Reversing the engine when digging in the anchor – how much anchor load does this correspond to?

[PilotWolf]

So genuine question.

How many use just chain or chain and warp?

The chain on the sea bed is what is really holding you, not the anchor - at least at my level.

PW

 

[Poey50]

So genuine question.

How many use just chain or chain and warp?

The chain on the sea bed is what is really holding you, not the anchor - at least at my level.

PW

Edit: I withdraw my reply to avoid a life-time ban.

 

[PilotWolf]

Haven't heard that one for a while! ???

Well it is a new anchor thread - first in a while.

W.

 

[NormanS]

I find it rather surprising that anyone would not make an effort to dig an anchor in.
If your anchor is going to plough a furrow across Newtown Creek with a few Hp's help then it's going to do the same in the oncoming gale. Wouldn't you rather discover this charactaristic while you still have time to deal with it?
Granted, if no gale is coming then probably no need to use full revs, but surely prudence and basic seamanship dictates that you ensure the anchor has got a bite? A pile of chain sitting on top of the anchor holds a boat deceptively well until the wind begins to whistle, as anyone who's sailed the Greek Islands with charter boats around can attest...

Re pull available under power, there is a rule of thumb amongst tuggies that approximates 1 ton of bollard pull per 100Hp. PBO's 2009 featherng/folding propellor test was run with 21Hp. The baseline standard 3 bladed prop provided 260Kg bollard pull in ahead and 180Kg in astern which corresponds quite well with that.
How that translates at the far end of an anchor rode is anyone's guess, but suffice to say it'll be a fair bit less. Quite how much force the previously quoted 30Kt wind exerts on an anchored boat is equally up for debate but I'd wager its substantially if not dramatically more than the engine's bollard pull

Me, I'll continue to dig in, thanks, and do so under full power if I think it's going to get boisterous.

Ah yes, but some of us have sailing boats, and enough common sense not to have a pile of chain sitting on top of the anchor. ?

 

[MathiasW]

So genuine question.

How many use just chain or chain and warp?

The chain on the sea bed is what is really holding you, not the anchor - at least at my level.

PW

Now that is definitely WRONG! It is the anchor that is holding you. The chain will only add to this a little by way of friction on the seabed, but you can work that out. It is less than the weight of the chain on the seabed. This means for a 10 mm chain with a weight of 2 kp/meter, it is just 40 kp for a chain of 20 meters on the seabed. This is nowhere near to what is needed to hold your vessel in anything but a very moderate wind.

 

[dunedin]

So genuine question.

How many use just chain or chain and warp?

The chain on the sea bed is what is really holding you, not the anchor - at least at my level.

PW

Myth number 4a.

There have been plenty of both mathematical calculations and photographs to show that the chain will be up off the seabed whenever the wind gets up to even moderately strong, let alone a gale

 

[PilotWolf]

Now that is definitely WRONG! It is the anchor that is holding you. The chain will only add to this a little by way of friction on the seabed, but you can work that out. It is less than the weight of the chain on the seabed. This means for a 10 mm chain with a weight of 2 kp/meter, it is just 40 kp for a chain of 20 meters on the seabed. This is nowhere near to what is needed to hold your vessel in anything but a very moderate wind.

Not even going to bother replying with facts.

W.

 

[MathiasW]

Not even going to bother replying with facts.

W.

Sure, I know, would be kind of difficult as there aren't any facts to support your view!

The chain is passing on the load on the vessel generated by wind and sea to the anchor. It will dampen peaks in the load when it is not fully stretched out. That is the essence of the physics that is happening.

I suggest you put your view to a test and remove the anchor, trusting the chain alone. But please do this experiment somewhere where you are alone at an anchorage...

 

[thinwater]

Oh dear. Here we go again.

I like chain, but much of my anchor testing is done with no (or very minimal) chain (eliminates catenary distractions), and I assure, the anchor holds 95% of the load if there is enough wind to matter. I'd be better off with a Mantus dinghy anchor and 100' of good clothes line than 100 feet of just chain.

