Forces on an anchored / moored vessel??

[bluedragon]

In our weekly anchor test and "mine is better than yours" threads, there is one inescapable conundrum...why is it those of us with so-called antiquated ground tackle designs are still afloat and not washed up on some distant lee shore?? Whatever one might think about all the "tests", clearly reality is saying something different.

Now, there could be a number of reasons for this, probably all working in tandem...most of us choose not to anchor in high risk situations...the importance of rode chcoice and scope....anchoring technique...BUT what I'd like to focus on in this thread are the FORCES being created on a boat at anchor (or indeed when moored) by tide and wind. There have been some surprising (for me) and contradictory statements in the yachting press about that recently. Could it be we are just not seeing the kind of loads that would cause even a cheapie Chinese clone to drag??

What I've observed for example is that in 30Kn of wind on the bow I can hold my 27ft/3 ton/narrow beam yacht off a pontoon with just body weight. When digging an anchor in with an 8HP engine in reverse, I can't pull against it. So is the latter the equivalent to more than 30Kn of wind? What effect does 3 kn of tide have? 2-3ft waves?? etc, etc. An understanding of all this is an under-rated part of ground tackle choice IMHO, and there seems little published. So, can any forumites point us towards a good discussion of the forces involved, and does anyone know of a simple means of measuring load on say a snubber (or mooring) line?

PS - Oh and what about the force it takes to lift chain off / drag it along the bottom? I suspect the anchor doesn't see any load at all for quite some wind / tide speed?

 

[Poignard]

[ QUOTE ]
and does anyone know of a simple means of measuring load on say a snubber (or mooring) line?


[/ QUOTE ]

A spring balance would do it. These are available in a range of sizes but what size you would need I couldn't say.

 

[Aja]

This Website tells you all you need to know.

And more.

Donald

 

[jimbaerselman]

The constant forces created by wind and tide on a vessel are trivial compared to the dynamic forces (excepting the case where you're anchored with a line ashore, and the wind is on the beam; med mooring in fact . . . which is partly why mediterranean experienced vessels pay quite a bit of attention to anchors!)

Pitching in a seaway creates a circular motion at the bow (seen from the side) which moves the whole vessel backwards and forwards by about the same amount as the difference between peak and trough of the oncoming waves. The anchor gear resists this motion as the vessel surges rearwards; these inertia forces are very much larger than any steady wind forces on the boat.

Another dynamic motion is created by the boat 'sailing' from side to side in winds above 20/25 kts. Short keel yachts are more prone this behaviour the long keel yachts. At each 'tack', the boat is brought to a halt and pulled through an angle, creating another high inertia force (a single anchor will also be swivelled and re-oriented at the same time).

These are the peak forces applied to an anchor, and in very demanding conditions they can be high enough to cause the anchor winch drive shaft to fail in torque. Use a snubber to take the strain!

Anchoring technique for demanding conditions is all about reducing these shock strains on the gear and ensuring the anchor is pulled in a consistent direction.

Reduce shock strains by having a long scope (6:1 or more if it's really hectic, with some 10 to 20 m of nylon snubber giving plenty of 'spring' - chain catenary alone is completely inadequate unless you're in depths over 20metres)

Reduce 'sailing' by hanging to a forked moor. You'll still 'sail' (though not so much) and each time the vessel snatches and tacks, at least the forces at the 'snatching' anchor will be aligned with he anchor.

And if you're med moored, lay a spring or anchor up wind!

For a more mathematical analasys (and some very good descriptions) see Alain Frayse's site - http://alain.fraysse.free.fr/sail/rode/

 

[craigsmith]

[ QUOTE ]
In our weekly anchor test and "mine is better than yours" threads, there is one inescapable conundrum...why is it those of us with so-called antiquated ground tackle designs are still afloat and not washed up on some distant lee shore??

[/ QUOTE ]Well I'm afraid there are a great deal many who are or have been... they immediately re-think their choice of anchor. The Spade for example is one of the most expensive anchors available, yet it is greatly preferred by many experienced long distance cruisers. Why?

[ QUOTE ]
Could it be we are just not seeing the kind of loads that would cause even a cheapie Chinese clone to drag??

[/ QUOTE ]Absolutely. For the majority of circumstances, you will "get away with it". It's when the chips go down and your luck runs out that you see the differences. One of the things we loved about the West Marine testing - it simulated tough conditions for the contenders.

[ QUOTE ]
What I've observed for example is that in 30Kn of wind on the bow I can hold my 27ft/3 ton/narrow beam yacht off a pontoon with just body weight.

[/ QUOTE ]Not entirely sure what you mean here - can't picture the pontoon and how much shelter it might be offering, and whether you mean pulling on the line from a standing position, or hanging on the line (which introduces a considerable leverage). Or you mean pushing, in which case again there are static forces at work between your body, the dock, and the boat which you couldn't really feel. Anyway, your 27' boat might be expected to exert less than 100daN (~100Kg) in static conditions in 30 knots wind.

[ QUOTE ]
When digging an anchor in with an 8HP engine in reverse, I can't pull against it. So is the latter the equivalent to more than 30Kn of wind?

