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

[NormanS]

Jonathan, it must be wonderful to be able to guarantee that the wind in your anchorages never exceeds 30 knots. ?
Sadly, that doesn't work for us.

 

[thinwater]

Jonathan, it must be wonderful to be able to guarantee that the wind in your anchorages never exceeds 30 knots. ?
Sadly, that doesn't work for us.

Then use more engine.

Whiner.

 

[MathiasW]

I think maybe you tripped over a conversion factor. I got 1820 pounds for your boat.

The theory is right. And in fact I have also confirmed your reasoning with a load cell.

Convert HP to ft-#/s, get your reverse speed in ft/s, and divide. The 60% efficiency factor is reasonable, and even as low as 35% for some outboards. We can quibble over prop slip and hydrodynamics, but you're not far off. For example, based on my PDQ:

HP = 19.8 (twin Yamaha high reverse thrust)
Speed in reverse 7.3 knots
=(50%)10890/12.4=439 pounds.

For my F-24
HP=4
speed = 5
=(35%)2200/8.50=90 pounds.

In fact, reverse thrust on the PDQ is about 440-455 and 80 pounds with the F-24. Very close. Certainly not bad compared with the guesswork to come.

The next question is windage and waves. This is different for each boat, but if you take ABYC H-40 table 1 storm value, and use about 1/3 of that, that is you rode tension at about 42 knots. From there, F~ V^2 with wind. The highest gusts don't matter quite as much as the sustained wind, because the motion of the boat and catenary damps that, waves depend on sustained wind, and because the table is based on sustained winds.

[I have assumed that you have either a long snubber or a lot of chain and are NOT snubbing up on the rode at all. Also that the boat is not yawing more than about 20 degrees each side of the wind. Finally, that you are in water deep enough that the waves are not too steep. if any of these are false, the load can be LOT more.]

But this is a broad aproximation. A good answer requires a load cell. In general, backing the engine hard is similar to a strong gale.

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... ?

 

[capnsensible]

Goody an anchor thread!

Anchors only drag at 0237 when it's raining. ........

Anyone else noticed that? .

 

[MathiasW]

As to my mistake in units - I suppose the appropriate swearing would be "Gesus Christ!"....

 

[westernman]

You do realise that CQR anchors have been on the go for rather longer than reliable "auxiliary" engines. That approach must have worked then.

I do anchor under sail from time to time.

With a CQR, the best approach seems to be to drop the anchor until it is just touching. Wait a little until you have drifted back a little bit. Then dump out a decent amount of chain. About 4:1 minimum.

Tom Cunliffe then recommends making a cup of tea. But I do things very differently.

I make a jar of coffee. And when I have finished that and washed up, the anchor will be dug in properly.

Just to be sure, I could fire up the engine and back down on it. But if the conditions are not going to get stormy over night, it is not necessary in anchorages which I know to have good holding. My biggest concern is that we have enough swinging room and that there are not other boats we are likely to collide with as the wind will almost certainly completely change direction a few times during the night.

Only ever dragged once and that was in poor holding on a stony bottom in windy conditions. I spent most of the night awake waiting for it to happen. Did not drag very far, but as soon as the anchor started moving you heard it loudly in the fore cabin. That kept me up most of the night although we did not need to re-anchor.

Another time, I was concerned that we were on the limit of the holding capacity. It was blowing a gale towards a lee shore. Good holding, all the chain in the locker out and plenty of room.

The only other boat in the bay dragged and ended up on their side on the beach. They got dragged off by the life boat (very expert coxswain) and no apparent damage. But we held solidly and did not move an inch. Although I hardly slept the whole night. I was ready to start the engine and to have it running slow ahead to take some load off the chain which was almost straight - no catenary but with a nylon warp as a shock absorber (needed because of the chop).

 

[noelex]

From a practical point of view for most yachts if you set the anchor in light wind by gradually ramping up to full reverse power and leaving this for around thirty seconds the depth of anchor set will be equivalent to an anchor that was simply dropped when the yacht has experienced about 25 knots of wind.

