Catamaran Anchor Bridle Design

[Ben]

I just tied in the anchor bridle on my Outremer 42 and I wanted to write up not just my choices but the reasons for them. Any comments and especially criticisms are welcome.

The bridle attaches to padeyes on the two hulls that are D=15 feet apart. Note that D is the important measurement, the boat's 23 foot beam is irrelevant. I use ¾ inch 3-strand twisted nylon line. This choice matches the strength of the line to that of my ⅜ inch chain.

For simplicity I use a single line with an alpine butterfly loop in the center, and the ends tied to the padeyes with bowlines. For extra security I cable tied the bowline tails. You can use a simple overhand knot instead of the butterfly but the butterfly is far superior for reasons you can google. Splices would be stronger than knots but are unnecessary as I explain below. There are many methods for attaching the butterfly to the chain and I will explain mine below.

For now just assume that the chain is pulling the butterfly forward with a force F. Let A be the angle that each leg makes with the crossbeam. Remembering your high school physics you can calculate that the force on each leg of the bridle is
f = F/(2*sinA). Note that as the bridle gets longer the angle A increases and so the force on the bridle decreases. That is: Longer is Stronger.

The unique choice of angle to match the bridle strength to that of the chain is A=45 degrees. Then f = F/Sqrt(2) = .7F. That is, the force on the bridle is about 30% less than the force on the chain. This approximately compensates for the loss of strength due to the knots and so matches the strength of the bridle to that of the chain. If you use splices instead of knots you can get away with a slightly shorter bridle.

With A= 45 degrees, the bridle length is then L= D*Sqrt(2) = 1.4 D. With my D=15 I get L = 21 feet.

To attach the butterfly to the chain I use a Prussik loop tied around the chain with a Klemheist knot then clipped into the butterfly with a carabiner. I find this very secure and very easy to tie and untie. In my experience ordinary chain hooks fall off in light winds. The Wichard hook has a retaining pin but the pin tends to get bent, leaving me hanging over the bow with pliers trying to get free. I have a Mantus, which looks very secure but is a big clunky thing so I never tried it.

For the Prussik loop I tie the ends with my favorite bend, the Zeppelin. You might use a double fisherman or something else. I could tie it to the chain with a Prussik knot but the Klemheist is superior here because the pull is only in one direction. The gripping power of the Klemheist knot is superior to that of a rolling hitch, which is nonetheless preferred by many other people.

To tie to up to a mooring I luggage tie the Prussik loop to the butterfly and clip the carabiner onto the Prussik loop. The carabiner is then passed through the attachment eye on the mooring, and clipped back onto the butterfly. Just unclip the carabiner from the butterfly and you are free of the mooring with all your hardware still attached to the boat.

[tomfl]

Just my quick observations.

It is fairly common to "adjust" the length of one of the bridle lines if there is any difference between the direction of the waves and the direction of the wind; not to mention any current so the bow points into the waves. Sometimes a few inches can make a difference, sometimes it may be a couple of feet or more.

Speaking of waves at times they can throw the boat one way or the other so one side of the bridle is slack and the other side is tight.

While I was a math major as an undergrad it never occurred to me to try and figure out how long or short the bridle should be. On my boat I use the bow cleats. I have noticed that if the bridle lines are too long they can rub against the hull resulting in a risk of chafe of the line and cosmetic damage to the paint/gelcoat. I have also seen cats where the cleats simply will cause chafe no matter the length of the bridle.

Bottom line for me is using a bridle is more of an art than a science.

[ausnp84]

So many knots I never knew existed!!

Thanks for the science on this one - I’d only ever gone for “longer is better” but not thought about the actualy calcs on it. I also use a chain hook but as noted, it just feels like it could slip off quite easily. Attaching the bridle with knots is a much better idea!

