Catamaran Anchor Bridle Design

[Ben]

Originally Posted by thinwater

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]

@thinwater By "strength" I mean Safe Working Load (SWL) sometimes called Maximum Working Load (MWL). As far as I know, this is the universally accepted design criterion for all hardware systems. Manufacturers expend a large amount of engineering effort to specify a SWL for every piece of equipment - chain, rope, blocks, padeyes, cleats, etc. They all have a SWL specification so that the user can quantify the strength. Of course the actual strength varies over the life of the equipment, but unless the piece has been used long and/ or hard it is customary for riggers to use the SWL that has been provided by the manufacturer to quantify strength. It would be unusual, to say the least, for you to do your own testing or even question the testing that has been done.

Similarly an enormous amount of engineering effort has been expended to estimate the forces on a boat at anchor. It is not necessary for us to delve into this research. The bottom line for all this research is that experts will recommend a chain size for typical boat of given size and windage. We have all used these charts in selecting chain for our boats. The recommendation is based on the forces in the most extreme sea conditions vs the strength of the chain. But the chart is all we boaters need to know, we can leave the underlying science to the experts.

Once we have chosen chain size from those charts the chain's strength, i.e. SWL, is known and sets the design criterion for the rest of the system, for instance shackles and swivels, as well as the bridle.

You are right that failure of the bridle might not result in loss of the boat. But in conditions that would snap the bridle you are then very likely to damage the windlass, and the enormous shock loads without a bridle may well part the chain or rip it from your boat.

I too have seen many severely undersized snubbers and the owners are happy with them. They simply have not encountered conditions severe enough for the snubber to fail. I wish them ongoing luck.

[Ben]

Originally Posted by Ben

@thinwater By "strength" I mean Safe Working Load (SWL) sometimes called Maximum Working Load (MWL). As far as I know, this is the universally accepted design criterion for all hardware systems. Manufacturers expend a large amount of engineering effort to specify a SWL for every piece of equipment - chain, rope, blocks, padeyes, cleats, etc. They all have a SWL specification so that the user can quantify the strength. Of course the actual strength varies over the life of the equipment, but unless the piece has been used long and/ or hard it is customary for riggers to use the SWL that has been provided by the manufacturer to quantify strength. It would be unusual, to say the least, for you to do your own testing or even question the testing that has been done.

Similarly an enormous amount of engineering effort has been expended to estimate the forces on a boat at anchor. It is not necessary for us to delve into this research. The bottom line for all this research is that experts will recommend a chain size for typical boat of given size and windage. We have all used these charts in selecting chain for our boats. The recommendation is based on the forces in the most extreme sea conditions vs the strength of the chain. But the chart is all we boaters need to know, we can leave the underlying science to the experts.

Once we have chosen chain size from those charts the chain's strength, i.e. SWL, is known and sets the design criterion for the rest of the system, for instance shackles and swivels, as well as the bridle.

You are right that failure of the bridle might not result in loss of the boat. But in conditions that would snap the bridle you are then very likely to damage the windlass, and the enormous shock loads without a bridle may well part the chain or rip it from your boat.

I too have seen many severely undersized snubbers and the owners are happy with them. They simply have not encountered conditions severe enough for the snubber to fail. I wish them ongoing luck.

@thinwater I'm curious: how would you size, say, a swivel for your anchor? I just match the SWL of the swivel to that of the chain and I"m done. Do you feel I am too simplistic and that this is a comlplex problem requiring knowledge of the force on the chain in various scenarios and the chain's fatigue history? I am sure you have solved this problem How did you do it?

[Palarran]

Originally Posted by tomfl

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.

Is it? I never have and really haven't heard anyone else doing that. It would be nearly impossible to adjust mine in any strong wind and if the wind is light, never saw the need.

[thinwater]

Originally Posted by Ben

... Similarly an enormous amount of engineering effort has been expended to estimate the forces on a boat at anchor. It is not necessary for us to delve into this research....

Show me a source. please. I know a few, but "huge effort" and not necessary stikes me as hand waving.


SWL, MWL, and WLL are three different things. Often they are used interchangeably, but this is incorrect. OSHA uses the WLL for nearly everything.

[thinwater]

Originally Posted by Ben

@thinwater By "strength" I mean Safe Working Load (SWL) sometimes called Maximum Working Load (MWL). [No, that is not clear. 3/8" chain and 3/4-inch rope are close in breaking strength but very different in WLL. I think you meant strength.] [SWL is basically obsolete. OSHA uses WLL, as do nearly all manufactures.]As far as I know, this is the universally [No, safety factors differ] accepted design criterion for all hardware systems. Manufacturers expend a large amount of engineering effort to specify a SWL for every piece of equipment - chain, rope, blocks, padeyes, cleats, etc. They all have a SWL specification so that the user can quantify the strength. Of course the actual strength varies over the life of the equipment, but unless the piece has been used long and/ or hard it is customary for riggers to use the SWL that has been provided by the manufacturer to quantify strength. It would be unusual, to say the least, for you to do your own testing or even question the testing that has been done.

