Some of my favourite anchorages are strongly tidal, a perverse eccentricity you may think. But I love the living feeling of the boat as she swings to the new tide, and the ever-changing view scratches my curiosity constantly and gives me a heightened sense of place.
Other than that, tidal anchorages only offer endless possibilities for trouble, especially those that are little more than tidal pools with steep banks. Swinging ashore, becoming all too intimate with your neighbours, pulling your anchor out and dragging as the tide turns—I've done them all. And so has virtually everyone else at some time or another, a fact that is widely recognized in Britain’s crowded harbours, where some, such as Dartmouth, insist that someone must always be aboard an anchored boat at the turn of the tide.
Colin, As usual, many good ideas to ponder, have in one’s armamentarioum, and hopefully, use rarely. Am I correct that the rational for attaching the kedge rode, for Alchemy (and I suspect many) this is rope, to the chain is to get it deep enough to not catch on underwater protuberances when swinging about at change of tide? I have always used a kellet (sentinel) to accomplish this, but there is something I like about attaching the kedge rode to the bower chain. I will have to think about this.
Thanks, Dick Stevenson, s/v Alchemy
Colin, Another alternative might be to attach the cheek block directly to the chain via a “D” shackle, maybe a long one. Most chains leave enough room and I notice that the snap shackle in your picture (a device I never fully trust especially if getting loads from different directions and out of sight) is attached to the cheek block with a screw pin. It might take a few seconds longer but be even more bulletproof.
I agree, it might be a good idea to simply use a shackle, but this is quick and easy. The snap shackle is quite hefty, and the whole set-up articulates well, but I’m still thinking of doin away with the snap shackle, using a simple shackle (as you suggest) and maybe a Spectra loop – I’ve got huge faith in the stuff.
And to answer your initial point, the attachment in that was does two things, pulls the joint (and so the two cables) down below keel level, and (b) acts as a swivel point. Works well in practice.
Hi Colin, The method you describe using a snatch block was outlined in one of the UK sailing magazines some time ago by Tom and Vicky Jackson of the SV Sunstone, but with the emphasis on lowering the block to just above the seabed and then pulling both bower chain and kedge warp tight up to the bow roller. If this is done evenly the block stays very low with the chain and warp tensioned against each other. This is effectively an ad hoc mooring with the advantage of the pulls on the anchors being at a good angle to maintain a solid set irrespective of loading direction. There is of course also minimal swing with this tensioned arrangement. I should add that I have no persoanl experience of use of the method, but I keep it in the armoury. Alan
Thanks very much for that. A snatch block seemed to me to be the best bet for reducing chafe on the kedge warp, although as I suggested in the piece, a warp reeve or hybrid of the two methods might be all that’s needed. As has also been suggested, a warpreeve in Spectra/Dyneema/Dux or Amsteel might do very well, with a properly installed thimble. What I like about either is the opportunity to adjust the kedge warp easily.
The one weakness of the warpreeve idea is that it can come loose hence I’d want to have some form of fixing it in place (as with the Wichard chain hook on our snatch block), for the very reasons that Tom and Vicky Jackson obviously identified, that being to have the attachment form a swivel. It certainly hadn’t occurred to me to lower the swivel to the seabed in the manner you describe, although of course at some stages of the tide (etc.) that would be the case. That seems to me like a really good idea, so hats off to them, and we’ll definitely try that out ourselves.
Hi Alan and Colin,
As part of the research I have been doing in connection with jackline loads, I have been looking at the load multiplication factor resulting from exerting a pull in the middle of a line between two fixed points. The results of this analysis are sobering. The load multiplier goes up exponentially as the the deflection of the line the goes down. Applying the same logic to Tom and Vicky’s set up gives me pause for thought since the load of pulling up on the attachment point between two anchors is going to be multiplied several times as it is exerted on the anchors. Add in some surge and a few good snatches of the boat on the chain and you are going to end up with some truly eye popping loads.
Bottom line, there needs to be plenty of deflection between the two anchors so tightening everything up so that the join remains on the bottom might not be a good idea.
Thanks to Eric Klem for first bringing up this important piece of engineering, which I for one, was shocked by.
Well, once again the numbers tell us a different story, and what looks, on the face of it like a perfectly sound idea may in fact note be so.
I know that whenever we have used this technique, we have simply allowed sufficient depth between the deck and the swivel to ensure that there was no chance of fouling the keel, which is the way that I was shown many years ago, and has always worked for me. Maybe we’ll stick with what we know to work!
I think that’s right. Better to leave more slack in the system as you originally described.
