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

[MathiasW]

The general advice for anchoring is to set the anchor, then drift backwards due to wind or current, and slowly pay out more chain until the desired length is reached. Then slowly crank up the engine in reverse to get the anchor dug in. „Slowly“ is important here, as the anchor needs time to set, otherwise it would only get dragged across the sea bed. The interesting question then is how much load have I tested the anchor with in this approach? To get an approximate answer, I did the following calculation: As physics dictates, work is force times distance, and so power is force times velocity. So I do the following measurement of my vessel: Whilst not at anchor and without any sails set, I put the engine in straight reverse and measure the velocity v at full engine power p. Ideally, this can be done with an apparent wind speed of zero, and no current or waves. The force f which is pushing the vessel through water is then simply given by f = p / v, and this should be the anchor load with which the anchor is dug in when using full reverse throttle when setting the anchor. The relevant power is not the nominal engine power, but the effective horse power after accounting for all losses in the system (within the transmission line from the engine to the propeller as well as the propeller itself). A simple way of dealing with this is to introduce an efficiency factor ?. In my case, this all amounts to – very roughly – 60% * 75 HP / 8 kn = 8042 kp, which is clearly way too much (being of the order of the chain breaking load for many chains), despite of an assumed efficiency of only 60%. So, something is wrong with this calculation, but I yet fail to understand where it has gone wrong. Perhaps the efficiency for reverse throttle is even much less than 60%? Some more food for thoughts can be found here and here.

Any ideas?

My thoughts on anchor loads and chains can be found here: Catenary Anchor Chain Length - Die Kettenkurve - Fun Facts - SAN (in English, no worries )

Cheers, Mathias

 

[fredrussell]

Neeves will be along in a mo with a typically short and succinct answer…

 

[Bobc]

Why bother digging the anchor in. As soon as the tide turns it will only unset itself and reset itself under the wind and tide load.

 

[MathiasW]

Why bother digging the anchor in. As soon as the tide turns it will only unset itself and reset itself under the wind and tide load.

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...

 

[geem]

Why bother digging the anchor in. As soon as the tide turns it will only unset itself and reset itself under the wind and tide load.

If you anchor is not dug in and ideally under the sand/ mud then you risk tripping the anchor or fouling it with your chain on change of tide. Dragging the anchor once you have gone to the pub is not great. Far better to set it and assuming you have a decent anchor like a Spade, it will turn through 180deg under the sand and remain set

 

[webcraft]

Why bother digging the anchor in. As soon as the tide turns it will only unset itself and reset itself under the wind and tide load.

A well-set anchor will turn 180 degrees without breaking out of the seabed.

- W

 

[westernman]

My way of thinking about it, (which may be completely wrong), is to think how many revs you need to hold your position (i.e. no way through the water in still water) against a wind of a certain force. E.g. a gale. Then if you back down with the same revs, then you are simulating the effect of that gale.

 

[KompetentKrew]

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

Perhaps I was doing it wrong, but every time I tried to reverse on my last anchor it would drag.

Singlehanded and with a manual windlass, resetting it was a huge pain in the ass so I soon stopped trying this and just let the wind set me. This held much more often than it did not, and quite happily in high winds and even gales.

No doubt I will soon learn the exact nature of my numptiness - I had already planned to try again when I get my new anchor; the last was a last-generation one, a Bruce, and at 15kg may have been under-specced for my 40' boat.

 

[MathiasW]

My way of thinking about it, (which may be completely wrong), is to think how many revs you need to hold your position (i.e. no way through the water in still water) against a wind of a certain force. E.g. a gale. Then if you back down with the same revs, then you are simulating the effect of that gale.

Yes, that is a good idea and I had also pondered about that one. Two complications I can see here: A) The propeller will be differently effective on forward and reverse, for multiple reasons. And B) one would need to know what HP a particular RPM corresponds to.

But it definitely is a way to to explore further...

 

[MathiasW]

Perhaps I was doing it wrong, but every time I tried to reverse on my last anchor it would drag.

Singlehanded and with a manual windlass, resetting it was a huge pain in the ass so I soon stopped trying this and just let the wind set me. This held much more often than it did not, and quite happily in high winds and even gales.

No doubt I will soon learn the exact nature of my numptiness - I had already planned to try again when I get my new anchor; the last was a last-generation one, a Bruce, and at 15kg may have been under-specced for my 40' boat.

I would put this experience down to not using enough chain to begin with. Have a look at my anchor chain calculator for what chain lengths to expect for your vessel: Anchor Chain Calculator

 

[Richard10002]

IIRC, When I was doing my own boat Yachtmaster Training Week, the instructor, (Rob at rusailing.com), suggested that 2000 revs in reverse was equivalent to 35 knots of wind. (Thinking about it, he might have said "more wind than you are likely to experience? ( Max revs was around 3000 revs, (again, IIRC))

This was a Volvo Penta MD22l, (2.0 litre Montego diesel engine), in a Moody 44, around 12 tonne IIRC.