Similarly an enormous amount of engineering effort has been expended to estimate the forces on a boat at anchor. [Please post a link. I know of a few, but "enormous is an exaggeration.]It is not necessary for us to delve into this research. The bottom line for all this research is that experts will recommend a chain size for typical boat of given size and windage. We have all used these charts [Which chart? What is the standard reffernce?] in selecting chain for our boats. The recommendation is based on the forces in the most extreme sea conditions vs the strength of the chain. But the chart is all we boaters need to know, we can leave the underlying science to the experts.

Once we have chosen chain size from those charts the chain's strength, i.e. SWL, is known and sets the design criterion for the rest of the system, for instance shackles and swivels, as well as the bridle.

You are right that failure of the bridle might not result in loss of the boat. But in conditions that would snap the bridle you are then very likely to damage the windlass [would there not be a secondary chain stopper? I always have.], and the enormous shock loads without a bridle may well part the chain or rip it from your boat. [The "chart" is, in fact, based on no-snubber, so that is not likely.]

I too have seen many severely undersized snubbers and the owners are happy with them. They simply have not encountered conditions severe enough for the snubber to fail. I wish them ongoing luck.

Just sayin', there are more factors. Matching WLL is a good starting point. But in that case, by your own arguments, you need splices.

[GordMay]

Originally Posted by thinwater

Show me a source. please. I know a few, but "huge effort" and not necessary stikes me as hand waving.
SWL, MWL, and WLL are three different things. Often they are used interchangeably, but this is incorrect. OSHA uses the WLL for nearly everything.

From ABYC H-40 “ANCHORING, MOORING, AND STRONG POINTS”
➥ http://www.epcomarineproducts.com/pdfs/H-40.pdf


Appendix 1: “Working Load Limits for Anchor Rodes”
➥ WLL for Anchor Rodes - ABYC Section H-40, Table 2 Cruisers & Sailing Photo Gallery


Table 1: “Design Loads for Deck Hardware”
➥ Design Loads for Deck Hardware - ABYC Section H-40, table 1 Cruisers & Sailing Photo Gallery

[Ben]

Originally Posted by GordMay

From ABYC H-40 “ANCHORING, MOORING, AND STRONG POINTS”
➥ http://www.epcomarineproducts.com/pdfs/H-40.pdf


Appendix 1: “Working Load Limits for Anchor Rodes”
➥ WLL for Anchor Rodes - ABYC Section H-40, Table 2 Cruisers & Sailing Photo Gallery


Table 1: “Design Loads for Deck Hardware”
➥ Design Loads for Deck Hardware - ABYC Section H-40, table 1 Cruisers & Sailing Photo Gallery

@GordMay Thanks for the references. In addition, ABYC is not the only organization addressing this. Noble Denton describes their efforts like thissee*https://www.dnvgl.com/rules-standard...delines.html):


"The Noble Denton rules and technical guidelines are developed through extensive research and development, the know-how and experience of our engineers and the close feedback of our worldwide network of technical experts. The rules and guidelines represent the current state of scientific development and industry practice."


For their guidlines on yacht anchoring you can download for free DNVGL-RU-YACHT-Pt3Ch8.pdf from their website. Then see Section 1 for anchoring and mooring.


We boaters just use simplified charts like the one at the end of this article from West Marine: https://www.westmarine.com/WestAdvis...An-Anchor-Rode.

[noelex 77]

Ben, you have obviously thought carefully about the strength issue of your bridle, but I wonder if your solution of 2 x 19mm nylon (the length is unclear) has enough stretch for your vessel when anchored in stronger conditions?

It is not normally practical to achieve adequate stretch and at the same time match the SWL of the chain, especially if you consider the nylon snubber’s strength in real world conditions, when wet, no longer new, and with some chafe.

So it is more common to size the snubber for the more important criterion of achieving adequate stretch, and accept that this will mean the snubber will be weaker than the chain. The snubber should obviously be strong enough to cope with normal anchoring loads, but the high reserve capacity incorporated into chain selection is not normally duplicated with the snubber, because this is impractical. The chain provides the ultimate reserve strength if the snubber breaks.

Snubbers do occasionally break so a secondary system (for example a chain stopper or short strop) for protecting the windlass is essential. Personally I would take this precaution even if the snubber had theoretically the same SWL as the chain.

[Ben]

Originally Posted by noelex 77

Ben, you have obviously thought carefully about the strength issue of your bridle, but I wonder if your solution of 2 x 19mm nylon (the length is unclear) has enough stretch for your vessel when anchored in stronger conditions?

It is not normally practical to achieve adequate stretch and at the same time match the SWL of the chain, especially if you consider the nylon snubber’s strength in real world conditions, when wet, no longer new, and with some chafe.

So it is more common to size the snubber for the more important criterion of achieving adequate stretch, and accept that this will mean the snubber will be weaker than the chain. The snubber should obviously be strong enough to cope with normal anchoring loads, but the high reserve capacity incorporated into chain selection is not normally duplicated with the snubber, because this is impractical. The chain provides the ultimate reserve strength if the snubber breaks.