So often the old ways we were taught were grounded in sound engineering, we just did not know that! And I was first taught the technique you describe in the post some (gulp) 50 years ago by Peter Pye. Having said that, your block system improves on Peter’s.
Hi John and All, I am not an engineer and not an opponent or proponent of either of the two tension variations we have been discussing. However, I do wonder whether the jackline model, of which I am aware but which I confess I cannot rationalise, can be extrapolated to the anchoring-cum-mooring situation we are considering. Going back to first principles, we know, do we not, that by changing the lead of a line by taking it over a single sheave does not change the loadings. Moreover, our anchors are not fixed, and if of the new generation they will likely shuffle round without fuss as the load directions demand. But of course in the tensioned (Sunstone) system that shuffle will be minimised. Perhaps it may help to think of the chain and warp combination as a riser. And we should remember that in anything like a blow, the reason for extending any anchor rode is primarily to keep the pull on the anchor shank as low as possible. The Sunstone system, which is after all only a variation of a well tried and tested mooring arrangement, ensures that low pull is maintained in all conditions. So in the final analysis perhaps it comes down to a preference for either a mooring or twin anchors in whatever situation prevails.
It seems to me that the different elements acting in both the tensioned and less-tensioned systems, including shock loadings, are numerous and their interactions complex. This is all the more reason to make sure the analysis method that is applied is the correct one. I suspect an hour or two over a beer or three with the right kind of engineer would not only be entertaining but possibly also enlightening. Finally, I should point out that the Jacksons emphasised the need for a seriously strong block for their method. I don’t think Colin’s Lewmar block would meet their specifications. Alan
I assure you that the engineering is right and that the loads on the anchors will be dramatically multiplied in relationship to the load going to the boat. I know it’s hard to visualize, but it is nonetheless true and no amount of beer will change it :-).
Here a professional engineer explains it better than I can.
Also reading this from a climbing manual may help.
Sorry, I couldn’t seem to reply to Alan or John so I am replying here. Alan, John is correct that by pulling the lines between 2 anchors tight, you increase the load between the two anchors potentially by a very large amount if they are really tight. The key is that you are changing the geometry of the system just like in that jackline model. It is true that the block is is simply a change of direction but it could also be a knot or shackle that doesn’t let anything slide as it is static. Anytime you put a side load on the middle of a taught line, you greatly increase the tension in the line. The reason is that line can only transmit tension force, not bending or compression. Therefore, all forces on a line are directly along it. If you put a side load in the middle of a piece of line, the total force in the line must be higher because the line needs to transmit that force in that direction which also means that it is transmitting a force in the perpendicular direction (these are the two force vectors that make up the total force). In a piece of line where you push sideways on exactly the middle, the tension is equal in all the parts once the line has been deflected 30 degrees from its endpoint. This is because at 30 degrees, each part of the line carries half of the load applied (2*sin(30)=1). 2 anchors (or more for that matter) work really well if the direction of pull is directly in line with 1 anchor and the the geometry means that there is no tension on the other anchor rode. I hope that this makes sense, it is a bit hard to explain in words, it is much easier to look at graphically. If you want to prove it to yourself or just have fun putting it to work, you can try it out in your yard. If you take a really stiff piece of line so that the deflections are low and get it really tight between an immovable object and a hard to move one, you will notice that as you push on the center and the object moves, you keep having to push harder to keep it moving as the angle in the line becomes greater. I was a whitewater kayaker for about 15 years and I used to use this technique all of the time while other people would be spending much more time trying to set up z drags which could not produce as much force as this. I have also used it to get stuck vehicles that are orders of magnitude heavier than me out of the mud/snow. This is not only a problem when using 2 anchors, it is a problem with any number. If you make a 3 point mooring, the scope in the upper section of the mooring needs to be greater than the… Read more »
Colin; Kudos again for an excellent, thought provoking, post. However I, as the least experienced yachtsman on this site, wonder why? The why being, why swing at all?
If the bower is set in opposite the greatest tidal surge ie upriver and the kedge downriver, then one does not necessarily have to swing at all.
In the Pacific Northwest we often stern tie to a tree, substantial rock i.e. something substantial on terra firma. We allow, of course, for tidal conditions which in this area are significant (7-9 feet) but not like Nova Scotia.
Finally we raise the keel so the underbody has the least resistance to water flowing underneath.
I realize I am missing something important in your reasoning and technique. Perhaps you could enlighten me as to why swing when almost stationary appears to be easier?
Thank you in advance.
There are a variety of ways to lie to two anchors, and each has it’s advantages and benefits, generally in fairly specific circumstances.