After setting in the above manner If the wind stays below 25 knots and does not change in direction the anchor will remain unchanged. If the wind rises above 25 knots the anchor will have to burry deeper to hold the yacht in position.

This rough rule of thumb holds true for most sailing yachts although there are exceptions. The biggest variable is the efficiency of the propeller in reverse

 

[roaringgirl]

I think you have a problem with the units. It's always easier to work with SI units, even if you have to convert them back at the end to relate to the original problem.

Relevant SI units:
Distance: metres
Speed: m/s
Energy (work): joules
Power: watts (j/s)
Mass: kg
Force: Newtons

 

[PilotWolf]

$h!t I've been doing it wrong all these years!

Stop forward motion - bow (assuming bow anchor) facing upwind/up tide, let it go knowing depth so having a reasonable amount of idea how much chain I need out, nudge astern as paying out 'guestimated' scope. It it holds great, if not let out a bit more! Monitor for dragging. Put kettle on and pass over anchor watch to junior.

W.

 

[MathiasW]

I think you have a problem with the units. It's always easier to work with SI units, even if you have to convert them back at the end to relate to the original problem.

Relevant SI units:
Distance: metres
Speed: m/s
Energy (work): joules
Power: watts (j/s)
Mass: kg
Force: Newtons

Yes, indeed, as admitted in an earlier post... I had forgotten about the little factor g to convert between N and kp... hence my pun, which I guess was hard to understand.

 

[thinwater]

I think you have a problem with the units. It's always easier to work with SI units, even if you have to convert them back at the end to relate to the original problem.

Relevant SI units:
Distance: metres
Speed: m/s
Energy (work): joules
Power: watts (j/s)
Mass: kg
Force: Newtons

That's funny. US engineers are trained to use both systems with equal facility, using whichever requires few conversions. The OP gave the boat mass in pounds, but you don't use the mass, so I guess that does not matter.

By doing the problem in traditional units I avoided some of the conversions. Easy. You do have to know the difference between pounds-mass and pounds-force. The rest are pretty transparent.

---

The real problem with the OP's question is that rode tension is extremely variable.

---

The reset question is a good one. There are several potential answers.

• If the anchor is set to full power in a good bottom, a tide change alone may not rotate it. It certainly would not rotate a Fortress.
• Not all anchorages are tidal, or so weakly that it hardly matters. See ^^. Many coves have no measurable tide. In many roadsteads it is very weak.
• Most of the time we ask this question it is because a storm is coming from a known direction or range of directions within a matter of hours.

If the tide is strong AND I am expecting a real blow, I will probably have two anchors placed in such a way that they do not reset. I anchor too many places where I don't trust reset.

 

[Neeves]

Jonathan, it must be wonderful to be able to guarantee that the wind in your anchorages never exceeds 30 knots. ?
Sadly, that doesn't work for us.

No-one can nor is guaranteeing anchorages where winds are 100% less than 30 knots. If such guarantees were available we would have less need for weather forecasts.

But the reality is most people sail in winds where 30 knots might be considered high and where anchorages are well known - and used as anchorages (as they have been for centuries - because they offer shelter. You choose to anchor in the outlying islands of the north west of Scotland and I'll wager the cruising ground is devoid of yachts, compared with, say, the Solent or Sydney harbour. You probably choose the N W of Scotland because you enjoy the solitude. NW Scotland is like Tasmania, a major undertaking to get there for most people - and once then it is subject to winds emanation from 1,000s of miles of uninterrupted sea - and those winds can be more than 'high' and harsh - one reason the location is devoid (almost) of inhabitants. (and of cruising yachts).

For most people the idea of an anchorage subject to 30 knots on a regular basis is alien and for this reason comments on anchoring practices cater for more benign conditions. You have honed your skills over decades, many have only taken to sailing recently - careers and families deny them the opportunity to venture to the Outer Hebrides - you and I are lucky (though I came to the harsh realities of anchoring in locations where 30 knots might be 'more' normal - later in life. We learnt by our mistakes - in the meantime I cater for the neophyte and would not suggest that their first long term cruise is to the NW of Scotland nor Tasmania.

You know all of what is to know of anchoring - you don't need suggestions, others are not so lucky.