Snubbers do occasionally break so a secondary system (for example a chain stopper or short strop) for protecting the windlass is essential. Personally I would take this precaution even if the snubber had theoretically the same SWL as the chain.

@noelex 77 Thanks you make some very good points. I've never seen windlass protection but it makes a lot of sense. You are also right that I need to consider shock absorption. For my A=45 degrees design choice and 15 feet between padeyes the bridal is 21 feet long. So I have two 10.5 foot legs of 3/4 inch nylon. Does this seem adequate as a snubber for a 10 ton cat?

[noelex 77]

Originally Posted by Ben

Does this seem adequate as a snubber for a 10 ton cat?

For severe conditions a starting point would be around 10m (30 feet) of perhaps 14mm (5/8 inch) nylon (owners of similar vessels will be able to give you a better idea of the required thickness). This will provide many times the stretch of your proposed 3.5m (10 feet) of 19mm (3/4 inch) nylon.

10m of snubber can be unwieldy for everyday conditions. A shorter snubber is fine for milder conditions, although if you can lead the snubber via turning blocks to the midship or rear cleats the longer snubber can be used routinely, which avoids having to make alterations in the middle of the night if conditions change unexpectedly.

Snubbers can in rare cases have too much stretch, so some experimentation is helpful. A starting point would be to rig both your 3.5m 19mm snubber and the snubber proposed above. In strong condition try alternating between the options. I think you will be surprised at the difference in shock loading between the two options. Even without a load cell, the reduction in peak loads on the anchor should be obvious. From here you can experiment with a bit more or less stretch to determine what works best for your vessel.

[Sos]

On our cat we have a long snubber that we can adjust. We have a bow roller on each hull with two cleats behind each roller. Very easy to fit the snubber and adjust leg length to suit conditions.

[Palarran]

Originally Posted by Sos

On our cat we have a long snubber that we can adjust. We have a bow roller on each hull with two cleats behind each roller. Very easy to fit the snubber and adjust leg length to suit conditions.

I thought about it after posting and it must depend on the size boat. Many new catamarans don't even allow you to access the bridle except the hook from the tramp or anchor locker.

[Ben]

Originally Posted by Sos

On our cat we have a long snubber that we can adjust. We have a bow roller on each hull with two cleats behind each roller. Very easy to fit the snubber and adjust leg length to suit conditions.

@Sos For versatility, I think we have a winner!

[Ben]

Originally Posted by noelex 77

For severe conditions a starting point would be around 10m (30 feet) of perhaps 14mm (5/8 inch) nylon (owners of similar vessels will be able to give you a better idea of the required thickness). This will provide many times the stretch of your proposed 3.5m (10 feet) of 19mm (3/4 inch) nylon.

10m of snubber can be unwieldy for everyday conditions. A shorter snubber is fine for milder conditions, although if you can lead the snubber via turning blocks to the midship or rear cleats the longer snubber can be used routinely, which avoids having to make alterations in the middle of the night if conditions change unexpectedly.

Snubbers can in rare cases have too much stretch, so some experimentation is helpful. A starting point would be to rig both your 3.5m 19mm snubber and the snubber proposed above. In strong condition try alternating between the options. I think you will be surprised at the difference in shock loading between the two options. Even without a load cell, the reduction in peak loads on the anchor should be obvious. From here you can experiment with a bit more or less stretch to determine what works best for your vessel.

@noelex77 Your point is very well taken.

To quantify all this I compared two snubbers of different lengths and diameters but the same elastic properties (i.e. Young's modulus) when both are subject to the same shock load. That is, they both stop a *boat lurching away at the same mass and velocity. Then the ratio of peak tensions in the two lines is
T2/T1 = (D2/D1) * Sqrt(L1/L2), where T is tension, D is line diameter and L is length. I don't have a reference for this formula, I just got it from Hooke's Law when the snubbers absorb the energy from the lurching boat.

Strength is proportional to cross-sectional area, so S2/S1 = (D2/D1)^2.

Here is what I get:

1.Going from a ¾" 10.5 foot line to a *¾" 30 foot line reduces the shock load by 40% with no loss of strength.

2. Going from a ¾" 10.5 foot line to a *⅝" 30 foot line reduces the shock load by 50% with a 30% loss of strength.

Interestingly, the loss of strength in Case 2 is more than compensated for by the reduction in shock load. This snubber is weaker under a steady pull, but stronger in resisting shocks.

Any comments and especially criticisms of this analysis are more than welcome.

[tomfl]

Originally Posted by Sos

On our cat we have a long snubber that we can adjust. We have a bow roller on each hull with two cleats behind each roller. Very easy to fit the snubber and adjust leg length to suit conditions.

Well I just have two cleats. Depending on how long the bridle lines are at times I have used a couple of feet of fire hose to eliminate chafe; and usually with some type of cloth to protect the hull.