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Old 31-03-2014, 22:17   #1
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How best to set an anchor?

Let's say I'm anchoring a sailboat (LOA = 28 foot) using a 15 kg Rocna on an all-chain rode in a bottom noted on the chart notes as a sand bottom.

A. My questions are:

(1) for how many minutes should I run the engine in astern so as to be sure that the Rocna embeds in the bottom and gives me reasonable confidence of staying put in, let's say, a 40 knot squall?

(2) at what engine rpm should I run the engine (further details below at C) when setting the anchor?

(3) if the chart noted the bottom to be mud, would your answers to #1 and #2 differ?

B. Theory and Guidelines (if CF or any other group of expert practitioners could so agree!)

The Vryhof Anchor Manual (2010 edition), hereafter VAM2010, presents what I take to be robust guidelines.

VAM2010 says:

i. "Drag embedment anchors are generally installed by applying a load equal to the maximum intact load" (p. 41). The maximum intact for a temporary anchorage is defined (on p. 40) as 1.0 times the quasi-static load and/or 0.8 times the total load. Quasi-static load is defined (on p. 39) as "the load due to the swell, wind, current and the frequency of the system" and total load is defined (also on p. 39) as the quasi-static load plus "the individual wave forces causing a high frequency motion". And adds "Generally the quasi-static loads will be equal to 50% to 90% of the total dynamic load". VAM2010 backs this up with a graph showing an anchor load curve with quasi-static load peaking at 2342 kiloNewton and total dynamic load peaking at 3895 kN.

ii. For a permanent anchorage (not a temporary anchorage, which is not further discussed), "the installation load should be held for the period specified by the classificiation societies" (p. 41). The specified periods are listed in a table (also on p. 41): Lloyd's specifies maintaining installation tension for 20 minutes; the American Bureau of Shipping specifies 30 minutes; and Det Norske Veritas specifies 15 minutes.

iii. In mud/clayey soils, "the penetrating anchor disturbs the soil and the soil temporarily loses strength. With time, the disturbed clay reconsolidates to its initial shear strength, this takes from a few hours up to 1 month. ... On reloading, the parallel soil resistance gains strength, it takes a larger load to move the anchor again. ... Observation on anchors for drilling rigs and theoretical considerations for a 3 to 4 week consolidation time demonstrate a typical set-up effect factor = 1.5" (p. 41)

Professor John H. Knox reckoned, from his practical experience measuring rode load with a strain gauge, a formula for estimating rode lode for a sailing vessel of a given LOA and a given wind speed: Load (rode) = 0.002 * LOA^2 * windspeed^2 with units of metres for LOA, knots for windspeed, and kilograms.force for load.

For a LOA 28 ft boat (8.53 m) and 40 knot wind gust, Prof. Knox would estimate a rode load of 233 kg.f (about 513.7 pound.force, about 2.29 kN). Probably about right.

C. Practical considerations

Led Myne, my 28 footer, has a Yanmar 3GM30F spinning a fixed 3-blade prop. My best estimates for bollard pull astern are: Maximum (3600 rpm) 173 kg.f (381 lb.f, 1.7 kN); and about half of that at 3000 rpm (which I regard as full throttle, while considering 3600 rpm as emergency throttle). Led Myne carries a 5/16" BBB chain rode, with a Working Load Limit around 861 kg.f (1900 lb.f). So my ability to install an anchor is no more than 74% of the likely maximum rode load/maximum intact load - I cannot reach 80% of total dynamic load, nor 100% of quasi-static load.

Can I compensate for that inadequacy by longer time of applying installation load? Anyone know of any real evidence and research?

Over to you as expert practitioners, especially the vendors and anchor makers among you. I know that one or more of you have designed and used devices to test anchors much more like my 15 kg Rocna than a Vryhof. I know that more than one of you have done considerable testing of anchors and have published your results.

Given the limited bollard pull of a sailboat auxiliary:

(1) for how many minutes should I apply astern thrust to set the anchor so I am confident of riding out a 40 knot squall?
(2) what engine rpm should I use (given that Yanmar admits that the cooling system of a 3GM30F cannot sustain more than 60 minutes at 3600 rpm)? and
(3) were I anchoring in mud/clay/silt, how would your answers to #1 and #2 vary?

Thanks

Al (my first post! Thanks for being gentle)
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Old 31-03-2014, 22:46   #2
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Re: How best to set an anchor?