[/ QUOTE ]Not necessarily because you're not comparing like-for-like. Apply the engine on the dock sometime in such a way that you can experiment same as you did for the windage. Or look up the static bollard pull for your prop at the RPM used.

[ QUOTE ]
What effect does 3 kn of tide have?

[/ QUOTE ]Very, very, little. Most over estimate the force of current, particularly at low speeds. Depending on the hull speed of your boat, it does increase very rapidly.

[ QUOTE ]
2-3ft waves?? etc, etc.

[/ QUOTE ]See the link to Fraysse's site posted above which goes some way toward modeling this sort of thing. Wave action can easily double the force present otherwise, but it depends heavily on the rode make-up.

[ QUOTE ]
An understanding of all this is an under-rated part of ground tackle choice IMHO, and there seems little published.

[/ QUOTE ]Not really as far as the anchor's concerned, because at the end of the day all that matters is: will it hold. That's all people care about, not analyzing every last little detail. As far as the rode is concerned, yes, but then most people just do what they see others doing, or read in a magazine, or on a forum...

Although, as this thread will show you, there is quite a lot published.

[ QUOTE ]
So, can any forumites point us towards a good discussion of the forces involved, and does anyone know of a simple means of measuring load on say a snubber (or mooring) line?
PS - Oh and what about the force it takes to lift chain off / drag it along the bottom? I suspect the anchor doesn't see any load at all for quite some wind / tide speed?

[/ QUOTE ]You can use a simple load-meter to measure force - put it inline in the rode. They are cheaply rented.

Try this article on catenary and anchor systems which almost directly answers your last question.

Note your last assumption is deeply flawed. The resistance from chain on the seabed is trivial. Apply any load to the chain attached to your boat, and the anchor immediately feels it.

 

[bluedragon]

OK quite a bit of reading to do. Craig - I disagree with your implied suggestion that understanding the forces is not important. Even once an anchor type is chosen (from tests, experience, opinion) there's still the question of sizing. I'm NOT a believer in "bigger the better" for a number of reasons. There's going to be an optimum size for every vessel and cruising grounds, and it would be nice to use some of the "test" load data in a more scientific way. If a chosen anchor already has a holding factor way above any likely load coming on it, then why consider bigger (or different). That's what I'd like to get at...

 

[craigsmith]

Not so much that it isn't important per se, just isn't important to many (most) people.

Of course we'd like every one of our customers to spend the numerous hours to study all the relevent material so they can make educated decisions... that would be in our interest too... but the world doesn't work like that.

 

[bluedragon]

[ QUOTE ]


For a more mathematical analasys (and some very good descriptions) see Alain Frayse's site - http://alain.fraysse.free.fr/sail/rode/

[/ QUOTE ]

Just had a quick run through this site. Very interesting! Needs more detailed study, but already I'm re-thinking the critical effect of the rode (type and length) in keeping the pull horizontal and minimising shock loads. The principle I already know, but the simulations gave me a numerical feel for it. Many thanks.

 

[bluedragon]

Craig - how do you calculate the recommended anchor sizes for your range? It must be based on likely loads. I'm not expecting a detailed breakdown (and trade secret formulae) but the principle of it. Take my case: 27ft yacht, 3 tons displacement, low freeboard, long keel. Lots of boat in the water, rather than on top. 40m chain extended by 30m nylon. No windlass..it all comes up by hand. Do you take any of those factors into account or is it just a simple LOA relationship ie. one-size-fits-all boats dependant on length.

 

[bluedragon]

PS - does anyone know who supplies anchor chain hooks (claws) in the UK? Seen them / used them in the Caribbean, but haven't noticed any in the chandlers here. Easier than a rolling hitch for a snubber line.

 

[gandy]

Do you mean one of these? I bought ours from Gael Force. Jimmy Green lists them as well.

http://www.gaelforcemarine.co.uk/ProductDetailsPage.aspx?product_id=18788

 

[wagenaar]

[ QUOTE ]

What effect does 3 kn of tide have?

Very, very, little. Most over estimate the force of current, particularly at low speeds. Depending on the hull speed of your boat, it does increase very rapidly.

[/ QUOTE ]
Like most objects in a current, be it a gas or a liquid, the force excerted by the current is proportional to the square of the speed.

The mathematics with respect to the forces excerted on an anchor are not the most important part of the picture, because, as is shown in the article you mention, its easy to determine the force an anchor will experience as a function of a constant load on the ship due to a non-varying wind and current, in a depth of x meter, y meter of chain and a scope of z. What causes the problems in actual anchoring are the varying forces, due to variation in windspeed and -direction, currents, most of all waves and the "sailing" of the ship behind the anchor. To me this is like fatigue testing and the consequences have to be determined by actual in situ testing. This is a normal procedure in airplane development, where the forces the components of a plane experience are determined, while the plane is in the air. The resulting load-picture is then used in the testing of components. As far as I know this type of testing has never been done for an anchoring system. The way to do such a test would be to measure the loads in an anchorrode with a tensiometer and then apply these, maybe increased 2-fold, to a dug-in anchor and see what happens. In particular such changes that occur that would cause the anchor to have to reset itself are to be taken into account. And including the resetting behavior.
Henk.

