Musings on anchor bridles
As I sat in the cockpit with our boat gently tugging at her mooring in Great Harbor, Jost Van Dyke, I took note of the catamaran just astern of us. The crew of the cat had rigged their mooring bridle at a very large angle, something we have done in the past as well.
Note the angle of the bridle legs.
They likely did so because the balls at that location have very long pennants but it made me think of some of the physics related to how rock climbers rig their anchors. A properly-rigged bridle actually splits the forces applied to the boat. This is in addition to helping to stop the boat from sailing around at anchor. Unbeknownst to many though, an anchor bridle with legs too short actually increases the forces applied to the boat, substantially in some cases. Take a look at the graphic below.
I ran my thinking past my friend Drew, a fellow PDQ owner and very experienced climber. With his permission, I have copied his reply to me below.
- Particularly at narrower angles, the strain will actually come onto just one leg when the wind suddenly shifts. Thus there is a point of diminishing returns at about a 30 degree angle. I’m sure you’ve seen this.
- If the bridle is too long it will ride on the bottom, and several bad things can happen: chafe (it’s fiber), snag, and the hook comes off. In deep water this is minor, but on the Chesapeake I seldom anchor in more than 7 feet of water. One cure, if you want the long lines for shock absorption (which matters more in shallow water) is to run the lines along the side deck and cleat mid-ships. I doubt it matters to you, but when you go very shallow, particularly if there is any risk of surge, it really helps. Of course, if the waves start to break and you’re in the impact zone, you’re hosed. No cat can handle waves breaking on the bow at anchor; too much area. Footprints lost their Gemini that way, as did some Aussie. Another reason not to anchor too near the beach.
- Longer bridles give more shock absorption, which is good in shallow water with all chain. But see the problem above; the apex drags.
The same math also applies to Bahamian moor. The angle should never exceed 120 degrees, because the force goes through the roof if the wind comes from the side. By the same token, if the wind shifts and you are on a shallow angle, the load will all be on one anchor (the reason 2 anchors are pretty pointless in most storms; better to have a big Rocna, no?!). In fact, this applies to any sort of anchor or rigging. Climbers, for example, aim for the 40-60 range; greater angle and the strain is more, lesser angle and the load is all on one if there is any swing. This is particularly bad for climbing anchors, because the anchor may shift in the crack.
So for many reasons, 40-60 degrees is a sort of sweet spot.
Great info. Thanks Drew! If you are interested in some excellent reading with very valuable technical info, do check out Drew’s blog!
I would say mine is in the 45-90 degree angle probably closer to 90…Good info with scientific reasons why…Good stuff
I think most people set it at a decent angle. Some just might not we aware of how it affects them.
Another place this comes into play is jacklines, those safety lines that make us feel better at night and when it’s nasty. Increase the mass to 500 pounds–the impact if you tripped from the cabin top, for example–watch the calculation go to 2850 pounds, and remember that the breaking strength of most jacklines is ~ 4500-5000 pounds when they are new. In practice jacklines are very dependable–the point is they need to be that strong.
Excellent point!
In cases in which you wanted a very short bridle perhaps a bridle with 3 legs could be used in which the center leg were composed of lighter cordage and brought back directly to a cleat in the windlass area. With careful tensioning of this center leg it could act as both a shock absorber and a load diverter such that the loads on the two side legs were acceptable.
It’s early now but that sounds complex. 🙂
Umm, you raise a valid point, and I don’t disagree with your results — but the diagram is a little misleading isn’t it? Even high school physics / mechanics tells you that the 100kg weight in the centre will end up with a horizontal and vertical component in the vectors applied at the bows – the arithmetic sum of which will still be 100kg. How can they possibly be more than 100kg? If you have 100kg “hanging there” ……regardless of the angles, the force applied is still 100kg.
I have not pulled out a scientific calculator to verify the formulae shown on this page but perhaps you may wish to:
http://www.chetwynd.info/other/anchors.htm
Mike – good link, but I don’t think its the same. The link you refer to is about using 2 anchors, both of which drag on the bottom. As they’re dragging towards each other, there is a horizontal load. In your case, as you look at a bridle, the bridle – if I’m not mistaken – is fixed to two immovable objects, your two bows. What about this? If you string a rope between the two bows and step on the rope, at one side, or in the middle, the amount of force you produce is still limited by your body weight. Wouldn’t you think? Anyway, it would make for a very good topic of conversation over a beer, next time we see each other face to face …….and that could be a while.
