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Beaufort scale & wind pressure page 1
To convert: 1
knot = 1.152 mph = 0.515 m/s = 1.85 km The wind pressure can be approximated by: Pressure = ½ 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
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? The 'Beaufort' Wind Scale
~ Rope strengths ~
Thanks to the following for
information used above:- 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.
Available from 'Rope, net & twine Ltd. Canada ~ Galvanised anchor chains ~ The following table is taken from the Australian PWB chain web site for your convenience.
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.
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.
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