Measuring Strain on the Anchor Rode

[Kings Gambit]

It appears that the loads in Stu's table are calculated by a standard formula: F= AxPxCd; where A= is the diameter of a tube x its length; (i.e. the sail boat hull), P= wind pressure, and Cd= drag coefficient, which appears to be near 0.85--the standard Cd of a long tube. For a 35 ft sail boat of 10 ft beam @15 kt we get F= (35x10) (0.762) (0.85) = 226 lb. (versus 225 lb of the table). So, apparently nothing in there for the rigging and other windage elements; perhaps they are insignificant. Noting in there for water pressure, etc. At the same time, A is probably over estimated and who really knows Cd?

 

[Kings Gambit]

I've done extensive testing using a 5000 pound digital strain gauge on my own boat. I've tested all my anchors, my boat in a number of storms and even how much just the chain can hold. What I found is that the ABYC guidelines, while a bit conservative, are pretty darn close.

What you will find is:

• Loads are never static. They fluctuate from 0.00 pounds to a spike then back down as rode stretches & relaxes.

• Shock loads in waves will give you the highest peak numbers. The peaks don't last very long but do induce high loads and heat from chafe.

• If you use all chain you will see very high shock loads during chop & waves some times exceeding 10X the average

• If you use a long snubber you will see much less shock loading.

• The strain gauge must be before the chock or the readings will be incorrect.

Thank you for this!

 

[thinwater]

Practical Sailor has published several articles on just this topic, discussing chain, rope, and the use of snubbers. I've spent many hours reading strain gauges. The difficult part is not the measuring, but determining the effects of scope, rode type, water depth, and waves.

I would not tie loops, I would use either a rope grab or gripper hitches. A small tackle is handy for getting the load onto the gauge (you really don't want it in-line in strong winds--you want to be able to retreat!).

 

[thinwater]

Here are those tables:

Anchor System Sizing Tables (Reply #6) & Why Swivels are a bad idea http://c34.org/bbs/index.php/topic,4990.msg30400.html#msg30400

Those are design loads, not actual loads. They include many concervative assumptions and are several times higher that actual in most cases. For example, it states a 35 boat has a 225-pound load in a 15 knot breeze, yet I'm sure I can pull the rope in hand-over-hand in 15 kn.

 

[thinwater]

I've done extensive testing using a 5000 pound digital strain gauge on my own boat. I've tested all my anchors, my boat in a number of storms and even how much just the chain can hold. What I found is that the ABYC guidelines, while a bit conservative, are pretty darn close.

What you will find is:

• Loads are never static. They fluctuate from 0.00 pounds to a spike then back down as rode stretches & relaxes.

• Shock loads in waves will give you the highest peak numbers. The peaks don't last very long but do induce high loads and heat from chafe.

• If you use all chain you will see very high shock loads during chop & waves some times exceeding 10X the average

• If you use a long snubber you will see much less shock loading.

• The strain gauge must be before the chock or the readings will be incorrect.

If the spikes are 10x the average, there is a problem with the anchor system design. A snubber is badly needed, as you suggested. I seldom see spikes over ~ 200% of the average, though perhaps 4-5 times the minimum.

 

[thinwater]

A simple way to get rough numbers, if you are curious, would be to attach a skinny nylon line (1/4"?) of known stretch with a camel hitch, pre-stretch a few times, place 2 marks 2' apart, transfer the load, and measure the stretch. You'll get within 20%

 

[Benny17441]

Now, how would we know that if those tables have never been checked in practice? Besides, I would not be surprised if the reported strains are systematically higher than what is actually measured most of the time. Not that averages would be useful in any kind of crises situation, etc. However, it'll give me something more to do while fuzting about the boat.

No need to measure data for 10, 20, 30 or even 40 knots of wind as in reality we do not carry a variety of anchors and when choosing a primary one and its rode we will choose a set-up capable of holding in at least at 60-70 knots of wind speed. I believed those tables have been thoroughly tested in practice by its many users and they do have an ample built in safety margin for those times when currents may induce higher loads than those of wind alone. Now the problems I foresee for your tests is that the wind does not blow always at different speeds on the same day and location and the effects of current are not always the same so by having to take measurements at different times and locations the results will not be uniform.

