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

MathiasW

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Bluewater
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, let me do 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 n. In our case, SV SAN, this all amounts to – very roughly – 60% * 75 HP / 8 kn = 8054 kN = 831 kp as anchor load when digging in. This is quite reasonable when comparing with wind forces in a light gale.

In fact, when I use my Anchor Chain Calculator tool, it turns out that for our vessel this corresponds to a wind strength of 41 kn in an unrealistically dead calm sea, or 35 kn in a choppy sea with a Velocity at anchor of 0.6 kn. When I remove the snubber, this number goes down to only 25 kn. 

 

221J

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CT
Did you try F=ma?  Time SV SAN doing 0 to 3 knots in reverse and assume the drag is zero because you are moving slowly.  Aim for the same rpm that you will use backing on the anchor.

I'm not going to say your way is wrong but there are parts that bother me.  The f needs to account for hull drag at 8 knots.  I think your real efficiency is closer to 25%.  

I back down on anchor to check the set too, but never at full throttle.  What do others do?

 

Rain Man

Super Anarchist
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We back down slowly until the anchor sets with about 3:1 scope out, then gradually increase the power to about 2/3 throttle over about 30 seconds and check that there is no movement.  Then ease out more chain to the desired 4:1 or more for the situation and put the snubber on.  Rum and coke or other frosty beverage next for the happy crew.

 
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Zonker

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Full throttle on our last two boats but one was only 21 HP and the other 27 HP with a very big ass anchor. If either popped the anchor out we would re-set. We would increase throttle gradually (1/4, 1/2, 2/3, full)

Exception 1 - muddy bottons where you want the anchor to slowly work its way down to firmer material

Exception 2 - thin layer of coral sand over hard pan. Just will have shitty holding but sometimes you just know you'll drag if there is a squall.
 

Very good rule of thumb. If you have a fixed 3 blade prop you'll get 20-25 lbs/HP in reverse. So... 75 HP x 25 lbs = 1875 lbs worse case. Lower with folding props, feathering props, 2 bladed etc.

If you have a 75 HP boat your boat is probably in the 45-55' range?  1875 lbs - that's probably closer to 60+ knots windage. So... I'd probably use 2/3 throttle or in that region. Make sure you have a snubber on with those sort of loads. You need to protect the windlass.

 

freewheelin

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WLIS
I don't think of backing down on the anchor as a test really. I think of it as an easy way to dig in the anchor once I have it in the right position. It will continue to work its way in from there over time as it settles and get more and more secure. The backing down is a quick way to get your position right and give it a head start, so the natural movement of the boat is working the anchor in vs out. Too much torque from the engine too quickly can alter the position of the anchor and be more harmful than helpful.

Another thing is you don't want to put too much stress on the windlass. And if you are lazy like me, you don't want to snub until you are hooked. Everyone has their own method, but I tend to put out about 4X slowly. On a mono, let the wind push you back. On a cat though, you will need to work the boat back to keep the bow in the wind. Then I take a pause and let it settle. Then back down about 1500 and see if I swing and have hooked. Then I swim on it in clear water, or clean up the boat. Then I will pay out the rest of the chain, snub, and bring it up to like 2500 and see how we set. Giving the anchor some time is key. They are not designed to be dropped right in to gale force conditions. 

If you are in clear water, and have an extra hand on board, I really recommend watching the anchor with your snorkel gear while your crew goes through the process. It helps to see it to understand it.

 

ropetrick

Super Anarchist
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Back her down until the rope is straight out from the bow. Ease 'er off and have a Margarita.

Capt'n  Ron said that's how they did it on the old Saratoga.

 

MathiasW

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Bluewater
Did you try F=ma?  Time SV SAN doing 0 to 3 knots in reverse and assume the drag is zero because you are moving slowly.  Aim for the same rpm that you will use backing on the anchor.

I'm not going to say your way is wrong but there are parts that bother me.  The f needs to account for hull drag at 8 knots.  I think your real efficiency is closer to 25%.  

I back down on anchor to check the set too, but never at full throttle.  What do others do?
Yes, indeed, efficiency is a big unknown. Generally, it is worse for outboarders. People who have made actual measurements have reported 35% for an out border and 50% for an inline engine. F = ma as such does not work, since there is no acceleration. Ideally, when the anchor is holding that is... ;)  

 

MathiasW

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Bluewater
If you have a 75 HP boat your boat is probably in the 45-55' range?  1875 lbs - that's probably closer to 60+ knots windage. So... I'd probably use 2/3 throttle or in that region. Make sure you have a snubber on with those sort of loads. You need to protect the windlass.
It is a 51/52'' trimaran. So 1875 lbs correspond to much less than 60+ kn, given our windage area... 

