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Beaufort scale & wind pressure

page 1

page 1 - Wind Pressure
page 2 - Beaufort Scale
page 3 - Rope strengths
page 4 - Chain strengths
page 5 - Rope & Chain guide

To convert: 1 knot = 1.152 mph = 0.515 m/s = 1.85 km
10 mph = 14.7 feet/sec - 10 mph = 4.41 metres/sec

The wind pressure can be approximated by: Pressure = x (density of air) x (wind speed)2 x (shape factor) - The shape factor (drag coefficient) depends on the shape of the body it impacts upon.

Upon a simple rough order calculation, wind pressure is proportional to wind velocity squared.  My friend 'Brian Robertson' tells me that the Newton's 'N' factor is the metric form of measuring force and torque: Thus with his help I have been able to compile the pressures in the two end columns using:

1 Newton = 0.224 lbs force - 1 Newton = 0.1019 kg force - I N/m = 0.737 lbs/foot

Beaufort
scale

 Windspeed max (m/s)

Approx
mph

Wind pressure (N/m2)

Approx
lbs sq/m
Approx
lbs sq/ft
 
0 0.2   0.03    
1 1.5   1.4    
2 3.3 7.5 6.8 1.5 .14
3 5.4   18    
4 7.9   39    
5 10.7   72    
6 13.8 30 119 26 2.47
7 17.1   183    
8 20.7   268    
9 24.4   372    
10 28.4 60 504 113 10.5
11 32.5   660    
12     > 660    

On the above figures, if your vessel 'presents' a frontal area to the wind of 15' beam x 10' high = 150 sq feet or 13.6 sq metres then, - - - at 6 on the Beaufort wind scale at 30 mph there is a total of 119 N = 2.47 lbs x 150 = 370 lbs load 'drag' factor imposed and, - - -

At 60 mph there is a load of 504 N = 10.5 x 150 = 1575 lbs load 'drag' factor imposed by the wind upon the above size anchored vessel.  And if your vessel is larger, say 25' beam and 18' high with a 'windage' area of 450 sq' - then in a 30 mph wind there is a pressure of 1110 lbs, and at 60 mph wind there is a 4750 lbs pressure, and to this must be added the even greater wave influences.

Many a large ship has 'foundered' on a rocky shore due to the failure of 'modern' anchors to hold them when they experience rudder problems or engine failure on windy days.  Having lived on the African coast I have seen many a ship drag its anchor due to wind pressure and suffer the consequence.

This coupled to the wave action shows the need to have the correct anchor type and size, and the correct chain size and length ensures that your vessel can remain 'at anchor.'

Note: The the above figures are simply informative in that they show the very great increase in anchor loading as the wind rises, but added to the figures above are the drag and lift factor of the waves that also increases proportionally as the wind rises, as do wave heights. 

Many an anchor will 'handle' the 'lighter' wind loading, but few 'boaties' seem to realise the lift factor created by rising wave action that is proportional to the wave size and the flotation capacity of the boat or the greater rise in pressure in higher winds.  This affects all craft to a great degree.

All these factors are but to assist you in realising that need for plenty of 'chain' to obtain a good and long catenary effect, as well as a safe anchor when the wind rises, and your craft is not anchored behind a sheltered headland. 

When you are anchored off an open 'beach' and your craft is subjected to wind and waves, then your entire anchor system must be 'capable' and strong enough to handle 'above' average conditions.  This need may also arise within seemingly sheltered bays that can become a 'frenzy' of waves.  Noah knows, and many of you may also have experienced this.

An insurance policy it taken out to 'cover' you for those very days.  So why not ensure that you craft is safe every day by having a 'Noah's Ark anchor' embedded in the seabed below so that your heart does not have to turn to 'frost' when the wind begins to blow?

page 2

The 'Beaufort' Wind Scale
For 'estimating' the wind speed

Beaufort
Scale

Wind speed
knots

Description

Sea Condition

0

0

Calm Sea like a mirror

1

1 - 3

Light Air Ripples but without foam crests

2

4 - 6

Light Breeze Small wavelets.  Crests do not break

3

7 - 10

Gentle Breeze Large wavelets.  Perhaps scattered white horses

4

11 - 16

Moderate Breeze Small waves.  Fairly frequent white horses.

