Cruisers Forum
 


Reply
  This discussion is proudly sponsored by:
Please support our sponsors and let them know you heard about their products on Cruisers Forums. Advertise Here
 
Thread Tools Search this Thread Rate Thread Display Modes
Old 01-04-2014, 17:35   #16
Registered User
 
FSMike's Avatar

Join Date: Aug 2009
Location: Bahamas/Florida
Boat: Solaris Sunstar 36' catamaran
Posts: 2,686
Images: 5
Re: How best to set an anchor?

Alan Mighty -
That was a pretty impressive post. I had half a headache when I quit reading it half way through. I had no idea you Aussies were so scientific.

Another anchor thread. Oh boy.
The following response is brought to you courtesy of a fairly good merlot.
I thought you might enjoy a somewhat less scientific approach to the problem.

I spend most of my time in the Bahamas and the southeast USA. I typically anchor in a sand or muddy bottom. I am usually single handing.

First thing I do in a harbor is to pull the necessary amount of rode out on deck. I then lower the anchor off the roller by 1 or 2 feet and cleat it off. I then move the boat to where I intend to anchor, bring her up into the wind, and when she is almost stopped I walk to the anchor and let it down to the bottom.
Assuming the wind is blowing I let her fall back 5-15 feet and snub the rode so as to pull the anchor straight and make sure no rode is on top of the anchor.
I then let the boat fall back 30-40 feet and repeat the process. Some times the anchor grabs at this point, but even if it doesn't the feel gives me a pretty good idea of what kind of bottom I'm dealing with, mud/sand/shell/rock etc. Of course in the Bahamas I can usually see it.

I repeat the process until I have out the necessary amount of scope for the conditions. If I'm anchoring in 10' of water (adjust as necessary for tidal fluctuations) plus 5' to my anchor roller a 7:1 scope equates to 105' of rode. When I get near that amount of rode past the roller I cleat off the rode and hold the rode up to waist height in between the roller and the cleat. The wind will normally push the boat hard enough so that when the anchor is hooked up the rode will be almost ripped out of my hand.

It is extremely rare for this method to not result in a well anchored boat.
If it doesn't work I move to a different location.

If there is little or no wind I use the engine, but rather gently. I learned long before the "new generation" anchors that it was often easier to get a CQR to set if you did it gently rather than applying max revs, which often caused the plow to plow. Letting anchors settle in gently usually results in a set which will hold better in that late night squall.

I have no idea how this works out scientifically, I only know that it works for me.

The primary ground tackle on my current boat consists of a 33 lb. Rocna, 90' of 5/16 chain, and adequate rope.

I guess I should say that your results may vary below the equator lol.
__________________
Sail Fast Live Slow
FSMike is offline   Reply With Quote
Old 01-04-2014, 17:55   #17
Registered User
 
Stu Jackson's Avatar

Join Date: Jan 2011
Location: Cowichan Bay, BC (Maple Bay Marina)
Posts: 8,658
Re: How best to set an anchor?

I agree with Mike. And if the wind isn't blowing, just drop the hook when you have the boat moving backwards slowly. The anchor will let you know when it gets a first set,then let out your intended rode, and use the engine or sails to back the anchor in finally.

Here's something a friend wrote:

Last summer on a friends boat he left me at the helm while he went to drop his CQR. I backed down, like I always do, gradually increasing to 80% throttle and the anchor dragged!

Here's how the conversation went "Geez that's never happened before","Really? Lets try it again",.

On the second attempt it had an initial bite (starting to burry) but when I applied power it broke free. "Your giving it to much throttle and ripping it out of the bottom", "it's an anchor!", "let me try", "ok".

So I now go up to let the anchor down & he puts the boat in reverse gets it moving and then puts it in neutral and we get an initial bite. "There see it's set", "No it's just starting to dig in it now needs to be set", "It's always held me before", "Have you ever experienced a 30 knot blow on the hook?", "No" "Well a 30 knot blow on your boat is the equivalent of roughly 900 pounds of pull on the anchor did you know that?", "No", "Did you know that the motor on this boat can barely re-produce 350 pounds of pull wide open?" "No", "Well let's let it set your way and in a couple of hours we'll simulate 20 knots of wind with the motor and see and happens", "You're on". You can probably guess what happened. Because we never properly set the anchor it dragged! We did get it to set that day using a 10:1 scope then shortening to 5:1. My friend could not beleive that the CQR could hold his boat using 80% throttle and was totally surprised by it! Scary I know.... From my experience I find a CQR likes a minimum of a 7:1 to set but it sometimes prefers more..

