Anchoring a Boat at Sea

Anchoring a boat seems to be a problem for a good many people, considering the number of anchors we pick up on the reef, and the people we pick up, before we chase down their drifting boats. Accordingly, here are some hard won tips for you relatively new boaters and divers, and even for some old boaters who have been lucky!

First there are the parts of an anchor. The long straight portion, ending in some kind of ring for attachment, is the shank. On the end opposite from the point of attachment of the anchor line, which is properly called the anchor rode, are the flukes. They are the portions of the anchor which dig into or hook on the bottom to keep the boat from drifting. Typically they will be elongate triangles, but on old anchors, may be heart shaped. On some anchors, but not as much today as in the 19th Century and before, there is also a bar across the attachment end of the shank, the function of which is to keep one or both of the flukes pointed at the bottom so that they dig in.

The anchor works on the principle that the shank will be nearly parallel with the bottom, placing the flukes so that they dig in. Here lies the whole problem with most peoples anchoring technique. If the shank rises up to 45 or more degrees from the bottom, the flukes cannot get a good bite and the anchor pulls out, allowing the boat to drift away, or to drag, plowing the bottom up, but resulting in a dive on a spot not of your choosing, and often out in the sand. To hold the shank low to the bottom, two main techniques are used, often together

The first is to add a length of chain between the shank and the anchor rode. The length and size of this chain is chosen to provide adequate weight on the shank to hold it down, and at the same time to keep the part of the anchor rode which is closest to the bottom from sawing thru on a coral head or rock ledge, a situation which you can avoid by anchoring in sand whenever possible. Do not succumb to the temptation to use small chain and only a short length. Better a little work weighing anchor than no boat when you come to the surface after a dive.

The second thing is to provide so much anchor rode that the catenary between the chain and the boat is gentle, and the pull on the chain and anchor is nearly parallel to the bottom. This means, not 110 feet of rode in 100 feet of water, but 250 to 300 feet-REALLY!! For anchoring where the boat must ride out rough conditions, untended, the U S Coast Guard recommends 500 feet of anchor rode. As divers, we sometimes set our anchors under a ledge of rock, since we don’t often dive on bare sand. In the latter instance we make sure the anchor is digging in well. For a fisherman or surface boater, anchoring becomes important when the weather deteriorates and there is danger of either being blown onto the beach, or out to sea.

A prime example of the result of too short scope was a recent attempt to dive the REBEL, a deep wreck off Pompano Beach. The first two attempts, with a good anchor and 8 feet of heavy chain, resulted in our dragging across the bottom at a great rate. For our third attempt, we added another hundred feet of anchor rode, and got ready to go over immediately after dropping the anchor, before it had time to drag very far. It turned out that the flukes dug in very well with the extra rode and we had a nice dive. We tried the TRIO BRAVO in somewhat deeper water, and dragged badly, never finding the wreck.

Many boats that I have been on do not have additional anchor rode aboard. Many, indeed, have far too little to start with. Ive pulled in boats that couldn’t anchor in 30 feet of water. Their anchor rode was about 30 feet long, and all their dock lines had been left at their berth. One man, whose life was saved by a Dutch tanker, far out in the Gulf Stream, at night, in the winter, lost an engine drifting over the Third Reef off S.E. Florida, and by the time he realized that he couldn’t get started, he was in 150 or so feet of water. He, too, had too little anchor rode to do the job. It almost cost him his life.

On one occasion, I watched a sailboat trying to anchor in about twenty feet of water. They tried and tried just about 100 yards ahead of where we were diving. They were clearly frustrated. I finally yelled at them and found out that their anchor would not hold. When we pulled up close it was apparent why. The rode was straight up and down. The anchor was pounding the reef to pieces but in no way could it get a bite and hold the boat. They were offended when we suggested letting out more line!

Let me relate one very valuable trick which has twice saved a boat that I've been on, in very rough seas, in the surf zone, with waves breaking all around us. When we lost an engine, we threw over an anchor with all the scope we dared, until the breakers, eight or ten feet high, were breaking just astern of the boat. As we dragged astern, I realized that we would soon be on the beach, in pieces. I snapped a divers weight belt over the anchor rode and pushed in down the line. Fortunately it worked its way down the slanting line rather rapidly until it snagged the shackle on the chain. Here it provided the additional weight required to let the flukes dig into the bottom, and we hung there until we could get the engine repaired, and head straight out to sea, to safety. The exact same trick saved the other boat, with only slight differences in the details.

Another common problem is putting a screw pin shackle to hold the anchor ring to the chain, or the chain to the rode. Screwed in finger tight, the screw pin can vibrate loose as the current causes the line to vibrate or strum. One classic example was an anchor I found in 60 feet of water, on the reef, with the purchase price sticker still on it. Within a foot of it was a brand new screw pin shackle, minus pin. I just hoped that the people who lost the anchor were still aboard the boat when they noticed it. If they were diving and came up to find the boat gone, they had a long swim to the beach. The cure is simple. Mouse the shackle. That is, use a piece of malleable stainless or copper wire thru the eye in the screw pin (that’s why its there), and around the leg of the shackle. Its nearly impossible to unscrew the pin, now, much less have it vibrate out.

For divers and fishermen, who anchor on the reefs, where it is not uncommon to get the tips of the flukes under a small ledge, or in a hole in the rock, I highly recommend an anchor which has a sliding ring attached to the chain. The ring, and hence the point of pull, can be at the extreme end of the shank, or can slide to the flukes end, depending on the direction of pull. It can almost always be removed by pulling in line till one is directly above or somewhat ahead of the anchor, then giving some slack to let the anchor ring slide to the fluke end. A slight tug at this time will pull the flukes out from under the ledge and the anchor is free. Solid shank anchors with the ring permanently fixed at the end, will stay caught. I have several good ones in my garage that were left that way, and picked up by divers.

Spend a little more for a good, recoverable anchor, put on a long stout chain, mouse both shackles, add more than enough line of sufficient diameter to take a little wear, and be sure to provide enough scope when anchoring. It also pays to examine your rode regularly, for fraying across the bow cleat will occur. Done routinely, these precautions will assure that your boat will always be there when you surface from your dive.

Gentle winds, flat seas and good visibility!!

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