5

17 - 21

Fresh Breeze Moderate waves, many white horses

6

22 - 27

Strong Breeze Large waves begin to form; white foam crests, probably spray

7

28 - 33

Near Gale Sea heaps up and white foam blown in streaks along the direction of the wind

8

34 - 40

Gale Moderately high waves, crests begin to break into spindrift

9

41 - 47

Strong Gale High waves.  Dense foam along the direction of the wind.  Crests of waves begin to roll over.  Spray may affect visibility

10

48 - 55

Storm Very high waves with long overhanging crests.  The surface of the sea takes a white appearance.  The tumbling of the sea becomes heavy and shock like.  Visibility affected

11

56 - 63

Violent Storm Exceptionally high waves.  The sea is completely covered with long white patches of foam lying in the direction of the wind.  Visibility affected

12

64+

Hurricane The air is filled with foam and spray.  Sea completely white with driving spray.  Visibility very seriously affected.

page 3

~ Rope strengths ~

.ROPE STRENGTH – AVERAGE BREAKING LOAD Kg

Dia.  mm

6

8

10

12

14

16

18

20

24

28

32

Circumference ins.

7/8

1

1

1 5/8

1

2

2

2

3

3

4

                       
Manila & Sisal

300

450

700

1100

1350

1750

2250

2750

4000

5100

6650

Italian hemp

450

600

750

1150

1400

1850

2350

2900

4200

5600

7250

Hardy hemp

390

640

1030

1470

2020

2870

3330

4470

5940

13600

Nelson polyprop.

670

1210

1760

2410

3250

4200

4840

6210

8430

10800

18840

3 str.  std.  p/ester

760

1420

2360

3200

3930

5090

7100

7770

11210

14640

Double braid

900

1590

2210

3180

4330

5660

7160

8840

3 str.  pre-stretch

1100

1830

2800

3550

5250

16 plait matt p/e

1490

2890

3770

5130

6700

8410

Marlow braid

1070

1820

3190

4320

5970

7840

9040

10190

3 str.  Nylon

1910

2720

3750

5100

6640

7920

9790

14260

18640

22600

Spectra/Kevlar

1480

3180

4510

6810

10000

11470

7x7 galv.  wire

2330

4150

6310

Thanks to the following for information used above:-
The Admiralty 'Manual Of Seamanship' Vol.  2 1951
Marlow Ropes Ltd.

Other 'sourced' information:

It may surprise you to find out that the safe working load for most kinds of rope as stated by the manufacturers is only 20% of the tensile strength.  The tensile strength is the 'breaking' load of new rope as given in the tables above.  Therefore, in the two highlighted 'green' tables you will see that 12 mm polypropylene has a breaking load of 2410 kilograms.  Thus its safe working load is only 500 kg approx.  when new.

You will see that 20 mm nylon has a breaking load of 9790 kilograms.  Thus its safe working load is only 2000 kg approx.  when new. In the maritime industry the Safe working load limit is set at one sixth of the breaking strain.

Now consider the fact that any time you tie a knot in a rope, you effectively cut the tensile strength in half.  The knot when tensioned cuts the line.  While certain kinds of knots damage the line less than others, the 50% loss of tensile strength is a good general rule to live by. If you join rope using a 'splice' it retains 90% of its strength.

DYNEX DUX 75

Pre-stretched braided rope having ultra high strength. Stronger than steel and suitable for rigging.
Also suitable for Para-anchoring (refer to Para anchors)

Dia Kg -100 mtr Min strength tons
     
7 3.4 7.9
11 8.3 18
16 18.6 46
21 32 64
33 67 124

Available from 'Rope, net & twine Ltd. Canada

page 4

~ Galvanised anchor chains ~

The following table is taken from the Australian PWB chain web site for your convenience.