He now understands that an anchor should hold your boat at wide open in reverse without moving. This is a guy who has been sailing for 25 years and admittedly dragged "perhaps 20 times but never with my CQR"! Once is to much! It's imperative the anchor gets "set" properly. Yes the CQR sets better in soft bottoms than in sand but not all boaters are lucky enough to always drop the hook in a soft bottom. So if you're in a hard bottom make sure to get it set. The CQR will set well but it may take more than one attempt. Don't ever be fooled by the "initial bite". With a CQR this is a situation where the anchor is laying on it's side with the tip just starting to dig in. Like the picture at the beginning of Sail Magazines article. If you stop there on any sort of wind or current shift the anchor will twist out. A CQR needs to be vertical and buried to the shank or it's not properly set. If it's properly buried it can sometimes survive a 180 shift without "breaking free". I suggest some of you begin diving on your anchors in a shallow spot to see what's going on down there I think you'd be surprised...


Not knocking any specific type of anchor, the story is about how to set ANY anchor.
__________________
Stu Jackson
Catalina 34 #224 (1986) C34IA Secretary
Cowichan Bay, BC, (Maple Bay Marina) SR/FK, M25, Rocna 10 (22#) (NZ model)
Stu Jackson is offline   Reply With Quote
Old 04-04-2014, 05:44   #18
Registered User
 
Alan Mighty's Avatar

Join Date: Mar 2014
Location: Moreton Bay
Boat: US$4,550 of lead under a GRP hull with cutter rig
Posts: 1,691
Re: How best to set an anchor?

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 available.

- #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

i. Importance?

(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 and vendor:
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 people:
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:
Marine 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.

Prof JHK:
* 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 2008
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 (undated)
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.
Alan Mighty is offline   Reply With Quote
Old 04-04-2014, 08:15   #19
Moderator
 
noelex 77's Avatar

Cruisers Forum Supporter

Join Date: Jul 2007
Boat: Bestevaer.
Posts: 12,571
Re: How best to set an anchor?

I agree for best setting in difficult substrates a gradual increase in power is best. This is what I generally employ.
Using the momentum of the boat and "jerking the chain" (did I say that ) is often employed with new generation anchors, but it is less reliable than a slow steady increase in force.

I gradually build up to full reverse (with a Maxiprop) and hold full reverse for 10-20 seconds. There is no backwards motion so the engine and prop are struggling I would be reluctant to hold full reverse for longer, but I have observed the anchor while diving and I am don't think holding full reverse longer would achieve much.
noelex 77 is offline   Reply With Quote
Old 04-04-2014, 08:40   #20
cruiser

Join Date: Dec 2013
Posts: 398
Re: How best to set an anchor?

My 2 cents,

1st. You can almost never apply to much power on setting an anchor, if it continually pulls out it's the anchor, scope. or bottom. Not the power, not even in a twin screw power boat.

2nd. Once at full throttle, if well set. The difference between 2 minutes and 10 minutes is not great enough to matter. If the anchor is set you shouldn't be moving anyway, not even creeping back slightly. If you are it's a poor holding bottom, scope, anchor, ect...

3rd. Going out and anchoring is the only way to become skilled, knowledgable about anchoring. Just doing it and seeing the results is the only way. No amount of research or net banter will take the place of anchoring in terrible conditions and dragging and then analyzing what went wrong.

4th. Yes you are over thinking this and all those cited sources don't help at all. See above #3.

5th. It's really just to long.
Horror Hotel is offline   Reply With Quote
Old 04-04-2014, 09:04   #21
Registered User

Join Date: Mar 2014
Boat: Sovereign 24
Posts: 72
Re: How best to set an anchor?

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

-Chris
Rocna has a whole knowledge base on Anchors, Chain, Rode, anchoring, etc. Rocna Knowledge Base
Phantoms is offline   Reply With Quote
Old 04-04-2014, 09:44   #22
Registered User

Join Date: May 2013
Location: Bermuda
Boat: Heritage West Indies 36
Posts: 931
Re: How best to set an anchor?

Alan Mighty. Having read the entirety of your posts and enjoyed them immensely, i think i can reasonably postulate that the answer to your question, 'who knows or could discover solution' is 'YOU!'
DefinitelyMe is offline   Reply With Quote
Old 04-04-2014, 15:32   #23
Registered User
 
Alan Mighty's Avatar

Join Date: Mar 2014
Location: Moreton Bay
Boat: US$4,550 of lead under a GRP hull with cutter rig
Posts: 1,691
Re: How best to set an anchor?

Errors in #18:

Vryhof anchor that dragged after rode veer dragged 7 metres (not one metre) and then reset. Engineering paper discussing incident is on net and can be found by search.

Website for Knox Anchors is knoxanchors.com, not knoxanchor.com.