I look forward to that chat. Just for clarification though, when the author says anchor, he does mean an anchor in a nautical sense. He is specifically talking about climbing anchors which are fixed, like our bow cleats. Happy cruising!
Well noted Mike. In fact this is the same physical principle we all take advantage of when we step with a foot on a cleated mooring line to get the boat closer to the dock instead of pulling the line directly.
Cheers,
Marco.
Apparently so.
I guess I have to go back and review some physics and especially, some math!
Better get a scientific calculator too.
http://www.chetwynd.info/other/anchors.htm
Excellent information. I had never put much thought behind the physics and loading on a anchor bridle.
I have no experience on cats (trying to learn more), but have seen several cats use a bridle with loops spliced into the ends (basically set length) where the loop is just placed over the mooring bollard on each hull when being used as an anchor snub, not seen it used on a mooring. From what you have seen, is that a common practise?
Drew’s comment on the jack lines is so true. Really need to take extra care of them, as you never know when you may depend on them to stop you disappearing. We always rinsed the jack lines in fresh water after a race, never leaving them on deck unnecessary.
To secure to a mooring we use dual dock lines, each one beginning at a cleat, looping through the mooring ball or pennant and then led back to the same cleat that it originated from. In this way we can adjust the length and the angle.
I can’t agree with the “We always rinsed the jack lines in fresh water after a race, never leaving them on deck unnecessary…” viewpoint. While that makes sense to racers, it doesn’t make sense to many cruisers. We use jacklines when…
* alone on deck,
* at night,
* when the water is cold,
* when it gets rough,
* when under spinnaker (short handed boats can’t turn around quick, and we consider a full
crew),
* and if there is risk of squalls.
You simply can’t be running around rigging jacklines in marginal conditions, alone, when there is deck cargo to be double checked and sails to furl or lower. The jackline needs to be there.
In other words, every day. The racer’s approach–with full crew and possibly chase boats–sounds to use like only wearing a seat belt when you need it… which is every day. How do you deal with UV? Either replace frequently or make them or something more UV resistant. Even for racers, jacklines that are there every day means you have practiced with them.
Ours are always rigged. For the most part, we had it that way on ZTC too. UV is no-doubt an issue.
Hi Drew, I didn’t mean to imply removing them while sailing, after race when you are back at the marina or mooring. 🙂 I have seen many race boats which leave all the safety head in deck until the next race. Taking care of equipment which potentially will save a life seems logical.
No need to learn celestial navigation when the misapplication of good old trigonometry can cost you the boat or live just as fast. 😀
The change in line loads are a bit counterintuitive but none the less very real. Thankfully the experiment to prove this is rather simple. Take a luggage or maybe fish scale, splice it into one leg of a bridle and attach a weight. Then observe the scale while pulling the bridle from parallel to a flatter angle.
I guess it’s time to exchange the good weather for more science in Mikes next video. 😉
I can’t actually tell if you’re agreeing with the post or not. 🙂
I did take out a fish scale and did as you suggested though. Although far from scientific, the force applied to the line did appear to be greater than the weight that I applied to it when the end points were separated at a great angle.
For the record, I agree.
It is just a bit counter inductive. Most will just remember the pulley principle where the effort gets reduced. For some reason virtually all pulleys use parallel lines. And the reason is in above math.
Btw, I also checked my math with an old scale before writing the prior comment. Science! *g*
Why did we never do such a relatively simple experiment to reinforce trigonometry lessons in school? It even contains fun with division by zero. We only did abstract stuff without any application… (I then studied electronics. Which contains lots of trigonometry you can’t see.)
There are always compromises. Two factors we all know but that were not mentioned.
1. A short bridle keeps the ball from hitting the hulls when the wind goes light and the cat drifts over the ball. This is the only reason I ever go that short.
2. A long bridle is better at aligning the boat with the wind. That’s why we have bridles in the first place. But how much is enough?