N

[chrisr]

we use a chainhook to attach bridle to chain.


however i agree about the risk of such slipping off, so usually take a few turns of the ubiqitious electrical tape around the hook.


this is an easy fix - provided i remember to take the tape with me when i go forward...


now to get my text books out and see what is what with some of the knots mentioned !



happy sailing,

[Dave_S]

I have been considering changing my bridle to nylon. Currently it is an unknown material, black with no detectable stretch.

I feel the stretch will give me a better response to swinging and reduce shock on boat and anchor. The guy who owned the boat before me would not have used this material without good reason which is why I haven't changed it out already.

???

[smj]

I’m not a fan of attaching the bridle to the chain with a knot. On a dark night as your dragging anchor towards the rocks the last thing I want to deal with is trying to untie a wet tight knot so the chain will make its way around the windlass gypsy. Chain hooks can fall of but I’ve had great luck with the Mantus chain hook.

[theDangerz]

Originally Posted by Ben

For the Prussik loop I tie the ends with my favorite bend, the Zeppelin. You might use a double fisherman or something else. I could tie it to the chain with a Prussik knot but the Klemheist is superior here because the pull is only in one direction. The gripping power of the Klemheist knot is superior to that of a rolling hitch, which is nonetheless preferred by many other people.

Thanks for the write up!
Mind if I ask what size line you're using for the prussic loop to equal the strength of the chain and bridle??

[thinwater]

Don't assume that 50% of the load is on each leg. it is normal for boats on Bridles to yaw as much as 30 degrees, and and odd gust can come from to side, placing most of the load on one leg.


Curiously, this is the argument against a bridle that is too long (more than 2x beam). It is too easy for one leg to go slack, and snatching back tight is quite damaging. Steady tension contributes less to fatigue.


Bottom line: A bridle should be designed with a assumption that one leg will occasionally carry the load.


---


Now, "what is the load on the chain" and "how strong should the bridle be" are questions that could each consume a chapter.


My favorite chain hook? My own invention, or more properly, my own variation.

[Ben]

Originally Posted by Ben

I just tied in the anchor bridle on my Outremer 42 and I wanted to write up not just my choices but the reasons for them. Any comments and especially criticisms are welcome.

The bridle attaches to padeyes on the two hulls that are D=15 feet apart. Note that D is the important measurement, the boat's 23 foot beam is irrelevant. I use ¾ inch 3-strand twisted nylon line. This choice matches the strength of the line to that of my ⅜ inch chain.

For simplicity I use a single line with an alpine butterfly loop in the center, and the ends tied to the padeyes with bowlines. For extra security I cable tied the bowline tails. You can use a simple overhand knot instead of the butterfly but the butterfly is far superior for reasons you can google. Splices would be stronger than knots but are unnecessary as I explain below. There are many methods for attaching the butterfly to the chain and I will explain mine below.

For now just assume that the chain is pulling the butterfly forward with a force F. Let A be the angle that each leg makes with the crossbeam. Remembering your high school physics you can calculate that the force on each leg of the bridle is
f = F/(2*sinA). Note that as the bridle gets longer the angle A increases and so the force on the bridle decreases. That is: Longer is Stronger.

The unique choice of angle to match the bridle strength to that of the chain is A=45 degrees. Then f = F/Sqrt(2) = .7F. That is, the force on the bridle is about 30% less than the force on the chain. This approximately compensates for the loss of strength due to the knots and so matches the strength of the bridle to that of the chain. If you use splices instead of knots you can get away with a slightly shorter bridle.

With A= 45 degrees, the bridle length is then L= D*Sqrt(2) = 1.4 D. With my D=15 I get L = 21 feet.

To attach the butterfly to the chain I use a Prussik loop tied around the chain with a Klemheist knot then clipped into the butterfly with a carabiner. I find this very secure and very easy to tie and untie. In my experience ordinary chain hooks fall off in light winds. The Wichard hook has a retaining pin but the pin tends to get bent, leaving me hanging over the bow with pliers trying to get free. I have a Mantus, which looks very secure but is a big clunky thing so I never tried it.