The Bahamian moor is usually only used where (a) you are lying in a pool with very restricted room to swing, (b) where there may be the additional complication of strong tides and/or crosswinds that might cause you to swing ashore or (c) where you are anchoring amongst moorings where your boat will need to have a similar amount of swinging room to avoid contact with other boats due to wind or tide.
As both rodes are attached to the bow the boat can simply swing around a fixed axis with none of the difficulties associated with setting the kedge from the stern (very high loading on the kedge from a less than favourable angle). Done right the boat has a very restricted arc of swing and the anchors both hold well.
The methods you describe all have their merits and are the best option in some circumstances – equally, the Bahamian moor is the best bet (in my experience) in the very specific circumstances I’ve outlined above.
Hope that all makes sense!
To add to what Colin wrote. The other huge benefit of Colin’s method, when weighed against anchoring fore and aft, is the reduction of the load on the anchors.
Matt Marsh, AAC technical correspondent has calculated that the load on our boat’s anchors is as much at 15 times higher when she is prevented from swinging head to wind. And that is before you add in the load multiplier that I discussed in answer to Alan’s comment.
Of course if the wind and current can be relied upon to act in the same direction as the anchors are laid out, then none of this matters. But all it takes is one gust from abeam to subject your gear to this kind of eye popping load. Colin’s method solves that problem.
Colin and John,
Thank you both for your additional comments. I will have to try it out someday in a quiet location when no one is watching to watch my bumbling about.:)
Fortunately my boat came with a dizzying array of rodes, warps, ground tackle of every size and shape. Now I just have to learn how to use it effectively.
You should see some of the things I have done in my time with multiple anchors—not pretty!
Don’t worry too much about it – I’m sure you’ll be fine, although, like you, I hate an audience to try out this type of thing!
Hi John, Many thanks for the links and I am grateful to Eric for the added detail. It is intuitive that there is a theta effect in “suspension” systems, and indeed Eric I have used it and a hi-lift jack on more than one safari to get Landrovers out of literally deep trouble. When we owned a smaller yacht we used it regularly to sweat-up halyards etc.. It is nice to be able now to put some numbers on the effect. The question for me remains its significance in the bahamian moor situation in which there are many players. A key one is the actual loading perpendicular to the anchor-anchor line. It is easy to see that even in the high tension mooring version the load required to induce a small theta is generally going to be small, that is, the multiplication will be from a small base. Increasing loads will in fact induce more favourable thetas and it is in this respect that our boat behaves differently from the climber on the rock face whose loading of the system remains constant when static. That said there will of course come a point where the loads start to pose a danger to the integrity of either a high tension or low tension bahamian moor. The difficulties inherent in calculating when this will occur in any given anchoring situation, using either a high-tensioned or less tensioned system, are daunting, at least for me. This makes a functional comparison between the two variants problematical. There is more than one theta to consider in each system, and moreover these exist in a three dimensional structure in which the vector ratios will differ between the high and low tension systems with differing effects on the structural components. So my sense is that once loads become dangerous in the tensioned system the first major concern would be the integrity of the snatch block if it were of a rating that would be used by most folk, rather than either anchor considerations or chain/warp integrity. However, concerns about the block can largely be put to bed by using one of the modern blocks on loops which have very high SWLs, even in those of modest size. Then given that the block is up to the task, once things get to the point where the other structural elements in the system come under threat, and assuming they have been properly specified for the boat, one should probably be doing something rather different in a different place. On the other hand, in the low tension system with increasing side loading things can get to the point where the main purpose of the bahamian moor has been defeated but where all of its deficiencies are getting more serious by the minute. In that situation what does one do? Many folk would just wish they were at home with a nice cup of tea. Some would increase the scopes further to reduce shock loading, but paradoxically to reduce thetas in… Read more »
I can only add my agreement to your final para – thanks to you all for such well presented arguments for and against this method, and most particularly the technical challenges it provokes.
Much to think about – and all of it good!
Lots of interesting thoughts, thank you. The one that really resonated for me is the fundamental issue of what to do when really bad weather is coming and the only alternative is an anchorage that is too small to swing in. Our approach, after years of being subjected to that quandary in the north, is as follows:
Having said that, if an anchorage beckons in a river we would use Colin’s method and enjoy the place, but we would always be ready to blow Dodge if really bad weather were coming on the basis of our opinion that any Bahamian mooring is intrinsically less secure that the options listed above.
One other thought. I suspect that the reason that things don’t break with the Jackson system is simply that the very large loads imposed cause the anchors to creep closer together, which, in turn, reduces the loads dramatically. Point being that if there is any significant load, particularly at an angle to the set of the two anchors, they probably end up anchored exactly as Colin does, without realizing it.
