of about 7. A snubber that is sized so that the total load on it will be 15% of its tensile strength will stretch 8%. Assuming worst case conditions, one half the 15% total load (787.5 lbs) is wind-induced, and the other half (787.5 lbs) is seastate-induced. The 787.5 lb wind component will pre-load the rope, stretching it 3% of the total available stretch of 8%. The stretch that remains available to cushion the 787.5 lb seastate heave and surge load is the the difference of the wind induced stretch (3%) from the total available stretch (8%). Thus, 5% stretch capability is available to absorb the heave/surge energy. Five percent is sufficient for the stretch that the surge can produce within a 15% SWL criteria.

Example 2:

A smaller snubber, one sized so that the total load on it will be 30% of its tensile strength (3150 lbs assuming 10,500 lb ABS, 5/8”, 3-strand nylon), will stretch the rope 13%. Half the load (now 1575 lbs) is wind-induced, the other half, also 1575 lbs, is seastate-induced. The 1575 lb wind component will pre-load the rope, stretching it 8% of the total available stretch of 13%. So taking away the wind induced stretch (8%) from the total stretch that is available (13%), the difference is 5%. So we can see that when the surge develops its full strength (1575 lbs), the stretch that remains in the rope (5%) is not sufficient for the load caused by the surge. In all likelihood, overloading the rope will cause failure of internal rope fibers.

Another way to look at the case of Example 2 is that “the line was too short.” When checking shock load, the rope will fetch up hard as the rope runs out of stretch. It will run out of stretch before the surge runs out of energy.

So if the rope is sized to allow load greater then 15% of the rope’s tensile strength, the result will be increased probability for chafing, melting and tearing of the rope’s internal fibers. This progressively decreases the safety factor chosen for the design point of the the system, and permanently damages the strength of the rope.

Summary:

This article is offered solely as an example. It contains averaged numbers and assumed design points. For any individual boat, all of this would need to be validated using manufacturer-published specifications and specific boat characteristics for wind loading. This is only a methodology for owner/operators to consider in analyzing their own ground tackle systems.

References:

By Jim Healy from his Blog Travels of the Monk 36 Trawler, Sanctuary

Disclaimer: Curtis Stokes and Associates does not necessarily agree or promote the content by the above author. This content is to be used only with the reader’s discretion.