Yup.
How many of you have ever moored or anchored without a bridle. Simply the line/chain over the bow roller to a strong cleat?
I do it every time. My cat does not sheer from side to side. There is no problem. Plus, it is easy.
Why complicate it?
Mike
Not even an option with the Leopard. It sails a lot.
Where does your anchor leave your boat? Starboard or port bow? Center-front of tramp? Center-aft of tramp?
Hey Mike, I’ve been searching for answers on the web for our particular problem, and this blog posting of yours actually came up although its really not the problem we are having, but might explain why the boat behind you had such a short bridle (I think that was your point, right?) But the problem we are having is with moorings….I hate them, and for good reason. Every time we hook up to one, when the wind stops, our boat rides up over the mooring ball, and the rope between the ball and the mooring rubs under whichever hull is the closest and proceeds to just wear away all our bottom paint. Happened again last night at St. John…..as soon as the wind stopped and the harbor was smooth, the ball went right between the hulls and the mooring was basically behind us as the boat couldn’t turn around because it was basically stuck “between our legs”. The only solution I’ve read about is to bring in the line that hangs off the mooring ball itself all the way as far as we can onto the boat. We have a cleat right next to our windlass and a roller up on our cross bar that we pulled the line over and got it right next to the cleat and looped it around several times with a spare line. What it does is keeps the ball from having enough slack to get on the other side of our hull and use the rope as a sander on our paint. The problem is that the park guy came by and said that we aren’t supposed to hook it up that way…we are messing up the mechanics of the way the mooring is supposed to be used. So, what did you guys do? I figure if a 60 foot mono hull can use that mooring, and they would weigh much more than my little 16,000 pounds, then I shouldn’t be messing up the load to the mooring any worse. Maybe the guy in your picture was just trying to keep the bridle as short as possible so that the ball wouldn’t be able to cross under his hull when ever the wind died down. Btw, we would normally hook up our long bridle to the eye loop on the mooring rope….but that would leave about 20 foot or so between our boat and the ball…and that’s where the problem is coming in.
(sorry if I submitted this question more than once…was having a glitch when I was submitting it)
Hi guys
In certain places with squirrelly winds, and on balls with very long pennants, what you’re describing can be a problem. There are a couple of places that we liked to stay in the BVI where this would always occur. It typically only happens when the wind dies or is extremely light though. If our local knowledge told us that it was likely to occur, we would rig our bridle lines directly to the ball itself (same two lines we normally use), ignoring the pennant altogether. We secure the boat to the pennant as we normally due and then either from the dinghy, or while swimming, change the lines, one by one, to the eye on top of the ball (or some other spot not likely to chafe our lines). The only downside is that if the boat spins around a lot, your lines could get tangled. This has happened to us on at least one occasion. Your park ranger likely wouldn’t approve of this either.
That’s what a swivel is for, to keep the bridle from getting wrapped up and tangled. I don’t suppose most cruisers carry one or use a double bridle much on a monohull. Where we were in Chicago last summer everyone used a double bridle hooked directly to the mooring ball in this way with a swivel as that was the specified equipment. A few folks left out the swivel and their lines would get twisted up to the point that they sometimes broke, and then the boat would drift away into other boats and eventually the seawall where it would often break up and sink if nobody was there to save it. A couple of boats end up sinking this way every year in the harbor we were at. 2015 was a bad year with 3 boats sinking in the harbor because of bridle failures.
A bridle of about 25 feet also acts as a snubber which can reduce the shock loads of an all-chain rode by 2/3. these are not just for cats, but monohulls too can benefit. If dragging on the bottom is an issue in very shallow anchorages then the bridle can be ran back to the midship cleats. To combat the fear of the chain hook coming off there are many improved hook designs that secure the hook to the chain in such a way that it can’t come off.
Mantus anchors recommends a long bridle, and also sells a fairly decent chain hook that won’t fall off the chain when it goes slack.
http://www.mantusanchors.com/snubbersbridles/
I’m lost… what part of this post has anything to do with bridles getting twisted?
For the record, I am, in general, not a fan of swivels. In almost all situations they are unnecessary, and add a potential failure point to the system.