Beaufort Confusions

[Gerry]

I was curious about the way our excellent Kiss wind generator works. At wind speeds less than 15 knots it generates barely one or two amps, at twenty knots it generates a steady five to ten amps, at twenty three knots it generates fifteen amps or more, and at wind speeds much above that it starts to overload and freewheel. A good friend explained that the wind exerts its pressure on an area, so its power does not increase linearly with wind speed but in proportion to the square of the wind speed.

This set me thinking about how our passion for quantification can lead to terrible over-simplification with catastrophic consequences. The Beaufort wind scale provides an example.

This classification system, though it is in use throughout the nautical world, suggests that the strength of the weather increases more or less linearly with wind speed. But if wind strength increase with the square of its speed then a twenty knot wind is not twice as powerful as a ten knot wind but four times as strong, forty knot winds are not four but sixteen times as strong, and eighty knot winds not eight but sixty-four times more powerful than ten knot winds.

Such misperceptions may have caught out some of the experienced sailors in the 1998 Sydney to Hobart race. Five yachts were lost, fifty-five people were rescued and six people died. The inquest (according to the book “Fatal Storm: The Inside Story of the Tragic Sydney-Hobart Race” by Rob Mundle) found that underestimating the difference in severity of a gale and a storm was a significant factor in the tragedy. Judging by Beaufort numbers alone, the forecast storm (Force 10) would seem to be only 25 per cent more powerful than a gale (Force 8). If judged by wind speed in knots, the storm (48 to 55 knots) would be only 40 per cent more powerful than a gale (34 to 40 knots). But by the correct measure – the square of the wind speed – the storm was twice as violent as a gale.

Is this right?

 

[Guest]

But any person should be able to say to themselves ... F5 is ok - maybe on limit for some depending on boat and location .... F6 is pushing the bounds etc.

OK - for newbies to boating I can see some logic in your post as they may assume wrongly as you suggest. But majority of boaters even newbies will check forecast ... look at seastate before venturing forth ..........

I have to admit to leaving berth more than once and poking noise towards harbour entrance ... found it a bit more than expected and gone home ....

Beaufort Scale was devised to provide a visual assessment of wind and seastate by viewing the sea etc. It was never scientifically determined ... and I think has done a fine job for many years .... No-one has really improved on it.

Onto your wind genny .... isn't that because it needs a minimum rpm to be able to start generating and overcome battery / system resistance etc. Once it's up to speed then the increase of ampage is better as now its overcome resistances etc.
As far as I know - it equates similar to an engine alternator as well .... for same reason.

 

[FWB]

From 'The Admiralty Manual of Navigation' Vol 1 1928 ...

Beaufort Number 1 ... 0.01 lbs/sq ft *
2 ... 0.08
3 ... 0.28
4 ... 0.67
5 ... 1.31
6 ... 2.3
7 ... 3.6
8 ... 5.4
9 ... 7.7
10 ... 10.5
11 ... 14.0
12 ... Above 17.0

* The pressure is stated as 'Equivalent pressure in pounds upon a circular disc of one square foot'

 

[Salty John]

The Beaufort scale was introduced to overcome the very situation you accuse it of failing to address: The fact that windspeed is a bad indicator of sailing conditions. In my opinion it succeeds admirably.
When you imagine a Force 8 you are supposed to think-"spindrift forms, clear foam streaks, waves could be as high as 25 feet". When you think of a storm Force 10 you are supposed to think-" long overhanging crests, great patches of dense foam streaks; surface of sea appears white; tumbling of sea is heavy and shocklike, waves could be 40 feet high".
Experienced sailors always "think Beaufort" because it addresses the conditions, not the wind strength.

 

[Krusty]

You are right about wind pressure being proportional to the SQUARE of windspeed, but I think that is well understood by serious sailors, Sidney-Hobart included.
And I don't see how the Beaufort scale is misleading: nowhere does it pretend to 'proportionality'. It was devised as a means of standardising commonly-used descriptions of wind in relation to the observable seastate; in open waters clear of land. It is only our modern preoccupation with numbers and the invention of wind instruments that has led to the introduction of measurable windspeeds.
It was, and still is, a useful guide for reefing. True; if the windspeed doubles, the pressure on a rig will quadruple, but the heeling-moment on the ship also depends upon the height of the rig, so reefing a square-rigged vessel from the topsails downward compensated, to some extent, for the quadrupled pressure.
Not so convenient for modern roller or slab-reefing triangular sails, perhaps.
But so what? We soon learn (sometimes the hard, uncomfortable way!) what sail we should be carrying in a ForceX when going to windward: and what we can get away with broad reaching!
Where's the problem?

