When is a knot not a knot?
dcohen
reported wind speed.
A short while ago I had been discussing light wind planing on the Starboard
website with Roger (Sailquik). I had described being able to plane on my
Starboard Free Formula 138 in 7-8 (metered) knots with a Retro 9.5m (for
those who are interested, I weigh 77 kg). Since this had been a measured
wind speed, we both accepted this as a valid reading for the bottom end
performance of the FF138.
This weekend, however, I began to wonder about the validity of wind speed as
a measure of wind strength and I am hoping that someone can explain this
further.
What happened was that yesterday I had been overpowered on my 5.4m
Supersonic/Starboard Carve 99 in just a 20 kt (gusting 25/26) seabreeze,
while just the weekend before I had been sailing in a 25 kts (gusting
28)seabreeze comfortably powered on my 6.0m Supersonic/custom ex-world cup
270 slalom. This experience was shared by my two sailing buddies.
The question is to what degree does air temperature and humidity affect wind
strength? We often talk of "heavy" air vs "light" air. A cold southerly in
Perth can be a fearful wind while a warm easterly has little power. Or is
this just mythology?
Derek Cohen (in Perth, Australia)
Frank Weston
You raise an interesting question. I think there's more to "windspeed" than
just what gets measured by an anemometer. Wind can have other attributes
that affect how sailors must deal with it. Gusts and shifts can have a big
influence on how a sailor perceives wind strength, and sea state is another
factor. I can imagine how a good Formula sailor could hold down a 10.4 in a
steady 20 kts on relatively flat seas. But with huge chop, gusts, and
shifts, a 7.5 might be all the same sailor could deal with. Some wind blows
steady and parallel to the water surface, but I can witness that some wind,
particularly after a cold front blows vertically, sometimes straight down
into the water and then radiates from a center. These bursts are usually
encapsulated within a more constant wind field. All make sailing more
difficult. A 20 kt thermal or warm front breeze might actually average 20,
lulling to 18 and gusting to 22, whereas a cold front breeze could well
average 20, lulling to 0 and gusting to 40. One 20 kt breeze is very
different from another.
Frank Weston
dcohen wrote in message <9uvvl6$g12$1...@yeppa.connect.com.au>...
charlesivey
I do not think it is a humidity effect, but rather the character of micro
bursts moment to moment -- the disturbed nature of the wind as Frank has
described. I know there are days I can handle a 9.5 with gusts into the
20's with no real problem, and other days when the same measured wind is
hard as heck with a 7.5. For us it is the pulsing nature of the wind even
more than the shifts, even though shifts are a part of it. Unless the
shifts are huge, at speed they are manageable (in fact much easier to handle
when sailing fast than slow), but when the wind has a piston like on-off
nature, you cannot commit yourself to one side or the other of the wind. We
get this piston effect where the frequency may be as quick as one cycle per
second (I do not know how) and when it happens it seems the wind is
tangential to the surface and then directed down at the surface and it
oscillates at the one hertz or so frequency. Then even a 7.5 is jerked
around in your hands and you dare not put all your weight into the harness.
I've seen days when it was hard to sheet in because you were oversheeted, no
luffing, no oversheeted, etc., all while the wind whips you around keeping
you from gaining balance. As Mike once said even the great Ian Boyd
declared some days not good for sailing -- and I assume due to such effects.
When they happen, these days are a bummer because you think there would be
no problem and it is like you are a beginner all over again. And yes this
can happen even with winds that average as little as 20 to 25. Don't you
guys agree?
So yes, I vote there are different natures to 20 knot winds -- some easy and
some hard.
CI
"Frank Weston" <fr...@weston-american.com> wrote in message
news:9v01hh$46d$1...@slb7.atl.mindspring.net...
Iain Cunningham
news:9uvvl6$g12$1...@yeppa.connect.com.au...
[wind 'feel']
> The question is to what degree does air temperature and humidity
affect wind
> strength?
