Time to weigh anchor, wash away any ball of mud caught on Rocna
15, and lay again in hope of better set.
I'll structure this interim thread summary:
A. Restate problem and review thread
B. Importance of problem, including identifying who already knows/could solve problem
C. Review concisely history
of answers to problem since 1981
A. Restate problem and review thread
* we cruisers lack scientific guidelines about combination of engine thrust/bollard pull astern and duration necessary to power set a drag embedment anchor in a known seabed to cope with a known windspeed load:
- most cruisers use drag embedment anchor, anchor that is (first) dragged along soil, so anchor (second) embeds or buries in soil. Burial in soil is what generates holding capacity.
- drag embedment anchors have to be set, by bollard pull astern, to bury them. Process called 'power set'.
- after embedment, anchor will stay buried and holding even with big veer of rode and considerable load.
- without embedment, anchor behaviour is unpredictable - after being loaded or being loaded in new direction of pull, it might embed and hold, it might not.
* in contrast, we know much about scope (e.g. 5:1 for adequate anchoring
hold; 10:1 for max anchoring hold; diminishing return above 10:1) and efficient anchor design (minor controversy: some are roll unstable (e.g. CQR) and still work
; Danforth, Fortress
, and the roll stable Bügelanker, Delta
, Spade, SARCA models, Rocna, Manson Supreme, Knox, & Mantus
all work to greater or lesser degree).
* in contrast to sailboat anchors, the combination of bollard pull astern and time of pull necessary to set and verify big anchors (e.g. Vryhof, US Navy
warships) is well known (by them, not us).
* science of power set should be easiest in sand. More difficult in mud/silt/cohesive soils, which need to be handled sensitively. Some other soils (gravel, thick kelp/weed, mixed strata, calcareous ooze, hard rock, unconsolidated coral
sand) much more difficult to the point that reliable power set is deep art, not science:
- Note well: Vryhof routinely sets in sensitive deep ocean silts, using prescribed bollard pulls and long pull times (ranging from 1 to 6 hours) to set and verify holding.
* problem is most acute for auxiliary powered sailing boats, i.e. generally with engine power designed from guidelines such as only 1 bhp for 1 foot of LOA
- auxiliary sailboats lack bollard pull astern equal to load from a 40 knot wind.
- which raises question about whether long duration pull astern at low power
can set and verify anchor hold (compare Vryhof specifying power set for 6 hours in difficult substrates compared to more standard set times of 30 minutes to one hour in more usual deep ocean silts)
- many pleasure powerboats have opposite problem of too much power. When power setting anchor, they have to apply astern power delicately. But still lack guideline of how much astern power is too much.
* we cruisers do have art of anchoring guidelines that deal with unknown/most any substrate:
- in contrast to a science of anchoring, involving hard data (e.g. in silica sand, use x rpm
astern, which delivers y kg.force of bollard pull, for z minutes, to verify holding power adequate to withstand load from wind of m knots from an arc
of n degrees either side of initial rode axis).
- art of anchoring guidelines for power set take form of algorithm of sitting back on rode by prevailing wind pressure (because most props produce side thrust/prop walk of ~14% that would otherwise lay rode in an arc
, not a line), then increase astern throttle in 200 rpm
increments while feeling rode for evidence of drag and checking transits for evidence of drag. Do slowly to accommodate unknown or mud bottoms (some advise letting anchor soak in mud and only very lightest power set), with suggestion of weigh anchor and re-set until no drag with taut (undefined!) rode.
Thread to date attracted 100% welcome contributions and zero unhelpful ones:
- #16 FSMike recognized unscientific nature of art of anchoring algorithm. Recognizing known unkown exists is progress! Thanks, Mike.
- #17 Stu Jackson anecdotally confirmed the importance of power setting. Including with a CQR - a roll-unstable anchor - but one that works in some substrates when power set.
- #9 Nigel1 commented on Vryhof power setting, of which he has personal experience. Thanks. A very few cases of Vryhof anchors handling considerable rode veer are known (and at least one has been published in scientific/engineering literature. Most Vyrhof sets are in multiple anchor arrangements just so rode veer is not an issue. From memory, Vyrhof anchor subject to rode veer dragged just 1 metre before self-resetting to acceptable hold. Vryhof thought such anchor behaviour (dragging 1 metre = 3 feet) was unpardonable, which is partly why case was published!!). Last I spoke to a person who had seen the Vrhof anchor database (about 2 years ago), I learned that Vryhof has good data (nature of bottom, exact specs of anchor and rode, power set procedure, verified holding capacity) for 9,000 anchor sets and excellent data (i.e. even more data points!) on about 1600 of them. I'd be v happy if we knew so much about anchors we cruisers use.