 

[graham]

[ QUOTE ]
What effect does 3 kn of tide have?


--------------------------------------------------------------------------------

Very, very, little. Most over estimate the force of current, particularly at low speeds. Depending on the hull speed of your boat, it does increase very rapidly.

[/ QUOTE ]

I know you have done a lot of testing so Im not disputing this but whenever I see fishing boats anchored around here in about 3 knots of tide,even in fresh wind they will usually be lying to the tide not the wind.

Ships at anchor seem to lie to the tide even in gale force winds.

 

[gandy]

Our boat will lie to the tide even in strong winds. I think this is because the tidal forces if the boat lies across the tide are pretty huge, so the boat can't swing away from that position.

In strong winds the boat rides forward over the mooring, showing that the windage is overpowering drag from the tide, but it doesn't swing round head to wind.

 

[bluedragon]

Yes, that's it!! Thanks.

 

[bluedragon]

Yup..I agree...didn't challenge this point, but it's my experience that it takes quite a bit of wind to overpower the tidal forces. But Gandy's explanation makes sense.

You know, I was asking for some theory to make sense of the empirical, but I wonder if the variables are so complex that it's just not possible.

 

[Poignard]

[ QUOTE ]
Our boat will lie to the tide even in strong winds.

[/ QUOTE ]

So will a Twister. I often find we are tide-rode when other sailing yachts with short keels are not. This is very noticeable just before slack water, when she is still lying to the last bit of tide whilst the others are pointing at the wind or, if there is no wind, pointing in all directions.

 

[HanaXAGOR]

[ QUOTE ]
PS - does anyone know who supplies anchor chain hooks (claws) in the UK?

[/ QUOTE ]

Try:

http://www.proboat.co.uk/item.asp?dep=1&prod_id=444&cat_id=24

 

[craigsmith]

[ QUOTE ]
Craig - how do you calculate the recommended anchor sizes for your range? It must be based on likely loads. I'm not expecting a detailed breakdown (and trade secret formulae) but the principle of it. Take my case: 27ft yacht, 3 tons displacement, low freeboard, long keel. Lots of boat in the water, rather than on top. 40m chain extended by 30m nylon. No windlass..it all comes up by hand. Do you take any of those factors into account or is it just a simple LOA relationship ie. one-size-fits-all boats dependant on length.

[/ QUOTE ]Our sizing chart considers LOA and displacement, which typically gives a fairly good idea of the boat. (Intended for monohulls). That chart is designed by considering the peak forces likely to be exerted by the boat in question in 50 knots wind, with a considerable degree of surge / wave action. M. Alain Fraysse has been most helpful here, as has other real world data.

No consideration of rode, other than we assume the scope is adequate - 5:1 or more, as expoused by most authorities and our anchor's own User's Guide. This is part of the "good seamanship" equation. We also assume it's fairly "normal", in that it isn't going to deal completely with shock loading.

We then compare those forces to test data of the anchors in real conditions, and what we know it can hold in moderate-to-poor-holding, assuming a decent set. Mix in a tiny bit of common sense and a lot of real life experience, and you get the final result. Which, to be honest, we got wrong the first time round (we recently adjusted the larger sizes, as they were proving too conversative).

There can be (and are) additional factors such as domestic maritime regulations and the requirements of classifications organizations (e.g. Lloyd's).

Then of course you are forced to choose from the sizes available; maybe ideally you would want an 8Kg anchor for your boat, but we only do a 6 and 10.

Of course factors other than LOA and weight can all be calculated, but there are only two dimensions on a sheet of paper /forums/images/graemlins/mad.gif. I could design a more sophisticated online interactive thing, for instance, which would ask you more details about the boat and intended conditions of anchoring, and the recommendation would be more accurate again, but frankly what we do is quite adequate. We get many enquiries from people who request our recommendation, rather than trusting themselves or the sizing chart, which is understandable, so I'm not sure there's much point in such a system.

For your particular boat, the chart says you want a Rocna 10. That happens to be the size in our video, and we ourselves have tested it in hard mud at holding over a tonne. The West Marine / Yachting Monthly testing suggests the 10 would hold nearly 1.5T in the same conditions as their test pulls. Point is, the recommendation is conservative. In good holding you could use a much lighter anchor - but then what about soft mud?

 

[craigsmith]

[ QUOTE ]
Our boat will lie to the tide even in strong winds. I think this is because the tidal forces if the boat lies across the tide are pretty huge, so the boat can't swing away from that position.

In strong winds the boat rides forward over the mooring, showing that the windage is overpowering drag from the tide, but it doesn't swing round head to wind.

[/ QUOTE ]Quite right, and the answer to the above posts.

If you are anchored in 3 knots current, and the wind happens to be 20 knots in the same direction, the wind will be exerting the vast majority of force on the rode.

If no wind, the current must approach the hull speed of the boat before you will have problems.

Naturally a cross wind will complicate things, so that your boat will be unable to present the optimum profile to either force. Of course it will find the best compromise on its own (no talk of stern anchors please), but the resultant forces will be higher by a considerable amount.