Tuesday, December 11, 2018

Actual Anchor Loads



I spent a LOT of time researching this topic, through the literature and by staring at a load cell for hours and hours. I used all sorts of rodes and snubbers, in all weather, on a number of boats.

ABYC (American Boat and Yacht Council) set standards for rodes and deck hardware 50 years ago, based on testing with boats on all-chain rode in open harbors and shallow water (but no breaking waves) and no snubber. In fact, these values are VERY close to what I got testing in similar conditions. This is very much a worst case situation and is something you should NEVER do, but is a good basis for the design of something that cannot fail. I don't think I've ever heard of a system meeting Table 1 requirements that was in good repair failing, outside the eye of a huricane (an even then it isn't the chain that breaks; they either drag or the roller and cleats fail.


Interestingly, every investigator reported 3-5 times less when they measured the strain on their boat (me too). Of course, they wisely anchored in deeper water (lower waves and more catenary), used rope, or used a snubber.

Looking only at the 60 knot row, which is very likely the most you will ever face without some protection from trees or shoreline, we see that the wind load is only about 20-25% of the ABYC load. Waves and snatching make up the rest. If you use a long chain in deep water (at least 20 feet), a long  snubber (35'-50'), or a rope rode, the load will be only 25-30% of the ABYC value.




For larger boats or those around coral and rocks, all chain is nice. Rig a LONG snubber, even if you have to cleat it at the mid-ships or stern cleats to keep the connection off the bottom.
My PDQ came with rope, but I changed to chain and a long snubber after a few years. I liked the increased security, but the main reason was that the windlass handled all-chain better than chain with a splice. 

In my mind, if your anchor is 35 pounds or more, you will be better served by a windlass and all chain, and if it is 15 pounds or less, manual handling and rope makes more sense. At 25 pounds, the decision is up to you, depending on pocketbook, fitness, and sensitivity to weight in the bow. I'd probably go with chain and a winch on a monohull or heavy cat, and manual with rope on a fast multihull.

 With smaller boats and mud bottoms, the rope is a better answer. It is much lighter, gives good shock absorption, and lasts as long or longer, if it is protected from chafe. You see, chain will fail from corrosion in 5-7 years, even if you don't anchor out, whereas nylon can go even longer if oversized a bit. So go oversize for durability and ease of handling. Finally, nylon can actually be too stretchy if over 100 feet are out, and oversize rope reduces this.
My 24-foot, 1700-pound trimaran only calls for 3/8-inch nylon rode, but I like 1/2-inch for better handling, durability, and reduced stretch. I did the same on my Stiletto 27.

I've even gone to a Dyneema bridle to reduce yawing and to reduce the sometimes-excessive stretch of all-nylon rode. Because the bridle does not distort under shifting loads, yawing is reduced.

[Images excerpted from Rigging Modern Anchors.]

10 comments:

  1. Excellent article. Couple of points of clarification.
    "Interestingly, every investigator reported 3-5 times less..." means they measured 1/5 to 1/3 of the table 1 values, right? And the "waves and snatching makes up the rest" is your (very reasonable) presumption?

    And the next couple of sentences means that with proper use if nylon ride or a long snubber, you can reduce the *max* load down to very near the wind-only load through the cushioning of dynamic loads?

    Also one question: do you know where in might find some estimates of the "back down" force as a function of rpm rpr a given engine and/or propeller pitch combination? Have you possibly measured that as well?

    Thanks again. Excellent stuff!

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  2. Table one is from ABYC. Yes, investigators measure 3-5 times less under real world anchoring conditions. If the boat is anchored with all-chain in very shallow water (no more than 5-6 feet), with long fetch and steep waves, and a good breeze (enough to straighten the chain) the snatch loads can reach those in ABYC Table 1. However, if the boat is anchored in deeper water (8-10 feet) and any snubber, the loads are much less, as indicate case situations, where waves are steep and the chain has been forced straight. Basically, circumstances you should never anchor it. In real world practice, I have never observed values higher than 1/3 of ABYC table 1.

    Yes, by using rope or a long snubber, the peak tension can be reduced to near the wind load only.

