Measuring Strain on the Anchor Rode

[Kings Gambit]

How could one measure the strain on the rode of a yacht at anchor? I was thinking of some sort of strain gauge applied to 3-strand nylon rode in say, 10, 15, and 20 kt of wind. It's true there are tables around for the generic yacht of a given size, etc., but what about one's own yacht? Would this work?

 

[woodster]

use a very expensive load test meter/scale if you wish to know but i think you are overthinking this ...just get recommended... size or one size larger and be done with it....sometimes a little faith goes a long way .....and along with inspections for chafe and broken strands...yes inspect the dock lines just like you would any other part of your running rigging

 

[justsomeguy]

Go to Maine Sail's forum and do a search on "anchor rode bar tight". It may give you some idea, but not necessarily in regards to your particular boat.

 

[rgranger]

Well this is purely academic but what if you tied two loops in your rode and then put a scale between the two loops, (see drawing). A 1/2 line has a cross sectional area of approximately 0.8 inches squared... so if you measured the pounds of pull between the two loops and divided it by 0.8 you would have a psi measurement.

But I think most line actually comes rated as simply pound working as a function of tensile strength so you would not even need to worry about the rode's diameter... just measure pounds on the scale.

BTW: The photo of the scale used here came from Northern Tool's website.

r

 

[rgranger]

And for the picky... yes... you would need to tie the loops above the water line...

 

[centerline]

there are charts online and in books that will tell you the approximate strain on a rode, based on wind and current speed, and the size and type of your boat....

 

[BudGates]

Assuming that you're looking for the maximum strain it will be at the bow of the boat. A simple tension gauge, such as a fish scale attached to the rode will give you all the information you need. Simply anchor as you would normally then attach the scale where it will take the load.

Having said that, maximum strain is much less an issue than chafe. Just be sure to inspect your rode like Woodstar said.

 

[centerline]

It's true there are tables around for the generic yacht of a given size, etc., but what about one's own yacht? Would this work?

what would this really tell you, and how would you use it?...
the problem is that there is such a wide range of ever changing conditions that would make it impossible to use a quick couple of readings in one or two random set of conditions for any purpose...

if you were to do exhaustive testing on "your" boat in all types of sea and weather conditions, you would be able know what the load is.
the tables that have been worked up will be as close as you would ever need....

but the answer to your basic question is, YES.... it can be done, and has been done, but just not with your boat... yet!

 

[Kings Gambit]

there are charts online and in books that will tell you the approximate strain on a rode, based on wind and current speed, and the size and type of your boat....

I realize that; thanks. I'm interested in getting the data for my own boat under a variety of conditions. Once I know, or decide, how to properly take the measurement, I will likely take many over the next few months. It would work better if the measurement is a relatively simple one to take. Maybe later I'll compare mine with the published estimates.

 

[Kings Gambit]

what would this really tell you, and how would you use it?...
the problem is that there is such a wide range of ever changing conditions that would make it impossible to use a quick couple of readings in one or two random set of conditions for any purpose...

if you were to do exhaustive testing on "your" boat in all types of sea and weather conditions, you would be able know what the load is.
the tables that have been worked up will be as close as you would ever need....

but the answer to your basic question is, YES.... it can be done, and has been done, but just not with your boat... yet!

Let's just call it research for the time being.

 

[Benny17441]

If you find the proper gauge and took enough readings it would work, but I do fail to see the usefulness of getting those results. For sizing anchors and rode the generic tables are more than adequate.

 

[Stu Jackson]

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

there are charts online and in books that will tell you the approximate strain on a rode, based on wind and current speed, and the size and type of your boat....

 

[Kings Gambit]

If you find the proper gauge and took enough readings it would work, but I do fail to see the usefulness of getting those results. For sizing anchors and rode the generic tables are more than adequate.

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.

 

[kito]

So what you are wanting to know is how much load your boat is putting on a line that is tied to a stationary object under various wind and current speeds in flat water? I bet one of our nerdy analytical engineers here where I work can calculate that on his computer using Mathcad. Probably just needs surface area, weight, wind speed etc

 

[BudGates]

The fact that there are tables and it can be fairly easily calculated isn't important. Kings Gambit wants to measure it for himself. We've already stated the best ways to take these measurements. Let's just let him do it.

 

[rgranger]

Now, how would we know that if those tables have never been checked in practice? ...

I'd like to see the numbers.... I'm betting wave action can cause a lot of fluctuation as well.

If you do the measurements, please post the results.

r

 

[capta]

Somehow, I think you are all under thinking this. Even a smaller 25 to 30 footer would probably take incredibly expensive and perishable (in salt water) equipment. We're not talking pounds here, we are talking TONS. Add a bit of swell, double the wind and all bets are off.

 

[Kings Gambit]

So what you are wanting to know is how much load your boat is putting on a line that is tied to a stationary object under various wind and current speeds in flat water? I bet one of our nerdy analytical engineers here where I work can calculate that on his computer using Mathcad. Probably just needs surface area, weight, wind speed etc

Yeah-- basically, and I'm sure your engineers could come up with a theoretical value. Lot of variables, however. Just musing, I'd say cross-sectional area of the boat facing the wind bow on, and athwart the wind as it yaws at anchor; dodger up/dodger down, plus the other stuff hanging on the boat's rigging, as well as the rigging itself, and the weight of the boat. Regarding the wind-- wind speed, temperature, humidity, etc., for the force component and drag coefficients. Then, the force of the sea water (e.g., wind waves) hitting the hull, etc. All of those would be assumptions. More assumptions, more errors--i.e., garbage in, garbage out, as they like to say in the computational world. You could do a hundred boats this theoretical way in a couple of hours with a mathematical model that would accept the basic parameters. But, you could actually measure it on only a few boats in practice (unless it's your life's work)!

Stu's table is labeled design loads. I imagine the only variable input parameters are wind speed, max beam, and length b/c that's what it says, and they are thumbnail calculations (estimates) only. For the Bavaria 38 @ 12.5 ft beam in 15 kt the load is predicted to be about 350 lb (not tons). I see 15 kt often at anchor. It should be easy to see if the boat pulls around 350 lbs on average or not. My bet is that it's less than that b/c I can usually pull the boat forward some with one arm tugging at the rode.

 

[shemandr]

Well this is purely academic but what if you tied two loops in your rode and then put a scale between the two loops, (see drawing). A 1/2 line has a cross sectional area of approximately 0.8 inches squared... so if you measured the pounds of pull between the two loops and divided it by 0.8 you would have a psi measurement.

But I think most line actually comes rated as simply pound working as a function of tensile strength so you would not even need to worry about the rode's diameter... just measure pounds on the scale.

View attachment 120963

Someone slept at a Holiday Inn Express last night.

Actually I'd like to see the raw numbers. I'm guessing the tables are way high.

 

[Maine Sail]

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.