Simviation Forums

Hi all,
My main hobby/sport is sailing, particularly racing. I'm a member of a S.E. England club, sailing on the coast.

A regular topic of conversation is "gutless wind" or wind with no power......The cups on the clubhouse roof may read 21 kts, we rush to get out and have a blast only to find that there is little power in the wind. It doesn't happen that often but sometimes the lack of power is quite remarkable.

We speculate as to the cause: could it be low pressure, is it temperature related, could it be humidity?.....is it a combination of factors?

Perhaps you real pilots could cast a bit of light on this phenomena, I guess it must effect aircraft quite considerably.

What causes this? and is it predictable?....any info on this would be appreciated.

A truly fascinating question!

As  keen Motorcyclist for many years I am equally interested in all aspects of; "Weather" and its various effects!

Looking forward to some comments on the subject!

Paul... [smiley=thumbsup.gif]...!

If moving air has sufficient mass and velocity to cause an instrument to read 21 kts, it's 21 kts, regardless of pressure. An anemometer doesn't use radar to track the speed of air molecules- there has to be X volume at X velocity for it to read X knots.

So my only guess is that the wind is simply blowing harder on the clubhouse than it is over the water.

Next time you think it might happen, bring an anemometer along on one of the boats, and compare it to the reading at the clubhouse... that is, if you don't trust your boats to tell you what's going on. 

It's also quite possible that tide or current plays a role in this scenario, but I'm not sure how you're determining the "lack of power" and how it's affecting your headway.  :-?

Perhaps you real pilots could cast a bit of light on this phenomena, I guess it must effect aircraft quite considerably.

I've lived near the coast in West Sussex all my life. I know nothing about sailing but used to do a lot of RC slope soaring which is dependent on wind speed & direction. No wind means you can't fly so we became very keen observers of wind conditions. The only effect on aircraft related to wind strength that I can think of is due to thermal activity. In some weather conditions it's possible to have a strong wind with no lift. What we called "dead air". A sudden drop in wind strength together with an increase in air temperature indicates a thermal & this is when you get the maximum lift. This doesn't last very long so we used to take the opportunity to gain as much altitude as possible. It might also explain the "gutless wind" that you describe.

Our beach is popular with wind & kitesurfers who I enjoy watching. Of course, they probably stay closer to the shore than you chaps & just like our slope soarers they're dependent on wind direction. A slight change of direction & they might have to pack it in for the day, even when a stiff breeze is blowing. Sometimes they move a little further along the coast to where the wind is working.

You don't say where your club is based but perhaps this is a local phenomenon caused by the terrain or when the wind is in a certain direction; for example, onshore or offshore. When you're out on the water what is the difference in altitude between your sails & the anemometer on the clubhouse?

Thanks Beaky and Hagar.
We're neighbours Hagar, We (my wife and I ) sail from Pevensey Bay

I appreciate the variation of speed at the clubhouse V's speed on the water, And yes tide does have an effect. The phenomena I'm attempting to describe seems to be an additional effect which tends to be noticed by the more experienced sailors.

If moving air has sufficient mass and velocity to cause an instrument to read 21 kts, it's 21 kts, regardless of pressure. An anemometer doesn't use radar to track the speed of air molecules- there has to be X volume at X velocity for it to read X knots.

Yes I understand that, but what effect does air density have on anemometer? My limited (so I may be wrong) understanding tells me that an anemometer will read 21 knots in a 21 knot stream regardless of density because reduced/increased density will also result in reduced/increased drag.......*This isn't the same for a boat as the drag from water remains pretty much the same regardless of air density*.

Perhaps if I rephrase the question. bare in mind although quite a competent sailor I know pretty much B****r all about real flying:

What atmospheric conditions effect the lift generated by an aircraft wing at a given airspeed....is it just air pressure/ altitude or are there other factors like humidity or perhaps something else?


*there are exceptions but trying not to over complicate things

Just spent a couple of hours googling. I found some answers.

It seems that a wind at sea level at a given speed (say 21 kts) can have a variation by as much as 10% density/weight.

The odd thing (for me anyway) was that increased humidity decreases the density of air :-? Water vapour is less dense than air by quite a margin apparently.......well I'll be

It seems you've answered your own question.

Just spent a couple of hours googling.

I've done a bit of googling myself. What do you make of this? --->Ride Like the Wind (only faster)

Just spent a couple of hours googling. I found some answers.

It seems that a wind at sea level at a given speed (say 21 kts) can have a variation by as much as 10% density/weight.

The odd thing (for me anyway) was that increased humidity decreases the density of air :-? Water vapour is less dense than air by quite a margin apparently.......well I'll be

Not quite true... although water vapor is lighter than the gases found in air, the amount of the gas, per unit of volume of atmosphere, is reduced when there's a lot of water vapor, and that's what matters to airplanes. Take an empty 1-gallon bucket (full of air, right?) and pour some water into it. There's now less air in that gallon of volume.
  And the hotter the air is, the more vapor it can hold. Hotter air, due to the reluctance of the agitated gas particles to stay close together, is less dense as well, so you can see how hot, humid weather can affect density altitude.
The problem for aircraft when it comes to high humidity is not so much with aerodynamics as it is with combustion. Non-turbocharged or supercharged piston engines in particular will have problems, because the engine needs a certain amount of oxygen per second to create the desired horsepower. It's getting the same volume of "air" all the time (for a given throttle setting), but there has to be a certain ratio of gas (specifically oxygen). Water is not useful at all to such an engine, regardless of its density. The fuel will only burn in the presence of oxygen. If the ratio of water vapor to gas starts to skew towards the water side, the problem is obvious. Leaning the fuel/air mixture only helps to a point, as many piston pilots have found when they tried to take off, leaned for as much power as they can, under density altitude conditions that had them starting out at an altitude equivalent to, or exceeding, the service ceiling of the airplane. 

Thanks for that link Hagar, it'll probably take me a while to get my head around that idea

Interesting reply beaky, the effects of water vapour on an infernal combustion engine just got more complicated for me. Nice to have a pilots perspective

.....Interesting reply beaky, the effects of water vapour on an infernal combustion engine just got more complicated for me. Nice to have a pilots perspective

From my experience of water vapour in the combustion engines of my Motor-Bikes, I observe that they run smoother, and generate more power, due to the added effect of generating steam in the combustion chambers, thereby increasing the compression ratio.
My normally aspirated (Carburettor) piston engines, love rain and damp weather conditions... ...!

I can't resist it any longer, wind with no Ooomphf, something my wife dreams of every day

Matt

.....Interesting reply beaky, the effects of water vapour on an infernal combustion engine just got more complicated for me. Nice to have a pilots perspective

From my experience of water vapour in the combustion engines of my Motor-Bikes, I observe that they run smoother, and generate more power, due to the added effect of generating steam in the combustion chambers, thereby increasing the compression ratio.
My normally aspirated (Carburettor) piston engines, love rain and damp weather conditions... ...!