But there are interesting topics here:

• Just how much does deep setting help if the wind shifts and is strong? Testing seems to show that good anchors stay mostly set and rotate. They do not flip out unless the test is rigged that way or in strong reversing tides.
• How do you chose between all-chain (with snubber) and combination rodes (rope and chain)? The answer, of course, depends on the boat and the area. I'm sure there is no one answer; a ship won't use rope and a kayak won't use chain.
• How do you size the rope? Chain is based on strength. Simples. Some complicated it with the virtues of weight, but I'll leave that alone as it is minor. But rope varies in stretch, and perhaps 200' of the smallest acceptable nylon is too stretchy. 50 feet of polyester double braid is not stretchy enough (with no catenary, it feels like steel cable). But what about oversized nylon or a long length of polyester brait? I did a small bit of testing and have my opinion, but I have not seen rope type comparison testing, just opinions.
• How much chain in a combination rode? Catenary effect (chain beyond 100' does not help that much)? Chafe (how much will likely be on the bottom). Just enough to reach the bottom, so you do not have it scraping the topsides during manual breakout (as little as 10')? Light chain (to save more weight) or heavy chain (so it can be shorter)?

 

[Sandro]

If I am not mistaken, the OP was asking for the force with which the boat pulls the anchor when setting it in by reversing.
The simplest way is bollard pull. Having access to a dynamometer (a kind of spring scale, cheap ones for sale on line), a bollard pull test reproduces exactly the conditions (propeller working with stationary boat) and renders dyrectly the wanted force value without any calculation.

 

[PilotWolf]

Sure, I know, would be kind of difficult as there aren't any facts to support your view!

The chain is passing on the load on the vessel generated by wind and sea to the anchor. It will dampen peaks in the load when it is not fully stretched out. That is the essence of the physics that is happening.

I suggest you put your view to a test and remove the anchor, trusting the chain alone. But please do this experiment somewhere where you are alone at an anchorage...

Plenty I just can't be ass'd to go and quote them.

Do you have any idea how much a ship's chain weighs compared to your yachtie anchor?

PW

 

[gordmac]

Ah, indeed! Thanks so much for taking the time to look into this and double check! My result was in kN and not in kp... Damn units... hasn't happened to me in a long while... ?

I think your number is Newtons ie 8kN.
What is a kp?

 

[MathiasW]

I think your number is Newtons ie 8kN.
What is a kp?

Indeed, it was in Newton - ever so slightly embarrassing.

kp is kilo pont, which is the same numerical value as kg on Earth, but just a unit of force and weight, rather than mass. The numerical factor between kp and N is 9.81, the gravitational acceleration on Earth.

 

[MathiasW]

Plenty I just can't be ass'd to go and quote them.

Do you have any idea how much a ship's chain weighs compared to your yachtie anchor?

PW

Yes, I happen to do, and it is still a nonsense argument. But of course, if you entertain a real ship's chain of size 100 mm or so on your vessel, then I rest my case.

Just do me a favour and do an experiment: Take your most favourite anchor chain onto a beach, say 100' ft of it, and pull it along the beach. You will be surprised how easy that is. Then compare that with the pull wind and waves will exert on your vessel...

You will find it hard to argue against basic physics laws.

 

[Richard10002]

Plenty I just can't be ass'd to go and quote them.

Do you have any idea how much a ship's chain weighs compared to your yachtie anchor?

PW

Are you saying that on, "ships", the anchor chain never becomes taut, such that it is all off the bottom?

 

[geem]

Plenty I just can't be ass'd to go and quote them.

Do you have any idea how much a ship's chain weighs compared to your yachtie anchor?

PW

So why do they bother putting an anchor on the end?

 

[thinwater]

Indeed, it was in Newton - ever so slightly embarrassing.

kp is kilo pont, which is the same numerical value as kg on Earth, but just a unit of force and weight, rather than mass. The numerical factor between kp and N is 9.81, the gravitational acceleration on Earth.

In the US engineers would call your kp "kilogram force," though normally they either use KN or kilogram (only if the context makes it obvious, but it's sloppy practice). Fractional KN is more common than newtons.

 

[thinwater]

Plenty I just can't be ass'd to go and quote them.

Do you have any idea how much a ship's chain weighs compared to your yachtie anchor?

PW

Don't feed the trolls....