    I feel like I've measured everything! Setting thrust is discussed in Chapter 5 of my book, "Rigging Modern Anchors."
    * Inboards, about 25 pounds/hp
    * Inboards with folding prop, about 20 pounds/hp
    * Outboards, about 10 pounds/hp
    * Outboards, high thrust (Yamaha 9.9 High Thrust, Mercury Big Foot), about 14 pounds/hp

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  3. Put another way, the ABYC Table 1 values are the worst possible scenario, that will occur only if you are anchored by all-chain, in shallow water, in nearly breaking waves with poor protection. Thus, it is a reasonable basis for a standard, because it can happen. But with any degree of planning, it won't. Simply using a snubber will reduce the load by half, which is a whole lot simpler than lugging twice the anchor around. More scope, deeper water, and protection from waves all play a part.

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  4. Thanks! Will have to go find your book! Are your numbers for force per hour based on exerted hp or engine rated hp? In other words, if I have a 40 hp in-board diesel (@3500 rpm) but pull down at 2000 rpm, at which speed the prop can absorb ~8 hp, which value would I use in your force/hp values? (Or I'll find it in the book.) Thanks again. Love data!

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  5. The figures were based on the RPM when the force dropped due to heavy cavitation. In fact you are correct; the engines were not at full throttle or full output. Obviously, there is considerable range in the figures depending on prop selection... except for the outboard numbers, since the prop selection is fixed.

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  6. You say you have gone to using dyneema for yoir bridle/snubber. I thought one of the advantages of a nylon bridle was that it stretches and smooths out shock loading, but with dyneema's limited stretch, doesn't the shock load increase? Just wondering how all dyneema snubber affects the horizontal working load in the table in the article.

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  7. The first table is ABYC and assumes all-chain. It is just there for reference.

    The second table (rope row) assumes a minimum amount of rope, which the Dyneema bridle scarcely diminishes (if in doubt, add 15 feet more rode).

    Nylon snubbers are VITAL with chain, but if the rode is already primarily nylon rope, substituting Dyneema for a small portion makes little difference in shock absorption but a significant difference in yawing, which is the more critical factor when anchoring using rope.

    Dyneema bridles are ONLY for use with rope rodes!

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  8. Hi Drew,

    A question on your first table from ABYC: I would expect that the values would double from 42 to 60 knots. The values you have published are 50% higher which I would expect to correspond to their design loads at slightly over 50 knots. It appears that your numbers match up with ABYC's "permanent mooring" column but I don't see any reference to the windspeed they are using to define a permanent mooring. Is this a typo in your table or am I missing something?

    By the way, I really appreciate all of the work you did in trying to understand what real loads are and how to minimize them. I was replacing some mooring gear recently and thought that I had better review my calculations from 10 years ago as well as some of the local mooring requirements and I was once again amazed at how poorly considered some of these requirements seem to me, now wonder every time we have a mild nor'easter the news is full of boats on the rocks. Maybe a future project for you? One of the hardest parts for me was deciding on chain size to keep the loads where I want them but I did finally come up with a calculation method that I am happy with.

    Eric

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    Replies
    1. I did not calculate those numbers, I simple extracted them from table ABYC H-40 table 1. I reformatted them and restored the original wind speed basis (an older version of ABYC H-40). You are probably correct, perhaps the value should have been 50 knots, but that information is lost to the sands of time, as the original research documents have been lost. My testing (only a few data points at 60 knots--it gets crazy!) supported the Load ~ V^2 relationship.

      Test results often diverge from the simple Load ~ V^2 rule of thumb; every boat reacts differently to gusts. Tests vary with regard to scope, fetch, water depth.

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    2. Thanks Drew. Yes, it is too bad that we do not know more about the origin of the numbers. I would love to see data that really allows us to understand the variability from boat design, wave height, etc but it just isn't practical. Thanks to the numbers that you and a few others have gotten, we can at least make moderately informed decisions.

      I am amazed that you even have data points at 60 knots. I have seen a true 60 knots sustained less than a handful of times and it is pretty interesting. It wouldn't be half as bad if you could just turn the noise off.

      Eric

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