This is a very good question that I will try to answer for you without
getting into the fluid dynamics too much, however I will refer to a good
text book on the same in case you wish to investigate further, the text
book is: Fluid Mechanics by Douglas, Gasiorek and Swaffield, published
by Longman Scientific and Technical (I have the 2nd edition, ISBN:
0-582-98861-6 (0-470-20469-9 USA only). We will deal with aerofoils in
INCOMPRESSIBLE flow since, if we bring compressibility into the model I
can't explain what happens without resorting to second order
differential equations. Ever try representing them in ASCII!?! (Plus
not everyone has the mathematical knowledge to be able to comfortably
cope with them - and I'm an engineer - not a maths teacher!)
I will try to give examples and analogies where possible to help
illustrate a point though.
OK, the first thing is to remember that warm air is less dense than cool
air.
Secondly, by referring to "steam tables" we can see that moist air is
less dense than dry air (this is why clouds are usually up in the sky
and not scraping along the ground) [To explain this requires a
knowledge of thermodynamics that I do not feel able to impart to you
easily so please accept this as a fact!]
Thirdly, by referring to "steam tables" we can also see that warm air
can hold more moisture than cool air. (A dew forms when the moist air
is cooled to the point where it can no longer hold all the moisture in
it as a vapour)
So in increasing order of density we have: warm-wet air,
cool-wet/warm-dry air, cool-dry air. (NOTE where I give a calculation
involving Density below the units are ALWAYS kg per cubic metre)
[Obviously the amount of water vapour and the temperature of the air
will determine which of cool-wet and warm-dry occurs where on the scale
so we'll simplify and just take the extremes]
Taking these extrems warm-wet would occur just before a summer thunder
storm and cool-dry would occur any time the air temperature drops below
0 C (32 F). [Ob-Legal bit: I would in no way suggest that anyone
should knowingly sail immediately prior to a thunderstorm, this is an
illustrative example only, likewise, sailing in sub-zero temperatures
should only be performed after taking appropriate safety measures]
[If you want to skip the maths bit that follows go down to the line that
has ***** in it!]
So moving on to some fluid dynamics (from FM, pp 297-299), the resultant
force on a body in a flow of fluid results from the combination of drag
and lift that the body generates. Lift is perpendicular to the relative
motion (in terms of the sail while windsurfing 'relative motion' =
'apparent wind') and drag is in line with the relative motion, but in
the opposite direction [drag is actually a combination of skin-profile
drag and pressure drag, the difference between the two is demonstrated
if you put your hand out of a car window held flat while at speed
[ob-legal bit: you should NEVER do this while driving, only while
travelling as a passenger!]. If your hand is horizontal the "backwards"
(relative to the car's motion) 'pull' you feel on it is mostly due to
skin profile-drag, if your hand is vertical the backwards 'pull' is due
to pressure drag, in between vertical and horizontal you get a
combination of the two (you'll also feel an up or down lift component
too depending on which way you've tilted your hand), which is where the
windsurfing sail lies.]
Lift, L = 0.5 * LiftCoEff * Density * VelocitySquared * ProjectedArea
(Equation 10.5 of FM)
Which can be simplified as follows:
1. VelocitySquared (the velocity of the sail relative to the wind (in
metres per second)) can be disregarded as you'll be sailing in exactly
the same wind speed in our two comparison conditions.
2. LiftCoEff (the Lift CoEfficient) can be disregarded as we'll assume
that you use the same rig on the two occasions we are going to consider
and have it rigged exactly the same!
3. ProjectedArea (the area of the sail when seen looking from the source
of the apparent wind (in square metres)) can be disregarded too as we'll
assume that you take the same, identical stance on both occasions in
question too!
To get: L = 0.5 * Constant1 * Density
And Drag, D = 0.5 * DragCoeff * Density * VelocitySquared *
ProjectedArea (Equation 10.4 of FM)
Again simplifying as above:
Gives: D = 0.5 * Constant2 * Density
The resultant force on the sail is given by, F = SquareRoot(D-squared +
L-squared)
Rearranging this and substituting for L and D gives:
F = 0.5 * Density * SquareRoot(Constant1-squared + Constant2-squared)
*****
Or to put it another way, if and only if everything else remains the
same for the two occasions, the force on the sail is directly
proportional to air density.