Apologies for not linking URLs. Newbie mistake. Complicated by my post being moderated, so I could not edit out typos in 30 minute grace period before post gets solidified.
Alan Mighty is offline   Reply With Quote
Old 05-04-2014, 04:25   #24
Registered User
 
Alan Mighty's Avatar

Join Date: Mar 2014
Location: Moreton Bay
Boat: US$4,550 of lead under a GRP hull with cutter rig
Posts: 1,691
Re: How best to set an anchor?

Quote:
Originally Posted by noelex 77 View Post
I gradually build up to full reverse (with a Maxiprop) and hold full reverse for 10-20 seconds. There is no backwards motion so the engine and prop are struggling I would be reluctant to hold full reverse for longer, but I have observed the anchor while diving and I am don't think holding full reverse longer would achieve much.
I had measured bollard pull astern on Led Myne, with fixed 3-blade, compared to sister ship with 3-blade MaxProp. 3-blade MaxProp delivered 10% (calculated from hard numbers, but only one trial of each boat) additional bollard pull astern compared to 3-blade fixed prop.

Throttle range of Led Myne's Yanmar 3GM30F is 880 - 3600 rpm. Yanmar admits thermal equilibrium cannot be maintained for significant periods at 3600 rpm (Yanmar quotes 60 minutes. I'm more conservative, because I don't renew flexible impeller or clean heat exchanger every month). So I similarly only use 3600 rpm for seconds at a time (eg 3 second kicks astern in a back-and-fill turn).

I think we need to be careful with loose terms such as 'full reverse'. And quote engine revs instead.

On Led Myne, I have naturally overthought (as opposed to normals who underthink their lives, bless their little hearts) and defined throttle positions:

* emergency 3600 rpm, consumes 6.5 litres/hour, not sustainable because of inadequate engine cooling;

* flank 3400 rpm, maximum continuous sustainable speed for thermal equilibrium;

* full 2900 - 3000 rpm, maximum sustainable speed for cruising when consumption of fuel is not limiting;

* standard 2700 rpm, recommended continuous engine speed to prevent coking of exhaust elbow.

* half 1500 - 2000 rpm

* slow 880 rpm, idle speed

My best guess for what's required to embed Led Myne's Rocna 15 in quartz sand is 3000 rpm for 5 minutes or greater, as noted above.

And I mean fully embed - shank is buried, shackle and some chain usually buried too. Retrieving anchor is not easy from that (I use trip line).

I have not yet quoted marine engineering literature that defines 'fully embedment', which is in terms of burying to depth proportional to fluke width. For Rocna 15 in mud, I calculate fully embedment figure is in order of 3 metres! Difficulty of measuring that means I'm only pursuing guidelines for sand. Shallower for sand (and not as well defined - engineering literature just says 'can be less than for mud'!!).

For Led Myne and quartz sand, bollard pull astern of 10 - 20 seconds only buries fluke, not shaft. Octopus can dig under anchor at that degree of embedment!

I welcome you trying 5 minute bollard pull astern at 3000 rpm next time you anchor on sand in conditions open to visual inspection. And compare that to 20 second bollard pull astern.

Al
Alan Mighty is offline   Reply With Quote
Old 05-04-2014, 22:09   #25
Registered User
 
Alan Mighty's Avatar

Join Date: Mar 2014
Location: Moreton Bay
Boat: US$4,550 of lead under a GRP hull with cutter rig
Posts: 1,691
Re: How best to set an anchor?

Questions flooding minds of concerned youth of today:

- What does "full embedment" mean?

- Can visual inspect, such as by diving on your anchor, reveal if your anchor has reached full embedment?

One of better bits of science on question is US Navy research done in 1980s, using Danforth/Fortress type anchors (and extended to include Bruce, Navy Stockless, and Vryhof). That research is used as font of wisdom by many later US Navy and non-USN marine engineering publications and probably still source of current USN guidance. I've not found anything better, including since introduction of Delta, Spade, Rocna, SARCA, Knox Anchor, Manson Supreme etc. I guess that 1980s research is 100% relevant to Fortress anchors. Until I see contradictory evidence, I consider it also relevant to anchors with 'new generation' marketing tags.

You should find one or more versions of the report by internet search.

* R. Taylor (Naval Civil Engineering Labs) & P. Valent (Brian Watt & Associates), Design Guide for Drag Embedment Anchors, 1984 (declassified and approved for public release 1986 under administrative number ADB080279, which you might like to use as search term).

- Taylor & Valent defined full embedment in terms of depth of penetration of anchor tip. And expressed depth of tip penetration (d subscript t) as a function of fluke length.

- that suggests Taylor & Valent did regression analysis after many trials. And so their d sub t formula is scalable to anchors of range of sizes.

- Rocna 15 fluke length is 470 mm (18.5"); fluke width is 415 mm (16.3").

- depth of soil must be greater than d sub t. That's relevant for mud over rock, mud over sand etc multilayer substrates ('difficult substrates').