- the contrast between #11 Cheechako, who power sets to 1400 rpm, and #12 cwyckham, who power sets to at least 2800 rpm if not more, is exactly what this inquiry is about! Thanks to both. #14 Cheechako added additional scientific datapoint: Cheechako's boat spins a MaxProp (much more efficient astern than the 3-blade fixed prop on Led Myne - I've compared the bollard pull astern of Led Myne with that of a sister ship) with which 1400 - 1600 rpm (for exactly how long?) will embed a Delta
in sand as confirmed by dive inspection
. Thanks again, Cheechako. Your MaxProp explains the difference from cwyckham. That's further point in database. Even better if we could add duration of bollard pull data.
- #3 cwyckham noted that few cruisers do 60 secs of power setting. I agree. I've seen 'internet anchoring gurus' say 10 - 20 seconds of bollard pull astern is adequate! I suspect, with limited bollard pull available on Led Myne (stated at #1) she needs 5 - 10 minutes at 3000 rpm for full power set in sand to equal 40 knot wind load and I've not worked out what's needed in mud (but my anchoring experience suggests soaking anchor in mud overnight does the job). Alternate power set in sand, as noelex wrote in another forum, would be a jerk (i.e. back on a slack rode). Or, as #5 Hydra (Alain) wrote, temporarily make the rode to the stern so the far superior bollard pull ahead can be used. Anyway, that's what this inquiry is about. Thanks.
- At #8 cwyckham noted the importance of powering down from a power set slowly, to avoid slingshotting forward. That's important and brings up a point no one else raised: Led Myne's Rocna 15 grips (in sand) hard and fast, so I power set with a nylon line as a snubber to absorb transient strain and take load away from my anchor windlass
. That's a big difference from power setting Led Myne's previous best bower, a CQR, which had to be embeded incrementally, as per 'art of anchoring' algorithm.
- #4 noelex (a known anchoring guru and not one in quote marks) noted full power set from his boat's engine (but without considering duration of pull) does not embed anchor to equal burial from a 40 knot wind load. Thanks noelex, that's precisely what I'm on about. What we now need is to find whether x or y minutes of bollard pull compensates for low power
- #10 Niagara Les is right - you can tell if an anchor is fully embedded by diving
on it. That's not always possible (cold, turbidity, crocodiles, night time, box jellyfish, when singlehanded and anchoring in winds I know would blow a dragging boat faster than I can swim etc). Reliable scientific guidelines would be answer.
- #2 robert sailor restated art of anchoring algorithm but with throttle recommendations: once the rode is taut, increment to 1500 rpm, then 2000 rpm, and finally to 2600 rpm (and, similar to cwyckham, noted to 'ease power off' which I take to be do so slowly). #13 DoubleWhisky (Tomasz) stated his more interesting algorithm with hard guidelines: including 30 seconds of 70% bollard pull astern followed by 60 seconds of full bollard pull astern for each ten metre of rode if shortening scope. Very interesting DoubleWhisky! Thanks. A valuable contribution. What I'd really like is that converted into science: data recording how much anchor buried in what substrate after 30 seconds of 70% bollard pull astern, followed by burial depth
after 60 seconds of 100% bollard pull astern.
B. Importance of problem and who likely knows or could discover answer
(a) for cruiser:
what value would you place on confidence knowing that backing on your anchor for x minutes at y rpm embeds anchor in sand so it will remain buried even if boat and rode veer 45 degrees eitehr side of rode lay and to withstand wind effects from gusts to z knots? or if you know that backing on anchor for x minutes at y rpm, after soaking anchor in mud for k minutes, verifies the holding capacity and may increase holding capacity?
(b) for anchor designer
what value do you place on having the unique selling point of certifying that your anchor design only needs power setting to x minutes at y rpm for sailboat with k horsepower and that motorboats with 2k horsepower should only power set at z rpm for m seconds?
(c) for "Bigger is Better" advocate (or opponent):
Dispel (or confirm) myth (truth) that anchors can be too big for particular engine to power set to appropriate burial depth
and holding power.
(d) for marine hull insurance
Win bonus for rejecting claims unless skipper
can prove with logbook entries they power set dragged anchor with appropriate shaft horsepower for appropriate time period to suit substrate.
(e) for university researcher:
engineering literature has a gap. Fill it with good research
, write thesis, collect degree, and gain publication credit as sole author.
If resulting algorithm (see iiie, below) can be simplified, embrace immortality by naming it after yourself.
Position yourself for career as anchor engineer/designer/vendor.
Consider writing definitive book on anchoring, informed by your literature review and research
. Earn pocket money
by contributing definitive articles (with photos and ready reckoner of anchor size vs engine bhp vs crankshaft rpm) on how to power set with underpowered sailboat and overpowered motorboat in each of Cruising World, Cruising Helmsman, Yachting World, Practical Boat Owner, Ocean Navigator, Trawler
World etc. Repeat each year for so long paper magazines continue to be published.