 

Zonker

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agreed.  Unless its one of these

image.png

 

MathiasW

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Bluewater
It is a NEEL 51  with quite some central cabin and the windage area calculated by the designers matches roughly what I have come to conclude it is...

 

weightless

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Assume a spherical anchor in a vacuum...

An ingenious person with a spring scale and, perhaps, some tackle to give it some mechanical advantage, some string and a couple of rolling hitches might get some numbers associated with forces on the anchor rode. For whatever they might be worth. As nearly as I can make out anchors that have set well once and drag later drag when something on the bottom lets go or they veer out of their set or there is some kind of shock load. All else being equal I get the anchor(s) to set well using the usual tricks and then contemplate the specifics of the environs and how the situation might evolve. Folks have been making this work without doing this kind of calculation with adequate reliability for a very, very long time. Is there a problem that needs to be solved or is this just for fun?

 

MathiasW

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Bluewater
Assume a spherical anchor in a vacuum...

An ingenious person with a spring scale and, perhaps, some tackle to give it some mechanical advantage, some string and a couple of rolling hitches might get some numbers associated with forces on the anchor rode. For whatever they might be worth. As nearly as I can make out anchors that have set well once and drag later drag when something on the bottom lets go or they veer out of their set or there is some kind of shock load. All else being equal I get the anchor(s) to set well using the usual tricks and then contemplate the specifics of the environs and how the situation might evolve. Folks have been making this work without doing this kind of calculation with adequate reliability for a very, very long time. Is there a problem that needs to be solved or is this just for fun?
Ah, you mean these folks that I saw getting smashed into the rocks the other day, whilst "at anchor"? Or those three months earlier?

Yes, people with lots of experience can rely on their experience to deal with something that they have already seen in the past. That's why it is called experience, as in having witnessed it in the past. But why should every single person have to wait for 30 years to acquire such an experience? Isn't it much better to get an understanding of the underlying physics and then make well informed decisions? Still learning every day, but having a head start? What could possibly be wrong with such an approach so that it should be sneered at as you do?

The history of scientific advances is littered with ignorants that have pointed at the lack of need to understand something better. You really should come to realise that medieval times are times of the past and this mindset is not adequate anymore!

Not claiming to be a Newton here, not at all, but it is not guys like you that made mankind advance as it did.

And btw - the joke goes as follows: A theoretical physicist is somebody who first designs a table with no legs, then with a single leg, and then with infinitely many legs...

 

zenmasterfred

Super Anarchist
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Lopez Island
I'm in the school of not backing down too hard, totally dependent on bottom, weather prediction, sea state etc.  I trust my seat of the pants WTF approach and trying to quantify something like anchoring safely w/ complicated equations takes the mystery out of seamanship.  As me mum used to say, "So far so Good" or in my words you are only as good as your last anchoring adventure.  Been able to stay off the rocks so far with a few miles under the keel.  Praise the Lord and Pass the Ammunition.

 
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Panoramix

Super Anarchist
There is also the old fashion way to do it :

Get in the anchorage downwind under sail, drop the anchor from the stern (end of the lines tied at the bow), when chain starts to tension, push the tiller. The boat will do an 180º turn and the anchor will dig in!

 

Rain Man

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Wet coast.
There is also the old fashion way to do it :

Get in the anchorage downwind under sail, drop the anchor from the stern (end of the lines tied at the bow), when chain starts to tension, push the tiller. The boat will do an 180º turn and the anchor will dig in!
We were at anchor at the north end of Jedediah Island and saw the most amazing anchoring job ever.  Guy came in to the narrow anchorage with skidlets jumping all over the deck like monkeys, mom at the helm.  Came in drifting out of gear, threw the anchor over the bow at just the right moment, and let out exactly the right amount of chain.  The anchor grabbed and spun the boat around so the transom was about 20' from the rocks as he walked back to the stern.  Then he casually jumped in the dinghy and rowed the shoreline ashore and tied it off.  Probably less than two minutes from start to finish.  

I chatted with him later - he'd built the boat from scratch many years ago and lived aboard with the family.  Very nice folks, and very accomplished at what some people find very stressful.

 

weightless

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What could possibly be wrong with such an approach so that it should be sneered at as you do?
Did I sneer? That wasn't exactly what I was going for. Oh well.

Anyway, you asked how much force there is on your anchor rode when you back down and proposed starting with thermodynamics or something. I suggested that it's not hard to measure the force directly. People have done this. People I know have done this. It is interesting. However, I'm not sure that it's easy to relate those numbers to how likely an anchor is to drag. I was, and am, curious if you are attempting to solve a specific problem or if you are just doing basic research. 

 
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El Borracho

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Knowing the backing-down force rather begs the follow-on questions of what will the weather offer and what is the wind and wave force in those conditions. 

 
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