I was trying to convert windspeed from miles per hour to knots and to then convert it to the beaufort Scale this evening, after wanting to judge the sailing cinditions for 'er in doors at the w/e. ( she's not a good sailor, and likes F0-4 with sun and a picnic.)

Unfortunatly last season every time we organised her to go afloat it was superb wind, F 4-6 where I just start to consider taking in a bit of sail on my long keeler.   Last w/e, Sun F0-3 she couldn't make it but this w/e, forecast Sat' rain F3-5 she can. I hope I'm still married by Sunday when it looks a bit brighter.

I've found this El Paso website that converts some of it for you

http://www.srh.noaa.gov/elp/wxcalc/windconvert.shtml

A quick rough conversion for Knots to MPH, 1 KT = 1 1/7 MPH

The problem with raw windspeed is that it doesn't really measure the pressure on the sails, this seems to be combination of wind speed, air pressure, air temperature and moisture level (dew point).

Have any of you noticed that on a nasty March/April day with wet air (rain or nearly rain) 18 - 20 kts of wind can be a complete handfull, wheras on a lovely summers day 25 knots is windy but just right.

Years ago I tried to make a table of which sail combinations one used in which wind strength (to get repeatability for racing) I found it was impossible to do.

On some days (esp early in the season with wet SW winds) we had a No3 Jib and a slab in the main at 20 Kts, wheras in Summer (nice dry hot SE wind) we could hold on to the Full Main (bladed off of couse) and No1 Genoa quite comfortably in 25kts. All measured on the same instrumment (B&G).

So I think it is a case of seeing how it feels (all very subjective) and going with what your instincts rather than the instruments tell you! I have a feeling that this is why aircraft have Pitot Tubes to measure the actual pressure rather than the airspeed! Perhaps we need Pitot Tubes at the masthead?

Yes, David, when I started sailing I did not believe an old salt who told me that winds of the same strength were "heavier" in the winter when the air was colder, but now I undertand exactly what he meant. And you are absolutely right about trusting your instincts and going by what you can feel rather than some theoretical diagram.

Brilliant website, Bob. I've always wanted to know when it was time to brail the top gallants!

Hi David,     Oh how i wish I had started sailing earlier.  It's all this sort of information that has come flooding in to me that has fascinated me and held my attentions for the last ten years. I have been messing about in boats since a toddler and feel so at home in most craft. One of the reasons I eventually ended up as a coach in a watersport at National and International level I suppose, is that I understood how boats move and react from outside influences. Sailing snippets such as those in your posting,  unfold a whole new bag of answers to the mysteries of sailing for me,  common sense though they may be to the experienced.    many thanks... Long may it continue!   Scotty

It's not just affected by temperature but by atmospheric pressure. David mentioned pitot tubes measuring actual pressure to give the speed of an aeroplane but what you would actually be measuring is "velocity pressure". If velocity pressure is calculated with air at 20 deg C and at 1013.25mbarg (normal atmospheric pressure for the purposes of many engineering and scientific calcolations) the velocity pressure would be 0 Pascals or Newtons per metre squared in still air and would increase as follows

1m/s = 0.6Pa

2m/s = 2.4Pa

3m/s = 5.4Pa

4m/s = 9.6Pa

5m/s = 15Pa

6m/s = 21.6Pa

7m/s = 29.4Pa

8m/s = 38.4Pa

You will see if you double the wind speed the velocity pressure is increased by x 4.

It doesn't quite end there because correction factors have to be applied for the density of air at different temperatures and atmospheric pressures for example in summer at low atmospheric pressure, say 30 deg C and 880 mbarg you would multiply the VP x 0.84, in the winter at high pressure, say 0 deg C and 1040 mbarg you would multiply the VP x 1.102. Moisture content of the air would also affect the density and correction factors but less so.

So you will see from this that the old salt was dead right!

Lovely

I didn't know there was a real scientific explanation/calculation.

I have been pontificating on this theory for years (usually in YC bars - and usually dismissed as a crank)great to be vindicated - Thank you Chas!

Cheers David!

It would be far more meaningful to measure velocity pressure as you really do need to apply the correction factors if you only measure wind velocity.

Velocity pressure indicates the ammount of force there really is on your sails; also, as I said, double the velocity and it's four times the pressure.

All the best

Chas

Just to give you all the idea of the actual force on your sails at various Beaufort Forces: 

Force 1 = 0.3 to 1.5 m/s - say 1m/s = 0.6Pa or Newtons per square metre

Force 2 = 1.6 to 3.3 m/s - say 3m/s = 5.4Pa

Force 3 = 3.4 to 5.4 m/s - say 5m/s = 15.0Pa

Force 4 = 5.5 to 7.9 m/s - say 7m/s = 29.4Pa

Force 5 = 8.0 to 10.7 m/s - say 9m/s = 48.6Pa

Force 6 = 10.8 to 13.8 m/s - say 12 m/s = 86.4Pa

Force 7 = 13.9 to 17.1 m/s - say 15 m/s = 135.0Pa

Force 8 = 17.2 to 20.7 m/s - say 19 m/s = 216.6 Pa

Interesting to see that the overall force on your sails increases by about two thirds with each increase in the Beaufort scale except for the very bottom of the scale.

Interesting that a Force 7 wind is equal in pressure to a force 6 plus a force 5!

Abd a force 8 is equal to a 7 plus a six!

Puts the odd force 10 into perspective I guess!

Ok ... another - this is Beaufort scale against Knots ...

http://www.r-p-r.co.uk/beaufort.htm

 Another that comparews various units of windspeed and gives equivalents based on Beaufort ranges ..

http://www.ikiteboarding.com/windspeed.html

The last I have printed of and pasted under my Navtex on board ... (and on the wall in my office !)

Another couple of observations about winter sailing especially inland.

As there are no leaves on the trees the gusts hit you so much more cleanly. Yes i know the speed is whatever it is but the visciousness of a gust is different.

Also 25 kts on a hot summers day is enjoyable and cooling but the same at 0º deg C  feels like survival conditions

At 0 deg C the air will be denser that at say 25 deg C - see my first posting on the subject. There's also chill factor.

Chas