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Old 14-08-2016, 06:05   #31
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Re: anchor bridle length

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Originally Posted by belizesailor View Post
I've yet to see you post any information actually supporting your position of 2.5. Calling others stupid does nothing to add to the credibility of your statement.

How much experience do you have actually anchoring/mooring cats?
The use of "stupid" was in response to another poster calling me stupid. Sorry you missed that.

As for the 2.5 times the beam, there is plenty on info on that. The best info available closely related to the topic is found in the discussions of Jordan Series Drogues. The principles are the same as are the design loads. Start here: Jordan Series Drogue
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Old 14-08-2016, 06:05   #32
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Re: anchor bridle length

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Originally Posted by kmacdonald View Post
Looks like they agree with me for the most part.
The article is actually about snubbers, not bridles. There is a difference, you know?

And the OP did not ask about storm conditions.
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Old 14-08-2016, 06:07   #33
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Re: anchor bridle length

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Originally Posted by belizesailor View Post
The article is actually about snubbers, not specifically about bridles. There is a difference, you know?
For all practical purposes the difference is in terminology only, you know?

The person that posted the link to snubbers thinks so also.
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Old 14-08-2016, 06:09   #34
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Re: Anchor bridle length

What the PD article did not say (edited for length-the study was long) or that has been overlooked in posts here is that:

  • If climbing rope is used the snubber can be shorter (more stretch).
  • The example cruisers often rig the snubber shorter, either by tying off to bow cleats or by anchoring the mid-ships cleats, to keep the apex off the bottom.
  • The failed cruiser snubbers were not listed. There were many examples of 6-15' snubbers that seemed strong enough (one in Sail Mag last month) but they failed because they were too short to absorb energy and were thus over loaded. In fact, the Sail example was quite obvious; if a snubber fails that is the same diameter as the rope rode suggestion, OBVIOUSLY it is the short length that doomed it, not the size (100 feet of rode would have been safe).
  • About 1.5x beam is generally needed to make the boat sit still. The rest is shock absorption.
  • The full length is often only used in winds over 20 knots, which will keep it well off the bottom.
  • There are also good reasons to use a very sort bridle. Sometimes I use only 75% of the beam, to keep a mooring ball from bumping. But the boat will hunt more.
  • Long bridles may seem to touch the bottom, but actual chafe is often limited. It takes VERY little wind to lift nylon plus 1 link of chain, perhaps only 2 knots or any tide. Only if there are vicious rocks.
  • Rig a back-up for severe storms. This is better than too heavy duty to absorb shock.
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Old 14-08-2016, 06:10   #35
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Re: anchor bridle length

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Originally Posted by belizesailor View Post
The article is actually about snubbers, not bridles. There is a difference, you know?

And the OP did not ask about storm conditions.
To design a bridle OR snubber disregarding storm conditions would be incompetent at best. Designs should consider worst possible case scenarios.
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Old 14-08-2016, 06:14   #36
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Re: anchor bridle length

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Originally Posted by kmacdonald View Post
For all practical purposes the difference is in terminology only, you know?

....
Yes, I know, because I've used both many times, on cats and monos.

More than just semantics. Like many things, if they were precisely the same thing then no separate vocabulary would have evolved.
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Old 14-08-2016, 06:33   #37
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Re: Anchor bridle length

Thinwater mentioned climbing rope. Those and arborist rigging line and bull rope make for great anchor lines, mooring pendants, bridles, AND snubbers. Some of them have polyester outer braid and nylon inner braid. Much better pricing than "marine" lines.
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Old 14-08-2016, 10:56   #38
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Re: anchor bridle length

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Originally Posted by StuM View Post
It's all about trigonometry. If you used the suggested 2.5 x beam for each leg, the angle of the bridle leg would only be 11.5 from dead ahead. The bridle would be 70 meters long and the attachment point to your anchor rode would be over 34 meters forward of your bow. Effectively, it would be no better no bridle at stopping you from sailing around at anchor. And in many (most?) cat anchoring situations, your bridle would be constantly dragging around on the bottom abd being abraded. It wouls also provide two more lines in addition to the anchor rode which could to snag on obstructions. That suggestion is just plain stupid!

I'm with Cheechako. You need a bit less than the beam for each leg if it permanenty attached to strong points. If it is detachable and you need to secure it to cleats, them a bit more that the beam for each leg.
I just calculated the hypotenuse of a triangle having half the beam as each side. To figure a ~45 degree angle of pull.
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Old 14-08-2016, 21:36   #39
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Re: Anchor bridle length

One trick distinction between snubber (could be single line) and bridle (2 lines) is how much of the load is on each leg. Do the bridle legs need to carry 100%? Probably not, since geometry will allow some sharing, and stretch will encourage additional sharing when the load is high enough to matter. They will also alternate, so the fatigue life is greater (fewer full load cycles). But they are designed for more than 1/2.

