Measuring/Estimating Windspeed from Deck-level

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Babylon

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I don't have a masthead-mounted wind instrument, and have therefore learnt to 'read' wind direction and guess its Beaufort force from the direction of ripples on the water, wave size, white-horses, etc, while the masthead wind-vane gives me a visual on (apparent) wind direction.

I do however have an inexpensive hand-held anenometer, but this can only give the deck level (apparent) wind-speed.

My question therefore is this: how can I 'adjust' the reading of the handheld to account for friction from the sea and topsides down at this level? (Standing in the cockpit with my hand held high, I'm assuming about 3m total above sea-level; all readings of course apparent, can mentally adjust to true.)

EG if hove-to (so apparent=true windspeed), handheld reads say 15kts (F4), what is the actual windspeed on the sails likely to be?
 
I find it is quite straightforward. If you can see your reflection clearly in the sea surface, hoist the iron topsail or just retreat to the bar. If you can feel a breeze on your head but your hat doesnt blow off, hoist full sail and off you go. If your hat blows off and lands on the ground within 10 feet, tuck one slab in the main and roll out the full genny but keep spinnaker in the locker. If your hat blows off and lands more than 20 feet away, tuck 2 slabs in the main and reckon on easing out the genny until you get to 7 knots boat speed. If your hat blows off and disappears into the surf crashing over the breakwater, retire to bar. If you can hardly walk into the wind, double up the mooring lines just in case it gets a bit breezy and then retire to the bar.
 
Thanks for the link Sarabande. Unfortunately this calculator's lowest level for a reading is 10m and above - my readings are taken at about 3m, which is pretty close to friction-disturbed airflow. What I'm looking for is a 'rule-of-thumb' to adjust my readings... ie something like 5%, 10%, 15% etc.
 
Beaufort's descriptions are very good as an indicator. But in the end - boat rounds up then reef. SWMBO complains about heel - reef. Reefed sails flap like Nora Batty's drawers - let more out.

The numbers dont really matter
 
I am fairly sure that the wind speed at 30ft is double that at ground level.

The only reason I remember this useless piece of information is that it so surprised me when I first read it!

Apparently this difference occurs as a result of the friction between the ground/sea slowing the wind down.

I spent most of my sailing life without an anemometer and on reflection the main disadvantage was being completely unable to corroborate my boasting at the bar afterwards. /forums/images/graemlins/grin.gif
 
Thats difficult to answer. I sail a 37ft cat and have a masthead instrument and one on the aft instrument bridge at about 10ft above WL . Mast is 50 ft above WL.
They both read apparent wind of course. When going downwind in light airs both read the same within a knats cock. As wind increases the masthead can show considerably more until at about 20knots (at the top) the lower reading can be as little as 12. Of course by that time I am doing about 9 Knots boat speed so read that as 29 and 21. To windward the lower reading shows higher sometimes due to the accelerated wind speed over the back of the sail so that's misleading. However, if close to land, say Alum bay with gert big cliffs, catabatic air currents can turn this entirely on it's head and I see higher wind speeds at the lower sensor than above. (Yes that surprised me too but it is so) I think therefore that increase of wind speed with height is only true for open water and any headland or high cliff will affect it. Sea breezes late afternoon in the Solent are often remarkably near the water and I have even had the situation when anchored in Norris Bay that at water level I was getting a breeze from the north but at masthead it was SW!
 
There is no single adjustment factor, it depends on the degree of turbulance, how much the air slowed by friction over water is mixed with the air from higher up. I think Frank Bethwaite's book High Performance Sailing had some useful measurements in it.
But measuring windspeed is an empirical science anyway, if you measure it the same way it will give useful information by comparison with previous info. One of the most useful applications of a measured wind speed is deciding which jib to go for approaching a leeward mark. You soon establish a speed which tells you 'No3 Jib'. Of course once you are beating it's obvious how windy it is!
 
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Beaufort's descriptions are very good as an indicator. ...

