Musings on anchor bridles
As I sat in the cockpit with our boat gently tugging at her mooring in Great Harbor, Jost Van Dyke, I took note of the catamaran just astern of us. The crew of the cat had rigged their mooring bridle at a very large angle, something we have done in the past as well.
Note the angle of the bridle legs.
They likely did so because the balls at that location have very long pennants but it made me think of some of the physics related to how rock climbers rig their anchors. A properly-rigged bridle actually splits the forces applied to the boat. This is in addition to helping to stop the boat from sailing around at anchor. Unbeknownst to many though, an anchor bridle with legs too short actually increases the forces applied to the boat, substantially in some cases. Take a look at the graphic below.
I ran my thinking past my friend Drew, a fellow PDQ owner and very experienced climber. With his permission, I have copied his reply to me below.
- Particularly at narrower angles, the strain will actually come onto just one leg when the wind suddenly shifts. Thus there is a point of diminishing returns at about a 30 degree angle. I’m sure you’ve seen this.
- If the bridle is too long it will ride on the bottom, and several bad things can happen: chafe (it’s fiber), snag, and the hook comes off. In deep water this is minor, but on the Chesapeake I seldom anchor in more than 7 feet of water. One cure, if you want the long lines for shock absorption (which matters more in shallow water) is to run the lines along the side deck and cleat mid-ships. I doubt it matters to you, but when you go very shallow, particularly if there is any risk of surge, it really helps. Of course, if the waves start to break and you’re in the impact zone, you’re hosed. No cat can handle waves breaking on the bow at anchor; too much area. Footprints lost their Gemini that way, as did some Aussie. Another reason not to anchor too near the beach.
- Longer bridles give more shock absorption, which is good in shallow water with all chain. But see the problem above; the apex drags.
The same math also applies to Bahamian moor. The angle should never exceed 120 degrees, because the force goes through the roof if the wind comes from the side. By the same token, if the wind shifts and you are on a shallow angle, the load will all be on one anchor (the reason 2 anchors are pretty pointless in most storms; better to have a big Rocna, no?!). In fact, this applies to any sort of anchor or rigging. Climbers, for example, aim for the 40-60 range; greater angle and the strain is more, lesser angle and the load is all on one if there is any swing. This is particularly bad for climbing anchors, because the anchor may shift in the crack.
So for many reasons, 40-60 degrees is a sort of sweet spot.
Great info. Thanks Drew! If you are interested in some excellent reading with very valuable technical info, do check out Drew’s blog!