Anchoring in coral

When anchoring, voyagers generally consider only the holding potential of the seabed. The chart reveals whether the bottom is sand, mud, rock, or coral and an anchor suitable to take hold in that bottom is set.

However, while sand, mud and rock are inanimate materials and suffer little damage from the ravages of anchor and chain, coral is a different story. It is a living organism and the vicious bite of a metal hook and the dragging of chain over it spell death to the coral polyps and the reef.

Mike Childers of the University of Hawaii can give a first hand account of the damage that casual anchoring does to a coral seabed: “On the morning of April 26, 1991, a team of University of Hawaii-authorized divers was doing research on the Puako Reef (Big Island of Hawaii). Before entering the water, a large number of snorkelers was observed swimming around a blue-hulled catamaran bobbing on the calm sea over the reef. It appeared to be a tranquil scene. The tranquillity was broken when, after the divers reached a depth of 45 feet, the saw the vessel’s anchor nestled amongst the living coral on the bottom. The catamaran, her hull visible through the clear water above, had dropped anchor atop the living reef. Upon observation, it appeared that the anchor had dragged for about 15 feet before it had set. The damage to the reef was extensive. The anchor had plowed its way through the living coral, leaving only rubble in its wake. The anchor now set, had its length of chain draped across the reef, chafing and abrading the corals, causing further damage.”

It was ironic that the vessel that inflicted this damage on the living coral reef was a tourist dive boat whose very future is dependent on the beauty of the tropic seas. It was killing off the principle attraction to its customers. If this sounds self-destructive, that’s exactly what it is, and it’s not restricted to commercial vessels. Every day of the year, voyagers carelessly drop anchors on living coral beds.

Individually, the damage done by one boat may be only a localized scratch on the coral’s surface. Collectively, though, it is a terrible mutilation of the delicate works of nature’s most amazing animal, the tiny coral polyp. Enough scratches and reef growth can be set back for decades, if not killed it off entirely. Coral grows at a very slow paceandmdash;0.1 to 0.2 inches per year for most corals – therefore, we cannot depend on a reef healing itself very quickly, if at all (See “Coral growth,” Issue No. 38). We must nurture what we have as an irreplaceable element in our underwater environment.

A great deal of voyaging takes place in the tropics, an area of extreme ecological sensitivity. Tropical sailing is much more than balmy breezes, moonlit nights, and sparkling seas. Boat operators need to take responsibility for preserving the natural growth of all kinds, above and below the water. When anchoring or mooring over a coral seabed, one must do so it in the most gentle manner possible.

So what is the answer? More careful anchoring by all of us and the use of permanent moorings where repetitive anchoring occurs.Coral seabed anchoring

A careful visual examination of a coral seabed before anchoring will usually reveal patches of sand where an anchor can be set. Maybe someone already anchored in the area has done the survey and can guide one into a sandy area where an anchor can securely set itself without destroying the coral growth. Such was the situation for my wife Betty and I on Horizon years ago when we first entered the beautiful landlocked Harbor of Refuge at Neiafu, Tonga. As we circled the large harbor looking for a suitable place to anchor, the skipper of an anchored sailboat rowed over in his dinghy and directed us to a sandy spot in the seabed that would provide a good anchor-set free of live coral. It was not only a friendly act, typical of a good blue water sailor, but it was an ecologically-correct act.

Let me suggest a few guidelines for voyagers to follow when anchoring in an area known to contain living coral reefs:

· With the sun at one’s back, visually examine the bottom to locate a sandy spot in which to set the anchor. This area should have sufficient sand around it to permit the anchor chain to lie across sand and not drag across coral.

· If one can’t adequately determine the nature of the seabed from the deck of one’s boat, don swim fins and snorkel/mask and dive overboard to search out a sandy spot for the anchor. After a long passage, the thought of a swim in warm tropical waters should be quite appealing.

· In case of murky waters, a quality depth sounder can be useful in identifying sandy seabeds.

· If one can’t find a location near shore, move to greater depths where coral doesn’t grow. This is a good reason to carry anchor rodes suitable for anchoring in 15 or more fathoms.

· If, after anchoring, one should hear the chain rode grumbling, one has anchored over coral. Weigh anchor and move to another spot since the chain is grinding away the life of the reef.

These guidelines are usually adequate for voyaging boats in their random wanderings throughout the coral-growing regions of the world, but what about the high density sailing that takes place close to home on weekends and holidays? And what about tour boats and charter boats that return almost daily to the same coral reefs? The answer is to install moorings that minimize the damage done to the coral seabed.Coral mooring design

Boat moorings are certainly not a new idea, but the techniques for making coral seabed moorings are. Forget moorings using anchors, concrete sinkers, engine blocks, and railroad car wheels. They damage large areas of the seabed because of their size and ability to slide across the bottom.

Consider, instead, an eyebolt cemented into a small hole that has been drilled into the seabed. Done once, it can be used over and over again. Certainly it is an ecologically attractive compromise that will allow us to moor our boats over living coral without additional damage to the marine organisms that originally attracted us to the sea.

In 1985, John Halas, a biologist with the Key Largo National Marine Sanctuary in Florida, developed a mooring design suitable for use in coral reefs. His design called for a diver to drill a four-inch diameter hole in the seabed. Into this is inserted an 18-inch-long stainless steel eyebolt fitted with an end plate. This eyebolt is cemented in place with a special concrete mix. Into the eye of the eyebolt he attached a more or less standard polypropylene riding line and float assembly with mooring pendant. The result was a mooring that was strong, permanent, and ecologically compatible with the seabed to which it was attached.

