Ground Rules: Anchoring in Three Dimensions

Anchoring involves more than dropping a chunk of metal overboard and fastening your boat to it. The best way to improve your technique is through practice, but this can be hard to come by if your boat is often on a mooring or dock.
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Anchoring involves more than dropping a chunk of metal overboard and fastening your boat to it. The best way to improve your technique is through practice, but this can be hard to come by if your boat is often on a mooring or dock. It’s good to remember that unless you are alone in an anchorage, setting the hook is not a solo endeavor, and those on boats around you have a legitimate interest in your anchoring skills, as do you in theirs. Doing it well will make both you and your neighbors safer and happier. 

Image above: If you have trouble visualizing how boats will swing in an anchorage, build a simulator of pins, thread, paper clips and a vertical piece of cardboard. Set up your model harbor, then rotate it to simulate a change in wind direction. Here, the wind shifts from north (left) to east (right). The three boats to the left will collide.

Anchor Securely

When anchoring in an empty harbor there are really only two issues—the anchor you are using and how you deploy it. Given a bottom suitable for anchoring, rule #1 is: if your anchor drags 100 percent of the time, you need a bigger anchor. Period. You can add scope, more or heavier chain, or switch to a different anchor design, but for staying put, nothing beats increasing anchor weight and blade area.

You can also try switching to a “better” anchor if you like, but when you do so double down on your bet and make it bigger as well. Dragging is so common some now consider it normal. Please! You surely expect your car to be where you parked it when you return. Don’t settle for less with your sailboat. If your anchor is big enough, dragging should be as rare as car theft.

The second issue is scope—the ratio of rode length to the distance from your anchor roller to the sea floor. If the water is 12 feet deep at high tide and your anchor roller is 4 feet above the water, you need to let out 64 feet of rode (16 x 4) to achieve a 4:1 scope. Inadequate scope is the second most common cause of dragging and often results from guessing at how much rode you have out. Therefore, rule #2 is: clearly mark your rode in 25-foot or 30-foot (5 fathom) increments. You cannot assess the security of an anchor set without knowing how much scope you have. How much is adequate? Under normal conditions—in winds under 40 knots—4:1 should be sufficient. If you drag with this much scope, refer to rule #1—you need a bigger anchor. Bear in mind that increasing scope does not decrease the total load on the anchor. It merely makes the load more horizontal. Doubling scope from 4:1 to 8:1 transfers around 9.3 percent of the total load from vertical to horizontal. Tripling scope shifts just 12.8 percent of the load. Meanwhile, doubling wind velocity increases the total load approximately fourfold, while tripling wind speed increases it by a+ factor of nine. Adding scope is prudent, but inadequate if you anticipate high winds.

No anchoring test I’ve seen in the last three decades has proved that increasing scope beyond 4:1 actually increases holding power. To the contrary, increasing scope sometimes may even reduce holding power. Once a rode’s vertical pulling angle falls significantly below an anchor’s blade or fluke angle—and at 4:1 scope the straight-line pull on the anchor is already down to just 14.5 degrees above horizontal—it does not tend to break out most anchors. Rather, it just alters their angle of penetration, the effect of which is unknown and probably unknowable given varied bottom compositions.

Many also believe that more chain increases holding power. Here is a test. Lay your anchor chain out in a straight line on your lawn, pick up one end and start walking. That is its holding power. Chain has catenary benefits, particularly in deep water, and it helps keep a boat from sailing around at anchor, but more chain on the bottom does not increase holding power.