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All responses are quite right. As I indicated in my OP, I like to sail by feel and rule-of-thumb estimation (and I don't even have tell-tales sewn into the sails). I was really just interested to know how 'out' my hand-held readings are likely to be.

As to 'what I would do with these readings', I tend note them in the log-book against my own visual Beaufort estimate of windspeed, compare them to the forecasted figures, and maybe later check what the actual recorded windspeed was.

I know last Sunday, for eg, that we were double-reefed beating back from Cowes to Southampton. I guessed the windspeed to be about F6 plus; later checking Bramblenet for the recorded speeds at the time showed 25kts gusting 35kts (F6 gusting 8). Its just about getting a little more confident in my guesses!
 
sorry for the false lead.

Ruggles (1970) The vertical mean wind profile over the ocean for light to moderate winds. J Applied Meteorology, 9, 389-395.

wind info clicky (2)


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You will also need info on t he changing angle of the wind relative to height (sometimes known as windshear angle).. Sailmakers build an allowance for the changing angle of attack as the sail height increases.
 
I think I would add a qualification to your approach. I think wind speeds can "gust", but I don't think the Beaufort Wind Scale Force can "gust". So the wind could be 25 gusting 35, but the Force would still be Force 6 - not F6 gusting F8.

I say this because (AFAIK) the Beaufort scale is intended to describe overall conditions rather than a momentary snapshot. It reflects both wind AND sea state. And sea state won't appreciably change with a 60-second gust.

So someone who thinks they have been out in an F8, just because they have been out in conditions in which the gusts reached the F8 envelope, will be unpleasantly surprised when they go out in a real F8.

In fact, using the Beaufort scale without instruments is exactly what mariners did for years, and indeed is the purpose behind standardising the observations with some objective criteria. Your use of a handheld device to confirm your observational assessment should do exactly what you expect - give you comfort and confidence in your assessment of the conditions.
 
I am not Welsh so don't need beer in me to make sense. When close inshore a gentle sea breeze off the shore on a summers afternoon can be from a quite different direction than the prevailing SW air current at 50 ft up. Perhaps meteorology in Swansea is different from elsewhere in the world but in general this is called a catabatic effect look you see??
 
Mike, not doubting you for a mo. You sound very sober to me.

I'm not Welsh, but we are so lucky to have some terriffic coasts, and I am sure we get winds going in different ways as well. Never seen them at 50' though.

I use the sea as an indicator for wind strength, but as said who cares. I just know when its time to reef or go home. Or stay put for that matter.

As an ex dinghy sailor, tell tales on the mast stays did the trick for me, as does the ensign now, along with the windvane on top of the mast. The wind on the face is also a good indicator.

Mj
 
It seems to me that most of the increase in wind speed occurs in the first 2 metres above sea level i.e. about where your face or hand held instrument will be located and so any estimate at this level won't be very different from the apparent wind felt by the sails.
 
When I used one of these gadgets I used to reckon the peak speeds I got in the cockpit were about the same as the mean speed at mast height for rough purposes.
 
Met reporting on ships - they had two methods .... the handheld as OP mentions, second was the mast mounted system. (Similar to yotties but at vastly different heights and difference of heights !)

I was always told that entries based on handheld were not accepted for detail and forecasting, but noted for tendency only. We used to transmit them just the same, but Met-Office decided value or not.
Various shipping Met publications did mention that deck-level readings were unreliable not only for speed but also direction if any items interrupted air-flow such as masts (!) vents etc. Uneven surfaces can cause vertical as well as horizontal deflection for speed and direction.

That's all before you get into Katabatic, Anabatic winds, Mistrals etc.

I have a professional Handheld meter - presented to me when a ship was scrapped ! I use that quite often but I stand well up from transom and in as clean air as possible. My reasoning is that major part of sails are down at that level and that is main effect to boat.

Later I do plan to fit wind mast head gear ... when I've saved some pennies..... maybe even NMEA 2000 to my Plotter !
 
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