Catamaran Anchor Bridle Design

[tomfl]

Originally Posted by Palarran

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.

You obviously have heard of it when you read my comment. I have seen it posted in threads here. Often in threads about wrap. If you need a long winded explanation here is a link.

As for adjusting in strong wind it is trivial to let a little line out; probably much harder to pull some in but I am way too lazy to try that.

[thinwater]

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

Exactly. But Does anyone here no the design basis of the rules? Not easy to find and a little hard to understand without knowing that. Multiple investigators have reported that actual loads are 3-5 times lower, because ABYC rules are based on specific worst case assumptions.

[thinwater]

Originally Posted by Ben

@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.

Both links are broken.

[thinwater]

Originally Posted by Ben

@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.

Is this the calculation you used? What was the calculated EN number?

https://rules.dnvgl.com/docs/pdf/DNV...CHT-Pt3Ch8.pdf

[Palarran]

Originally Posted by tomfl

You obviously have heard of it when you read my comment. I have seen it posted in threads here. Often in threads about wrap. If you need a long winded explanation here is a link.

As for adjusting in strong wind it is trivial to let a little line out; probably much harder to pull some in but I am way too lazy to try that.

Your right, I have heard of it, for monohulls. Which is what your link describes. As for reading about cat sailors adjusting their bridle length here, I honestly don't recall that. I'd again say that it has to be a size of cat situation. My bridle isn't on a cleat, it's attached to the front crossbeam. From observation most of the, lets say above 40', cats have welded eyelets at the cross beam and the bridle is connected to that with a shackle.

As to the OP's question about bridle design and stretch, this is just my opinion, the the stretch of my bridle has very little effect to dampen the pull of the boat compared to the weight and friction of the chain moving through the water and off the bottom. If I wanted to dampen the movement more, rather than worrying about stretch, I'd A) anchor in deeper water, and B) let much more chain out after attaching the bridle. For B, you can let out enough chain in order to create a belly, or a lot and create a sort of kellet. If your in 30' of water, let an extra 40' of chain out after the bridle. Of course I'd only do this if in some serious wind - over 40 knots. We also never anchor, even in mild conditions, without a short snubber in the anchor locker to protect the windlass. It's just too easy to put on and we use it constantly when setting the anchor anyway. Which is another subject and point, but we don't attach the bridle to set the anchor, just the snubber. It's much easier to observe if the anchor is dragging that way, for us.

[Ben]

Originally Posted by thinwater

Is this the calculation you used? What was the calculated EN number?

https://rules.dnvgl.com/docs/pdf/DNV...CHT-Pt3Ch8.pdf

I personally did no calculation. 3/8" chain was recommended by the simplified charts, which also recommend 3/4" line. The rule of thumb is that chain size is doubled to get the matching line diameter. Tables of SWL show a rough, though not exact match when line size is double chain size. The exact numbers depend on the specific brands of line and chain, but I never thought that important for choosing size.

How did you choose the size of chain and line for your boat?

[Palarran]

Originally Posted by Ben

How did you choose the size of chain and line for your boat?

I know this wasn't directed to me, but it was simple. I asked Noelex. Without a doubt he knows as much as anyone about anchors, chain, sea bottoms, and how to make it all work.

[Ben]

Originally Posted by thinwater

Both links are broken.

They work for me. ??

[Ben]

Originally Posted by Palarran

I know this wasn't directed to me, but it was simple. I asked Noelex. Without a doubt he knows as much as anyone about anchors, chain, sea bottoms, and how to make it all work.

My method was even simpler, I just looked at a chart like that one from West Marine I referenced. My boat is in the 41-45 foot range. It is light, but has high windage so I used the heavy boat recommendation: 3/8" chain, 3/4" line. Done!

Apparently this is too simplistic for thinwater. I look forward to hearing about all his complex calculations to solve this problem.

[thinwater]

Originally Posted by Ben

I personally did no calculation. 3/8" chain was recommended by the simplified charts, which also recommend 3/4" line. The rule of thumb is that chain size is doubled to get the matching line diameter. Tables of SWL show a rough, though not exact match when line size is double chain size. The exact numbers depend on the specific brands of line and chain, but I never thought that important for choosing size.

How did you choose the size of chain and line for your boat?

The Noble Denton table does not go as small as our boats; the minimum chain sized is 1/2-inch. That is why I wondered. Even the "yacht" guide is really for ships and assumes less surging and yawing, I think. (This is based on the mathematical small weight given profile area in the calculation--20 times less.)

Typically ABYC H-40 table one, adjusted for multihulls. As for rope working load (ABYC does not use strength), you will find it interesting that ABYC uses a different basis for rope than for chain, because the force with rope is much less. This is also true with the Noble Denton table.

In fact, I have measured wind and surge loads for each of my boats. The result compares well with ABYC tables, assuming I anchored in the worst possible places (shallow, no snubber).

[Ben]

Originally Posted by thinwater

The Noble Denton table does not go as small as our boats; the minimum chain sized is 1/2-inch. That is why I wondered. Even the "yacht" guide is really for ships and assumes less surging and yawing, I think. (This is based on the mathematical small weight given profile area in the calculation--20 times less.)

