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Hobbo View Drop Down
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Post Options Post Options   Quote Hobbo Quote  Post ReplyReply Direct Link To This Post Topic: Wind
    Posted: 11 Jun 06 at 4:21pm
The anamometer picks up local conditions and what the wind "appears" to be like right? So therefore it is elementary whther it is reading 16 knots in colder or denser airs as the anamometer is reading the force going into it, accounting for the local air pressure. Therefore i think the effect of the wind would be the same in both - in your scenario twenty knots, however the true windspeed in the denser air may not be that speed the denser air is just casuing it to appear faster.

That's jsut my line of logic, may be complete bollocks and i'd be happy to be proved wong  
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gonzo View Drop Down
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Post Options Post Options   Quote gonzo Quote  Post ReplyReply Direct Link To This Post Posted: 11 Jun 06 at 5:17pm

One  example i have heard is that an aeroplane that takes off in dubai mid summer must have a higher take off speed to get the lift than it must in the uk where the air is denser.

So for colder denser wind do we get more power per knot of wind than in a hot country with lower density.

 

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Ian99 View Drop Down
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Post Options Post Options   Quote Ian99 Quote  Post ReplyReply Direct Link To This Post Posted: 11 Jun 06 at 6:12pm
The effect is far more noticeable with altitude. Try sailing at nearly 6000 feet above sea level and wind speeds of 25+ knots become far more manageable.
A decent anenometer should measure the actual speed of the air movement and shouldn't be affected much by air density - there's only the resistance of the bearings providing any kind of load.
If however, you use beaufort scale to measure wind strength, a force 5 will always feel the same regardless of altitude, temperature, humidity etc. The beaufort number is classified by the effect the wind has on the sea (or objects such as trees on the land). The wind speeds in knots at sea level on a British summers day (I'm guessing they used a British summers day!!) which have this effect are only there for convenience.
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gonzo View Drop Down
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Post Options Post Options   Quote gonzo Quote  Post ReplyReply Direct Link To This Post Posted: 11 Jun 06 at 6:31pm

thanks for that it has cleared some of it up.

So an anamometer built in the uk set to measure wind on a summers day at sea level in the uk is not accurate anywear else.

And yes i agree the beaufort is what on what you can see not a measurement.

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Post Options Post Options   Quote Prince Buster Quote  Post ReplyReply Direct Link To This Post Posted: 11 Jun 06 at 8:18pm
yeah i suggest doing a search mate.....there's loads of this stuff on the forum that's been discussed before.....
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Medway Maniac View Drop Down
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Post Options Post Options   Quote Medway Maniac Quote  Post ReplyReply Direct Link To This Post Posted: 11 Jun 06 at 9:26pm

I read Ian99 as saying that a decent anemometer will measure speed not force. As such, it would be equally accurate anywhere.

Of course, it depends on how you define 'decent'. I'd have thought that a good rotating turbine-style of anemometer would have such minimal bearing drag that the turbine would always effectively spin at a rate proportional to the windspeed, regardless of the air density, i.e. it will be accurate everywhere.

An instrument that depends on the pressure force exerted by the fluid - e.g. a vane with a pointer that moves against a spring or gravity, will depend on density and therefore be less accurate. That said, anybody who's used an anemometer will know that the fluctuations are usually so great, gust to lull, that such inaccuracy would be hardly noticeable and thus irrelevant, or so I'd have thought...

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Post Options Post Options   Quote Skiffman Quote  Post ReplyReply Direct Link To This Post Posted: 11 Jun 06 at 9:27pm

no offence really to Julian beithewaite because he has designed some of the best boats in the world, but from past experiance i do not think you would think hes god if you met him.

At the 29er Worlds in 2005 julian made up a rule that we had to measure the spreader hieght that wasnt in the rules, and he also said that you weigh a 29er with the pole in which is not correct (but julian said so, so it was done). There was hardly any boats underweight and most were 1.5kg over which is about the weight of the pole.

back to the topic, i agree that when it is warmer the wind is not as powerful for its speed as a cold UK winter breeze.

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tack'ho View Drop Down
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Post Options Post Options   Quote tack'ho Quote  Post ReplyReply Direct Link To This Post Posted: 12 Jun 06 at 10:00am
When thinking about this just remember being hit by a fly doing 30 mph hurts a lot less than being hit by a HGV doing the same speed!
I might be sailing it, but it's still sh**e!
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49erGBR735HSC View Drop Down
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Post Options Post Options   Quote 49erGBR735HSC Quote  Post ReplyReply Direct Link To This Post Posted: 12 Jun 06 at 1:48pm
Haha, that reminds me of one of the boats our uni crashed at BUSA, we were trying to work out the extent of the damage and reckoned the forces involeved were on par with crashing a Transit van into a wall doing about 30mph. Back on topic, its all good to consider the pressure differences but with the size of the sails on our boats and the slight differences involved with varying pressure, is it worth really getting too caught up on the subject. Yes there will be a difference but its not as apparent as for air-crafts because they have massively more efficient wing sections. My theory is that I will consider all aspects before going out on the water but we generally try and head out early to make sure the boat is feeling right and get a general idea for what is happening wind and tide-wise in the race area. I won't be thinking about the fact that one day is warmer than another on the course, but I will be picking up constantly feed back from the boat to see if its tuned properly, whether she could be doing with more power or slightly less.
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gonzo View Drop Down
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Post Options Post Options   Quote gonzo Quote  Post ReplyReply Direct Link To This Post Posted: 12 Jun 06 at 2:47pm

And all your tuning efforts are worthless when you go arse over tit in a gybe and end up swimming or have to spin after hitting the mark.

My veiw spend more time on your boat handling skills than worrying about which country you well get most wind.

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