With this in mind we can state that on our "freezing" day (denser air)
we get a larger force acting on the sail than we do from sailing
immediately before a midsummer thunderstorm (less dense air).
> We often talk of "heavy" air vs "light" air. A cold southerly in
> Perth can be a fearful wind while a warm easterly has little power. Or
is
> this just mythology?
Looking at a map of Australia, imagine putting (or draw if you want to)
two circles on it the size of the 'average' anti-cyclonic weather system
so that Perth lies at the northern quadrant of one circle and the
western quadrant of the other. By then tracing one or other of these
circles to simulate the path of the wind due to anti-cyclones over
Western Australia you can try to apply the above to your situation (by
thinking about where air will be humidified, dried cooled and warmed to
get its end state as it hits Perth - I'd do this for you, but my WA
geography is woefully inadequate for that type of task). [However, in
addition to/inspite of the above, I think you'll find that the Darling
Range provides a nice natural windbreak for the city when easterlies
occur and acts as half of a funnel for southerlies.]
Sorry it took so long to explain all that, I fear I've not done a great
job of it either (at least the fluid dynamics and thermodynamics parts)
and the 'tone' may be seen by some to be patronising, but I really don't
know how much base knowledge I can assume here! I also apologise for
any typos above, but my spill chicken doesn't appear to be working. If
anyone has any further questions on the above then please post them as
I'm sure you won't be alone (*I've* got questions on the above - but not
enough time to answer them!) As for me, I'm off to the pub!
Before I go: should we start a FAQ? - this question and variants of it
seem to pop up regularly, we do also have other FAQ's: What size sail
is best for x, where is best in <insert month>, what board is best to
learn y on, what fin size is best for z, etc. I'm sure you can think of
more examples.
ttfn!
Iain (this typing's thirsty work!)
--
Remove "my.pants" to e-mail me!
isobars
One could elaborate on that until the cows blow away, but why bother? The fact
remains that wind over yonder and wind right here can vary dramatically, short
and long term, even when over yonder minus right here is small.
Mike \m/
charlesivey
>
> F = 0.5 * Density * SquareRoot(Constant1-squared + Constant2-squared)
>
> *****
>
and min/max differences one could reasonably expect -- I suspect they are
much less a factor than the pulsing and direction changes in disturbed flow.
CI
Mike F
north of the Columbia's centerline. It then rolls off a cliff, leaving us
with a wind pattern of square wave shape ... single-digit wind speed in the
15-20-second lulls, solid 30-40 mph or even more in the 10-15-second gusts,
and no seconds in the transitions between the two. You rig a 3.x sail, try
to sail in the gusts, but a hundred-yard sprint before hitting another lull
is a rare treat. Meanwhile the current (described by some as a Class V
rapids in the spring runoff) is taking you down the river at a dizzying
speed.
Average wind was, say, 20-25 mph, but so what? You rig for the average and
you still can't plane in the lulls or sail in the gusts, or you rig for the
lulls (what ... a 7.5?) and get injured in the gusts, so you're better off
rigging for the gusts. The worst part is that this is common unless the
winds are purely thermal, with no synoptic basis to drive the wind direction
north of the centerline, and strong thermals aren't common that far east. At
least in the Hood River stretch winds that wide-ranging ramp up and down so
one can actually sail for minutes at a time and have some fun; this
sometimes isn't possible below the cliffs further east when the wind
direction goes much north of the centerline. In either place, both the wind
speed range and the shape of the wind speed variations are important to
determining sail and board size -- not to mention the risk of shoulder
separation.
Then there's the correlation of wind on the water to wind at the sensor.
That can vary dramatically with wind direction, season, the cause of the
wind, obstructions, etc.
And THEN there's the terrain. Sailing overpowered is much easier on flat
water than in swell or harsh chop.
