Calculating Forces on Anchor Rode

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bristol27
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Calculating Forces on Anchor Rode

Post by bristol27 »

I am trying to determine the forces (in pounds) at work on an anchor rode. My specific questions are in bold, though the extra text may be useful for better understand the source of my questions. (Also sorry for the

The strength of the anchor rode should be sized based on the rode's ability to resist the forces of a 'worst-case load scenario'. This scenario could could be described as: a fully loaded cruising yacht, anchored in a soft, moderate holding bottom, with 50 - 64 knot winds and associated surge.

With the environment conditions in mind, the forces acting on an anchor rode include:

Vessel Displacement - The maximum load that could pull on the anchor rode would be the full displacement (cruising weight) of the vessel. In my case, that weight is ~9000 lb. To put this in perspective, this would be like trying to hoist the vessel by it's anchor rode without it snapping. After speaking with a Yale Cordage technician, they mentioned the loads on an anchor rode would never be the full displacement of the vessel. They thought the anchor rode well below the full weight of a vessel's displacement.


Wave & Wind Action - The force of winds and waves create a total load on an anchor rode. Research indicated that a 30', 5 ton Catalina was estimated to have 443 lb. of force being applied to the entire boat in a 40 knot wind with 3 feet waves (this figure includes topsides, hardware, rigging etc.). Since a 3/8" line is suggested to have a 249 lb. safe working load, this road size would be unsafe. A better choice would be 5/8" line which has a safe working load of 691 lb.

It's not clear how wave nor wind forces were calculated above, but an idea for calculating windage might be:

Area Exposed to Wind (inches) * Strength of Wind (pounds per square inch) = Total Windage Force (pounds)


It should be possible to calculate the total force (pounds) on an anchor rode during a worst-case scenario loading by using the vessel's mass and wave and wind action described above. The idea is that by using these forces, it should be possible to calculate the minimum WLL of an anchor rode component. As long as an anchor component can resist the forces during the worst case scenario loading described above, it should (hypothetically) never fail.

Below is the equation I believe could be used to determine minimum WLL of an anchor rode component:

Force of Wave & Wind on Vessel (lbs.) + Force of Vessel Displacement (lbs.) = Minimum WLL of Anchor Rode Component(s) (lbs.)

Below is an estimated calculation for my 27' cruising sailboat.

500 lbs. Force of Wave & Wind on Vessel (50 knot winds & swell) + 9000 lbs. displacement = 9500 lbs. Minimum WLL for Rode Component


There is a problem with the above result, however. The 9500 lb WLL is a higher strength than the WLL of any component I've researched thus far. For example, 5/16" G43 High Test (HT) chain has a WLL of 3900 lb. The chain is the highest strength component of the entire anchor rode, but yet it isn't even near the 9500 lb WLL that my calculation requires.

I think where I've gone wrong is in the calculation of the 'Force of Vessel Displacement'. I say 9000 lb. Perhaps, this should be a safety factor of total displacement. By taking my vessel displacement, and reduce it by a factor that better matches the WLL of the anchor rode components available. This ratio would be 1/5. So instead of 9000 lb, it would be 1800 lb of force placed on the anchor rode, by the vessel's mass.

If it's not the loaded displacement of a boat, how can the vessel's displacement be calculated in terms of force (pounds) acting on an anchor rode? In other words, what is the actual Force of the Vessel's Displacement acting on the anchor rode?

Perhaps the displacement of the vessel doesn't have such a large impact as I assume above? Maybe the water helps to absorb a lot of the load forces?



Below are some other references for sizing an anchor rode's strength:
  • A poster on cruiser forum said that the USCG suggests that the rode WLL is 1/8 of displacement.
  • Yale Cordage documentation mentioned that both wind and wave forces impart measurable loads (energy) on the hull and superstructure of a vessel. This energy must be absorbed either by movement of the boat through the water, or by the anchor system’s ability to absorb this energy.
  • A Yale Cordage technician mentioned that water adds some reduction in the force of the full displacement of the boat.

I should note that I am aware that chafe and the anchor letting loose are the primary reasons for a boat to break free at anchorage. These calculations are simply meant to give a more measurable figure on the forces at work on an anchor rode.

Also, I do not want to oversize the components, as oversizing adds unnecessary weight and cost to an anchor rode. This said, an oversized component will have a greater safety margin against breaking.
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Re: Calculating Forces on Anchor Rode

Post by okawbow »

I rode out a tornado on 2, 14 lb danforth anchors with 6' of 1/4" chain and 1/2" nylon rode. I was in a Cheoy Lee 31 ketch, anchored fore and aft in a river. The boat was knocked down to the spreaders twice, but the anchors held, and although the rode was stretched like a banjo string, it held also. One anchor had the shank bent 90 degrees.

Later in the trip, I switched to a 33# Lewmar claw anchor and 5/8" nylon rode with 35' of 3/8" chain. That setup held well in many different bottoms from Mobile to Maine.

Check out the absloute holding power of your anchors and size the rode about double. Good 1/2" rode would be plenty for your 27, but you might sleep better with 5/8".

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Re: Calculating Forces on Anchor Rode

Post by Carl-A259 »

I would always choose on oversizing the components since you are never guaranteed the conditions or the availability of replacements when you have a failure. but that's just me... you know belt and suspenders!!!! 1/8th the displacement.... I don't think he understood the USCG on that
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Re: Calculating Forces on Anchor Rode

Post by CharlieJ »

one important consideration- if you use too LARGE a rode, it cannot stretch, thereby negating some of the advantages of nylon, and the ability to stretch is a MAJOR factor in anchor rodes.

I used to cruise a 35 trimaran( 3 years). Disp about 10,000 loaded ( three aboard, our only home). Windage though, was like two monohulls. I used 5/8 nylon on that, riding out several 60-70, and one 90 knot blow.

On my 25 foot sloop, 7500 pounds loaded, I used 1/2 nylon.

I would think one size above 1/2 would be totally sufficient. Add at LEAST one boat length of chain- more is nicer
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Re: Calculating Forces on Anchor Rode

Post by earlylight »

My Sabre 34 MK I is about 12500 fully loaded plus another 350 for two adults. My rode consists of 75 feet of 5/16 inch stainless BBB chain spliced to 200 feet of 9/16 inch premium three strand nylon. We have rode out a 75 knot blow with 5 foot seas that lasted about 10 hours in a semi-protected anchorage with no problems.
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