For the Prussik loop I tie the ends with my favorite bend, the Zeppelin. You might use a double fisherman or something else. I could tie it to the chain with a Prussik knot but the Klemheist is superior here because the pull is only in one direction. The gripping power of the Klemheist knot is superior to that of a rolling hitch, which is nonetheless preferred by many other people.

To tie to up to a mooring I luggage tie the Prussik loop to the butterfly and clip the carabiner onto the Prussik loop. The carabiner is then passed through the attachment eye on the mooring, and clipped back onto the butterfly. Just unclip the carabiner from the butterfly and you are free of the mooring with all your hardware still attached to the boat.

@tomfl For simplicity I just use a fixed bridle, for which Outremer has provided padeyes. I thought most people were fine with this but would like to hear from others who also adjust each bridle leg in response to sea conditions.

@Dave_S Yes, definitely nylon! The bridle acts as a snubber and you need the stretch to avoid shock loads.

@smj "I’m not a fan of attaching the bridle to the chain with a knot. On a dark night as your dragging anchor towards the rocks the last thing I want to deal with is trying to untie a wet tight knot"

Your criticism of knots is spot on *for those who use a rolling hitch and even more so for the icicle hitch. But I also own a Mantus and I believe the Klemheist knot is easier to undo than the Mantus, even wet and blindfolded. For the Mantus you have to undo the gate and jiggle a chain link through the zigzag channel. https://www.mantusmarine.com/mantus-chain-hook/
For the Klemheist you just unclip the carabiner and unwrap the 3 wraps from around the chain. https://www.animatedknots.com/klemheist-knot

This is just my opinion I am not trying to sell anything to anyone. Whatever works for you.

@theDangerz *I use ½" *line for the Prusik loop. Since the two sides of the loop pull on the caribiner the strength is doubled, giving the loop the strength of a 1" line. *This is overkill since the chain is only as strong as ¾" line, and it more than compensates for the loss of strength from the knot.

@thinwater You are right that we should consider asymmetric loadings where one leg takes more of the load than the other. But remember that the strength of the line is matched to the strength of the chain, so one leg alone is sufficiently strong to take the entire load, if we disregard any weakening from the knots.

"Don't assume that 50% of the load is on each leg" In fact even in the symmetric case the load on each leg is MORE than 50%. For the A=45 degree angle each leg pulls with a force 70% of the force on the chain. That is, the combined force put out by the bridle components is 40% more than the force on the chain. This is because the legs each waste an extra 20% pulling against each other.

I'll answer your last two questions in reverse order

Q#2: "How strong should the bridle be?
A#2: "The bridle should be exactly as strong as the chain". If the bridle is less strong than the chain then it reduces the strength of your entire anchoring system. If the bridle is stronger than the chain that extra strength won't help you when the chain parts.

Q#1: "What is the load on the chain?"
A#2: This is the important question when sizing the appropriate chain for your boat. But once the chain has been chosen it is irrelevant for designing the bridle. Because… See A#2.

[thinwater]

Originally Posted by Ben

...
@thinwater You are right that we should consider asymmetric loadings where one leg takes more of the load than the other. But remember that the strength of the line is matched to the strength of the chain, so one leg alone is sufficiently strong to take the entire load, if we disregard any weakening from the knots.

"Don't assume that 50% of the load is on each leg" In fact even in the symmetric case the load on each leg is MORE than 50%. For the A=45 degree angle each leg pulls with a force 70% of the force on the chain. That is, the combined force put out by the bridle components is 40% more than the force on the chain. This is because the legs each waste an extra 20% pulling against each other.

I'll answer your last two questions in reverse order

Q#2: "How strong should the bridle be?
A#2: "The bridle should be exactly as strong as the chain". If the bridle is less strong than the chain then it reduces the strength of your entire anchoring system. If the bridle is stronger than the chain that extra strength won't help you when the chain parts.