Mike \m/
"charlesivey" <charl...@home.com> wrote
jfe...@ix.netcom.com
alright, so i'm picking nits, but
> 1. VelocitySquared (the velocity of the sail relative to the wind (in
> metres per second)) can be disregarded as you'll be sailing in exactly
> the same wind speed in our two comparison conditions.
is right to a first order. but, as you show, a dry cold air flowing
over a sail will generate more force than a wet moist air. we might
expect that this will lead to a greater boardspeed, in the same
windspeed, in the former case compared to the later.
so, there will be a greater apparent wind in the case of the cold dry
air. in other words, the flow over the sails will not be of exactly the
same velocity, and the velocity squared terms will not be equal, even
though the true windspeed is equal.
jeff feehan
Iain Cunningham
news:S_PQ7.54625$Sx.15...@news1.elcjn1.sdca.home.com...
This was one of the questions I had, and I was hoping that no-one would
ask! However, just considering dry air (to make things easy) to start
with:
The temperature change from 1.85 C to 26.85 C (temperatures for which I
am able to read data directly from the tables) results in a change in
density from 1.284 kg/cu. m to 1.117 kg/cu. m, which doesn't sound like
a lot, but would mean the same sail giving 86.99 % of the 'drive' at
26.85 C that it does at 1.85 C.
Now we can take into account the 0.0158004 kg/cu. m change in density
between air with a Relative Humidity of 0 % and air with a RH of 100 %
at 26.85 C (this was calculated after an evening at the pub and should
be regarded as possibly suspect) which results in a density of 1.101200
kg/cu. m. So we're now at a difference in density and hence drive of
85.76 %.
As can be seen from the above, temperature changes in the range
considered have larger effects on density than humidity changes (unless
I did cock up the calculation).
Factor in the changes in drag on the sailor (don't ask me to do this,
please?) due to the changes in air density, and if you happen to have an
accompanying change in water temperature along with your air temperature
change you also need to take into account additional skin-profile drag
on the board and additional total drag on the skeg (both due to
increased water viscosity with a decrease in temperature). That 14.24 %
decrease in drive starts to feel like it has actually become closer to
20 % (I admit that we're out of the realms of my calculations and so I'm
guessing here, but a 2/5 increase due to the previous two items makes
things easy, and I'm all for that). After all, the force that the
sailor feels is the result of the backwards drag of him/herself, the
board and fin acting against the forwards drive of the sail. OK, it's
not huge compared with the change given by the difference between gusts
and lulls, but given that kind of difference, on a cold mid-winter (dry)
day, 20 knot gusts would feel the same as 25 knot gusts on a hot summer
(humid) day against the same sail - the difference between comfortably
over-powered and wet hair?
Also, please remember that the above is for a simplified situation. The
real situation (involving compressible flow) could give rise to a much
greater (or lesser) difference than that discussed above. Unfortunately
I have none of the specialised tools required to analyse the
compressible flow situation at my disposal and so I can't help with
that. (Of course if anyone has access to CFD software...)
Hope this sheds a little more light. My head hurts now.
Goodnight all,
Iain
Iain Cunningham
news:3C1417...@ix.netcom.com...
> hi iain,
>
> alright, so i'm picking nits, but
>
> > 1. VelocitySquared (the velocity of the sail relative to the wind
(in
> > metres per second)) can be disregarded as you'll be sailing in
exactly
> > the same wind speed in our two comparison conditions.
>
>
> is right to a first order. but, as you show, a dry cold air flowing
> over a sail will generate more force than a wet moist air. we might
> expect that this will lead to a greater boardspeed, in the same
> windspeed, in the former case compared to the later.
>
> so, there will be a greater apparent wind in the case of the cold dry
> air. in other words, the flow over the sails will not be of exactly
the
> same velocity, and the velocity squared terms will not be equal, even
> though the true windspeed is equal.