Climbing rope is a broad designation. In all cases that I am aware of, the sailors are using UIAA single dynamic ropes, which are quite different from static (polyester) and hybrid (polyester + nylon) ropes. Unless these are evaluated individually from a stress/strain basis, I'm not sure we know how they perform as compared to 3-strand and UIAA dynamic ropes. My understand based on evaluating a few arborist-type ropes is that there is a HUGE range in energy absorption, depending on intended use by the arborist. That said, they are built for abrasion resistance and there are probably some excellent choices out there. I would look for a rope with UIAA fall ratings.
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Old 14-08-2016, 22:00   #40
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Re: Anchor bridle length

The design load for a series drogue uses 70% of total load for each leg of the bridle. They still recommend using the same diameter as the drogue line or greater. For the bridle attachments points they use 70% for each leg attachment.

A cleat is rarely adequate and makes a bridle with proper attachment points appealing for anchoring also. That would eliminate the chafe issue and pulled cleats in hurricane conditions. The design is fairly simple but some may not care for the looks of a horizontal chain plate bolted to the transom.
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Old 15-08-2016, 14:49   #41
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Re: Anchor bridle length

The question is "how long should an ANCHOR bridal be on a catamaran". You first need to know what purpose an anchor bridal on a catamaran serves to be able to find the answer. There are three reasons for an anchor bridal:
1. To attempt to keep the anchor rode central to the beam of the cat and to try to make the pivot point well in front of the bows.
2. To attempt to keep the load equally on both of the hulls from the forward most possible point and keep the anchor rode load off the windlass.
3. And to prevent the rode (be it chain or rope) from chaffing the hulls.

The third point is the one you need to think about - chain rubbing on the bow/hull area will damage your anti-fouling and then start grinding into the hull. With rope rode chaffing the hull, you will soon lose your anchor.

Points 1 and 2 will look after themselves, irrelevant of the length of the bridal legs. But, to try and prevent point 3 happening, you need to ensure your bridal keeps the rode away from the hulls, especially a cat that is swinging (sailing) with the wind and tide, and thus you need to ensure that the length of each leg of the bridal is around 80% to 90% of the distance between the bows of the cat. This is especially important with cats that have the anchor deployment position (and thus the anchor rode pivoting from the same position) well behind the crossbeam, such as the configuration used on Leopards and many other such cats.

Hope I have put this in an understandable format.
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Old 15-08-2016, 15:22   #42
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Re: Anchor bridle length

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...The third point is the one you need to think about - chain rubbing on the bow/hull area will damage your anti-fouling and then start grinding into the hull. With rope rode chaffing the hull, you will soon lose your anchor.

Points 1 and 2 will look after themselves, irrelevant of the length of the bridal legs. But, to try and prevent point 3 happening, you need to ensure your bridal keeps the rode away from the hulls, especially a cat that is swinging (sailing) with the wind and tide, and thus you need to ensure that the length of each leg of the bridal is around 80% to 90% of the distance between the bows of the cat. This is especially important with cats that have the anchor deployment position (and thus the anchor rode pivoting from the same position) well behind the crossbeam, such as the configuration used on Leopards and many other such cats.

Hope I have put this in an understandable format.
That is an interesting point. Occasionally, in freak wind/tide situations the bridle will rub the hull. But other than mooring balls (which I mentioned), I've never noticed damage, since this only happens in very light conditions.

But worth mentioning. Thanks. On those occasions it is simple enough to shorten up.
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Old 15-08-2016, 16:53   #43
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Catamaran Anchor Bridal & Snubbers

Catamaran Anchor Bridal & Snubbers

We wanted a bit more stretch in the anchor bridal, so we made this one with 20 foot legs and used 5/8 three strand and 19-1/2" long rubber mooring snubbers to prevent snatch loads (wind gusts) from tugging on the anchor.

Added a piece of 42 inch long x 1-1/4 inside diameter clear hose as chain chafe guard. Purchased 43 feet of 5/8 three strand and two rubber mooring snubber at a total cost of $138 usd.

Have been using this anchor bridal & snubbers for years now and the snatch loads from wind gusts is gone and she rides the anchor so much smoother no matter what the wind is
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Old 15-08-2016, 18:43   #44
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Re: Anchor bridle length

Very interesting thread. i was trying to figure this out myself to figure out the lengths of each leg of pendants from mooring ball to cleat on the bow.

Mathematically, I figured one of 2 things. 1. make each leg equal to the beam of the boat, essentially forming an equilateral triangle. 2. Make the distance from the mid point of the beam to the mooring ball be equal to half the width of the beam, e.g. each leg is equal of the 2 right triangles formed by the bridle. So the leg is 1.5 (rounded up a bit) times the half the beam or easier, .75 * beam.

The anchor bridle that came with the boat from the factory is about 1.2x the beam.

Cotemar, great stuff as usual.
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Old 15-08-2016, 20:53   #45
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Re: Anchor bridle length

There is one notable advantage of using a long (about 1 boat length) bridle which is tied off short for mooring balls and in light winds; when it blows you can ease out more scope without retrieving the bridle. For example:

5 feet water, 3 feet bow = 9 feet. 25' chain + 9' bridle = 34' = 4:1 scope.

Now ease out the spare 20' of bridle (adds more than 20' because of the trig).

25' chain + 31' = 56' = 7:1 scope.

Big improvement. I have done this many times when the wind picked up at night and I didn't trust the soft mud. Easy to shorten again if needed.

My opinion is that both groups are right; you need short for mooring balls and crowded places, and long for big wind and bad bottoms.
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