Anchor Rode Strength

[O C Diver]

There have been several threads recently on chain strength and size for given vessels and strength for anchor swivels. So it got me thinking. I know the strength rating for my chain, shackle, and snubber hardware, but how much do I need?

Let's assume my anchor is buried or wedged between 2 rocks where it can't come out. How much force can wind, waves, and current put on an anchor rode. Assuming we have a good snubber system to eliminate the shock loading, how many tons or pounds of force could a rode expect to see on a 40 to 50' vessel? Can anyone offer a link to some testing that was done or maybe an article from a respected organization like Practical Sailor?

Ted

 

[MurrayM]

Try googling/entering into search engine of your choice:

rode windage saiboat vs trawler

That will bring up some links to information...I’d do it, but am busy in backyard today.

 

[DDW]

You can make at least a first order estimate based on the frontal area of the trawler and the dynamic pressure of the wind. The coefficient of drag comes into the equation as well, but most trawlers aren't exactly streamlined, so call it 1 for this estimate.

Dynamic pressure = 0.5 * m* v^2, or about 1 lb/sq ft at 17 knots. Any other speed can be calculated as 17 * (V/17)^2 with V in knots. 34 knots would be 4 lbs/sq ft.

Drag = area * Cd * Dp, but we take Cd as 1 so Drag = area * Dp.

On a 45 trawler the area is about beam * air draft which will be conservative. Say 16 * 16 or 256 sq ft. At 34 knots wind that is 1000 lbs drag (256 x (34/17)^2). At 50 knots, 2200 lbs drag. At 60, 3200 lbs.

Even with a snubber, there is shock loading, also most trawlers don't just sit there they tack back of forth at anchor, exposing more frontal area. If you double that to be safe, you need 4400 lbs working load in 50 knots.

I think any reasonable sized chain is unlikely to break due to wind alone. If your snubber is ineffective (breaks, or is stretched enough to bring the chain tight) the shock loading can multiply by orders of magnitude very rapidly. The ability to absorb the energy of a surge is proportional to the stretch of the rode squared, and chain has very little stretch.

 

[O C Diver]

You can make at least a first order estimate based on the frontal area of the trawler and the dynamic pressure of the wind. The coefficient of drag comes into the equation as well, but most trawlers aren't exactly streamlined, so call it 1 for this estimate.

Dynamic pressure = 0.5 * m* v^2, or about 1 lb/sq ft at 17 knots. Any other speed can be calculated as 17 * (V/17)^2 with V in knots. 34 knots would be 4 lbs/sq ft.

Drag = area * Cd * Dp, but we take Cd as 1 so Drag = area * Dp.

On a 45 trawler the area is about beam * air draft which will be conservative. Say 16 * 16 or 256 sq ft. At 34 knots wind that is 1000 lbs drag (256 x (34/17)^2). At 50 knots, 2200 lbs drag. At 60, 3200 lbs.

Even with a snubber, there is shock loading, also most trawlers don't just sit there they tack back of forth at anchor, exposing more frontal area. If you double that to be safe, you need 4400 lbs working load in 50 knots.

I think any reasonable sized chain is unlikely to break due to wind alone. If your snubber is ineffective (breaks, or is stretched enough to bring the chain tight) the shock loading can multiply by orders of magnitude very rapidly. The ability to absorb the energy of a surge is proportional to the stretch of the rode squared, and chain has very little stretch.

Ok, I'll buy that. So with 50 knot winds, even in a protected anchorage, there's going to be some seas. How much force does 3' seas add to the chain load?

Ted

 

[psneeld]

Dont agree, ...

You have to take into account shape, but add in drag.

If a boat was 16 feet square of flat area (as in Class A RV) then OK. But the hull is very aerodynamic on many boats and a curved fly bridge is better too. I am just gessing , but you might be able to halve that calculation.

Based on height times beam, the calculation for my boat at 13×13 and that's not including bimini or mast/rigging in 17 knots of wind is over a 160 pounds of pressure. Yet I can reasonably pull my boat along a face dock in those conditions. I am guessing because of true flat frontal area, not just beam times height.

My real world experience says the same, even horsing around at anchor in 25 to 35 knot winds, no way is it in the thousands of pounds of pull. I have salvaged many a boat and have seen line, cable and chain under scary tension....not so much at anchor.