Yes, this is true, but I couldn't think of a way to get this into my
explanation and still keep things simple. All I could think of was that
rather than saying "sailing in the same wind speed" I could say "sailing
in the same apparent wind speed", but apparent wind is very rarely
measured, so I dealt with true wind speed as this is what most people
measure and so are familiar with. To do this I assumed that the
difference in VelocitySquared's were negligable in order to reduce them
to a constant term (if not, you have to iterate until the forces are
resolved). As you point out (well spotted by the way) this is not true,
but neither is the over-riding assumption of incompressible flow over
the sail which will probably have a much greater influence on the end
results (oh for some CFD software...). But, as I just stated, at the
time of writing I wanted to simplify things as much as possible and also
put things in terms that people are familiar with (there are quite a few
other unstated assumptions in my original post, such as equal water
viscosity and density in both situations, see my reply to CI for a short
(and simplified!) discussion of the effect of this changing).
Moving on, I'm surprised that so far no-one has picked up my biggest
mistake - I didn't reference the 'drive' from the sail to the
sailor/board, which (I think) was the whole point of the question: What
affects wind strength (a trivial answer in windsrufing terms is:
Nothing if you aren't sailing!). I also touch on this (glaring)
ommission in my reply to CI.
Next time I'll try not to be in such a rush to get to the pub, and maybe
I'll tidy things up a little more.
Still, it's the first time I've used any Fluid Dynamics or
Thermodynamics in about seven years (i.e. since completing my degree),
so I'm quite happy that so far there's only been one flaw in my response
pointed out.
Iain
Martin Allen
Good to hear from someone who has some real in depth technical knowledge,
and can give those without the results in plain English.
Martin
Frank Weston
force is being measured, it doesn't matter if the air is wet, dry, cold,
hot, dense, or less dense. The force exerted on the anemometer to give a
reading of say 20 kts will be the same relative force with which a
windsurfer must deal no matter what the physical properties of the air.
Asking if the density, temperature, or humidity of the air affects air speed
measurements is sort of like asking what weighs more: A pound of feathers
or a pound of lead? 20 kts is 20 kts.
There have been long and contentious debates on this subject here before. I
won't get into it again, other than to alert you to the idea.
Frank Weston
Iain Cunningham wrote in message ...lots of words that I've deleted.
jfe...@ix.netcom.com
>
>
> but neither is the over-riding assumption of incompressible flow over
> the sail which will probably have a much greater influence on the end
> results (oh for some CFD software...).
assumption of incompressible flow is not a problem.
jeff
(Pete Cresswell)
>Don't most anemometers essentially measure the "force" of the wind? If the
>force is being measured, it doesn't matter if the air is wet, dry, cold,
>hot, dense, or less dense.
Depends on what most anemometers use as the interface. I'd think
that spinning cups measure speed....My trusty Dwyer looks to me like
it measures force as to the spring-loaded plate-style handhelds that
were being sold some years back.
-----------------------
Pete Cresswell
Iain Cunningham
> Don't most anemometers essentially measure the "force" of the wind?
If the
> force is being measured, it doesn't matter if the air is wet, dry,
cold,
> hot, dense, or less dense. The force exerted on the anemometer to
give a
> reading of say 20 kts will be the same relative force with which a
> windsurfer must deal no matter what the physical properties of the
air.
This is partially true, at least in the case of the simple anemometer
(but other things affect the reading too), however there are instruments
available that measure the other things (such as humidity and
temperature) and so could/should be capable of compensation. Thus
avoiding the effects of different air densities.
If you're interested in the type of bearings now starting to be used to
give higher accuracy in anemometers, take a look at:
http://www.birdprecision.com/PDFs/Vee%20Bearings.pdf
If you are lucky enough to have one of the fancy electronic anemometers
mentioned above (that I have been salivating over for a while) that also
tell you humidity and temperature then the circuitry and firmware could
correct for these factors and display true wind speed (pay attention
Santa!)
If you've not seen one yet, try:
http://www.r-p-r.com/environmental_meter.htm
and even
http://www.r-p-r.com/weather_tracker.htm
if you want to really go to town!