Q#1: "What is the load on the chain?"
A#2: This is the important question when sizing the appropriate chain for your boat. But once the chain has been chosen it is irrelevant for designing the bridle. Because… See A#2.

Actually, the engineering is far more complex than this.



For instance, the working load rating of nylon, for a given strength, is about 3-5 times lower than steel, depending on who you ask. So matching the strength of the chain is... arbitrary unless the reasoning is explained.

Knowing the load on the chain is the ONLY way you can estimate the fatigue life of the snubber. And the load on the chain is not related to the strength of the chain, unless the sizing basis is clearly stated, load testing has been done, or calculations have been made. It's actually quiet complicated. The load on the chain depends heavily on how much the boat yaws, depth of the water, exposure to waves, and perhaps most importantly, the stretch characteristics of the snubber.They all make important differences. A long snubber can reduce the load on the chain by 3-5 times vs. all-chain in some circumstances. This is based on testing, not guesswork.

So long as the chain is stoppered to the boat, a bridle which is weaker than the chain does NOT weaken the anchoring system. Not sayin' you whether you want a thin snubber, just sayin' it isn't a show stopper and I know several well-published cruisers that have circled the globe with snubbers that were 1/2 the strength of the chain. In the case of a cat, I would rather the bridle did not fail, because yawing will then become extreme.


Knot weakening does matter. The snubber will fail from either fatigue or chafe, and if it is fatigue, it will most likely be at the knot in my experience. So yes, the bridle is 40% weaker, without the benefit of increased elasticity. There is no reason not to splice a highly stressed line. It's not hard and there are good reasons anchor rodes are always spliced.

[If you want high cycles, you can't load nylon past about 10% of BS]

[Tornadosailing]

On my bridle I used 3/4 three strand nylon, and installed rubber snubbers in each leg. I used a super cheap 316 stainless chain hook, but I had it modified (stole the idea from someone smarter than me) with a small bridge across the back and a small knob on the tip of the hook. I have a piece of shock cord threaded inside the gap under the bridge on the back, and tied so it can be pulled over the knob on the tip. After the hook goes on the chain I simply pull the elastic across the opening and hook it on the small welded knob. This keeps the hook on the chain, and there's no electrical tape.

[Tornadosailing]

Originally Posted by smj

Chain hooks can fall of but I’ve had great luck with the Mantus chain hook.
Attachment 194992

Mantus stopped making that design in anything larger than 1/4 inch. No idea why, but the new design has a little strap that breaks when you pull it backward such as launching or retrieving the bridle.

[GordMay]

Originally Posted by Tornadosailing

Mantus stopped making that design in anything larger than 1/4 inch. No idea why, but the new design has a little strap that breaks when you pull it backward such as launching or retrieving the bridle.

The Mantus website says that their Chain Hook is Available for 1/4,5/16, 3/8 and 1/2 inch chain.
Is this the "new" design?
➥ https://www.mantusmarine.com/mantus-chain-hook/

[Tornadosailing]

Originally Posted by GordMay

The Mantus website says that their Chain Hook is Available for 1/4,5/16, 3/8 and 1/2 inch chain.
Is this the "new" design?
➥ https://www.mantusmarine.com/mantus-chain-hook/

No, that's the old design. Look again at the top of the page on your link. It says in RED BOLD CAPITAL LETTERS that it's only available in 1/4 inch, and then they give nice descriptions of the part they no longer sell.

This is the new one.

https://www.mantusmarine.com/m2-chain-hook/

[thinwater]

Originally Posted by Tornadosailing

Mantus stopped making that design in anything larger than 1/4 inch. No idea why, but the new design has a little strap that breaks when you pull it backward such as launching or retrieving the bridle.

The plastic lock works great with one hand in the 1/4-inch size. Can't be beat in that size. In larger sizes the lock is hard to work and breaks. It just didn't scale-up well.