Hurricane season is on us. It's time to assess the strength of your
anchoring system.

Wind drag on a boat is a function of the projected area at right angles
to the wind, the square of the wind speed, the density of the air, and
the dimensionless coefficient of drag which depends on the shape of the
boat. Drag coefficients have been determined from wind tunnel tests.
Some representative drag coefficients are:

Open parachute (or efficient spinnaker) = 2.0
Hollow hemisphere, concave to wind = 1.7
Flat rectangular plate = 1.28
Wires, cylinders, masts = 1.0
Cargo ship, wind dead ahead = .95
Fishing trawler, wind dead ahead = .9 to 1.05, depending on
superstructure.
Streamlined passenger vessel = .70
Recreational trawler = .70 to 1.0, depending on superstructure.
Sphere = .47
Hollow hemisphere, convex to wind = .38
Modern automobile = .26 to .35
Airplane = .09

Using Area in sq. ft., wind Velocity in knots, and the U.S. Standard
Atmosphere for air density, the equation for Drag in lbs. can be
written as:

Drag = .00339 x Coefficient of Drag x Knots^2 x Area

In a 20 kt. wind, a boat with an area of 100 sq. ft. at right angles to
the wind with a drag coefficient of 1.0 will have 135.6 lbs. of wind
pressure on its surface. A conservative way to estimate frontal area is
to multiply the beam by the height of the superstructure. An even
simpler way is to multiply the beam by 3/4 of the beam. By this
calculation, a Willard 30' trawler has about 100 sq. ft. of area. A
Nordhavn 40 has 160 sq. ft. of area. A Nordhavn 47 has about 195 sq.
ft. of area. A Nordhavn 72 has 330 sq.ft. of area.

The strain on the anchor rode in hurricane force winds is far greater
than most boaters imagine. Remember that the strain rises as the square
of the increase in wind speed.

For each 100 sq. feet of area:
20 Kts = 136 Lbs.
40 Kts = 542 Lbs.
60 Kts = 1220 Lbs.
80 Kts = 2170 Lbs.
100 Kts = 3990 Lbs.
120 Kts = 4882 Lbs.
140 Kts = 6644 Lbs.
160 Kts = 8678 Lbs
180 Kts = 10984 Lbs.

The breaking strength of proof coil 5/16 chain is 7600 lbs., of 3/8
chain is 10,600 lbs.
The breaking strength of 1/2 nylon is 7500 lbs., of 5/8 nylon is 12,200
lbs., but nylon rodes should be loaded to no more than 1/4 breaking
strength to assure adequate stretch with a margin of safety. At first
glance it appears that a 3/8" chain, typical of many trawler yacht
anchoring rodes, would be sufficient to hold a boat with 200 sq. ft. of
area in a Category 2 hurricane but all is not what it seems. Long
before the chain broke, it would be stretched bar tight. Any transient
forces of waves or surge would be transferred directly to the anchor,
which, in all probability would be **** out of the bottom. Either
than or the cleats would rip out of the deck.

Assuming an adequately sized anchor, an effective rode will spread
transient forces of wave and wind out in time to reduce the forces
below the anchor's horizontal holding power. This attenuation is a
function of the rode's elasticity gained either through stretch or
catenary forces. A heavy all chain rode permits a shorter scope than a
rope, wire, or composite rode. But to equal the elasticity of a nylon
rode a chain would have to be too heavy for most recreational boats to
carry.

Stretch. while an enemy in most tasks involving rope, is an asset in
climbing and anchoring. The rock climber wants a rope that will stretch
if he/she falls to attenuate the shock. Boat anchoring also profits by
rope stretch. The one time elastic limit of new laid nylon rope is
about 25%. This will occur when the rope is loaded to about half its
breaking strength. Stretch it much further and the fibers will
permanently deform or break. Stretching to this point will weaken nylon
rope and it should be replaced as soon as possible. Nylon loses about
20% of its ultimate strength when wet. But wet nylon will handle
transient shock loads even better than dry nylon. The water acts as a
lubricant, permitting fibers to slide over each other and minimizing
internal heating and friction.

It is important to remember that the safe working load of a rope is
dependent on its use, not its ultimate strength. The SWL is the
breaking strength of a rope divided by a safety factor, generally from
2 to 12, depending on use. Tasks that involve overhead loads in working
areas require the highest safety factors. A 3/8" nylon rope with a
breaking strength of 3000 lbs, used in an overhead lifting situation,
might have a SWL of 200 lb. Dacron rope, roughly equivalent in breaking
strength to nylon, might have a SWL of 600 lb. in the same application
because of its lower stretch. The safety factor is generally determined
by lawyers not engineers.

In anchoring situations, where we want stretch to attenuate transient
shock of wind gusts and waves, a safety factor of 3 or 4 might be
appropriate. Thus for a boat with 100 sq. feet of projected area, an
appropriate anchor rode for a storm anchor sized to resist a 60 kt.
wind would be 1/2" nylon at a minimum. A 5/8" rode might offer a
greater safety margin but would offer less stretch in lighter wind
conditions. Naturally all cleats must be sized and secured to handle
the maximum shock loading expected. Because of gusts and wave action
this may be two to three times the load listed in the wind force table.



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