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Old 10-12-2009, 20:39   #76
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Well, I for one, have learned and or have been reminded of a few things..following this thread...

The whole Mass/Density discussion was very informative imo...good reminders..

To me...the B scale is another tool in an arsenal. It's at the very least a standard / a common language...used to transmit information to a broad population.

Use it, don't use it......it's up to each individual what they do with any information/tool they receive...or how they convert it for their own use. It provides a baseline.

A force 7 might be an exhilerating day on the water.in many places....put a force 7 north wind on the gulf stream and it's another story entirely.........

my 2 cents..fwiw
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Old 10-12-2009, 21:22   #77
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Altitude and temperature have an impact on density. There was a thread here a couple of years ago about lake sailing at high altitudes. My point then was that indicated speeds result in the same performance.

Less dense air would impact the wind speed indicator with less force so 5kts sea level might be 3kts at altitude. However performance would be the same for 5kts sea level indicated and 5kts indicated at altitude because it takes the same "force" to move the mechanical indicator.

The same holds true for airplanes - You don't have a higher indicated take off speed at altitude because you generate the same lift with the same indicated pressure. True Airspeed - what we would call SOG - is higher and ther are formula and tables to calculate TAS for density.

However for airplanes there is a significant issue in that the horsepower generated by the engine is less and the amount (mass) of air moved by the prop at the same take off RPM is less results in much slower acceleration to indicated take off speed.

I don't think the sail cares what altitude or density it is at as long as the indicated windspeed is the same.
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Old 10-12-2009, 22:12   #78
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I think density should have impact. More air - more power to the sail, more power transferred into the water as waves.
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Old 11-12-2009, 01:35   #79
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Originally Posted by barnakiel View Post
Yeah. Quite interesting to learn how different sailors tell how much wind there is. In my case it is the kind of sound the wind will make in the rigging. As a rule, I will say it is blowing e.g. 25, then look at the anemo to find it is only gusting 20.

b.
I've been working at sea for over 30 years, and old enough to have learnt the Beuafort Scale. I can look out of the bridge window at the sea state and say yep, that a F6 to 7 and OK to work alongside a rig, or run anchors. The
anemometer on top of the mast is telling me its F 7 to 8,and the one on top of the oil rig is telling them its F9, and too much wind to work
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Old 11-12-2009, 01:40   #80
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I once got caught out in late October in a force nine NW in the Southern North Sea (Blackwater to the Orwell with a seven forecast). I was singlehanded in an engineless Folkboat. Several years later, I managed to persuade the Met Office to fax me the relevant reports, which they were, at first, unwilling to give me - something to do with not wanting to get caught up in any possible insurance claims.
Thats a comfort, I'm taking my boat from Ipswich to Liverpool in early January
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Old 11-12-2009, 15:13   #81
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Can you explain how to do this or point to where I can otherwise read it?
Thanks.

Or PM me as to not cause more thread drift then I've already just done.

Regards,
Extemp.
Block the air intake, crank, remove the obstruction. As explained in Calder's book.
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Old 11-12-2009, 15:25   #82
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And THAT is where you and so many others go wrong and don't understand it. Your wind instrument does NOT show wind force. It shows wind speed.
Much force I sense with you, my old Jedi. Wind force ....

;-)

OK. Anemo shows the wind speed. Mine too.

But going into force vs. temp vs. density is really one step too far (unless I am wrong): I mean - given the spread of ocean weather say between the tropics and the arctic - how much wind force difference would we face say at B8 ??? A rough figure PLS.

Your young Padawan,
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Old 11-12-2009, 23:43   #83
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The density of air does not change that much from the temperature.

0 deg C, 1.29
30 deg C, 1.16

That's about 10% difference between really cold and really hot. The same difference would happen between 20 knots and 21 knots of wind. Or between 10 and 12 knots. Hardly a huge difference to a sailor.
Sorry, wrong again ;-) The wind hits your sails which have a surface area. This means that the difference gets squared. It the air would be twice the density, the power transferred to the sails is quadrupled. In your example, if the density is 10% more, the power will be more than double that: 21%. So 20 knots becomes 24.2 knots. 35 knots becomes more than 42 knots. These are substantial differences.

I think that answers Barnie's question too.

Quote:
Originally Posted by Ex-Calif
Less dense air would impact the wind speed indicator with less force so 5kts sea level might be 3kts at altitude. However performance would be the same for 5kts sea level indicated and 5kts indicated at altitude because it takes the same "force" to move the mechanical indicator.
Erhmmm... no. The resistance of the mechanical indicator is so low that it doesn't matter how dense the air is, it will always turn at the same speed as the windspeed. It has no sensors to measure the force, because those would create friction and slow the indicator down, resulting in a mis-reading for wind speed.