 

[Kings Gambit]

No need to measure data for 10, 20, 30 or even 40 knots of wind as in reality we do not carry a variety of anchors and when choosing a primary one and its rode we will choose a set-up capable of holding in at least at 60-70 knots of wind speed. I believed those tables have been thoroughly tested in practice by its many users and they do have an ample built in safety margin for those times when currents may induce higher loads than those of wind alone. Now the problems I foresee for your tests is that the wind does not blow always at different speeds on the same day and location and the effects of current are not always the same so by having to take measurements at different times and locations the results will not be uniform.

If I take ten accurate measurements on ten different days near the same spot in say 14-16 kt of wind each time and get a measured average load 15% lower than the design load offered in Stu's table at 15 kt--there's no issue. The number in the table tells me for a sail boat of the size I own what loads my ground tackle should be capable of handling (in their opinion) at a given wind speed; the numerical average of my ten reads tells me what loads the boat actually experiences in 15 kt. There's no right or wrong here. If I ignore the table and end up on the beach b/c I chose tackle too light for my anchoring jobs, then--it's my ass! I think I understand all of this much better now.

And by the way, I do carry three sets of ground tackle with three different anchor types. However, considering some of the discussion I've followed in this forum, I'm considering exchanging one type I'm carrying now for a Rochna.

 

[Bill Roosa]

Have you thought about using trigonometry? Draw a free body diagram with the chain as a bent line and a force applied at right angles to where the bend occurs. calculate what the angle would be for a given load in the chain and a constant and repeatable force applied to the chain at a right angle. Alternately you could calculate the force in the chain given a fixed angle of bend and a known applied force causing the bend. you can make a table of angles and/or applide forces for given chain loads and check under actual conditions.
Or you could use MS's "bar tight" assessment
example: if the bend you can make in the rode is 120 degrees measured angle from rode to rode on the opposite side the force is applied to and the force applied is 10 lb then the tension in the rode would be 10 lb also.
summing forces perpendicular to the line of the rode with the applide force being the positive direction
Fa=Fr*cos(60)+Fr*cos(60)=2*Fr*cos(60). Solving for the Force in the rode (Fr) gets
Fr=Fa/(2*cos(60))=10/(2*0.5)=10/1=10
You can use the formula for various applied forces (Fa) and angles where the angle is 1/2 of the rode to rode angle as described above.
a more realistic example:
I can apply 25 lb to the rode and get a rode to rode angle of 175 degrees
Fr=25/(2*cos(175/2))=25/(2*.0436)= 287 lb

 

[Bill Roosa]

high school trig. Most useful class I ever took.

 

[Bill Roosa]

I'm thinking a simple angle measuring device (protractor) and a calibrated hand and arm. Lean over the bow and attach the not so expensive angle measuring device to the rode (or do this right after you anchor) and then use your calibrated hand and arm to apply a known force perpendicular to the rode. measure the angle through a couple of surges and retire to the salon to consult with your calculator. Probably want to practice on the hard to get your hand and arm calibration or use a spring scale if you want to go the more expensive route.

 

[Maine Sail]

If the spikes are 10x the average, there is a problem with the anchor system design. A snubber is badly needed, as you suggested. I seldom see spikes over ~ 200% of the average, though perhaps 4-5 times the minimum.

Keep in mind that was based on "all chain" no snubber as many a lax sailor will do when they assume fair weather. I never do this and only did it to see the results. In flat water you'd not see this but I anchored our boat in a fairly exposed anchorage with a fair swell and chop n top of that and did capture a spike at about 10X what it was averaging. Based on my testing I went back to a mixed rode as it greatly reduced the spikes..

 

[Kings Gambit]

I'm thinking a simple angle measuring device (protractor) and a calibrated hand and arm. Lean over the bow and attach the not so expensive angle measuring device to the rode (or do this right after you anchor) and then use your calibrated hand and arm to apply a known force perpendicular to the rode. measure the angle through a couple of surges and retire to the salon to consult with your calculator. Probably want to practice on the hard to get your hand and arm calibration or use a spring scale if you want to go the more expensive route.

Interesting, but I can't say I had thought of it, or that I ever would have thought of using trigonometry in that way to take the measurement. I'll fiddle around with it and see if I can master the calculations, etc.

Thanks!
KG

 

[Stu Jackson]

Based on my testing I went back to a mixed rode as it greatly reduced the spikes..