Also, post coming on the load multiplier effect as it relates to jacklines.
Just for fun, I tried to see what the loads would be like in an example system. To make the math easy on me, I did the whole thing backwards from the load on the anchors back to the load on the boat so sorry that it is harder to understand. The assumptions that I made were that you had two anchors, each with 100′ of rode leading to a snatch block on the seabed with another 20′ of chain and line then leading direction to the boats bow. I also assumed that at 0 load, there was 0 slack in the system which should be a reasonably bad scenario. I assumed that the chain was heavy and rigid and that a 5/8″ piece of 3 strand nylon was used. The geometry solved for was the worst case scenario with a force perpendicular to the line/chain going between the anchors.
By putting a 4000 lb load on each anchor, the 120′ of nylon stretches 6% or 7.2′. This means that the angle of pull on each of the anchors changes by about 15 degrees. Therefore, the force being transmitted to the boat is a bit over 2050 lbs. With the stretched rode, the effective scope of the piece of line/chain leading up to the bow is 2.35:1. This means that the horizontal component of the force which is transmitted is about 90%. Therefore, the horizontal load on the bow is about 1860 lbs which is equivalent to a 40 foot sailboat in a gale.
In one sense, the load multiplication is not that bad, it is only about 2X. But in another sense, you are generating 8000 lbs of holding power with your anchors and only using about 1/4 of it. If you were using something that deflected less, than the loads would be a lot worse. Also, I would not want to put a piece of 5/8″ line through that much load for any length of time. I can’t imagine doing this with 2 pieces of chain. Interestingly, at lower loads, the load multiplication factor would be a lot greater due to the lower deflections.
Colin, I hope that I haven’t derailed your post too much but I thought that it might be helpful for us to see what real loads would look like on all of this. Thank you for the good post as always.
I am sure that everyone who has read this post has appreciated the sheer breadth of knowledge that has been brought to bear on the subject by all contributors. And it is always instructive to challenge our assumptions via the crucible of numbers and science. Which is, I believe, why people visit this site.
So you haven’t in any way hijacked the post – quite the opposite, so thanks!
Because this article is a bit dated. I was wondering if you still use the Wichard chain hook, or now us a different method, of connecting the snatch block to the bower chain? Also wondering about the snatch blocks longevity.
Generally, although I’m here pretty much 365 days a year, AAC writers only monitor comments to posts for a couple of weeks after original publication date. This is only fair since they are paid on a per-post basis only.
In a perfect world we would pay them an annual fee to monitor and answer comments on older posts, but unfortunately our revenue stream does not run to that. Also they are all very busy with other things.
The result is that you are stuck with me.
My advice would be to use a dynema loop, with a prusic hitch, rather than the Wichard chain hook. I am now using these all over our boat and loving them.
As to the snatch block, I just had one of those Lewmar snatch blocks fail on me (at the swivel) after very little use, so I don’t recommend them. Rather I would get a Harken snatch block. (In 40 years of using their kit I have never had a Harken fitting fail on me.)
I’ve had two Harken fittings fail on me just recently, though both were relatively old (5 and 8 years respectively). An HR shackle blew up at loading below its SWL, and an ESP becket block blew out at the pin – the aluminium body actually fractured. This was also below the SWL. So, any fitting from any manufacturer can fail.
Perhaps, get rid of the snatch block and replace with a low friction ring. A bit like the original thimble idea, but better running.
Sorry to hear about the Harken failures. In all my years of using their gear (some 40) I have never had a failure, so surprising, particularly since, if memory serves, Harken SWL is one third of break load. How do you know for sure that it was below the SWL? Impact loads, if there was any slack in the system, are hard to calculate but can be many times the calculated continues load, I think.
I agree low friction rings are great, that said it would require threading the entire anchor rode through it. That’s the big advantage of snatch blocks: you can clip them on wherever you want. That said, that very capability is, as you point out, a potential weakness. I have had three fail on me over the years, one Lewmar, and two Nicro, all quite old, but never a Harken. However, all those failure were at the trunnion, not the opening mechanism so not sure there is anything wrong with snatch blocks per sec.
Anyway, I would never be without a couple of snatch blocks since there are times when they are the only fitting that will do the job.
Possibly someone has already brought this up- are you still using a snubber on your bow rode? It seems you would have to apply this before attaching the kedge rode.
Unlikely Colin will come up: https://www.morganscloud.com/2013/11/10/aac-comment-guide-lines/
That said, both Colin and I always use a snubber on an all chain rode so I’m pretty sure the answer is yes.