Also
http://www.airflow.com/instr/ta4.htm
actually states that it performs temperature compensation. (The two
above don't but I'd be [moderately] surprised if they didn't)
> Asking if the density, temperature, or humidity of the air affects air
speed
> measurements is sort of like asking what weighs more: A pound of
feathers
> or a pound of lead? 20 kts is 20 kts.
True, but as you say right up at the top ^^^, it may not actually *be*
20 knots if you measure it accurately. When you're out on the water a
dry-cold air at a true speed of 20 knots will deliver more power to your
rig than a 20 knot warm-wet air. And this is what I /think/ the OP was
getting at. I may have misunderstood. It wouldn't be the first time,
it won't be the last.
> There have been long and contentious debates on this subject here
before. I
> won't get into it again, other than to alert you to the idea.
OK, I'll steer clear of it too then, other than to point out that this
kind of argument (about the true wind speed) could be a thing of the
past if we all get those fancy meters for Christmas. I'm sure eveyone
has had enough engineering for now!
> Iain Cunningham wrote in message ...lots of words that I've deleted.
Don't blame you!
Can you tell that there's not much wind at the moment?
Iain
Craig (gsogh) Goudie
Well sorta. It depends on the bearing friction. More bearing friction more
force driven, less normalization, less bearing friction, more a function of
velocity, more normailzation. If the bearing were frictionless, the anamometer
would increase velocity until the particle velocity were reached, and then
continue
at that velocity. Hot wire air velocimeters are probably
most "velocity" acurate. These would have to be normalized for density
and humidity, to correlate sail size. A weighted hinge, or venturi meter
needs no normalization for humidity, density, or sail size.
To rig appropriately, use force indicators (like white caps, and trees, and the
wind
on your skin)
-Craig
Frank Weston wrote:
--
Craig (Go Short or Go Home!) Goudie
Sailing the high desert lakes of Utah on my:
RRD 298, Starboard 272 and Cross M 8'2" with
Sailworks/Naish Sails and Rec Composites Fins
Sailing the Gorge on my: 9'1" RRD Freeride,
8'3" Logosz Squish, 8'0" Hitech IBM with
Sailworks/Northwave Sails and Curtis Fins
Mike F
AD.
anamometers?
> Well sorta. It depends on the bearing friction. More bearing
friction more
> force driven, less normalization, less bearing friction, more a
function of
> velocity, more normailzation. If the bearing were frictionless, the
anamometer
> would increase velocity until the particle velocity were reached, and
then
> continue
> at that velocity. Hot wire air velocimeters are probably
> most "velocity" acurate. These would have to be normalized for
density
> and humidity, to correlate sail size. A weighted hinge, or venturi
meter
> needs no normalization for humidity, density, or sail size.
>
> To rig appropriately, use force indicators (like white caps, and
trees, and the
> wind
> on your skin)
>
> -Craig
I vote for Craigs reply to be the final anemometer answer, and for it to
go in the FAQ if it hasn't already (assuming it's correct of course - it
sounds good to me though!).
Cheers
Anton
windwanker
the wind on ,maui when its perfect cross shore,will can get you planing very
easily and its always suprises me how small a sail one can use.
but when the wind turns northerly and onshore its much more dificult to get
planing even though the breeze feels stronger.
the same logic applies for any location.
as long as you are comparing wind from the same direction variations in
temperature and humidity are negligble.
"dcohen" <dco...@vianet.net.au> wrote in message
news:9uvvl6$g12$1...@yeppa.connect.com.au...
Justin_Boland
on the next they are opposite. this can greatly change the amount of power
felt. some tides can add up or take away 7-10 knots of wind speed depending
on direction. could be the fin and not the sail or wind.
"windwanker"
<windw...@thebeach.com> wrote in message
news:3c158d7a$0$3949$afc3...@news.optusnet.com.au...
Justin_Boland
on the next they are opposite. this can greatly change the amount of power
felt. some tides can add up or take away 7-10 knots of wind speed depending
on direction. could be the fin and not the sail or wind.