Don't over think this stuff Allen. To begin with, you have a substantial anchor for your size of boat so good move on your part. Find where you want to anchor stop the boat and ease out the rode as you slowly back down. Allow for the height of the stem head above the water as well as your depth and pay out enough rode to give you about a 5 to 1 scope. Continue to back down very slowly until the rode is tight. Give the helmsman a signal to take the rpm up to around 1500 and pick some land marks on shore that you can eye ball your position to let you know that you are holding. If you are and I expect with your anchor in sand you will be signal the helmsman to take the rpm up to 2000 and once again check to ensure you are holding well. Finally take the rpm up to 2500 and when your holding is confirmed ease the power off and attach a snubber line to the chain to take the direct load off the windless. If you are expecting a high wind you can remove the snubber and let out more chain(in the neighborhood of 7-8 to 1 will hold you quite easily in 40 knots, actually the original 5 to 1 would hold you as well but a little extra rode is good insurance. Many boats tend to sail at anchor and can get some high snatch loads when they wander from side to side. A simple small riding sail attached to the back stay will allow your boat to lie quietly and reduce the loads on the anchor. Do some reading about where to drop an anchor around other boats so you don't look like the typical charter pilot that we see all to often.
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Old 31-03-2014, 23:28   #3
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All good advice. A minute of high throttle with you not moving backwards will be more than most do.
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Old 01-04-2014, 00:52   #4
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Re: How best to set an anchor?

Quote:
Originally Posted by Alan Mighty View Post

(1) for how many minutes should I run the engine in astern so as to be sure that the Rocna embeds in the bottom and gives me reasonable confidence of staying put in, let's say, a 40 knot squall?
Welcome to CF Alan. That's brave starting your first post in the anchoring forum

You have been given some good advice. I usually slowly build up to full reverse and hold that for 10-20 seconds.
The biggest beginner mistake is not to recognise you are moving backwards and the anchor is actually dragging when the reverse is applied. Learn to use transits, these are the best method and will detect even a couple of metres movement (as secondary check you can observe, or feel, the anchor rode and/or use the GPS)

However, even after a full reverse set (assuming the set was in light wind) I notice when diving and observing the anchor it will set deeper when the wind exceeds 30-35 knots. This suggests to me that the reverse set is only applying the equivalent force of 30-35 knots of wind.

An anchor that is well set will normally just dig in a bit deeper as more force is applied, so this is not a great concern, but even with full reverse you are not testing the equivalent force of 40 knots of wind.
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Old 01-04-2014, 14:04   #5
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Re: How best to set an anchor?

To increase the tension in the rode, would it be possible to lead it to the stern, rig a bridle and run the engine in ahead, in true "bollard pull" configuration? Since the propeller flow wouldn't strike the hull, the force on the rode should be higher.

This would have the added merit of having the shaft pushing on the gearbox, instead of pulling: sometimes, the shaft escapes from the coupling (don't ask how I know).

Alain
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Old 01-04-2014, 14:41   #6
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Re: How best to set an anchor?

As you're slowly backing down and paying out rode, remember to slightly restrict the speed the anchor chain is going out. Just for a moment or two. This will straighten your hook and chain so that it is positioned to dig in. Not difficult with a Rocna. The idea is to slowly apply setting pressure so she'll dig in properly instead of skipping across the bottom.

Once our anchor is set I'll back about half to dig her in. If I'm in a confined area with weather or current concerns then I'll use more throttle. Plus I'll let the boat drift forward on the weight of the rode then back her down with some momentum into the chain. Not too much. But enough to put the hook in its grave.
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Old 01-04-2014, 14:43   #7
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Re: How best to set an anchor?

Rocna may have some Rocna-specific guidelines, too. I know Fortress and Supermax do...

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Old 01-04-2014, 14:48   #8
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Re: How best to set an anchor?

Small lessons we learned the hard way were to look what's behind us before powering up, power up slowly, hang on it for a while with no movement, then power down slowly (prevents slingshotting across the anchorage from the chain dropping to the bottom).
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Old 01-04-2014, 15:16   #9
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Re: How best to set an anchor?

Having deployed more Vryhof anchors than I care to remember, best advise, before attempting to dig the anchor in, let it soak for a while.
So, lay out the anchor and chain, then go for a brew, then use the engine.

Bit of background info, a typical Vryhof anchor weighs 12 to 15 tonnes. For insurance purposes, a pull of about 150 tonnes is put on the anchor for an hour. Remember, the Vryhof manual concerns anchors which are designed to take a pull in one direction only
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Old 01-04-2014, 15:22   #10
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Re: How best to set an anchor?

Hi Al,

For what it's worth, it may not be always desirable, or possible, but my best night sleep comes only after diving down to have a peek!

A little off topic, but here in the Great Lakes, diving down early in the season gives you with the opportunity to see what a thermocline looks like (and shortly after, feels like!).

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Old 01-04-2014, 16:01   #11
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Re: How best to set an anchor?

