Resonating bridle on catamaran

[torrmundi]

Hi,

I’ve been anchoring in high-current spots lately and I’ve found that my bridle is pulled nearly straight down by the chain’s catenary. The two legs can be resting on my bob stays holding the bowsprit. That’s a chafe issue, which I can somewhat solve by releasing the halyard tension and letting the bobstays go slack(ish). But in a high current, the bridle legs can start to resonate, like a violin string, “plucked” by the current. Does anyone have any ideas how to stop this? Under high tension, the vibrations and noise runs into the cabins at night.

[thinwater]

Originally Posted by torrmundi

Hi,

I’ve been anchoring in high-current spots lately and I’ve found that my bridle is pulled nearly straight down by the chain’s catenary. The two legs can be resting on my bob stays holding the bowsprit. That’s a chafe issue, which I can somewhat solve by releasing the halyard tension and letting the bobstays go slack(ish). But in a high current, the bridle legs can start to resonate, like a violin string, “plucked” by the current. Does anyone have any ideas how to stop this? Under high tension, the vibrations and noise runs into the cabins at night.

• Shorten the bridle and get it out of the water.
• Add weigh mid-leg.
• Slide webbing over the bridle leg.

[torrmundi]

Originally Posted by thinwater

• Shorten the bridle and get it out of the water.
• Add weigh mid-leg.
• Slide webbing over the bridle leg.

Thanks, we'll try shortening the bridle. But we can't get it out of the water without losing the point of having it. That's too short. The webbing might work, interesting damping solution. I'll need to source some 1.5" or 2" tubular webbing. I'm not keen on adding kellets or other weights to the bridle, that not a KISS solution.

[thinwater]

Originally Posted by torrmundi

Thanks, we'll try shortening the bridle. But we can't get it out of the water without losing the point of having it. That's too short. The webbing might work, interesting damping solution. I'll need to source some 1.5" or 2" tubular webbing. I'm not keen on adding kellets or other weights to the bridle, that not a KISS solution.

There are 2 reasons to have a bridle. One is shock reduction; if the chain is pointing down you have catenery. Another option to increase shock absorption is to move the anchor point to the mid-ships or stern cleats. Lots of stretch. Neeves has written about this n Practical Sailor and I can tell you that it works.


The other reason is yawing. There are other ways to reduce it:

• Riding sail
• Hammerlock mooring (second anchor just scraping--does not need to be on the bridle, just from the bow)
• Some board down. If the boards are up and the rudders are down you have really moved the CLR far back.

[Steve_C]

Its interesting that you are in a high current situation and your bridal legs are pulled straight down? That would seem to be the opposite of what you would expect with high current.

What scope are you using? I am genuinely curious because when I have anchored in high current areas, my rode (and bridle) is usually raised due to the increased tension.

let us know how you solve this.

[mikedefieslife]

Not if the current is against him and the wind from the stern.

Happened often to me in rivers in Portugal. The chain and bridle would be well under the boat.

[chris mac]

Our cat does this as well in wind against current. We seem to hover sideways over the anchor with the bridal straight down. When the current turns back to the other way(same as wind) we stretch out again
For us it doesn't cause a problem

[torrmundi]

It was pulled straight down, 3/8" G4 in 28 feet of water. My bridles legs are 50', about 4' of line for cleating (46' left for shock absorption), 5' of line used to reach the bow, then descending 5' to water. The current was (guessing) about 3-4 knots, not that high, but enough to resonate my bridle. I have not been able to test shorter in-water bridle yet. I can move to my mid-ship cleats (aft 18.5', so 50-4-5-18.5 = 23 feet for anti-yaw).



The current was on the bow, wind astern. Sometimes I overran the bridle & anchor, sometimes I was not. 125' of chain in 25 feet of water.


I have an aluminum Fortress as 2nd anchor. I can give it a try, but I expect it will interfere with the bridle at some point.



The rudders and fin keels are fixed, can't raise them.

[CDR Smith]

Your bridle legs sound waaayyy to long to me, but perhaps that length is necessary for your beam. I dunno. Also, if they're vibrating like a violin string, they may be made of too small a cord. I think heavier cord would be less likely to vibrate. I'd make them shorter and thicker. You might also try attaching some short webbing "flags" to the underwater lengths which may provide enough dampening to moderate the vibration. Good luck. Try the flags first. That'd be a cheap thing to try.

[Chotu]

No wonder my cat has always been an absolute beast at anchor. I always keep the boards up to keep growth off them.

It sails like it’s doing the cans at an afternoon club race whenever I’m at anchor. I need to put some board down and see what that does.

Always something new to learn. Thanks for this post.

Originally Posted by thinwater

There are 2 reasons to have a bridle. One is shock reduction; if the chain is pointing down you have catenery. Another option to increase shock absorption is to move the anchor point to the mid-ships or stern cleats. Lots of stretch. Neeves has written about this n Practical Sailor and I can tell you that it works.


The other reason is yawing. There are other ways to reduce it:

• Riding sail
• Hammerlock mooring (second anchor just scraping--does not need to be on the bridle, just from the bow)
• Some board down. If the boards are up and the rudders are down you have really moved the CLR far back.

[Jetx]

From a past life, I think the humming is caused by vortex-induced vibration, which can happen any time a fluid is flowing across a cylinder. Anything you can do to change the resonant frequency should help... changing line diameter, length or stiffness, or adding weight as already suggested.

In some engineering applications, the solution is to add helical ridges or strakes to the cylinder (like you see on smokestacks, or old car radio antennas), which disrupt the alternating vortex shedding pattern. So if you're using braided line for your bridle, switching to 3-strand, which is naturally continuously "straked", might solve the problem. Then again the relative dimension of the line and the ridges may not be right, so might not help... but worth a try.

[meirriba]

Are you perhaps sailing over the chain?
If this is a result of the current against wind, perhaps you can deploy a stern kedge (Bahamian moor)

[SV Bacchus]

I bought a new bridle from Mantus when my old one gave up on me down at Union Island, Eastern Caribbean. The new bridle has about 5 or 6 feet of webbing at about the 1/2 point and I never knew why. The bridle is 29' long, almost twice as long as what it replaced.

After reading this I now remember seeing those webbings just poking out above water, most is below the water line when the current is running strong. Now I believe I know why, thanks to this thread. I never have felt or heard the mentioned vibrations when the bridle is stretched out.

[thinwater]

Originally Posted by torrmundi

It was pulled straight down, 3/8" G4 in 28 feet of water. My bridles legs are 50', about 4' of line for cleating (46' left for shock absorption), 5' of line used to reach the bow, then descending 5' to water. The current was (guessing) about 3-4 knots, not that high, but enough to resonate my bridle. I have not been able to test shorter in-water bridle yet. I can move to my mid-ship cleats (aft 18.5', so 50-4-5-18.5 = 23 feet for anti-yaw).



The current was on the bow, wind astern. Sometimes I overran the bridle & anchor, sometimes I was not. 125' of chain in 25 feet of water.


I have an aluminum Fortress as 2nd anchor. I can give it a try, but I expect it will interfere with the bridle at some point.



The rudders and fin keels are fixed, can't raise them.

A fortress anchor will NOT work as a hammerlock. Too light, will "fly" in current, won't bit at hyper low scope. Must be a Claw or Rocna/Spade type.

[thinwater]

Originally Posted by Chotu

No wonder my cat has always been an absolute beast at anchor. I always keep the boards up to keep growth off them....

Oh yeah, that will make her dance!