How to size the thickness/strength of a bridle

[workmaster2n]

I've got a 12 meter (40 foot) catamaran. She supposedly weighs 8.5 tonnes, but let's call it 10 to be safe. She came with an anchor bridle which is made out of a decently sized 3 strand line. I'm in the process of making a mooring bridle, but I'm not sure how to choose the proper line in terms of strength of the line.

Is there a rule of thumb for how strong each arm of the bridle should be? If the boat weighs 10 tonnes, does each bridle need to be able to support 10 tonnes? Or is it more like a 10 tonne boat only exerts 50% of it's weight on a mooring/anchor chain?

I guess this could be extended out for all anchoring/mooring situations. How strong does the weakest link in an anchoring/mooring setup need to be in relation to the weight of the vessel?

In case anyone is interested, the reason for the separate anchor bridle is the anchor bridle is fixed length and fixed to eyelets on the bow, whereas I want to be able to adjust the length of the mooring bridle (particularly in low wind conditions so I can trap the can and not have it bang into me all night long).

Thanks!

[chrisr]

have a look at what is the breaking strain of your anchor chain. it will be a whole lot less than the weight of your boat

eg our cat is 7.5mt and our 8mm G40 chain has a breaking load of 4300kg.

look also at the breaking load of any swivels and shackles in the chain system

there is obviously no point in having a bridle which is any more than the already weakest part of the system

but on the other hand, how much force do you think is put on your anchor ? for my boat (a relatively low windage / moderate displacement cat) i figure the maximum load i am likely to see is somewhere in the vicinity of 2000kg.

extreme conditions (> cyclonic) may be more but you've gota stop somewhere

our bridle is 14mm nylon with a breaking strain of 4000kg. there are 2 legs so load is halved but weathering & fraying will subtract as well

hope this helps

cheers,

[workmaster2n]

Thanks for that info chrisr!

I'm new to boats, so all of that is really great info. I don't know the grade of my anchor chain, but I do know it's 10mm. Looking that up online, it looks like that puts me in line for 7-11 mtn in terms of breaking strength. That gives me a starting point to go off of.

Out of curiosity, how did you come up with 2000kg of pulling force on the anchor? Is there a way to check or is it just something that is built up with experience?

If I'm reading your setup correctly, it sounds like each arm of the bridle is pretty much equal to the breaking point of the chain, so all things being equal, if you lost a bridle arm, the system as a whole would still be in the 4000kg breaking point range?

Thanks!

[s/v Jedi]

Originally Posted by workmaster2n

Thanks for that info chrisr!

I'm new to boats, so all of that is really great info. I don't know the grade of my anchor chain, but I do know it's 10mm. Looking that up online, it looks like that puts me in line for 7-11 mtn in terms of breaking strength. That gives me a starting point to go off of.

Out of curiosity, how did you come up with 2000kg of pulling force on the anchor? Is there a way to check or is it just something that is built up with experience?

If I'm reading your setup correctly, it sounds like each arm of the bridle is pretty much equal to the breaking point of the chain, so all things being equal, if you lost a bridle arm, the system as a whole would still be in the 4000kg breaking point range?

Thanks!

We are 25 metric tons, our chain is 6,600 pounds breaking strength and our single line bridle is 5/8” nylon 3-strand. We managed to break 1/2” during a 70 knot squall but never the 5/8”. During hurricanes we use a 3/4” bridle and that has survived everything incl. sustained hurricane force wind and gusts to 100kts.

In your case I would make each leg strong enough to hold the whole boat as often the forces go to just one leg. I don’t think you will ever be able to break a 1/2” nylon 3-strand bridle.

The reason for going small diameter is so that you get more stretch.

[workmaster2n]

Originally Posted by s/v Jedi

During hurricanes we use a 3/4” bridle and that has survived everything incl. sustained hurricane force wind and gusts to 100kts.

The reason for going small diameter is so that you get more stretch.

That's a heck of a lot of wind and thanks for the pointer about smaller size giving more stretch!

[s/v Jedi]

Originally Posted by workmaster2n

That's a heck of a lot of wind and thanks for the pointer about smaller size giving more stretch!