Deploy the anchor . Let the boat settle down at netral/idle for a couple minutes. Start pulling in reverse slowly , then increase rpm to maybe 1400. Line things up on shore and if you are not moving.. you're done. One minute is enough if you are not moving. If you are moving, it's likely you are not going to solve it by anything other than pulling up and reanchoring.
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Old 01-04-2014, 16:20   #12
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Re: How best to set an anchor?

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Originally Posted by Cheechako View Post
Deploy the anchor . Let the boat settle down at netral/idle for a couple minutes. Start pulling in reverse slowly , then increase rpm to maybe 1400. Line things up on shore and if you are not moving.. you're done. One minute is enough if you are not moving. If you are moving, it's likely you are not going to solve it by anything other than pulling up and reanchoring.
Why only 1400? I bring it up to twice that. I basically try to pull back as hard as I can.
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Old 01-04-2014, 16:31   #13
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Re: How best to set an anchor?

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Originally Posted by ranger42c View Post
Rocna may have some Rocna-specific guidelines, too. I know Fortress and Supermax do...

-Chris
You are right, and they were previously available from "Rocna knowledge base" on their site. Now, after changing of ownership it is unhappily missing.

After buying my Rocna iIexperienced some problems with setting it properly, so I looked at the instructions published. When following them all problems were gone.

The trick with Rocna is setting it very gently at the beginning to allow her to dig herself properly.

I'm applying following procedure (with accordance to Rocna's advice):

  1. I'm putting out the amount of chain roughly even to the depth + height of the bow roller over the water, just enough for the anchor to touch the bottom. I'm doing my best to keep the boat in place (necessary revs on engine only and I have a bow thruster - it helps).
  2. I'm backing slowly, trying to do it at the same speed as the anchor winch is putting the chain out. The aim is to put out the chain in straight line, but to not tighten it.
  3. I'm putting out the maximum possible scope for the given place - often much more than a final amount of scope.
  4. After putting the chain out I allow the chain to tighten, but not too much. Depending on weather it can be necessary to use some rvs backward (if ther is no wind), just to wait (if the wind is light) or even to apply some revs forward (if the wind is strong). If I'm not in a hurry for some reason, I like to give my Rocna not less than five minutes to start digging in.
  5. After several minutes I'm tightening the chain more and more, adjusting the revs accordingly, and still taking my time - not less than five minutes to the same revs I'm using as cruising revs, while motoring forward (for me it is 60 % of maximum revs)
  6. The above described procedure allows normally the anchor to be in working position, initially dug in, in position not further than several metres from the initial "touch down".
  7. With the anchor initially set, I'm starting the "power setting" procedure - several periods of high revs backward, starting from about thirty second burst at about 70 % of maximum revs and finishing with about three minutes on full revs backward.
  8. Often the scope of chain is quite a bit more than intended final scope, so I'm taking up the chain, stopping it, and giving the minute of full revs backward for each ten metres of taken up chain.
  9. After taking up enough of chain to achieve the intended scope I'm giving the last minute of full revs backwards, putting the engine to the idle for several minutes to cool down the turbo and after putting on the snubber I'm ready.
  10. If setting the anchor alarm I'm using the waypoint taken at the "touch down" position, with the circle of 120 - 150 % of the final scope.
This procedure is applicable to setting the Rocna when anchoring out. It is a little different when med - mooring, but - as I understand it - OP's question was related to anchoring out.

And - welcome to the Cruisers Forum, Alan

Cheers!

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Old 01-04-2014, 16:34   #14
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Re: How best to set an anchor?

You can certainly do more. I've been very sucessful with about that, maybe some more. The only time I seem to drag is in a wind direction change with lots of wind. I tend to have Max Props too though and the pull in reverse is very good with those, If one is using a std two blade etc maybe more will help, or maybe you're just cavititating. Many anchors are "plows"... meaning yes they will move some with enough pull, (sand) just like plowing a field.. The Bruce move like a freight train. I jsut dont see the point in pulling too much. You just may pull it out and the bottom isnt going to change unless you head for a new anchorage! I've snorkeled my acnchor a lot in the sand, 1400/1600 will bury a Delta completely.
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Old 01-04-2014, 17:10   #15
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Re: How best to set an anchor?

Answers:

1) I let the anchor drop and do not pull back on it till the wind has pulled the boat back. Then I gently pull back for a few seconds. Then neutral till the boat goes forward, then back again. Kinda tapping it in. Then when I think its set I come back on it for one solid minute at....

2) 2,000 RPM thats a fair damn bit of power!

3) Mud: I might tap it in a few more times if its not setting. Or in very soft mud I drop it, go below for half an hour to let it settle then come up and tap it in


Now I'm gunna read everyone elses answer
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