Can Your Gear Handle Mother Nature?
Although there is a plethora of details involved in anchoring, the entire process is predicated on the fact that the gear needs to be able to resist the loads that Mother Nature will demand of it. While determining what this load can be is fairly easy, it isnt always as straightforward as it might seem.
FORCES AGAINST THE BOAT
For anchoring, the load that is of concern is the force of the wind against the boat, thus on the ground tackle. From this figure, the other loads, which also need to be considered, can be calculated. Fortunately, these figures are readily available, and, they, or the formula for calculating them, can be obtained from the ABYC, naval architects, boat designers, anchor manufacturers, and from some books, such as Earl Hinzs The Complete Book of Anchoring and Mooring.
WINDAGE
If the figure for the wind load on the ground tackle is based on a typical sailboat, as is often the case, boats with more windage, such as many trawlers, houseboats, and multihulls, will have loads on the ground tackle, depending on the boats windage, often 150 percent more than those on the sailboat.
SURGE LOADS
When there is moderate protection from seas, slightly more than half of the load carried by the ground tackle will be created by the wind; surge loads (seas and waves) produce the remainder of the load. But this portion of the load can vary greatly depending upon the amount of protection that the boat has from the seas.
If the boat is well protected from the seas, there may be little, if any, additional load on the ground tackle over that produced by the wind. But, if the boat is anchored in a location where there is very little or no protection from the seas (and waves), this figure will be higher, 150 percent higher, possibly more, especially if coupled with winds of long duration.
An easy way to calculate the load on the ground tackle, if it isnt already included, is to just double the winds load, then modify this amount if the protection from seas is greater or lesser than moderate.
WIND GUSTS
While typical wind gusts can exceed the sustained wind speed by 130 percent, some gusts can come close to doubling the speed of the sustained wind. So, when sizing ground tackle, ignoring the loads that wind gusts can contribute can be a dangerous practice. A good rule of thumb would be to size each piece of gear so that its working load limit (WLL) equals 130 percent of the load that the sustained wind will produce; or 150 percent, if an extra margin of safety is desired.
For anchors though, these figures may prove inadequate, as anchors need to be sized to provide enough holding power to resist the highest load that will be encountered and that includes the infrequent gusts, which can double the force of that produced by the sustained wind.
OSCILLATION
A boat not having the freedom to keep its nose into the wind, such as when anchored fore and aft, can, depending on the direction of the wind, have more load on its ground tackle than if the boat could keep its head into the windoften 150 percent more.
DURATION
Winds of long duration allow the seas and waves to develop more fully than do winds that are of shorter duration. Because of their larger size, these seas and waves have more power; plus, the longer the wind blows, the more time it has to wreak havoc.
Gear chosen for conditions of longer duration will need to be heftier than what may otherwise be adequate for winds of shorter duration. How much heftier? A good rule of thumb to follow: if the item has gone past the halfway point of its WLL, or for an anchor, its holding power, it would be more prudent to go to the next size up.
CURRENT
As mentioned in The Complete Book of Anchoring and Mooring, the load on the ground tackle of a 40-foot boat in a 5-knot current amount to about 300 lb., about the same load that a 15-knot wind would produce, surge loads included. A wind-driven current from a 60-knot wind with unlimited fetch is approximately 1.2 knots, creating a rather insignificant drag on the boat. But this load is not to be confused with the surge loads or loads that are substantial.
CONSERVATIVENESS
How conservative to be when computing the load on the ground tackle is a personal decision. However, it should be noted that those who oversize their gear seldom have the problems that often plague those who dont.
Since the term oversize seems to be viewed so often in a negative light, maybe switching to the term big enough instead would be better!
THE KEY
Having calculated what the highest loads will be on the ground tackle, the key to sizing all of the components, from the belaying point all of the way out to the anchor, is now at hand. It is this figure that is the minimum WLL that each piece of gear will need, and it is also the minimum holding power that the anchor will be required to have.
CAUTIONARY NOTE FOR CHOOSING ANCHORS
Many charts for sizing anchors are predicated on the anchor being used in hard sand. Sticky clay has only two-thirds of the holding power of hard sand, while softer bottoms, like soft sand, mud, and loose gravel have only one-third that of hard sand. So an anchor must be sized to have the holding power that the calculations suggest for the type of bottom in which it will be used. This may require an anchor that is two to four times bigger than what a chart suggests.
EXAMPLE
The Complete Book of Anchoring and Mooring states that in 30-knot winds, a 40-foot boat can have a load of 1,200 lb. on its ground tackle. Double the wind speed, and the force against the boat quadruples, so, at 60 knots, the force against the boat is 4,800 lb.2.4 tons!
But, it is what may be missing from these figures that can make a difference. In this example, these figures are for a typical sailboat with moderate protection from seas and with the boat having the freedom to oscillate. These figures also include the load on the ground tackle that results from surge loads, but do not include any loads caused by wind gusts. In addition, these figures are calculated conservatively. Should the boat be a trawler or multihull, anchored fore and aft, or if it will have much less protection from the seas, a prudent mariner would increase these figures by 150 percent for each of these factors if they are applicable. To handle wind gusts, another 130 percent (or maybe 150 percent) should be included, but for the anchors holding power, 200 percent would be the more appropriate multiplier to use.
When these figures are contemplated, it is no wonder that anchoring in protected waters with big, hefty gear comes highly recommended, and if the boat can be located so as to not receive the full force of the wind, so much the better. But, if this cannot be done, then these figures illustrate the need to have gear that can perform as Mother Nature will demand of it.
MINIMUMS, NOT MAXIMUMS
Finally, recommendations given for ground tackle are minimum recommendationsa starting point, not the stopping point. If in doubt, get bigger, stronger, heavier, or longer, but think long and hard before doing the opposite.