Yes, it sounds like hurricanes are easy so I thought about it and remember we were once hit by the eye of a cat.5 hurricane and the snubber did not survive that. Also, my 5/8” anchor shackle was bent and some chain links cold-welded to the stainless steel anchor sprit.

Only thick steel survives direct hits from major hurricanes. When I mentioned that our 3/4” nylon snubber survived everything, it means category 1 hurricane direct and cat. 2-3 passing at 30-100nm distance.

Sustained hurricane force winds and gusts to 100kts was accurate, but we have been in 120kts sustained and that is where only chain has a chance of surviving.

[chrisr]

Originally Posted by workmaster2n

Thanks for that info chrisr!

I'm new to boats, so all of that is really great info. I don't know the grade of my anchor chain, but I do know it's 10mm. Looking that up online, it looks like that puts me in line for 7-11 mtn in terms of breaking strength. That gives me a starting point to go off of.

Out of curiosity, how did you come up with 2000kg of pulling force on the anchor? Is there a way to check or is it just something that is built up with experience?

If I'm reading your setup correctly, it sounds like each arm of the bridle is pretty much equal to the breaking point of the chain, so all things being equal, if you lost a bridle arm, the system as a whole would still be in the 4000kg breaking point range?

Thanks!

respectfully : the chain shown on the (american) jimmy green website is all high tensile. good gear but if you don't know what grade you have, in australia, i think you'd be safe to assume it's your standard G30 grade L

for 10mm this has a swl of abt 1200kg and a breaking load of about 4 times that (maybe)

there are a heap of web resources that can give you data on anchor chain loading

there some good info here

https://www.practical-sailor.com/sai...d-on-your-rode
https://www.mantusmarine.com/mantus-...e/rode-sizing/
https://www.passagemaker.com/trawler-news/hold-fast

there are so many variables (boat type / size / weather / sea) that it is next to impossible to standardise anything. the The ABYC Horizontal Working Load (lbs) Table has a go but all these will need a lot of adapting to your boat and situation.

i agree that each arm of the bridle will be close to the chain in BL...when NEW. however imho the rope ages a lot quicker than the chain so you eat into this safety margin quickly and need to replace it a lot more often.

cheers,

[captstu]

My 43' Voyage came with a plaited 18 mm anchor bridle. The boat lurched in a rough anchorage - so I went down to 5/8" bridle - it also was spliced to hard points.

I made a second 3/4" bridle for use in heavy weather - I only needed it once.

A hurricane hit that was predicted to be strong so I secured the Voyage and went to a safe house well inland.

The storm hit and the eye passed over the anchorage causing a sudden veer when the boat saw a 180 degree wind change. The anchor was a 50 kg Bruce with a FX-17 Fortress secured to the trip point on the front of the Bruce. The Bruce came out of the bottom during the veer, the Fortress bent into a pretzel but held.

All four strands of both anchor bridles (one shackled on, one bent on with a rolling hitch) snapped.

The chain broke its security lashing, pulled the winless up from the moundings and the entire mess jammed in the hawse pipe - AND HELD THE BOAT WIthOut OTHER DAMAGE.

I replaced the 5/16 ht chain, replaced the bridles, repaired the windless mounting and reset everything exactly as it was in an equally strong storm ever hit again.

Almost every other boat in the anchorage was stacked on shore - most totaled.

I was lucky - but I was also well prepared. A good combination.

Lession, it isn't the strength of the chain or line that is the ultimate test. The force is the MV(squared) with it mostly driven by the wind as it suddenly changes direction.

The most important think I learned is the primary bridle makes the boat ride uncomfortably at anchor if it is to large.

[Hudson Force]

5/8" three-strand nylon sounds appropriate for your application. The great majority of snubber and bridle failure is to to chafe and not the breaking strength of the undamaged line. When subject to hurricane conditions, my practice was to have a second and third anchor bridle in place to take the load if the primary failed. 'never got past the second in my experiences, but I haven't anchored out in anything more than Cat I and in very well selected locations.