Typically ABYC H-40 table one, adjusted for multihulls. As for rope working load (ABYC does not use strength), you will find it interesting that ABYC uses a different basis for rope than for chain, because the force with rope is much less. This is also true with the Noble Denton table.

In fact, I have measured wind and surge loads for each of my boats. The result compares well with ABYC tables, assuming I anchored in the worst possible places (shallow, no snubber).

So what size chain and line did you choose, and what were the reasons for those choices?

[smj]

Originally Posted by Palarran

Your right, I have heard of it, for monohulls. Which is what your link describes. As for reading about cat sailors adjusting their bridle length here, I honestly don't recall that. I'd again say that it has to be a size of cat situation. My bridle isn't on a cleat, it's attached to the front crossbeam. From observation most of the, lets say above 40', cats have welded eyelets at the cross beam and the bridle is connected to that with a shackle.



As to the OP's question about bridle design and stretch, this is just my opinion, the the stretch of my bridle has very little effect to dampen the pull of the boat compared to the weight and friction of the chain moving through the water and off the bottom. If I wanted to dampen the movement more, rather than worrying about stretch, I'd A) anchor in deeper water, and B) let much more chain out after attaching the bridle. For B, you can let out enough chain in order to create a belly, or a lot and create a sort of kellet. If your in 30' of water, let an extra 40' of chain out after the bridle. Of course I'd only do this if in some serious wind - over 40 knots. We also never anchor, even in mild conditions, without a short snubber in the anchor locker to protect the windlass. It's just too easy to put on and we use it constantly when setting the anchor anyway. Which is another subject and point, but we don't attach the bridle to set the anchor, just the snubber. It's much easier to observe if the anchor is dragging that way, for us.

I believe your right, it’s probably a boat size thing. Ive adjusted my bridle numerous times to face the bows into the swell when the swell doesn’t align with the wind. Makes for more comfort. At 56’ I’m guessing your boat doesn’t feel a side swell like a smaller catamaran.
I’m also a firm believer in a good amount of nylon snubber or bridle to take the shock load of both the ground tackle and attachment points on the boat. At a certain point no matter how much chain you let out it will get bar tight and the loads on the anchor and attachment points are huge. Witnessed this many times.

[tomfl]

Originally Posted by smj

I believe your right, it’s probably a boat size thing. Ive adjusted my bridle numerous times to face the bows into the swell when the swell doesn’t align with the wind. Makes for more comfort. At 56’ I’m guessing your boat doesn’t feel a side swell like a smaller catamaran.
I’m also a firm believer in a good amount of nylon snubber or bridle to take the shock load of both the ground tackle and attachment points on the boat. At a certain point no matter how much chain you let out it will get bar tight and the loads on the anchor and attachment points are huge. Witnessed this many times.

Agree about adjusting the bridle can make the boat more comfortable. Have to say you set up SMJ very well for that.

I am still convinced that attaching the bridle to cleats is a better idea than eyes on the crossbar. Not just for the ability to adjust the bridle length but I am not sure a welded eye would be as strong as a cleat. Some of the previous posters mentioned the eyes were in different places than on the cross bar which would probably be even a weaker place to attach the bridle.

Even so I would not like to be anchored in this with any setup.

https://www.youtube.com/watch?v=9d8jDLdfJ7Y

[Palarran]

Originally Posted by smj

I believe your right, it’s probably a boat size thing. Ive adjusted my bridle numerous times to face the bows into the swell when the swell doesn’t align with the wind. Makes for more comfort. At 56’ I’m guessing your boat doesn’t feel a side swell like a smaller catamaran.
I’m also a firm believer in a good amount of nylon snubber or bridle to take the shock load of both the ground tackle and attachment points on the boat. At a certain point no matter how much chain you let out it will get bar tight and the loads on the anchor and attachment points are huge. Witnessed this many times.

Well, if you have enough chain, and let enough out, it takes a shiatsu amount of force to go "bar tight". Personally I've never had a problem with shock loads except when anchored in shallow water with too much chain out. Then the boat starts to hunt, but seldom actually snaps to a stop. This past year in the Bahamas we did a lot of shallow water anchoring. You could see the anchor easily from the front of the boat, 4 to 1, no more, even in depths where I had 2' under the keel. So my anchor is less than my boat's length away. In general though, and again this is probably a boat size thing, I prefer to anchor in 30+ feet of water is possible, even if it exposes me to more wave action.

The best example I ever experienced was a 50 knot meltemi where I was anchored in 90 to 100 feet of water with 340' of chain out. We literally didn't move at all even though the chop was serious around us. The droop of the chain essentially kept us stationary.

I do have to ask though, how is your bridle attached to your boat where it allows you to adjust it?

[Ben]

Originally Posted by tomfl

Agree about adjusting the bridle can make the boat more comfortable. Have to say you set up SMJ very well for that.

I am still convinced that attaching the bridle to cleats is a better idea than eyes on the crossbar. Not just for the ability to adjust the bridle length but I am not sure a welded eye would be as strong as a cleat. Some of the previous posters mentioned the eyes were in different places than on the cross bar which would probably be even a weaker place to attach the bridle.

Even so I would not like to be anchored in this with any setup.

I didn't see a snubber in that video, and the chain was bar tight. Unless I missed something, it seems to contradict everything in this thread.