Resonating bridle on catamaran

[torrmundi]

Originally Posted by 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)

Yes, sailing over the chain sometimes, although not when I had the resonant situation.



I have done that in St. Augustine - I had to abandon the moorings since I wasn't acting like a monohull, and the mooring field was very tight. I went to an anchor and within 20 minutes had done many circles, sometimes at up to 3 knots. So I put out a stern anchor. That stopped all the bad motion very effectively. The next day I dropped the stern rode, then tried to recover. It was too shallow for my boat, so I tried diving it and digging it up. It was completely buried, every bit of it, by 8-10 inches of silt. I got a dinghy with 20 HP and a stern bridle to haul it up. With two men in the dinghy, it roared in circles about the anchor (no way would it pull straight) for 20 minutes before coming up. Wow, not what I expected.

[NevilleCat]

Just curious; what would happen if you only used 1 arm of the bridle? Or set one arm to a stern cleat? I have done that on occasion to get the boat turned into swell but it also tends to sail off to one side and stay there. Might be enough to create a different angle or tension to stop a harmonic vibration?

[Manateeman]

Member Jetex is correct. It is vortex induced vibration. There are several methods to reduce it. You might find an acceptable solution by looking at oceanography equipment.

[torrmundi]

Originally Posted by NevilleCat

Just curious; what would happen if you only used 1 arm of the bridle? Or set one arm to a stern cleat? I have done that on occasion to get the boat turned into swell but it also tends to sail off to one side and stay there. Might be enough to create a different angle or tension to stop a harmonic vibration?

Maybe; not sure what would happen in opposing wind and currents. It might double the tension on one leg if the boat sails to one side, reducing vibration.

[torrmundi]

There really is a lot more to this subject than I expected! Vortex induced vibration is indeed a big subject in the industrial marine area, as long undersea pipes have the same issue as my bridle, but with worse consequences. This paper is a good primer on the subject, with effectiveness of various mitigations: https://www.comsol.nl/paper/download...ndenAbeele.pdf. Since I do have braided lines, switching to a triple-strand might be similar to a helical strake that decorrelates the vortex over length, thus preventing establishment of vortices with uniform lift vectors that oscillate. Or I could add a section of several feet of freely rotating splitter plate or foil section that could rotate with the flow, like a rotating mast. It does require installation of the device onto the bridle, and a thrust bearing is also needed to keep it in place. Obviously, that's extra steps in deployment, so it's not preferred. Adding a bit of material like a kite's tail near the water surface can be done. It does not require a thrust bearing, is self-aligning to a point (it can wrap up), and might even be able to deploy with the bridle.

[thinwater]

^^ Yeah, that is a bit of what I was getting at with both shortening the bridle (changes frequency) and the webbing (disrupts periodic vortexs). 3-strand is definitely worth trying, although I have experienced this sort of vibration with 3-strand. I suspect hosrtening will work best.


A stern anchor or Bahamian moor that is tight puts a LOT of strain on the ground tackle. I'm not surprised it was deep. Better, rig the anchors at about 120 degree angle.