How to size the thickness/strength of a bridle

[team karst]

Traditionalist. ☝️

[fxykty]

Originally Posted by meirriba

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.

And to be clear, the force is not anywhere close to the displacement of the boat! So your boat’s displacement of 10t is not relevant. Take a look at the maximum holding power of your anchor and match that for your storm bridle. For more usual anchoring you can use a much lower force figure.

With an all chain rode it takes about 25 knots to remove most of the catenary, so above 25 knots you will be relying solely on your bridle to absorb the shocks. That helps establish a ballpark force range for your regular bridle.

[Wandering1]

We sail a 44ft heavy (18 tonne) cruising ketch and anchor in the Caribbean normally 9 months of the year
For anchoring we have two snubbers. One for normal anchoring and one for high wind/wave anchoring. Being a monohull, we have no need for a bridle. We need a device to take shock off the anchor. A snubber needs to be sufficiently lightweight to stretch otherwise it won't take any shock load. Our normal snubber is 9 metres of 14mm anchorplait. This attaches to a middle deck cleat and runs across the deck via a block and over the second roller on the bow. A long length of hose acts as chafe protection over the roller. This lightweight snubber provides comfort in most situations.
If wind us forecast over 30 kts we would swap to the heavier snubber. This is 12 metres of 16mn three strand. We don't run this over the deck but fix this to the forward cleat. The heavier snubber doesn't stretch as much so more length is required. We back both snubbers up with a short length of dyneema through the chain incase a snubber fails. This ensures we don't put load through the windlass.
If you have a lightweight boat prone to veering a snubber is not just for comfort. It will vastly reduce the load on the anchor at the end of the veer. it will stop you sailing you anchor out!
It has to stretch. Don't be surprised with a 10m snubber if you see 1m of stretch. This would be about right when the wind is blowing some or waves are making the boat pitch a lot.
Happy anchoring

[MathiasW]

Originally Posted by fxykty

With an all chain rode it takes about 25 knots to remove most of the catenary, so above 25 knots you will be relying solely on your bridle to absorb the shocks. That helps establish a ballpark force range for your regular bridle.

That really depends on the water depth and cannot be stated in this generality as done here. The deeper the water, the harder it is for the catenary to disappear - provided there was enough chain to begin with.

[MathiasW]

Originally Posted by Wandering1

We sail a 44ft heavy (18 tonne) cruising ketch and anchor in the Caribbean normally 9 months of the year
For anchoring we have two snubbers. One for normal anchoring and one for high wind/wave anchoring. Being a monohull, we have no need for a bridle. We need a device to take shock off the anchor. A snubber needs to be sufficiently lightweight to stretch otherwise it won't take any shock load. Our normal snubber is 9 metres of 14mm anchorplait. This attaches to a middle deck cleat and runs across the deck via a block and over the second roller on the bow. A long length of hose acts as chafe protection over the roller. This lightweight snubber provides comfort in most situations.
If wind us forecast over 30 kts we would swap to the heavier snubber. This is 12 metres of 16mn three strand. We don't run this over the deck but fix this to the forward cleat. The heavier snubber doesn't stretch as much so more length is required. We back both snubbers up with a short length of dyneema through the chain incase a snubber fails. This ensures we don't put load through the windlass.
If you have a lightweight boat prone to veering a snubber is not just for comfort. It will vastly reduce the load on the anchor at the end of the veer. it will stop you sailing you anchor out!
It has to stretch. Don't be surprised with a 10m snubber if you see 1m of stretch. This would be about right when the wind is blowing some or waves are making the boat pitch a lot.
Happy anchoring

I very much like this! A good snubber / bridle should stretch by 1 metre or more in heavy wind, if it is to provide any relief for the shock loads induced by waves and gusts.

Here is a tool I created to see how effective snubbers can be, particularly in shallow water and large waves and gusts (estimated by vessel velocity at anchor). An EXCELLENT snubber is defined by me as stretching by 1.6 metres at 8 Beaufort of wind (40.2 kn). In Expert mode you can define the snubber stretch yourself.

https://anchorchaincalculator.com

play around with your vessel characteristics and use, i.e., a vessel velocity at anchor of 0.6 kn. Stronger wind, so perhaps 30 kn. Then use different snubbers at anchor depths of 3, 5, and 9 metres and compare anchor loads. You will find a massive difference!

[fxykty]

Originally Posted by MathiasW

I very much like this! A good snubber / bridle should stretch by 1 metre or more in heavy wind, if it is to provide any relief for the shock loads induced by waves and gusts.



Here is a tool I created to see how effective snubbers can be, particularly in shallow water and large waves and gusts (estimated by vessel velocity at anchor). An EXCELLENT snubber is defined by me as stretching by 1.6 metres at 8 Beaufort of wind (40.2 kn). In Expert mode you can define the snubber stretch yourself.



https://anchorchaincalculator.com



play around with your vessel characteristics and use, i.e., a vessel velocity at anchor of 0.6 kn. Stronger wind, so perhaps 30 kn. Then use different snubbers at anchor depths of 3, 5, and 9 metres and compare anchor loads. You will find a massive difference!

I’ve been playing around with the website but can’t figure out how to apply the bow load figures in daN or kp to selecting a snubber/bridle line with break force in kg. Is there a reasonable conversion that can be used?

[MathiasW]

Originally Posted by fxykty

I’ve been playing around with the website but can’t figure out how to apply the bow load figures in daN or kp to selecting a snubber/bridle line with break force in kg. Is there a reasonable conversion that can be used?

yeah, sorry, I need to add to this tool...