[meirriba]

Originally Posted by workmaster2n

I've got a 12 meter (40 foot) catamaran. She supposedly weighs 8.5 tonnes, but let's call it 10 to be safe. She came with an anchor bridle which is made out of a decently sized 3 strand line. I'm in the process of making a mooring bridle, but I'm not sure how to choose the proper line in terms of strength of the line.

Is there a rule of thumb for how strong each arm of the bridle should be? If the boat weighs 10 tonnes, does each bridle need to be able to support 10 tonnes? Or is it more like a 10 tonne boat only exerts 50% of it's weight on a mooring/anchor chain?

I guess this could be extended out for all anchoring/mooring situations. How strong does the weakest link in an anchoring/mooring setup need to be in relation to the weight of the vessel?

In case anyone is interested, the reason for the separate anchor bridle is the anchor bridle is fixed length and fixed to eyelets on the bow, whereas I want to be able to adjust the length of the mooring bridle (particularly in low wind conditions so I can trap the can and not have it bang into me all night long).

Thanks!

I would refer only to one point of your post.
Once you decide what is the force you expect to counter with the bridle, you should use a line where each leg of the bridle can withstand the total. Not 50%!.
While in most cases the pulling force will be split between the two legs, in many cases the wind will twist the boat and the full force will be on one side only until the boat swivels back head to wind.

[MathiasW]

There are a couple of online tools and mobile apps that help you calculate the anchor load, once you know the windage area of your vessel, or have at least an estimate for it. Some even allow to include swell in the analysis.


Free, but very powerful online tool by Bjarne:
http://svamanda.dk/anchor/intro

Free, but very simple Anchor Chain Simulator for Android:
https://play.google.com/store/apps/d...ae.anchorchain

Not free, but rather sophisticated AnchorChainCalculator for iOS and Android:
https://play.google.com/store/apps/d...haincalculator
https://apps.apple.com/us/app/anchor...r/id1533741243

The latter is by me .

All of these tools will calculate the anchor load given wind force and windage area of vessel, and some, like Bjarne's tool or mine, will also cover the effect of snubbers and bridles. (I am currently working on extending my snubber model to be non-linear if needed, which is more realistic.)

If you want to read more about anchor chains, snubbers and bridles, here is a long write-up by me (in English, despite the German words in the link... ):
https://trimaran-san.de/die-kettenku...atiker-ankert/

There are some links to the work done by Jonathan Neeves on snubbers and bridles, which you definitely should read, if this topic interests you.

Cheers, Mathias

[Cheechako]

I had a 5/8" 3 strand bridle for my 42 ft catamaran. In a hard blow it's surprising how much even that stretches.
I previously had a 1/2" bridle. It developed "knuckles" after a hard blow.
For a mooring ball, I might want 3/4" due to possible chafing etc.

Your cleat size may drive if you can use 3/4" though.

[phorvati]

Originally Posted by workmaster2n

I've got a 12 meter (40 foot) catamaran. She supposedly weighs 8.5 tonnes, but let's call it 10 to be safe. She came with an anchor bridle which is made out of a decently sized 3 strand line. I'm in the process of making a mooring bridle, but I'm not sure how to choose the proper line in terms of strength of the line.

Is there a rule of thumb for how strong each arm of the bridle should be? If the boat weighs 10 tonnes, does each bridle need to be able to support 10 tonnes? Or is it more like a 10 tonne boat only exerts 50% of it's weight on a mooring/anchor chain?

I guess this could be extended out for all anchoring/mooring situations. How strong does the weakest link in an anchoring/mooring setup need to be in relation to the weight of the vessel?

In case anyone is interested, the reason for the separate anchor bridle is the anchor bridle is fixed length and fixed to eyelets on the bow, whereas I want to be able to adjust the length of the mooring bridle (particularly in low wind conditions so I can trap the can and not have it bang into me all night long).

Thanks!

Size the bridal to the morring block. Or oversize. It's easier to do tucks on a larger 3 strand rope.

[team karst]

I use the forward cleats to adjust my bridal on moorings or settled wx. Mark them with a sharpie. Storm setting is all out.
5/8” on my 38 cat; FWIW

[s/v Jedi]

Originally Posted by team karst

Mark them with a sharpie.