It would be difficult to cater for the neophyte and the long in the tooth, old (or young) salts without the script becoming long and tedious ......

Jonathan

 

[Neeves]

One of the biggest dangers is simple complacency and thinking its good seaman ship to sit out adverse conditions - bragging rights being part of the badge of honour.

The recent Storm in the Scillies is an example where yachts dragged and were driven onto beaches but the wise on hearing the forecast 'upped sticks' and moved to locations with shelter - and slept soundly. There was a recent post, I don't recall by whom, mentioning that in a similar incident in the Caribbean those that took heed of the forecast a sailed 60nm slept well but those who thought to sit out the forecast were surrounded by mayhem.

Bragging rights don't impress me

Jonathan

 

[westernman]

If the wind is really strong, there is no dishonour in running the engine to take most of the load off the anchor. You do have to keep a watch, because when the wind dies down ....

 

[NormanS]

Then use more engine.

Whiner.

I've no idea what you're getting at with your comment. Sadly, I think it's meant to be derogatory. If not, I apologise.
The fact is that in some of the more exposed parts of the world, winds are often strong, and there is a lack of vegetation, which might otherwise reduce the wind speed. Helpful comments are of course always welcome.

 

[Neeves]

I've no idea what you're getting at with your comment. Sadly, I think it's meant to be derogatory. If not, I apologise.
The fact is that in some of the more exposed parts of the world, winds are often strong, and there is a lack of vegetation, which might otherwise reduce the wind speed. Helpful comments are of course always welcome.

Norman - as a regular member often subject to anchorages with strong winds (and no vegetation to reduce the power of the wind) your knowledge would be of much more use - if it were shared. Possibly you can summarise, given that you dislike lengthy prose, what your anchoring practices (and ground tackle) are with detail of the characteristics of the anchorages.

Tasmania does have trees but in some parts of the island they are only 150mm high - adapted to the winds to which you allude - but not much use as shelter. In other parts of Tasmania there are monster trees - valued for their application in boat building. Huon Pine being top of the list.

Lagarostrobos - Wikipedia

Jonathan

 

[Neeves]

Returning to the focus of the thread:

I made these measurements on our cat, same presented area as a 45' AWB. I took measurements on different occasions in the same location at different scope. There was no chop but the wind did veer, so the tensions measured at peak were due to veering and windage. I was using 8mm chain and a 15 kg Excel anchor. I removed the bridle (or 2 snubbers) and replaced them with a dyneema bridle - I wanted to measure worst case scenario. The worst case I measured was a shnatch load of 650kgs - this was enough to scare the pants off me )its like driving into a brick wall) - and I abandoned the testing

Anchor Testing and Rode Loads - Practical Sailor

I've posted this before - but its worth reading, maybe again

Safety at Sea: Surviving a Powerful Storm in the Med

If you keep in the back of your mind that the developed tension under engine approximates to around 100kg per 10hp of engine size and bear in mind that 30m of 8mm chain at a 5:1 scope will require around 70kg of tension to lift all the chain with the link at the anchor just kissing the seabed - for a 45' AWB and check with Mathias spread sheets - you can develop a fairly decent approximation of the characteristics of your yacht.

It is a very in-exact science - there are many variables - you will arrive at an 'order of magnitude'.

Also bear in mind that in a good holding seabed most 15kg modern anchors will hold - under ideal test conditions around 2,000kg. This hold will fall in less than ideal conditions - but reports of modern anchors dragging are less common than dragging of reports of earlier models. It does appear there is sufficient safety margin for anchors if you opt for an anchor defined by the manufacturer on their spread sheets.


Realise wind is unstable - look at a wind farm, even a wind farm at sea and all the propellors are pointing in different directions. The average is the wind direction, the same wind direction in the forecast (hopefully) but the wind direction varies even in a wind farm. The location of wind farms at sea is checked and chosen for the stability of wind direction - even after much surgery the wind direction even in as small an area as the wind farm - varies. If you race, or have raced and helm then you are constantly altering course as the wind direction varies and you use this to determine when to tack. If you watch aircraft landing - they wobble all over the place. Your yacht is subject to the same variations - so you cannot ignore the idea that your yacht will yaw and sail at anchor. The windage of your yacht determines the tension in the rode - the yaw defines the maximum snatch loads.


