Most of us realize that as the size of the boat gets bigger, beamier, has more top hamper, or as the wind speed rises, the anchor must be bigger. However, few of us are aware that an anchor should also have a built-in ‘safety factor.’ Classification society standards recommend that anchors have twice the holding power needed.

Size

In sand, unless the manufacturer states differently, the size suggested by the manufacturer’s sizing chart is usually adequate, but only in wind speeds less than 25 knots; for gale-force winds, the anchor should be at least one size bigger, and for storm-force winds, the anchor should be at least two sizes bigger. Keep in mind that even in short-duration squalls, wind speeds can easily reach storm-force levels. These are minimums, don’t hesitate to go bigger.

Bottom Composition

Taken from Naval Shiphandling, R.S. Crenshaw, Jr., the following is a list of the coefficients for various types of soils, each in comparison to firm sand:

As seen in the list, sea beds vary in density, and an anchor will have less holding power, the softer the bottom. The only way to compensate for this decrease in holding power is to use a bigger anchor. Go up at least one size in each category from that used in sand, and even bigger if the bottom is very soft.

• Stiff, dense clay- 1.5 the resistance of sand
• Firm sand- 1
• Sticky clay- .66 the resistance of sand
• Soft mud- .33 the resistance of sand
• Coarse, loose sand- .33 the resistance of sand
• Gravel- .33 the resistance of sand
• Hard (rock) bottom- 0

Fluke Angle

An anchor’s fluke angle is designed to excel in a particular type of bottom, thus it will hold less optimally in other types of bottoms. An anchor with a fluke angle designed for sand, as are most general-purpose anchors, but used in a soft bottom will need to be at least one more size bigger even than that noted above.

Conversely, anchors with a fluke angle designed for ‘mud’, but used in sand will set poorly, if at all, and hold even worse. Do not use these anchors in sand¾navy-style anchors and the myriad of like designs fit into this category.

With an anchor that has adjustable fluke angles, adjusting the fluke angle to its ‘mud’ position enhances the anchor’s holding power in ‘mud’, but there remains the question¾“even though the anchor holds better, will it be enough?” If not, here, too, a bigger anchor must be chosen.

Since there are so many boats that drag due to anchors that are too small, these points bear repeating:

• The higher the wind or the more area the boat presents to the wind, the larger the anchor needs to be.
• The softer the bottom, the anchor needs to be even bigger than that for sand.
• An anchor with a fluke angle for sand needs to be bigger still, if used in soft bottoms.
• Adjustable anchors big enough for sand, but adjusted to the ‘mud’ setting, may need to be bigger still, to have the necessary holding power in ‘mud’.
• Anchors designed for ‘mud’ should not be used in sand.

Rudy and Jill have been living aboard and cruising since 1997.

Their book, “ANCHORING-A Ground Tackler’s Apprentice” is available as hard copy or e-book from Waterway Guide Media, LLC.

Their popular PowerPoint presentations:

‘Avoid the Art of Dragging

‘The Nuances of Anchoring’

‘How We Cruise Simply’

‘Navigating the ICW-Tips & Tricks’ are available to boating groups just ask [email protected], phone 850-832-7748, or go to trawlertrainingabc.com (no text messages, please).