Guest blog by Captain John
Captain John Jamieson, a regular contributor to the Daily Boater, is author of Seamanship Secrets and publisher of the popular boating education website SkipperTips.com
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| Photo Courtesy of The U.S. Coast Guard. |
Editor's note: In his last column, Captain John discussed the math of safe anchoring for a peaceful night at anchor "on the hook". This week, he puts his equations to the test. Enjoy...
The other day, a reader sent in a query as to why I recommend 7:1 scope on anchor rode as opposed to 3:1. After all, less rode means less hassle with having all that line messed up by gooey mud, slimy bottom crud, and other unknowns beneath the sea.
Check this theory out quick and easy with a small anchor. The anchor type isn’t important - even a tiny grapnel will do. Conduct this experiment on land in a sandy area (or soft ground). Attach 20 feet of small diameter rope to an anchor shank. Dig the anchor flukes into the ground just a bit. No need to bury them all the way. Walk to the end of your 20-foot anchor line. Turn around and face the anchor. Pull hard on the rope rode. What do you notice?
Your anchor will dig into the seabed and hold well. That’s because most of the pull on the anchor rode will be horizontal. And that’s the secret to keep your anchor dug deep. You want to do everything possible to keep the rode as parallel as possible to the seabed. That’s another reason to use a length of chain on the bottom of a rope anchor rode. The chain protects the line from chafe (wear) and it provides weight to keep the rode more parallel to the bottom.
Now, let’s simulate what happens with just 3:1. In our experiment, we will assume what the 20 foot piece of rope at the end represents a scope of 7:1. So, about half of that length would represent a scope of close to 3: 1. Walk slowly toward your anchor. Keep tension on the rope rode as you go. Watch the angle that the rode makes with the anchor and sea bed. Stop when you get to a point about half of the distance to the end of the rode.
Look at the shank of your anchor. Note that the rode now has an upward (vertical) pull on the end of the anchor shank. Any vertical pull on an anchor shank has the potential to break out an anchor and cause it to drag. The shorter the rode relative to the water depth, the less chance that your anchor will hold. And that’s the main reason that you want to avoid short scope. Long scopes give your anchor greater holding power over a broader range of wind or sea condition.
Remember, you will not always anchor on a flat plane of water. Wave action, ground swell, boat wakes all can cause your boat to lift and fall in position. Think of any lift as vertical motion. Enough of this can cause your anchor to break free of the sea bottom.
But what about vessels with all-chain rode? Sure, those with heavy chain might use shorter scope of about 5:1 in light to moderate conditions. But 7:1 will always be better. And, in storm conditions or in unprotected areas with heavy ground swell, you need 10:1.
Follow these easy boat anchoring tips for worry-free anchoring for you and your sailing crew or partner. You will sleep better at night when you choose to use enough scope to keep your boat anchor dug deep into the sea bottom - wherever in the world you choose to cruise!
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