If you know your snubber characteristics from a data sheet - great! For now, you need to do a little calculus to translate the data sheet figures to the tool:

I assume the data sheet will say, the snubber stretches for ds % at a load of Ls, where this load is often given as a fraction of the breaking load. Like 15% or 30% of it.

In the first step, you need to work out the total stretch of your snubber at this nominal load. So, if the snubber is 8 metres long, it will be dst = 8 * ds. (make sure you punch in 15% as 0.15, etc.)

Then use the tool - with your vessel characteristics already punched in - to work out the anchor load at 40.2 kn of wind (upper end of 8 BFT), 0 anchor depth, and 0 vessel velocity at anchor, so no swell. Take a note of this value and call this value al8BFT.

With this you work out the snubber stretch @ 8 Beaufort that the tool expects in Expert Mode as:

snubber stretch @ 8 Beaufort = al8BFT / Ls * dst

So, it is simply the proportional stretch of the nominal stretch, using your vessel anchor load as reference.

Admittedly not the easiest for now, but this is where it is...

Thanks for coming back to me on this!

Cheers

Mathias

[Allan Corder]

Hi all. I'm a new member. Sorry to join in with further question and thanks for previous info. I have a 38ft (11.5m) cat weighing 7 to 8 tons. She has 27kg Manson supreme bower, 20 m of 10mm chain spliced to 60 m of 16mm 3 strand nylon. I want to improve my bridle arrangements (don't want to rig one each time I anchor) and improve general security. I plan to add a high strength shackle to the slot on the Manson Supreme and then use 12 mm high grade 3 strand nylon for the bridle arms. Do others think that I should go to 14mm or even 16 mm. Thanks for any thoughts. Allan, Toucan.

[team karst]

I just replaced my 5/8” 3 strand bridal. Similar boat. It sustained wear at two points. At the chain hook, from abrasion on the bottom; and at the whisker lines when wind counters the current. I am happy with the 5/8.

[Shrew]

My vessel weighs around:

9.9 metric tons
11 US tons
22,000 lbs

I use a 5/16" (8mm) G40 chain and 5/8" (16mm) 8-plait snubbers.

[Shrew]

Originally Posted by Allan Corder

I have a 38ft (11.5m) cat weighing 7 to 8 tons. She has 27kg Manson supreme bower, 20 m of 10mm chain spliced to 60 m of 16mm 3 strand nylon. I want to improve my bridle arrangements (don't want to rig one each time I anchor) and improve general security. I plan to add a high strength shackle to the slot on the Manson Supreme and then use 12 mm high grade 3 strand nylon for the bridle arms.

I'm not clear what you mean by "I don't want to rig the snubber each time"


I'm not clear what you're describing. Are you planning on adding the snubber directly to the shackle on the anchor slot??

Personally, I wouldn't use the anchor slot as my anchor rode attachment. That slot is handy as a second trip line to release the anchor if it gets fouled.

I would not want the vessel to reverse in tide or wind and pull the anchor out backwards when the shackle slides down the slot to the wrong end.

[phorvati]

City of Newport has pretty good guidelines.
You would think that a city with type of tradition/history and concentration of vessels in one small harbor would have figured it out by now.


https://library.municode.com/ri/newp...PL_CH12.28CIHA

[captstu]

Everything is a trade. Several years ago my 43' Voyage catamaran rode out a hurricane that destroyed most of the sister ships in the anchorage.

I had a tandem anchor, a 50 kg Bruce with a 37# Fortress shackled to the front - looking toward the boat, the Fortress was first, 6' of chain, the Bruce, 100' of chain and the bridle, 5/8" nylon. The veer drove the boat over the anchor tripping the Bruce, the Fortress held. The resultant sudden stop broke both the primary and storm bridles, pulled the windless out of its mount (once the slack was lost with the broken bridles), and the whole mess jammed in the hawse pipe and held the boat.

The chain (5/16" G4) stretched significantly but held the boat.

So, had the bridle been stronger, I believe the attachment points (beam end factory eyes for the main and midship cleats for the storm bridle) would have broken, causing significant damage. Breaking the bridle saved the boat from a worse situation.

So, if you upsize your bridle, ask yourself: 1) will the anchor hold (My Fortress was destroyed and replaced under warranty). 2) will the chain survive? 3) are you lucky

[alamoana]

48’ cat with 3/8” G43 chain, Rocna 70#, 5/8” Octoplait nylon bridle that broke once, swapped the 5/8” for same. I do have a 3/4” octoplait bridle for stronger winds. I believe you are fine. Like pointed above, if too strong something else will break or you will have very little elasticity that you need!

[Allan Corder]

Hi all. Thanks for all the helpful comments. Just to clarify, the Manson Supreme has a slot along the shank, for tripping the anchor if necessary. I am not 100% happy with the chain to nylon splice on the main rode, so I am going to get a super strong shackle for the slot and attach the bridle arms to this, so I have 2 points to the anchor. Long bridle arms, I know and unconventional yes. More to tangle up yes, but if I keep lines taught when weighing anchor, shouldn't be a hassle and I've got ready made up trip lines. After considering your comments and the sailing I do and consulting with Richard Woods, I've decided on 12mm nylon...good and stretchy. When I get the boat back in the water, I can report back but that won't be very soon. ps. Manson Supreme is a great anchor. Thanks again.