 

[Stemar]

I rather liked a French version which included:
F8 Small children blow away
...
F12 Their parents blow away

 

[Salty John]

Wonderful!

 

[Gerry]

Right. The problem seems to be our modern over-reliance on numbers taken out of context.

Talking to some Americans at the local yacht club, they tell me that one of the reasons that Katrina was so devastating was that people failed to appreciate the difference between a category five and a category three hurricane. There were people who had the time and transport to get away but were expecting a twelve foot storm surge not the thirty foot surge that swept tens of miles inland.

The trouble is we are not experienced square-rigger captains and we take our numbers literally.

 

[machurley22]

Lots of good Beaufort scales on Frank Singleton's Beaufort page.

 

[macd]

I can't imagine anyone in the Sydney-Hobart suffered from this 'simple' confusion. In the case of that particular race, I think pretty well everyone was surprised by the intensity of the storm and the seas (more than the wind strength) it gave rise to.
You might equally suggest that our use of a linear system of speed measurement in a car is misleading, since it doesn't reflect the kinetic energy involved (and, come to think of it, you might have a point).
Non-linear scales of this sort, such as the logarithmic Richter for earthquakes, are widespread, and a practical way of describing non-linear phenomena.

 

[Rob_Webb]

I was bought up on Beaufort but since moving to NZ have got used to living with windpseed quoted in knots. And I now regard the Beaufort scale as a quaint tradition which the rest of the world doesn't use for a good reason.

The problem I would find going back to Beaufort is the very point made by everyone here i.e. wind pressure increases non-linearly with speed. Hence 16 knots feels a damn site harder than 11 knots. But they are both within the range of a so-called Force 4.

So for me, when I listen to weather reports in the area I'm sailing, and I hear the average wind speed quoted as 12 knots, it gives me a much clearer idea as to the right sails to prepare. Whereas if I simply heard Force 4, I could find myself undercanvassed or overcanvassed because the actual wind turned out to be at one end of the range or the other.

And it's even worse when they say "F4, gusting F5". That tells me that the wind speed could vary between 11 and 21 knots. A huge difference. And much too broad to be useful. Whereas being told "15 knots, gusting 20" is much more helpful.

 

[Captain_Chaos]

Gosh, your weather forecasters must be really clever to be able forecast wind speeds to the nearest knot; or perhaps in the southern hemisphere the wind blows at a constant speed so that when they tell you its 16 knots you can set the appropriate sails and lounge in the cockpit for the rest of the day knowing that the winds will be 16 knots because the mester on the radio said so..

So you are told that the average wind speed will be 12 knots; doesn't that mean that the wind speed will at times be greater than 12 knots and at other times less than 12 knots? It is a long time since I did statistics at school but my recollection is that the use of an average in decision making can be misleading. eg 5 hours wind at 5 knots, 5 hours wind 25 knots gives you an average of 15 knots..hmmm. Perhaps there is a more scientific calculation, but that makes the end product even more unusable imho.

I do not understand why quoting an average speed is any better than the beaufort system which itself indicates a range of wind speeds.

Giving an average speed could be misleading if that average is distorted by gusts. Surely it is more meaningful to state that the winds are for example F4 occasionally F5, rather than quote a single number.

 

[Krusty]

Interesting! I think this is another example of 'horses for courses'.
Local forecasts for Orkney, broadcast by Orkney Harbours radio, use the windspeed in knots, 'gusting to... ' ; for 6-hour periods of the next 24. That is what tugs and pilots need for tanker-berthing, and I find it useful since wind pressure is the main consideration when day-sailing in relatively sheltered waters.
However, when about to depart on passage for the Hebrides or Shetland or Norway it is the sea-state to be expected that determines which headsail to hank on, which to lash to the rail, and the reefs to pull down; and some other 'ready-for-sea' prep: so the Beaufort scale rules!
It may be antiquated, but not obsolete! Maybe that goes for me too!