Yes, start getting above 50 knots and things get serious, but in a lifetime of boating, seeing those kinds of winds have been at a dock. If at anchor, my chosen anchorage has protection from continuous, straight line winds on purpose, and few serious waves.

So I am completely comfortable without battleship dround tackle that if needed us one day in every 10 years of anchoring, if I get caught.

As I posted before. It's different if you are a long distance cruiser and expect to anchor in storms.

 

[MurrayM]

https://www.passagemaker.com/trawler-news/hold-fast

 

[rslifkin]

Wind speeds are also lower closer to the water level, so in 60 it winds, the lower part of the hull is probably only feeling 40 or 50 kts. That further reduces the loading compared to the basic calculations.

 

[Nomad Willy]

Yes rslifkin,
Ground effect .. perhaps we can call it water effect.
So sailboats and built-up trawlers w the oxygen tent should use higher performance ground tackle. Essentially no doubt.

Re air drag there’s so many variations of shapes and sizes of boats and their cabins and all the little things that add up like stowed dinghies, roof overhangs, deck railings, exhaust stacks, TV antennas ect. The numbers one might attach to boats re these very proliferate appendages insure that any attempt to numberize values of air drag may be not much different than wild guesses. Many however attempt to numberize wind drag but to an objective person it probably looks rather silly.

But live real world testing could come fairly close. Imagine a boat tethered w a line or cable with the other end attached to a tension scale. This w the scale on a mooring float with little in the way upwind. If a wind velocity was measured about 6’ above the surface and compared to a given vessel some real-world numbers could be generated. Obviously there would be much guessing and speculation even w this kind of study. Especially how far aloft to measure the wind speed.

So just observing what kind of boats carry what king of ground tackle may be about as good as the closest thing to a scientific undertaking one can find.
Documented failures could tell a lot if one got serious. IMO

 

[MurrayM]

...My real world experience says the same, even horsing around at anchor in 25 to 35 knot winds, no way is it in the thousands of pounds of pull. I have salvaged many a boat and have seen line, cable and chain under scary tension....not so much at anchor...

This from Practical Sailor:

In sharp contrast to the ABYC data is the data published in the table of the third edition of Robert Smith’s 1983 book called Anchors—Selection and Use (see table at end of article).

Smith told PS (a year or so before he died in June of 1997) that he obtained his data from testing in a level area of the Columbia River that has a fine sand bottom and a fetch of four miles to windward. He used several boats, both power and sail. With a 30-foot boat anchored in 30 knots of wind, the ABYC says the load on your anchor system will be 1,200 pounds. Using Smith’s figures, the load will be only 341 pounds. The figures, if used to help you determine what size anchor and rode to use, will produce radically different choices.

One more example: if you have a 30-foot sailboat and get caught on a night when a 60-knot squall goes through the protected harbor in which you’re anchored, the ABYC says the pull on your anchor rode will be 2,800 pounds. Smith says it would be 765 pounds. The ABYC figures are about 3.5 times greater than Smith’s. When asked to explain these wild discrepancies, Smith said, “I don’t know how they got their figures; mine are real, developed by actual testing.”

https://www.practical-sailor.com/sails-rigging-deckgear/the-load-on-your-rode

 

[MathiasW]

Great links, particularly to the work of Robert Smith! I have always doubted the ABYC figures. My reckoning is that they wanted to include dynamic load effects due to swell as well. The catenary formulas work with a fundamental parameter which is the ratio of anchor load to chain weight per unit length (in water). Now, this anchor load could be due to wind, so proportional to the windage area of the vessel, but it could also stem in parts from dynamic loads such as swell. Now, how much of an impact a swell has on anchor load depends on the anchor depth. In shallower water it increases dramatically and is only kept in check by very good snubbers. I have now come to believe that the ABYC data just make assumptions about the swell load in shallow water and I do not trust them anymore.

In case somebody finds this useful:

I have worked on a mathematical modelling of the anchor chain and snubber / bridle. In which situations do they work best, or not at all? Nothing is perfect, as always, but understanding this model may help this community to make better choices for anchor gear as well as anchorage site choices.

For instances, it answers the question under what circumstances the chain should be chosen thicker but longer, or when the opposite might be the smarter choice.

This information can be found here:

https://trimaran-san.de/die-kettenkurve-oder-wie-ein-mathematiker-ankert/

And no worries, it is in English, not in German...