Very good point. A good friend of mine sailed his C34 from Vancouver to Mexico, and reported this about his anchoring system:

I forgot. The Rocna. All 20kg of it with 100ft of chain. The rest of the world can debate all they like. When I pull into a place like Bodega Bay at midnight and the fog is so thick I can't see the jetty 50 feet away to make an entrance, I drop my hook in the rolling ocean swells with the surf crashing (Foster says it's like staying in a cheap Best Western beside the highway), and I sleep. And in the morning I have a windlass to pull the beast up and I wouldn't trade it for anything. (I also wouldn't add more chain - this works perfectly in 25 to 30 feet of water - you let all the chain out and you tie off nylon at the preferred scope and don't bother with snubbers and chain hooks and all that stuff...)

This was our best upgrade.

 

[thinwater]

Keep in mind that was based on "all chain" no snubber as many a lax sailor will do when they assume fair weather. I never do this and only did it to see the results. In flat water you'd not see this but I anchored our boat in a fairly exposed anchorage with a fair swell and chop n top of that and did capture a spike at about 10X what it was averaging. Based on my testing I went back to a mixed rode as it greatly reduced the spikes..

Bingo. I did that test as well and got exactly the same answer you did; good agreement with ABYC tables. I actually busted a 2500-pound load cell in 10 knots when a wake caught me by surprise in 6 feet of water and straighten the chain. Wham. Fortunately the load was on a Dyneema bridle and not over the roller. The average load was only running ~ 25 pounds at the time.

And that is how you bust chain and ruin bow rollers.

 

[RoyS]

As Maine Sail stated, shock loads will be off the chart. It is during a shock load that your gear will break.

 

[Maine Sail]

And that is how you bust chain and ruin bow rollers.

Trawler with a fouled prop, lee shore, all chain rode & no time to deploy the snubber before this happened. Also broke the windlass, a very, very expensive old bronze Ideal... Yet another reason I went back to a mixed rode I can deploy it very quickly if needed and I need no time to deploy the snubber.....

 

[centerline]

I'm guessing the tables are way high.

some would think the general numbers as posted to be high, and other believe them to be low.

but when one thinks about the boat setting in a fluid, which is a damper to the movement of the boat, it will help to realize that it does not take as much to hold a boat as one might think.

when the boat is "tied to the ground" during wave action, and there is a wave that hits the boat, even though the wave itself is moving forward, the water in it is not. the water is only a stationary medium for the "wave" to travel thru... (the same as a sound wave traveling thru the air. the air does not move, but only the wave, OR, a sound wave traveling thru water)...
YES...there is a slight forward movement to the water (12-18" in a 20-30ft wave), but it returns back to its original position as the wave passes

so whether the boat is anchored or adrift, with the boat setting in the dampening fluid, being free to move in any direction, and only as fast as its mass can be accelerated thru the dampening fluid, this will determine how much energy the boat/mass can generate.
so with the boat being tied to the ground, and its movement limited, there is only so much movement that will take place, and it will only take place at a pace that is controlled by the damping effects of the fluid in which it is setting.

now... if the boat were able to get up some momentum, such as when someone were to immediately pay out another 2-300ft of rode during a 50kt blow, and the boat was to suddenly catch a wave and surf, when the boat hits the end of the rode, there is going to be a tremendous shock load that is much higher than one will ever see.... it could be much higher than the actual weight of the boat.... but this will rarely if ever happen in a real life situation because the boat will never get up wind, or up wave far enough for this much momentum/potential energy to get built up...

water current has a stronger effect on rode strain than wind does, which is why the numbers as posted are as high as they are... and higher winds do create some surface current....

as posted by Maine Sail, using all chain will produce higher shock loads, but only because the dynamic load/momentum will be brought to a halt suddenly and severely at the end of the chain, where as with a nylon rode, as the load begins to quickly increase on the rode, it stretches out as it slows to boat to a stop. by doing this it actually LESSENS the built up energy in the momentum, because it ABSORBS some of it it before transferring the remainder of it to the cleat.... (a chain transfers all the energy directly to the cleat) and then the nylon springs back to its original position waiting for the next wave of energy...

shock loads can cause damage, maybe not all at once, but its like hammer blows.... even enough of the little blows can be enough to inflict a change in whats being hammered against.

the nylon rode as it stretches, does create heat, and the closer it stretches to its breaking strength, the more heat it generates... and like a rubber band, the higher the tension, the easier it will chafe/wear/cut/break....
so if properly protected from chafe, it is a great tool to use to lessen the loading on the boat and ground tackle, and the more nylon vs chain that you have out, the lower the peak load will ever be.