Setting a Mooring That Will Not Drag

By Gus Marchetti, Salvage master. Reviewed by Dana Whitcombe, technical reviewer.

Commercial Diving Contractors Alabama | Commercial Diving Contractors Alaska April 16, 2011. Mobile Bay. The anchor chain of the 200-ton crane barge was jumping in the stopper like a snake. I came out of the rack swearing. The mate pointed at the GPS: we'd moved three hundred yards since midnight. We were dragging toward a jack-up rig that had a crew on deck. The problem was simple. Soft mud over shell, a Bruce knockoff anchor, and scope at barely three-to-one because we were trying to keep the barge off the shipping lane. You can't bend geometry. That fluke pulled out like a fork out of mashed potatoes. I've been setting moorings for twenty-two years, on wreck removals and marine casualties from the Gulf to the Strait of Hormuz. Dragging is the failure you learn to fear—it's slow, dumb, and almost always your own fault. Here's what I do now, every time, so it doesn't happen again.

Read the bottom before you drop

Mud holds force a different way than sand, and rock holds force in a third way entirely. A Danforth fluke digs into mud and sand, but it skates on rock and grabs uselessly in grass. A Bruce or a Delta will set on rock or coral, but it's heavy and awkward in sloppy mud. That means you don't get to have a favorite anchor—you get to have the right one. I take a core whenever I can. We have a four-foot length of two-inch pipe that we drive over the side, then pull and inspect. You can also read a chart, talk to local shrimpers, or just drop a lead line and feel the bottom. Six ounces of grease on the lead tells you if it's soft or hard. If you're in the Gulf, chances are you've got mud; off the Pacific Northwest you've got glacial silt and rock. Know what you're hooking into before you pick the gear.

Don't cheap out on scope

Scope is just the length of your anchor line or chain divided by the distance from the water surface to the bottom. A 5:1 scope means for every 10 feet of water depth, you need 50 feet of line from your anchor to the boat. That's the minimum for a standing mooring. I run 7:1 whenever there's weather on the way. The reason is angle. The flatter the pull, the more the anchor buries. At 3:1 your anchor is pulling upward and it just plows. At 7:1 it's practically parallel to the bottom, so it digs in. The problem is space. If you're in a crowded harbor, you can't always get the scope you want. That's when you set a second anchor out to the side, or run a shore tie. Never stretch the geometry to fit the space—stretch your system instead.

Set it in stages, not in one go

Most people drop the anchor, let out some line, and call it good. That's how you get dragged. You need to put the gear under load in a controlled way, so the fluke has time to bury. I do it like this: lower the anchor to the bottom, then veer to the full scope. Then put a slow, steady pull on it—usually a half-astern with the main engine or a moderate pull on the winch. Watch the line. If it's twitching. Good. Take it up another few hundred pounds if the winch allows. Leave it there for ten minutes. Then check your position against a fixed object or a transit. If you're holding, repeat with more load. If you're not, pull the anchor and set it again somewhere else. This isn't complicated, but it takes patience.

Watch the wrong things

A GPS position tells you where the boat is, not where the anchor is. The anchor could be dragged, and your water track might show you're holding if the GPS is moving slightly with you. I watch the chain. If the chain is lifting off the bottom, you're losing holding. The angle of the line at the waterline is a better telltale than any instrument. And once you're set, leave a trip line on the anchor. A buoy at the surface tells you exactly where the fluke is. If you need to move in a rush, you've got a line to pull it up. I still go by the old rule: set it slow, load it twice, and stand watch like you don't trust the ground, because you don't.

Frequently Asked Questions

What if I can't get 5:1 scope because the harbor is too tight? Set a second anchor at an angle to the first, so they're pulling in different directions. Or use a shore tie to a pier or tree if you're in a river. You're still getting the flat pull, just from a different point. How long should I let it set before I trust it? We let a working mooring sit under load for at least an hour before we put people or equipment on it. If weather is coming, I double that and re-check the position. Is a bigger anchor always better? No. You need the right type for the bottom and the right scope. An anchor that's too big might not penetrate the bottom properly, especially in stiff clay. Match the anchor to the holding ground, not to your ego.