(f) for literate cruiser-researcher:
Gap exists in field of knowledge. Earn currency by publishing, and re-publishing every year in the future, definitive article (with photos and table of anchor size vs engine bhp vs crankshaft rpm) on how to power set any anchor in sand, mud, and relevant other substrates. See (e) above. Contemplate yet another definitive book on anchoring.
ii. Who knows or could discover solution?
(a) Professor John H. Knox would know. I expect Prof JHK to celebrate 87th birthday in November 2014. Given age, further publications not likely. A sympathetic interviewer with access might deliver goods.
* invented Anchorwatch, strain gauge load cell for measuring rode load (see anchorwatch.com);
* developed several algorithms and guidelines for anchoring (see back issues of PBO, Yachting Monthly);
* developed guidelines for testing anchors; and
* invented one of best anchors on market (see knoxanchor.com, note the sharp penetration points) with great design and excellent metallurgy (I'd have one, but shipping
costs to Moreton Bay are swingeing).
(b) Vryhof and most anchor designers
Most anchor designers work with test anchors of 10 or 15 kg and then scale up.
Vryhof has a database from 9,000 thousand commercial
anchor sets done by anchor handling tenders (AHV or AHT) using its big drag embedment anchors with design lives of ~20 years in deep open ocean in all possible soils. On demand, Vryhof can model most any anchoring problem and, if necessary, design a special anchor for job - and specify exactly what bollard pull to set and verify and for exactly how long. AHT contractors then lay, power set, and verify the anchor hold (I know of a case with power set and verification using 240 tonnes.force bollard pull for 6 hours! Anchor didn't move. Vryhof engineer
smiled.) to its precise specs. Vryhof and AHT contractors hold their database as commercial-in-confidence. Of course!
I guess Brian Sheehan of Fortress
Anchors, Rex Francis aka congo of Anchor Right Australia
, Dr Kutsen of Mantus
Anchors must know and have real data.
At <http://www.cruisersforum.com/forums/f2/lightweight-anchor-set-from-brian-sheehan-fortress-anchors-1383.html>, Brian Sheehan discussed Power Setting:
'7. You should then very slowly start to increase the engine rpm while in reverse. This slow, steady pull on the anchor will cause it to dig deeper and deeper, and as this happens the holding power is going up and up.
'You should now be able to sit there with the engine in reverse with a fair amount of power and the boat is not moving back. This is because the anchor is holding more than the pull from the engine.
'This technique is called "power setting" the anchor.This is standard procedure for the US Navy
. It applies equally to all anchor types, and there are no exceptions.'
Brian did not discuss embedding in different soils, nor address problem of under-powered auxiliary sailboats, nor define how to prolong power setting to compensate for underpower.
With Rex Francis's anchor testing rig, Rex would be able to do science easily (and has probably done it, but perhaps without recording the data points). In Rex's "Essential knowledge when anchoring" brochure, Rex wrote 'In crowded anchorages
"Power Set" your anchor at 5:1 scope then shorten scope as required ...' If only Rex had defined how to "Power Set" in terms of how much propeller
power for how many minutes for what substrate!
(c) US Navy
I've read all USN's anchor research I found published since 1984. USN definitely has algorithms for successful anchoring. I doubt if USN bothered to consider question of underpowered auxiliary-engine sailboat trying to anchor - when Navy Academy sailed Navy 44s, I think anchoring was regarded as non-routine procedure requiring special approval.
(d) university researchers
I've read PhD theses and published research papers. Most all have concentrated on scope and anchor design. A search will find you a marine engineering thesis that reads 'I set the anchor by hand'. In other words, problems of power set ignored. Some used centrifugal devices to model anchor set, again ignoring questions about setting, to research dynamic behaviour of anchor under load.
(e) expert cruisers/gurus
Evans Starzinger has done superb science on load testing, see: <http://www.cruisersforum.com/forums/f116/load-testing-results-118009.html> that changed way I look at safety
harness and knots in cordage. Working with 55 kg Rocna (and similar size anchors) Evans determined if set with 1000#.force (455 kg.f) embedded anchor veered by big angles remained buried and holding. Now if Evans tried a range of loads to set, different bollard pull times, and different size anchors, half the job would be done.
Noelex (in 2013, another forum) noted slowly increasing load was needed in difficult substrates but not in (quartz) sand, where a jerk would likely set a new gen anchor. No figures for engine revs, no discussion of time to power set with an auxiliary. I guess noelex knows intuitively, likely has not quantified answer.
Jonathon Neeves (in 2013, that same other forum) noted importance of soak time for some (?old gen anchors) and hard power set was inappropriate in mud/fine silt. No figures for engine revs or for time compensation, but Jonathon has done hard figures before.