"windwanker"
<windw...@thebeach.com> wrote in message
news:3c158d7a$0$3949$afc3...@news.optusnet.com.au...
KentB
thermodynamics for a few years since graduation. I use them every day and I
would dream of trying to calculate the proper size sail. His points about
humidity is a very small effect is right on. Temperature effects due to air
density difference is also right. We should let him slide on trying to
include compressibility effects - should not make as much difference as
humidity unless he is sailing several hundred miles per hour (> Mach 0.6).
We will not get into aeroelastic effects (where the sail geometry changes in
response to the wind loads) but even though sails are made to take advantage
of these effects. But what is left out of Iain's analysis is the
temperature effect on the wind.
One of the South Padre Island windsurfing web sites had a great page about
the effect of water to air temperature difference on wind. I think they
must have removed that page because I can't find it now. Basically, if the
water is a few degrees cooler than the air the air near the water doesn't
hardly move so the wind at the surface is roughly zero. If the water is a
few degrees warmer than the air, that air rises and is replaced by air that
is moving. This keeps the wind speed up until very near the waters surface.
The effect is that on cold water, some of the lower portion of your sail is
not powered when the water is cooler than the air, which effectively reduces
your sail size. If you have plenty of wind, it doesn't matter because you
still have enough power. But if you are comparing the 20 knot warm wind
yesterday before the front to the 20 knot wind today after the cold front on
the same board and rig, you might notice the difference.
The water/air temperature difference assumes that there is nothing to block
the wind - like waves, trees, or land mass. These wind shadows would
probably have a larger effect than water/air temperature differences.
The bottom line - engineering analysis can get you in the ball park then it
is trial and error. If your rig feels too small - it is! Go rig a bigger
sail.
"Iain Cunningham" <iain.cu...@my.pants.ntlworld.com> wrote in message
news:tV4R7.22686$GU1.4...@news6-win.server.ntlworld.com...
Mike F
ago. We saw winter winds on warm days blow 30-40 knots all day in low clouds
just over our heads (and at NOAA sensors on land nearby), while we remained
totally skunked. This happened often once the lakes got cold. We've even
seen the prevailing wind run 30+ from the west all day over our heads ...
and 10-15 from the east on the water ... way too many times.
Mike \m/
"KentB" <KWR-Se...@nospamengineer.com> wrote in
Derek Cohen
so much interest and thoughful information. I only wish that I had taken
mechanical engineering at university instead of mathematical statistics, and
that the gap between doing so was less than 25 years! My head hurts but I've
enjoyed it all. Many thanks to all who responded.
Derek Cohen (in Perth, Australia)
dcohen <dco...@vianet.net.au> wrote in message
news:9uvvl6$g12$1...@yeppa.connect.com.au...
Frank Weston
KentB wrote in message ...
>The bottom line - engineering analysis can get you in the ball park then it
>is trial and error. If your rig feels too small - it is! Go rig a bigger
>sail.
True words, obviously spoken by someone with experience.
If only 50% of the problem is understood, even if that 50% can be analyzed
and defined with 99% certainty, any conclusions drawn will still most likely
be only 50% accurate.
Frank Weston
Mike LaRonde
banner but a big heavy sucker on a large sturdy pole, but not an immovable post.
and not on a roof top or you'll be fooled
If I can't maintain its shape don't bother sailing
If it can just barely fly full, then its 8.0-9.0
If its full and slightly ripping, maybe a 6.5-7.5
When it is steadily ripping in the wind, making noise, its time for a 5.4-6.0
When the pole starts to move about 6" at the tip, then a 4.7-5.2...a pole with a
tie line stretching down may start to make a rythmic noise as the line hits the
pole over and over. this sound may start earlier if its not tied tight enough but
it won't be as regular.
When the pole is seriously shakin then 4.2-4.5 At this point the tip is moving
about 1 foot, and the sound of the flag is louder than the pole, like its about to
tear off.
Any windier than that and the owner of the flag usually takes it down, so I'm not
sure.
Mike