How to make sure your boat stays put
by Bill Sandifer
Not long ago I spoke with a sailor who had a 27-foot Sabre moored on a 250-pound mushroom mooring anchor as recommended by the local yacht club and the municipal harbormaster. During a big storm, his boat dragged and was destroyed on the rocks. He asked what kind of mooring to use in the future. Before offering suggestions, I did some research about mooring systems. Not much information was available.
|Mushroom anchor |
Most of the information we rely on seems to be based on the recommendation of some trusted source, such as a harbormaster or a local yacht club. Even Chapman's Piloting, Seamanship and Small Boat Handling has just a limited discussion of "permanent moorings" and offers a small table showing the recommendations of the Manhasset Yacht Club of Long Island, New York. These recommendations are good for their area but should not necessarily be trusted in other areas.
In deciding on a permanent mooring for your boat and location, consider the type of sea bottom, current, exposure, prevailing winds, water depth, tides, and swinging room. Also, try to get some local knowledge.
When I was a kid in Oyster Bay, New York, I had a 16-foot outboard boat on a mooring. I had a lot of discussions with Conrad, the local mooring guy, who set all the larger moorings in the harbor. He advised me on the type of mooring anchor to use and where to put it. His sole business -- other than a little winter fishing -- was setting and retrieving moorings. He charged a moderate sum to set a mooring in the spring, equip it with a new pendant every year, and pull it out in the fall before the harbor iced over.
He knew by long experience what size mooring to use for what size boat. He took into consideration the windage of each boat and used a heavier mooring anchor for boats with more windage. Conrad knew the composition of the bottom and the best place for each boat. He was a qualified harbormaster long before the term was coined.
There are three basic types of permanent mooring anchors in common use. These are mushroom, pyramid, and dead-weight anchors. In addition, some moorings use three anchors set at 120 degrees to each other with chain leading to a center point, and some moorings use a helix screwed into the bottom.
Mushroom anchors – The sailor whose boat had dragged had used the correct sources to arrive at his mooring system, but it was nevertheless not heavy enough or well enough dug in for the big storm that destroyed his boat. Digging in is important; otherwise, a mushroom anchor may skid and roll along the bottom. It's not common knowledge that a mushroom anchor must be tipped over and "set" in the same way that one "sets" a conventional anchor. One improvement for mushroom anchors is a new variety of these anchors with weights placed on the shank to help them lie horizontal.
|Pyramid anchor |
|Granite block |
Pyramid anchors – A relative newcomer to the field is a pyramid anchor similar to the old lead sinkers we used when bottom fishing as kids. Due to their shape, these anchors must lie on their sides and present a flat edge to the sea bottom when pulled against. My mooring in Mississippi for a previous boat was a triangular version of this anchor and worked very well (see article in Good Old Boat, January 2002). I set it in mud, but it would have done as well in soft sand or silt. I do not think it would have been as successful on hard sand or weeds.
Dead-weight moorings – One other reliable mooring used with hard bottoms is the dead-weight type. Some folks in Maine use granite blocks with a hole in them as mooring weights. These serve well but are very heavy . . . on the order of several tons. These weights must be set by a barge and crane or tripod. If enough wind is blowing they can still drag, but even when they drag they offer considerable resistance and drag slowly enough for the boatowner to make the necessary adjustments or move the boat to a safer mooring.
Three-anchor mooring – Another way to make a permanent mooring is to use three conventional anchors set in a circle at 120 degrees, each shackled to a common center chain. This system ensures that there is always an anchor upwind of the boat, no matter which way the wind blows. Since it takes three normal anchors of a large size to make it effective, this system is more complicated to set and more expensive to install. Each anchor in the set must be large enough to hold the boat in storm conditions.
Helix anchors – The helix screw anchor was developed for hard sand or weeds and other difficult-to-penetrate bottoms. This is a bigger -- a lot bigger -- version of the screw-type dog anchor found in pet stores. The principle is the same. The helix screws into the bottom and is prevented from pulling out by the wide flukes of the screw bit. These anchors must be set by commercial firms and are usually left installed in the bottom from season to season. Small versions of the helix can be set by divers in shallower water.
The first thing to remember when considering the weight of a mooring is that everything weighs less in water than it does in air. It may not float in water but it definitely weighs less. It's a question of how much less.
|Helix anchor |
Underwater weight can vary quite a lot. Some materials weigh 50 percent less in water than they do in air; some may weigh 30 percent less. In water, concrete loses almost one-half of its weight, granite loses more than one third, and iron loses an eighth of its weight. These are significant losses, particularly if the anchor is of the dead-weight type.
Since iron, as used in mushroom anchors, weighs 12.5 percent less in water, a 300-pound mushroom really weighs 262.5 pounds when submerged. Cinder blocks, cement, and old engine blocks are subject to the same laws of physics. Whatever weight you use must be proportionally heavier to give you the desired weight under water. This is why the experienced Maine mariners use several tons of granite to hold their boats on the hard and rocky bottom.
When I was a kid I built a dock using large granite rocks from the beach. I could not lift these rocks when they were exposed at low tide, but I could clear the sand from around them and, as the tide came in, I was able to move them to my construction site under water. I could never bring them above the water's surface, but I successfully built a pier with them. The hardest part was finding small enough rocks to build the above-water part of the pier. I did it and filled the center of the pier with cement. The pier is still there all these years later.
The weight of a mushroom anchor must be calculated as "dead weight" until it has dug itself into the bottom. The holding power of a mushroom that is not set is, at best, equal to twice its weight. When properly set, its holding power can increase to 10 times its weight, but once it's broken out of the bottom again, it will not reset. Instead, it will slide or roll over the bottom.