(f) harder, but not impossible, is group of cruisers reporting their anchoring data (bollard pull and time needed to embed anchor size in known substrate) to establish database (see some of partial data cited in summary of thread to date in (A) especially contributions from Cheechako, cwyckham, and DoubleWhisky. Noelex's datapoint (full power set does not equal 35+ knot wind load) is also useful datapoint - data on unsuccessful embed shines light on what extra might be needed, e.g. longer time of bollard pull astern.
(iii) What's necessary to do science on power setting?
(a) easiest substrate is sand, silica or quartz sand (not coral
grit sand or shell fragment sand);
(b) use strain gauge load cell or some other means to calibrate bollard pull astern at range of engine rpm;
(c) determine minimum bollard pull and time of pull to embed new gen anchor to necessary depth;
(d) determine whether under-powered auxiliary craft, with power output in range I've quoted for 15 kg anchor, can compensate for its underpower by prolonging pull astern;
(e) verify (c) and (d) through repetition;
(f) repeat (b) through (e) with range of engines, different props, and anchor sizes, then do regression analysis to result in algorithm any cruiser, knowing engine power, prop type, and anchor size, can use;
(g) repeat (b) through (f) for range of substrates of sort marked on nautical charts
(h) repeat (b) through (g) for select overpowered motorboats as you wish.
C. History of answers to problem since about 1981
* Earl Hinz The Complete Book of Anchoring and Mooring
1981 p. 208: back down at idle speed, increase throttle in 200 rpm increments until half throttle. Bollard pull of 300 - 400# (136 - 181 kg.f).
- look back at #17 Stu Jackson's story suggesting that rode load from 30 knots would be (depending on boat size) about 900# (408 kg.f). So Hinz's suggestion is manifestly inadequate if goal of power setting (as in Stu's story) is to apply load equal to that expected from wind to embed anchor and verify holding capacity of soil. Note at #1 my estimation that 40 knot wind load on 28 ft Led Myne would be 514# (233 kg.f).
- as noted above, several participants in this thread have updated Hinz 1981 algorithm with superior versions. I reckon duration of bollard pull needs to be addressed.
* Peter Neilsen, Anchoring
(Captain's Quick Guides) 2007
p. 8: to make doubly sure the anchor is well dug in, back down with the engine at half throttle for 30 seconds. The boat should move forward on the rode when you ease the throttle.
- Nielsen gives (1) throttle setting and (2) duration of pull. I think 30 secs at half throttle is inadequate. I think 30 seconds at _full_ throttle is start of ballpark! For Led Myne, I've lately been using 5 - 10 mins at 3000 rpm. And I think I could extend that to 5 minutes at 3600 rpm and still be inside ballpark.
* Alain Poiraud & A & G Ginsberg-Klemmet, The Complete Anchoring Handbook
p. 115: The best way to make sure your anchor will embed is by pulling on it hard. Often skippers put the boat in reverse for just a few seconds. But to be sure the anchor is set you must put a reasonable strain on the rode for a reasonable length of time. If anchoring a sailboat, don’t hesitate to run your engine at full speed in reverse to assure proper setting. Even with the highest possible rpm, most modern sailboat engines offer about the equivalent load of 25 to 30 knots of wind.
- Poiraud, peace be upon him, failed to define what he meant by 'a reasonable length of time'.
* Rocna User's Guide
pp. 2 - 3: Your Rocna is designed to set as quickly and reliably as possible. Typically it will bury itself within one meter of where it lands. This performance is so dramatic that care must be taken during your normal anchoring procedure, since it will likely take up more abruptly that you are used to. If you reverse your boat too speedily and are not using a chain stop, you risk damaging equipment
. ... As mentioned in the “About Your Rocna” section, your new anchor may set much more quickly than you are used to, particularly if you are most familiar with plows. Take care when reversing your boat under power and do not build up too much speed, as the anchor will grab quickly and the resulting shock could damage equipment
or injure personnel.
* Professor John H. Knox, 'Anchoring with new generation anchors', Knoxanchor.com (undated but likely 2012)
Once the required amount of chain is deployed it should be tensioned gently at first to allow the anchor to engage the seabed, then more vigorously using the engine to ensure that it is well dug in, a sustained pull at full engine revs will test this convincingly. A very important part of any anchoring cable, especially when using NGAs, is an adequate spring to absorb shock loads. Typically this should be of nylon and roughly the length of the yacht. It can be conveniently secured with a strong stern cleat and led over the bow roller where it can be shackled onto the main chain. If an efficient spring has been deployed, then driving the yacht in reverse and bringing it to a jerk stop will ensure that the anchor will resist even storm conditions. A good modern anchor should start embedment immediately and develop its maximum hold or UHC after it has ploughed a 5 to 10 metres.