The secret to the mooring was drilling a hole in the seabed and cementing the eyebolt in place. Halas pioneered use of a hydraulically-powered rotary drill to cut a four-inch diameter core out of the relatively soft coral. It took about 30 minutes to drill the 18-inch long hole. After breaking out and removing the core, Halas then cemented his 3/4 inch diameter stainless steel eyebolt assembly in place with Portland cement grouting tamped into the drill hole.

In 1988 the University of Hawaii Sea Grant program adapted Halas’ idea to install a mooring in heavily-used tour-boat and dive boat areas. Faced with seabeds made of dense coral, black lava, and cemented volcanic ash (whose strength and competence is about equal to sandstone), a smaller diameter hole was envisioned to shorten the drilling time. It was found that a 3/4-inch, chevron-pointed, carbide-tipped drill bit in a Stanley HD-45 rotary-impact hydraulic drill could bore a 7/8-inch diameter hole, 12-inches deep, in solid lava in less than 10 minutes.

The Stanley HD-45s drilling action is a series of vertical impacts of the drill bit interspersed with rotational movements of several degrees. This motion chips away at the rock which then floats out as a powdery cloud from the hole. Although one might expect the drilling noise to be quite loud, it was actually so quiet that curious fish were enticed to come over and see what was going on. Typical drilling times for 12-inch deep holes were: dike lava, 10 minutes; porous lava or cemented ash, five minutes; dense, solid coral, 30 seconds.

The mooring eyes were made from 3/4-inch-diameter, type 304 stainless steel rod, with an eye on one end of a 12-inch straight shank. The end opposite the eye was threaded for about six inches to provide better shear strength in the grout. The full 12 inches of straight shank is immersed in the grout with only the eye showing.

Because the Hawaii drill holes are much smaller, a different grouting scheme was needed. Two quick-setting grout materials manufactured by Quickcrete were found satisfactory: No. 1126 Hydraulic Cement or No. 1245 Anchoring Cement. The quick-setting cement was mixed on the boat with fresh water, forming a thick slurry. It was then injected by the diver into the seabed hole (from the bottom up) using a homemade two-inch diameter PVC syringe. The eyebolt was then inserted into the fluid grout, sinking slowly until the neck of the eye was flush with the top of the hole. It was gently rotated two or three revolutions to thoroughly wet the bolt shank. Setting time for these cements was 30 minutes for an initial hardening, and 24 hours required for final cure.Rigging

Alternative ways of rigging the eyebolts for use have been suggested. They range from the complete mooring buoy and pendant scheme of Halas to nothing but the exposed, bare eyebolt, possibly fitted with plastic streamer to enhance visibility for quick recognition. Dive boats can effectively use the bare-eye technique, while pleasure boats might employ a floating surface buoy.

In between these two extremes is the submerged mooring buoy (four to six feet deep) that does not interfere with surface traffic and can still be reached without deep diving. An unblemished appearance for the ocean’s surface is sometimes critical when moorings lie offshore of expensive residential property or vacation hotel sites in Hawaii.

Setting eyebolt moorings in coral is not a single-person task. There is equipment to procure and the multi-manning of it during the setting process. It is best done by a cadre of experienced scuba divers who are prepared to set a whole series of eyebolts in a relatively small area. Having the eyebolts manufactured, procuring the grouting, renting (or buying) the hydraulic equipment, and bringing the gear out to the site all require manpower and dollars which can be justified in the placement of several moorings at one time.

The hydraulic power supply and drill can be rented, or for extended use can be purchased. (The University of Hawaii finally purchased its equipment at a price of $6,300 so that it would be available for continued mooring emplacements around the state.) Exclusive of the hydraulic equipment, the cost of materials for a mooring is about $100 per mooring eye with labor provided by volunteer scuba divers interested in preserving the ecology of their diving domain.

Having confidence in the eyebolt’s mooring strength is critical to convincing boaters that they are as safe to use as conventional anchors. Hawaii researchers built a test fixture using a hydraulic truck jack. The test fixture was capable of a maximum vertical pull of 8,000 pounds, but a test limit was placed at 4,000 pounds. This was determined to be reasonable for use by vessels as large as 12-passenger dive boats in normal sea and wind conditions. None of the test holes gave up their eyebolt moorings under the 4000-pound test loading. It is recommended by the university that all eyebolt moorings be tested on installation and at least every year until experience has proven their long-term integrity (which may vary depending on the seabed composition).

Other regular maintenance operations should include:

· Monthly: Inspection and cleaning of buoy and other surface elements.

· Quarterly: Inspection of down line and all visible underwater elements.

· Semiannually: Replace buoy through line and pickup lines.

· Annually: Replace shackle pin in eyebolt shackle.

· Biennially: Replace down line.

It would appear, offhand, that the overall cost of the coral eyebolt mooring is no more than that of a conventional mud mooring using a mushroom anchor, yet it is far less destructive to a coral seabed than a boat’s anchor. This technique allows us to preserve and protect our wonderful tropical marine habitat, while at the same time still enjoying it – a fortuitous union of man and nature.

Earl Hinz has voyaged extensively in the Pacific and written six books and numerous articles on marine topics.

By Ocean Navigator