 

[NormanS]

If you think that I'm going to negotiate a tricky entrance among the rocks, under sail alone, to get into one of my favourite secluded and sheltered anchorages, and then start the engine to dig in the anchor........
Where has seamanship gone? It's perfectly possible to anchor under sail, as previous generations have shown, by dropping the anchor in the chosen spot, while still underway. The boat's momentum will dig the anchor in just as well as an engine. Psssst, the anchor won't know what's pulling it. (There's an element of devil's advocate here). ?

 

[NormanS]

Neeves will be along in a mo with a typically short and succinct answer…

 

[geem]

If you think that I'm going to negotiate a tricky entrance among the rocks, under sail alone, to get into one of my favourite secluded and sheltered anchorages, and then start the engine to dig in the anchor........
Where has seamanship gone? It's perfectly possible to anchor under sail, as previous generations have shown, by dropping the anchor in the chosen spot, while still underway. The boat's momentum will dig the anchor in just as well as an engine. Psssst, the anchor won't know what's pulling it. (There's an element of devil's advocate here). ?

We usually drop our anchor going forward at about 1.5kts SOG. As soon the anchor hits bottom ( free dropping on the clutch) we turn hard over. This uses the windage of the boat quite effectively to drag more chain out of the locker and ensure that we don't pile chain on top of the anchor. It keep the anchor chain taught. Once the boat has pulled the chain tight we engage reverse at tickover revs and slowly increase revs to 1500rpm. Since our engine only revs to 1900rpm it's a fair percentage of the boats horse power. Its seems to be enough to set the anchor when I check it with our glass bottom boat? or snorkeling.
A few days ago I anchored off a rocky headland. The anchor wouldn't hold at 1500rpm and we dragged back some. I put mask and snorkel on to take a look. Bottom looked like sand but the anchor was only slightly in with a drag mark about 15 ft across the sea bed. I dived down and there was about 4" of sand over flat rock. No way that was going to set but I wouldn't have known unless we had pushed the revs up to 1500rpm.
We reanchored about 50 ft back and the anchor buried completely.

 

[bedouin]

The simple answer is that there is no way to tell in the general sense. A propeller trying to pull back a moored (or anchored) boat is a very different situation from a propeller pushing the boat through the water. You can't translate from engine revs to engine power, and nor can you translate from engine power to static pull.

That said my usual technique is slowly to build up to my normal cruising revs.

 

[westernman]

If you think that I'm going to negotiate a tricky entrance among the rocks, under sail alone, to get into one of my favourite secluded and sheltered anchorages, and then start the engine to dig in the anchor........
Where has seamanship gone? It's perfectly possible to anchor under sail, as previous generations have shown, by dropping the anchor in the chosen spot, while still underway. The boat's momentum will dig the anchor in just as well as an engine. Psssst, the anchor won't know what's pulling it. (There's an element of devil's advocate here). ?

You need to dig in some anchors very slowly. That approach won't work with some. For instance a CQR.

 

[MathiasW]

The simple answer is that there is no way to tell in the general sense. A propeller trying to pull back a moored (or anchored) boat is a very different situation from a propeller pushing the boat through the water. You can't translate from engine revs to engine power, and nor can you translate from engine power to static pull.

That said my usual technique is slowly to build up to my normal cruising revs.

Very true, but one should be able to outline a sketch how to work out the number. Have a look at the links I had provided. They do a lot of calculations around the topic of propelling vessels.

And yes, the water flow is different when at anchor in reverse and when freely moving backwards under engine. Still, the latter will require a force that should not be orders of magnitude different to what the engine can do when at rest at anchor. A factor 2 I could understand, but not a factor 10, which my naive calculation seems to be off reality...

 

[NormanS]

You need to dig in some anchors very slowly. That approach won't work with some. For instance a CQR.

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

 

[thinwater]

The general advice for anchoring is to set the anchor, then drift backwards due to wind or current, and slowly pay out more chain until the desired length is reached. Then slowly crank up the engine in reverse to get the anchor dug in. „Slowly“ is important here, as the anchor needs time to set, otherwise it would only get dragged across the sea bed. The interesting question then is how much load have I tested the anchor with in this approach? To get an approximate answer, I did the following calculation: As physics dictates, work is force times distance, and so power is force times velocity. So I do the following measurement of my vessel: Whilst not at anchor and without any sails set, I put the engine in straight reverse and measure the velocity v at full engine power p. Ideally, this can be done with an apparent wind speed of zero, and no current or waves. The force f which is pushing the vessel through water is then simply given by f = p / v, and this should be the anchor load with which the anchor is dug in when using full reverse throttle when setting the anchor. The relevant power is not the nominal engine power, but the effective horse power after accounting for all losses in the system (within the transmission line from the engine to the propeller as well as the propeller itself). A simple way of dealing with this is to introduce an efficiency factor ?. In my case, this all amounts to – very roughly – 60% * 75 HP / 8 kn = 8042 kp, which is clearly way too much (being of the order of the chain breaking load for many chains), despite of an assumed efficiency of only 60%. So, something is wrong with this calculation, but I yet fail to understand where it has gone wrong. Perhaps the efficiency for reverse throttle is even much less than 60%? Some more food for thoughts can be found here and here.