Take the case of an average 40-foot vessel in a 64-knot blow. The wind load on this boat will reach 5,500 pounds. While a 500-pound mushroom might seem to be enough for this boat, a 500-pound mushroom (438 pounds actual underwater weight) fully dug in will resist 10 times its actual underwater weight, or 4,380 pounds. If the load goes over 4,380 pounds, the mushroom will pull out of the bottom and offer little resistance as the boat drags it along the bottom. This is what happened to the Sabre 27.
In a comparison test held in Vineyard Haven harbor, the helix showed outstanding holding results. A 350-pound mushroom set 5 feet deep in mud required 2,000 pounds of force to pull it out. A 500-pound mushroom set in a sand bottom required 1,700 pounds to break it out. A 3,000-pound concrete block set in mud required 2,100 pounds to break it out. A 6,000-pound concrete block on a sand bottom required 3,200 pounds to break it out.
|"The first thing to remember when considering the weight of a mooring is that everything weighs less in water than it does in air." |
Compared to these, an 8/10 helix set in soft clay mud required more than 20,800 pounds to break it out. The "8" in this description refers to the bottom helix, which is 8 inches in diameter. The "10" refers to the top helix, which is 10 inches in diameter. Typically, shaft thickness is 1 1/2 inches in diameter. The shaft length varies depending on bottom soil conditions. Boats moored over a firmer soil will get by with a shorter shaft. Looser soils will require a longer shaft. In looser soils, there may be more than two helixes on a shaft.
The helix is strong, long-lived, environmentally friendly (as it does not drag over the harbor bottom), and some insurance companies give a premium discount when you use one as your mooring anchor. When the contractor installs a helix, he can accurately predict the amount of resistance needed to pull it out. This gives a great degree of assurance to the boatowner.
Of the five mooring systems, I like the helix best. But pros who set helixes are not available in every harbor. The pyramid would be my second choice, the three-anchor combination next, and the weighted mushroom fourth. I do not consider the un-weighted shank mushroom to be reliable unless it is carefully set by a knowledgeable person.
For the sailor whose boat had dragged, I suggested a 500-pound pyramid anchor for his intended 30-foot replacement boat. This mooring should have a length of heavy 3/4-inch chain equal to the high-water depth plus 5 feet. Connected to that, 1/2-inch chain should be at least twice the high-water depth. The mooring buoy should be connected to the lighter chain with a swivel. Then a minimum of 10 feet of 3/4-inch three-stranded nylon pendant with chafing gear should finish the mooring by also connecting to the swivel. If there is insufficient swinging room for this mooring, the pyramid should be 50 percent heavier for safety reasons.
Overkill? Perhaps. But it's cheap insurance for a boat. A three-to-one scope will let the angle of pull from the mooring to the cleat on the boat's deck be reasonable. Anything less than three-to-one and the angle will get too steep and create more lift than straight-line pull. Remember that lift will break out the best mooring anchor. When calculating the amount of scope, the height of the bow must figure into the equation. Some boats have a bow 3 or 4 feet above the waterline. If a boat is moored in 20 feet of water at high tide and the bow is 4 feet above the waterline, it needs 3 times 24 feet for a correct three-to-one scope.
One of the ways to alleviate a short scope problem is through the use of a kellet. This is an additional weight shackled to the anchor chain about 10 feet from the anchor to help decrease the angle of pull in all but the most severe storms.
A kellet provides a catenary to absorb some of the shock. In addition, a three-stranded nylon pendant will act as a shock absorber and decrease the shock on a mooring. A safety pendant, 30 percent longer than the everyday pendant, will provide extra safety if the primary pendant fails. If this occurs, the longer pendant will increase the scope in a blow just when it's needed.
If you have a year-round mooring, it will have more time to dig in and acquire greater holding power. In areas where the water freezes each winter, such as Maine, a winter stick can replace the summer mooring buoy for better ice protection. A winter stick is simply a length of 4 x 4-inch lumber sufficiently long to remain well above the surface of the water during the winter. There is little for the ice to grab on to, so a winter stick is suitable for marking your mooring all winter long. In some areas, a group of moorings are left over the winter and located once again in spring because they are chained together with the chain led to a piling.
The ways to maintain a mooring are limited only by your imagination. My mooring is left fully submerged all year long and shackled in winter with old stainless-steel rigging wire led to a tree near the water's edge. Each year when spring arrives, I can easily find the mooring, check the lighter chain, replace that if necessary, check the heavy chain, and lubricate the swivel.
A proper mooring requires a bit of work and additional maintenance expense, but the end result -- a secure mooring for your boat -- is worth the effort.
Note that the Manhasset Yacht Club mooring recommendations below are based on 20 feet nominal water depth with a minimum scope of 3.5-to-1 to as much as 4.5-to-1. There is no 2-to-1 or 2.5-to-1 in the table, regardless of boat size.
If I were to moor in similar conditions, I would follow the Manhasset Bay Yacht Club recommendations, except that I would want to use a helix, if available, or a pyramid, if a helix were not available.
Finally, even with the best mooring in the world, there is always the danger of some other boat dragging down on your boat and both dragging away. Choose your mooring neighbors carefully and help the other sailor with his selection of a mooring. This way both boats will be safer.
Helix Mooring Systems
P.O. Box 723, Belfast, ME 04915; 207-338-0412;
Toll-free: 800-866-4775; Fax: 207-338-0415
|The author: |
Bill Sandifer (Mooring systems, Page 29) is a contributing editor with Good Old Boat and a marine surveyor and boatbuilder who has been living, eating, and sleeping boats since the early '50s. He and his wife, Genie, sail an Eastward Ho 32.