- Taylor & Valent do not explain how to measure d sub t after anchoring. With Rocna 15 in embedment posture, tip is 361 mm (14.2") below plane of top of roll bar (anchor shackle and top of shank are just below that. Sometimes in quartz sand, resistance of burial caused by shackle and chain means top of shank and top of roll bar are in same plane.

- for sand, Taylor & Valent (Table 6.1, p. 20) offered simple relationship d sub t = fluke length. For Rocna 15, d sub t = 470 mm (18.5"), so roll bar top should be 109 mm (4.3") below surface of sand (i.e. roll bar, shank, and shackle should all be buried).

- for mud, Taylor & Valent offered a couple of ways of defining full embedment (I'll only mention simplest). I've not found a way to use any. Especially because anchoring in mud: (1) requires long soak time; (2) water is usually turbid or anchoring disturbs bottom so water becomes turbid; and (3) don't have probe to find anchor tip and measure d sub t. The simplest definition is: (Table 6.1, p. 20; for Danforth) d sub t = fluke length * 4.5. For Rocna 15, that means tip depth of 2.1 metres (83").

- Taylor & Valent offer much more. They conceptualized rode in three parts: 'cutting chain' (embedded in substrate); 'sliding chain' (resting on substrate and offering resistance to pull - but T & V noted better to put money into fluke area than thicker chain); and 'suspended chain' (the catenary). Lots of math. And of course definitions of full embedment in terms of how many fluke lengths the anchor will drag as it embeds. I've tried to estimate drag length. Found it not useful, so will not waste your time.

Summary for the 'too long, couldn't read' crowd:

* full embedment in sand means you should not see any part of your anchor above the sand surface. Roll bar, shank, shackle all under sand. Octopus should not excavate under anchor. For mud, even deeper - tip of Rocna 15 should be 2 metres down, roll bar should be 1.6 metres (63" or 5 foot 3 inch) below surface.

* for 'dive on your anchor' advocates: Full embedment means all buried. In clean sand, roll bar should be more than longest finger length below the sand. You really going to dive and then feel around? If not, perhaps consider bollard pull astern and duration of pull as ways to (a) power set anchor and (b) verify holding capacity of soil.

* for 'better to underthink problem' advocates: if that helps you sleep better when at anchor, feel free. After all, Lord Buddha did counsel 'do not cling'. I prefer non-Buddhist anchors that do cling.

Al
Alan Mighty is offline   Reply With Quote
Old 05-04-2014, 23:28   #26
Moderator
 
noelex 77's Avatar

Cruisers Forum Supporter

Join Date: Jul 2007
Boat: Bestevaer.
Posts: 12,571
Re: How best to set an anchor?

It is good to think about, and recognise what constitutes a good set.

In warm (shark and crocodile safe) waters it is not unsusual to dive and look at the anchor. I (or my wife) does that almost every time we drop anchor. After a sail a swim is a just what is needed anyway.

Many other boats including even charter yachts do the same thing in these waters. The puzzle is they invariably do nothing even if the anchor is completely unset.
I think someone has told them diving and taking a look is a good idea, but failed to mention you are inspecting the set not just checking the anchor is on the bottom

However, when looking at the set of the anchor the substrate needs to be taken into account. In firm substrates you will not completely burry the anchor on engine power alone. (Danforth type anchors are the exception because they not very tall)

I have a new underwater camera and hope to take some photos this year of different anchors that are well and poorly set.
There will be no photos of Sarca anchors, even those with unbent tips.

A good addition to any anchor is small float with 1-2m of line attached to the tripping hole. This float stays underwater. It means when the anchor is completely buried you can still find it and the amount of rope showing gives you some idea of the depth of the set. You can even mark the rope if you are keen. The rope also makes it easier to retrieve a stuck anchor. It is hard to dive and attach a rope to the right spot to pull an anchor out backwards. A line with a loop, floating above the bottom makes it easier and it is a couple of meters less to free dive down.

If you anchor in very shallow water where there is a risk of the float catching other boats just shorten the rope (I usually just cable tie it to the shank.). If you regularly anchor in shallow water use a shorter line
noelex 77 is offline   Reply With Quote
Reply

Tags
anchor

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Delta Anchor, Set in Sand, with Pictures doug86 Anchoring & Mooring 44 26-09-2012 19:29
Specific 'Two Anchor Set' Question Therapy Anchoring & Mooring 9 27-08-2010 03:12
Anchor set-up question. Therapy Multihull Sailboats 6 09-04-2009 13:14
how do you set up your anchor rode? Ram Anchoring & Mooring 48 01-05-2008 01:05
Lightweight Anchor Set - from Brian Sheehan (Fortress Anchors) GordMay General Sailing Forum 2 14-12-2004 17:43

Advertise Here


All times are GMT -7. The time now is 19:30.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2021, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2021, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.