Manufactured from the highest quality lower tensile steel, PWB Anchor Grade L Chain is the premier chain for general lifting purposes, and the preferred choice for application in marine environments.  Ideal for anchoring and mooring applications, Grade L chain’s short link configuration matches the clear majority of marine jockey wheel indentations, providing a neat fit to prevent jamming and ensuring safe and reliable pulling power.  The WLL column is the working load limit. 
Chain Size WLL S R F Self-colour Galvanised Link Marking
(mm) (tonnes) (mm) (mm) (mm) Part No M/100kg Part No M/100kg
6 0.40 6.3 18.8 21.6 40073 116.0 40074 110.0 PWB-L
8 0.64 8.0 23.8 27.0 40075 74.4 40076 67.8 PWB-L
10 1.00 10.0 29.8 34.3 40077 45.4 40078 43.2 PWB-L
13 1.70 12.7 38.0 43.0 40079 29.6 40080 27.8 PWB-L
16 2.58 16.0 47.8 54.9 40081 17.8 40082 17.5 PWB-L
20 4.03 20.0 59.2 68.5 40083 11.3 40084 11.1 PWB-L
22 5.06 22.0 64.9 77.0 40085 9.5 40086 9.525 PWB-L

The following note is from Kinugawa Chain manufacturing company 'records' Japan.

"The history of the anchor chain is old.  The following description is left in the ancient documents.  "Caesar broke 'Venetian' off the Brittany Peninsula south shore in B.C.  At that time, their ship was connected with the iron chain." It is understood that an anchor chain has already been used for 2000 years  from this description. 

English blacksmith acquired a patent in 1643.  (Phillip White) The English navy thought that it was intended to be adopted, but wasn't actually used.  The ship named 'Ann Isabela' in the sea with the tremendous drift ice in 1808 used an anchor chain, and then it was anchored.  It became reputed that the anchor chain of this ship wasn't cut off but held, though rope of some Tomari ships in the area was cut.

Incidentally, this anchor chain was created by Robert Flinn.  The shackle was developed in the same year, and then Swivel was developed in 1811, too, and the Stud was devised in 1813.  The use of the anchor chain proceeded rapidly by these ideas as did its development because the inconvenience of chain disappeared, and its strength became stable and the connection of the chain became easy.

page 5

'Ark' anchors 'rope & chain' size guide.
The larger anchors will usually only have all chain rode

 
Anchor
size
Recommended
rope size - mm
rope
safe length
Recommended
chain size
chain
safe length*
         
Ark - 2.5 kg 8 20 mtr 6mm 6 mtr
Ark - 5 kg 10 40 mtr 6 mm 15 mtr
Ark - 8.5 kg 12 50 mtr 8 mm 40 mtr
Ark - 23 kg 12 60 mtr 10 mm 60 mtr
         
Ark - 33 kg 20 75 mtr 13 mm 80 mtr

Ark - 55 kg

 

 

16 mm

100 mtr

Ark - 65 kg

 

 

16 mm

120 mtr

Ark - 85 kg     20 mm 150 mtr
         
Note: safe length* - Is the minimum safe length I recommend when travelling in open waters.  It is 'obvious that protected anchorages having little wind, little current, and low waves, requires very little chain length.  Longer chain is recommended for anchoring off unprotected anchorage's in wind speeds in excess of 35 knots.  You can never have 'too much' chain length.  

Note: The above figures are for bad weather and strong current situations as a guideline only.  Your choice, your experience, your personal decision at all times in all things.  Ignorance and 'blame' leads you to spiritual destruction.

Note: In my opinion in a high wind situation, it is better to use all your available chain and rope on one line of great length on one anchor, rather than using two anchors with less chain and rope on each.  The more chain 'out,' the more catenary 'action' that takes the strain off the anchor as well as the 'bow' of the vessel, and the extra chain also adds 'weight' and drag to the 'anchoring' effect as such if much of it is lying on the seabed. 

There are definitely situations where you need to use a scope of 10 or 15 to 1, especially when you are 'hiding' behind a low lying sand island and the wind is being funneled across it between two hills.  For the 'reported' 50 knots and gusting when being 'funneled' can rise to 70 knots or more, and if your vessel is only in 10 to 20 feet or water, you will still need plenty of chain and rope even if there is 'little' wave action. 

Too little rode length in these conditions means that your 'line' will be already stretched 'taught' when 'caught' by a strong added gust, and this will 'jerk' your vessel with unbelievable force with possibly bad consequences.

Note: In most cases, meteorological wave height and wind speed forecasts may be exceeded by 50% or more.