Jonathan

 

[Bobc]

Hmm, so you just drop the anchor and let it lie?

At least, when you dig in the anchor with reverse engine you will have an idea how good the holding power of the sea bed is.

And in the case of Spade anchor, which we have been using for years, it never has failed to adjust to changing wind directions and reset itself. So, I fully trust that aspect, but I still would like to know what the holding power is at some point...

I've had my current boat for 19 years now, and in that time, once the anchor has grabbed and set, I've never had it drag using this method, not even when sitting-out hurricane Tomas in Antigua.

We used to laugh our heads off in the Solent (especially in Newtown Creek) when boats would come in, chuck their anchor out, and throw it into reverse, only to find that that they have ploughed right across the creek and then they have to lift their anchor up and try again. On one occasion, we watched someone do this half a dozen times and then give up and go to Yarmouth instead.

If you let the anchor set on its own, it is less likely to plough a trough, and once it is dug in, it will keep digging itself in slowly with the wind, tide, and motion of the boat. There is absolutely no need to use reverse other than maybe in tick-over to help it get a grip in light wind conditions.

 

[Neeves]

I've had my current boat for 19 years now, and in that time, once the anchor has grabbed and set, I've never had it drag using this method, not even when sitting-out hurricane Tomas in Antigua.

We used to laugh our heads off in the Solent (especially in Newtown Creek) when boats would come in, chuck their anchor out, and throw it into reverse, only to find that that they have ploughed right across the creek and then they have to lift their anchor up and try again. On one occasion, we watched someone do this half a dozen times and then give up and go to Yarmouth instead.

If you let the anchor set on its own, it is less likely to plough a trough, and once it is dug in, it will keep digging itself in slowly with the wind, tide, and motion of the boat. There is absolutely no need to use reverse other than maybe in tick-over to help it get a grip in light wind conditions.

I have sat and watched people lose swim suits, towels (and even some folding chairs) not to mention beer cans in anchorages. More often than not the offending articles are never retrieved. We have heard of people catching crab pots and supermarket trolleys - and we once caught a 5kg gas cylinder.

If you anchor catches any of these items your anchor will not set - if you simply chuck the anchor over the side in relatively calm conditions you will not know it is fouled unless you try it. You can wait for the wind to pick up, but that might not happen till 2am. It does seem ever so simple to put the line in reverse.

Modern anchor can have a sharp toe - sharp enough to catch more natural items, seaweed or a shell - again - instead of waiting for the wind to pick up - reverse is so much predictable.

These anchors might 'set' but will never offer reliable hold as they simply cannot set deeply - how are you going to find out - unless you test them to near maximum of your yacht's ability to check the hold? Both of these anchors are under the bridge deck of two different catamarans, a common location for the bow roller. You cannot easily see that the anchors have been fouled.

 

[NormanS]

I have sat and watched people lose swim suits, towels (and even some folding chairs) not to mention beer cans in anchorages. More often than not the offending articles are never retrieved. We have heard of people catching crab pots and supermarket trolleys - and we once caught a 5kg gas cylinder.

If you anchor catches any of these items your anchor will not set - if you simply chuck the anchor over the side in relatively calm conditions you will not know it is fouled unless you try it. You can wait for the wind to pick up, but that might not happen till 2am. It does seem ever so simple to put the line in reverse.

Modern anchor can have a sharp toe - sharp enough to catch more natural items, seaweed or a shell - again - instead of waiting for the wind to pick up - reverse is so much predictable.

These anchors might 'set' but will never offer reliable hold as they simply cannot set deeply - how are you going to find out - unless you test them to near maximum of your yacht's ability to check the hold? Both of these anchors are under the bridge deck of two different catamarans, a common location for the bow roller. You cannot easily see that the anchors have been fouled.

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These are interesting photos. It must be a nightmare to clear weed etc from an anchor on a catamaran like that. How do you get access to clear it? Do you have to partially unrig the trampoline netting?