Guys, pick up a book if you don't believe us posters here, this is a well documented thing and known for ages.

cheers,
Nick.
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Old 12-12-2009, 06:00   #84
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DOWN CHANNEL

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Thats a comfort, I'm taking my boat from Ipswich to Liverpool in early January
Down Channel in January! Rather you than me! Having said that, I had one of the best sails of my life going the other way in early February - Left Southampton at midnight and thirty hours later, we were hove to off the Deben Bar, waiting for the tide to make enough to cross. SW 5 - 6 the whole way, just right for a Nich32.
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Old 12-12-2009, 06:40   #85
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Originally Posted by galleyslave View Post
Down Channel in January! Rather you than me! Having said that, I had one of the best sails of my life going the other way in early February - Left Southampton at midnight and thirty hours later, we were hove to off the Deben Bar, waiting for the tide to make enough to cross. SW 5 - 6 the whole way, just right for a Nich32.
Shame I cannot make the trip this week, near perfect conditions, barometer is sitting at 1040, force 3 to 4 around the UK coast. Instead I'm stuck in Aberdeen harbour propping up the habour wall.
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Old 12-12-2009, 08:02   #86
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Sorry, wrong again ;-) The wind hits your sails which have a surface area. This means that the difference gets squared. It the air would be twice the density, the power transferred to the sails is quadrupled. In your example, if the density is 10% more, the power will be more than double that: 21%. So 20 knots becomes 24.2 knots. 35 knots becomes more than 42 knots. These are substantial differences.
...
cheers,
Nick.
Er, you might want to pick up a fluids dynamic book, force is "area * density * velocity squared".

But you are correct that the anemometers that use on sailboats measure true wind speed, not wind force.
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Old 12-12-2009, 11:20   #87
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Sorry, wrong again ;-) ... This means that the difference gets squared. It the air would be twice the density, the power transferred to the sails is quadrupled. In your example, if the density is 10% more, the power will be more than double that: 21%. So 20 knots becomes 24.2 knots. 35 knots becomes more than 42 knots. These are substantial differences.

I think that answers Barnie's question too.
Well, it sort of addresses my question, but I believe the answer to how much force is generated is 1) different, 2) not exact (in the sailing boat environ, unless we sail a laboratory):

The first reason is that the kinetic energy of moving air is expressed by Ek=(m*v2)/2. (Note - only the speed is squared, not the mass). Thus the a 10% change in air mass would correspond with 10% change of energy. Anyway, cooler air = more mass = more energy. (See zydecotoad's post)

The second reason is that the warmer air will contain more water (humidity), and the more humid air should be lighter than the more dry, colder air. Here again, cooler air = less vapour content = less mass

... and so on.

We can for now safely leave aside the factors of air pressure (more pressure, more density) and the fact that we are far more likely to encounter higher pressures in areas where air humidity is lower and so is the temperature ...

I hope you are getting my drift - the wind instrument IS a freaking accurate measure of what is going on in the air. It is probably the best tool available onboard (other than our brains) tool to base our decision making on. I know it does not show the wind force. But if we know (cognitively or intuitively) how the wind speed relates to the force it exerts on the boat then we can make better decisions than by using the B scale and disregarding the m/s knots or whatever measure of the wind speed we may (so easily) get.

Long live the B scale! And just watch that anemo ;-)

barnie
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Old 12-12-2009, 23:36   #88
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It was force 10 out the door this morning but after I swabed the deck and finished my honey do list it settled to a force 0 and the first mate was all smiles...I even got a hot meal out of the galley it calmed down so much...I'm at anchor right now and monitoring the swing room...but not a breathe of wind so far...and shes a good bottom...I think the storms past and its gonna be a cozy night I'm thinking......sometimes sailors just get lucky ..
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Old 13-12-2009, 06:23   #89
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It was force 10 out the door this morning but after I swabed the deck and finished my honey do list it settled to a force 0 and the first mate was all smiles...I even got a hot meal out of the galley it calmed down so much...I'm at anchor right now and monitoring the swing room...but not a breathe of wind so far...and shes a good bottom...I think the storms past and its gonna be a cozy night I'm thinking......sometimes sailors just get lucky ..
Those are Force levels we can all understand.

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Old 13-12-2009, 09:25   #90
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Maybe we should use a "Modified" Beaufort and leave out the wind speed.
I would argue that knowledgeable mariners are already doing precisely that.

I'll give you an example of where the Beaufort system provides more useful data than an anemometer would: I'm Faculty Advisor for a university sailing team. We race three to six boats (Collegiate Flying Juniors) at once, and depending on the regatta each boat will compete in two or three short races before coming in to exchange crews. It would not be legal for us to put someone out in a chase boat and report wind speeds. (A violation of the rule on "outside help.") However, we can stand on shore and observe sea state. Thanks to the Beafort scale we can do this objectively. We use this data to help decide whether to put a lighter or heaver crew with each skipper. For example, I know that Skipper A does better with Crew A in Force 3, but that he'll do better with Crew B in Force 4. And once it reaches Force 5, we yank Skipper A and put Skipper B in with Crew B.
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