Anchor Precisely

 To anchor accurately and securely in a crowded anchorage you need to know how much rode you have deployed; mark your rode at regular intervals to help you measure

To anchor accurately and securely in a crowded anchorage you need to know how much rode you have deployed; mark your rode at regular intervals to help you measure

Can you anchor your boat exactly where you want it? In constant wind and current-free conditions, you start by positioning your boat where you want to end up, then motor forward into the wind a distance equal to the amount of rode you expect to let out, plus a few extra feet so the anchor can slide a bit before holding. Say the water is 21 feet deep and your bow roller is 4 feet high. With your rode fully extended on 4:1 scope, your boat will sit about 100 feet (25 x 4) behind your anchor. Allowing 10 more feet for your anchor to get traction, a 40-foot boat, for example, should motor forward just under three boat lengths before dropping anchor. Thus rule #3: determine how much rode you will deploy before you anchor. Positioning your boat accurately is essential in crowded anchorages and knowing where to put your anchor is the only way you can do this consistently.

 To keep from fouling your anchor, your boat should be stopped or moving slowly backwards when you drop it. Once it reaches the bottom, drag the rode a moment to pivot the anchor’s shank in line with the bow, then let the rode run as the boat moves backwards. Back down at idle speed or let the bow blow off with the wind. When the length of the rode paid out is about twice the water depth, snub it just long enough so you can feel the anchor tugging, then let it run again. This helps the anchor dig in. Keep snubbing and releasing the rode every 15 or 20 feet. You can also pay out additional scope to maximize the anchor’s initial angle of penetration. More scope lets you “tickle” the anchor into the bottom longer. With scope of 6:1 or 7:1, run your engine in reverse and slowly bring the revs up to around 3/4 throttle. In all but the most challenging bottom conditions, if your anchor is big enough, it should dig in rather than slip. Once you are satisfied with the set, shorten your rode to your predetermined length.

It is easy to misjudge your anchor placement, so if after you settle on your rode if you are unhappy with your position, move. Doing it again shows you are prudent, not inept. It hones your skills and will improve your comfort level—and that of those around you—for the remainder of your stay. 

Anchor Courteously

Unlike cars parked neatly side by side, anchored boats occupy large circles centered on their anchors. Anywhere these circles overlap, there is risk of collision. So rule #4 is: don’t be a harbor hog. Whether you arrive first or last, limit your scope. The area of a circle circumscribed by a boat sitting on 10:1 scope is more than 4 times that of a boat on 4:1 scope. That means a cove that might safely harbor 40 boats will have space for just 10. The social obligation to claim no more space than you need outweighs the negligible benefit (if any) of maximizing scope.

Ideally you should anchor outside the swinging area of other boats, but this is rarely practical in a popular anchorage. How do you stay clear of nearby boats? Placing your anchor astern of another boat is relatively safe as long as your scope does not significantly exceed theirs. Even if you drop your anchor abeam or slightly forward of a nearby boat, you should swing clear as long as your bow ends up behind their stern. If you anchor forward of a boat, be sure your anchor is also forward of theirs and is some distance to the right or left of it.

Bear in mind, if you arrive in a calm, or during or after a squall that has disrupted the normal wind pattern, or if there is significant current, all the boats may not swing in unison. It is wise to wait a while to see how things play out before leaving the boat unattended. The shapes of boats around you are also a factor. Shallow boats or those with high profiles can skitter about in any breeze, while deep draft boats will be more affected by even modest currents. So it’s a good idea to anchor near boats similar to yours whenever possible.

Be sure you leave adequate space for both safety and comfort between your boat and those nearby. No one likes to have another boat anchor too close. If you end up closer than you intended, move away. If your new neighbor expresses concern and there is room to anchor farther away, move. There is no downside to this. More separation is always better, and your neighbor will appreciate it.

Also, if you watch movies in your cockpit, play loud music, practice your bagpipe, run a generator, have an obnoxious wind charger or smoke anything, do not anchor directly upwind of an occupied boat. Sure the wind can change, but then Murphy and his law share the blame. When picking a spot also consider your own tranquility. You can’t complain of a neighboring boat with clanging halyards or yapping dogs if you willingly anchored next to it.

Much of this, of course, is common sense. If you arrive in a new anchorage and lay out your anchor competently and without drama, ending up a discreet distance from your new neighbors, you’ll have put on a satisfying display of seamanship. It’s also a very nice calling card.

Photos by Don Casey

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