 

[Neeves]

I find it rather surprising that anyone would not make an effort to dig an anchor in.
If your anchor is going to plough a furrow across Newtown Creek with a few Hp's help then it's going to do the same in the oncoming gale. Wouldn't you rather discover this charactaristic while you still have time to deal with it?
Granted, if no gale is coming then probably no need to use full revs, but surely prudence and basic seamanship dictates that you ensure the anchor has got a bite? A pile of chain sitting on top of the anchor holds a boat deceptively well until the wind begins to whistle, as anyone who's sailed the Greek Islands with charter boats around can attest...

Re pull available under power, there is a rule of thumb amongst tuggies that approximates 1 ton of bollard pull per 100Hp. PBO's 2009 featherng/folding propellor test was run with 21Hp. The baseline standard 3 bladed prop provided 260Kg bollard pull in ahead and 180Kg in astern which corresponds quite well with that.
How that translates at the far end of an anchor rode is anyone's guess, but suffice to say it'll be a fair bit less. Quite how much force the previously quoted 30Kt wind exerts on an anchored boat is equally up for debate but I'd wager its substantially if not dramatically more than the engine's bollard pull

Me, I'll continue to dig in, thanks, and do so under full power if I think it's going to get boisterous.

It depends on the prop and there are also as many choices of props as there are anchors - so the data is an approximation (as is the hold of your anchor). The hold of your anchor is the tension in the rode - that's how hold is measured. Apply 200kg of tension then your anchor is holding 200kg (provided it is not moving), no more no less. If your anchor is big or small - it is still holding 200kg. The ultimate hold, the figure sometimes quoted, is the maximum hold before the anchor drags, moves continuously through the seabed. A larger anchor has an ultimate hold greater than a small anchor of the same design - if you choose an anchor sized by that defined on a manufacturers spread sheet you will never ever come close to the ultimate hold of the anchor (with certain caveats over the seabed characteristics).

As an aside I don't believe that anchors of the correct size drag because they have exceeded their ultimate hold (as measured under ideal conditions). Most anchors seem to drag because they are fouled or because of veering or horsing (chop). Some of this is operator error - deploy more rode and you will have rode on the seabed and then chop is not the same issue etc etc.

If you read back you will find I have actually measured the tension and our folding Volvo props return approximately 100kg of tension for every 10hp in reverse. I think the numbers, measured, are irrelevant if you use an outboard. If you measure a tension of 100kg just forward of the bow roller and the chain is all off the seabed then the tension at the anchor is also 100kg. With 100kg of tension and 30m deployed of 8mm chain at a 6:1 scope all the chain will be off the seabed.

For other scopes, other chain sizes there are numerous catenary calculators to allow determination of tensions under different conditions. But to save you searching Mathias has a neat spread sheet calculator which might be more useful as he has developed the calculator specifically for leisure marine applications. His calculator will also underline the value of using a snubber.

The idea that chain offers any material hold is laughable. Lay 50m of 8mm chain on the seabed and you can pull it by hand - I know I've done it (in clean sand its one way to remove surface rust). Its difficult to get it 'started' but once started you can pull it for ever. A grown fit man can pull roughly his own body weight. The whole idea is a joke. It is obvious that people who expound the idea have never tested their ideas. But hey its a forum - what can you expect.

If you try the 50m of 8mm chain on the seabed, pull it along the surf line, you can also try pulling it sideways. When you try to pull it sideways then you find the frictional resistance of the chain - and the chain will defeat you. This underlines why having some chain on the seabed is advantageous in a veering wind - the frictional resistance of the chain protects your anchor from tension angle changes caused by your yacht veering. In a crowded anchorage this is of little use - and then a snubber comes into its own.

Jonathan

 

[Slowboat35]

If I am not mistaken, the OP was asking for the force with which the boat pulls the anchor when setting it in by reversing.
The simplest way is bollard pull. Having access to a dynamometer (a kind of spring scale, cheap ones for sale on line), a bollard pull test reproduces exactly the conditions (propeller working with stationary boat) and renders dyrectly the wanted force value without any calculation.

It seems that the definition of "dynpmometer" has become either confused, blurred or changed recently. The only definition I know for it is a devece for measurung the power output of a rotating shaft, yet Amazon are selling ranges of spring balances and other simple weight-measuring scales as "dynomometers" which surely can't be technically correct.
Bollard pull is measurd with a spring balance or a strain gauge of some kind - nothing is rotating there - is it?