Any ideas?

My thoughts on anchor loads and chains can be found here: Catenary Anchor Chain Length - Die Kettenkurve - Fun Facts - SAN (in English, no worries )

Cheers, Mathias

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.

 

[Neeves]

There is a general rule of thumb, and PBO or YM had an article on the topic (indirectly) - I think they were testing props, that suggests the tension in the rode is roughly 100kg for every 10hp. I've tested this, measured with a load cell, and its about right.

So a 20hp engine with a typical prop will develop about 200kg of tension.

This is going to vary with the prop and the motor - but its a good place to start.

To lift all the chain off the seabed , 8mm chain, at 5:1 scope in around 5m of depth + 1m to bow roller demands a tension in the rode of about 70/80kgs. Thats the last, or first link just toughing (or not touching) the seabed. For our cat, which has the approximate same windage as a 45' AWB that equivalent to 17 knots of wind (same scope/chain etc.

We have 2 x 20hp engines and at max revs can generate about 400kgs of tension - this is roughly equivalent to 30 knots of wind (flat water).

I've ,measured the tension of the rode at 30 knots and measured the tension developed by the engine using the same rode 8mm - so its 'about right' for our cat. I've measured windage of our cat vs a 45' AWB on the basis that both the head on and beam windage contribute to yawing - and the consequences of yawing.


What you find when making these measurements is:

As you reverse up the catenary straightens, but the yacht has momentum. As the catenarary straightens the rode 'absorbs' energy, but the yacht moving backward has energy. At a given point the energy in the yacht balances the energy in the rode and the yacht stops, now the yacht has no momentum - the energy stored in the chain is larger than the energy of the yacht (as it is not longer moving and only has the energy from the engine) - the catenary pulls the yacht forward. At some point the energy left in the catenary is insufficient to overcome the opposing energy of the engine and the sequence re-starts.

You can develop a sort of yo-yo effect - engine + momentum drives yacht back, catenary stored energy overcomes absence of momentum - yacht moves forward. Energy of engine wins out yacht moves back etc etc.

Basically you can only offer, say, 400kg of tension on the anchor momentarily.

You can 'add' more tension by taking a run at it - giving the yacht more momentum. However this means that you might snatch load the anchor - and to me it seems unnecessary.

Mathias will convert my ramblings into a more technical dissertation - maybe?

We use a 15kg anchor and it will develop a hold, in a decent sand seabed, of about 2,000kg - straight line pull with a large winch. The 400kg of tension we can develop is not really significant in relation to its potential. However most of the time reversing up is more than the tension you are likely to experience at anchor (you choose places to anchor with shelter and 30 knots of wind is not shelter). 9 times out of 10 your reversing or power set is more than you will experience. But it also tells you your anchor is holding (to in our case 400kg) and you can hope the seabed is fairly consistent and that if the wind does exceed 30 knots then your anchor will set more deeply.

A caveat - as mentioned seabeds are not consistent. We know of one with 10cm of nice, clean even sand under which is rock pebbles about 50mm in diameter. You still get hold - but its less than you would expect for a given depth of anchor burial - but if you tension the rode to 400kg - you have 400kg of hold. The rode does not know if its even sand or pebbles. Setting in soupy mud is another issue - rev up the engine and your anchor will simply swim through the mud - leave it to soak and settle - you have a greater chance of hold developing (but you would be better to use a Fortress).

The deeper you can set your anchor - the better. Its the anchor that provides hold, not the chain. Your anchor is designed to develop hold, your chain simply joins anchor to yacht (it could be piano wire (if it were strong enough). It would be better to be piano wire as it would allow the anchor to dive more deeply. If you have oversized chain for your yacht - it will reduce depth of burial of your anchor. If you use a big swivel it will reduce the depth of burial of your anchor.

However none of this is useful if you don't know the windage of your yacht - and there is no standard data as all yachts, even the same when bought, are different. Everyone has real estate mounted on the transom (solar panels, Bimini) everyone has a different headsail arrangement. The data I offer is approximate (as is the hold of your anchor in different seabeds).

Take care, stay safe

Jonathan