Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 14-12-2009, 15:36   #106
Senior Cruiser

Cruisers Forum Supporter

Join Date: Aug 2009
Location: between the devil and the deep blue sea
Boat: a sailing boat
Posts: 17,314
OK, I believe what Jedi says on the water speed vs. air speed is true (it sounds intuitive and intuitive is more often correct than not).

Suppose he is. Now you tell me how does he get the water to the top of the mast???

QQ: What is the device to measure the energy of the wind? Is it OK/enough to re-calc the data from the Baro, the Anemo, the Hygro and the Thermo? The formulas do not look too complicated. Maybe we could ask this to be implemented in that Open Source thing called OpenCPN? Or ask say MaxSea and the Deckman to implement such a virtual "WIND FORCE" meter?

Now someone with guts and some venture capital will overhear this thread and build the gizmo and sell it to us. Bet.

b.
__________________

__________________
barnakiel is offline   Reply With Quote
Old 14-12-2009, 19:38   #107
Do… or do not
 
s/v Jedi's Avatar

Cruisers Forum Supporter

Join Date: Feb 2009
Location: in paradise
Boat: Sundeer 64
Posts: 9,198
Ha ha ;-) how about you catch a liter of air in a bottle and weigh it with a scale and do the math after that ;-)

But then again, you could just look at the effect the wind has on the sea (linear relation ship with the wind energy all free and without math) and accept the wisdom of Mr Beaufort!

cheers,
Nick.
__________________

__________________
s/v Jedi is offline   Reply With Quote
Old 15-12-2009, 01:25   #108
Moderator Emeritus
 
Ex-Calif's Avatar

Cruisers Forum Supporter

Join Date: May 2007
Location: Singapore
Boat: Maxi 77 - Relax Lah!
Posts: 11,514
Images: 4
Quote:
Originally Posted by s/v Jedi View Post
When the indicator shows 5 knots, it's 5 knots no matter how dense the air is. Even when you put it in the water like a water mill and it shows 5 knots then that's the speed of the water flowing by.
So let's run with this as air is a fluid much like water.

Imagine hypothetically that you have a garden hose with a nozzle that adjusts from mist (like misting plants) to steady stream. The water exits the nozzle at 30 mph. You point the "mist" mode at the paddlewheel and read 15 mph. You adjust to steady stream and read 30 mph. You have just demonstrated density. Now turn the velocity up to 60 mph and use mist and you read 30 mph. You've traded velocity for density and the wheel sees 30mph.

Quote:
Originally Posted by s/v Jedi View Post
The indicator shows wind speed regardless of density.
I agree 100% - And the sails don't care about density either. If 5kts indicated is blowing across the sail they will perform the same regardless of altitude and temperature.

Quote:
Originally Posted by s/v Jedi View Post
Oh no no no, you asked if the boat would sail faster or slower, not if the anemometer would turn slower or faster. So, my reply was about boat speed, not about the windspeed indication because you told me it would show 5 knots in both situations.
If 5 kts is indicated, regardless of density, the performance of the sail, hence the performance of the boat, will be the same. If 3 knots boat speed results from 5kts indicated at Sea Level it will result in 3 kts at that 10,000 foot lake.
__________________
Relax Lah! is For Sale <--- Click
Click--> Custom CF Google Search or CF Rules
You're gonna need a bigger boat... - Martin Brody
Ex-Calif is offline   Reply With Quote
Old 15-12-2009, 05:21   #109
Certifiable Refitter/Senior Wannbe
 
Wotname's Avatar

Cruisers Forum Supporter

Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: Van DeStat Super Dogger 31'
Posts: 7,331
Quote:
Originally Posted by s/v Jedi View Post
.......... and accept the wisdom of Mr Beaufort!

cheers,
Nick.
Rear-Admiral Sir Francis Beaufort FRS, FRGS - please...
__________________
All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence
Wotname is offline   Reply With Quote
Old 15-12-2009, 06:04   #110
Moderator Emeritus
 
Pblais's Avatar

Join Date: Mar 2003
Location: Hayes, VA
Boat: Gozzard 36
Posts: 8,700
Images: 15
Send a message via Skype™ to Pblais
Quote:
Less dense air would impact the wind speed indicator with less force so 5kts sea level might be 3kts at altitude.
The dense air will provide more acceleration than the less dense air. f = mA. Velocity would be measured as one half of acceleration times the Time squared. This is offset by the drag and windage of the boat. The air density would then reduce the force to move the boat but not effect the drag unless water temperature starts to play. On a displacement hull the drag increases with velocity as well.

When air density is low you see it best in say Salt Lake City, UT on a summer afternoon at the airport. They start bumping passengers off the small commuter flights because the air density is insufficient for a safe take off / landing. Your luggage can be delayed and they don't carry mail. Heat and altitude both combine to make a difference that needs to be considered for the lift of the smaller planes. Most pilots carry a circular slide rule to compute "standard elevation". At about 90 F and 6,000 ft it matters for a small plane larger planes can compensate for ti so you won't see anything different even though they do consider it.

It really takes an increase of both components of heat and altitude to make a problem that effects how you operate the plane. For practical sailors at or near sea level the seas state will more than obscure the difference in air density due to temperature alone.

It's one of those things that just because it is true does not mean it matters. When the boat is heeled over 50 degrees and the sound of the Admiral is splitting your brain in two, I'm not thinking "I wonder what the air density is?" The difference in extremes of temperature create other issues that really are significant to the extent they demand more consideration that air density.
__________________
Paul Blais
s/v Bright Eyes Gozzard 36
37 15.7 N 76 28.9 W
Pblais is offline   Reply With Quote
Old 15-12-2009, 07:33   #111
Registered User
 
muskoka's Avatar

Join Date: Jun 2006
Location: Sai Kung, Hong Kong
Boat: FP Lavezzi 40 / Hatteras 48
Posts: 775
Quote:
Originally Posted by Ex-Calif View Post
I agree 100% - And the sails don't care about density either. If 5kts indicated is blowing across the sail they will perform the same regardless of altitude and temperature.

If 5 kts is indicated, regardless of density, the performance of the sail, hence the performance of the boat, will be the same. If 3 knots boat speed results from 5kts indicated at Sea Level it will result in 3 kts at that 10,000 foot lake.
Actually the physics are simply that less dense air applies less force than denser air even if winds are moving at the same speed. Sails translate wind energy (density & velocity) into boat speed (propulsion energy less drag). So 5 knots of wind at sea level will propel the same boat faster than 5 knots of wind at 10,000 feet. Though in fact, the difference would be pretty slight.

NASA has explored the idea of solar sails for propulsion of space vehicles. These would get their energy from the 'solar wind' moving at the speed of light. If density of 'wind' wasn't an issue then these sails could basically be the size of your average yacht - however the theoretical size of the sails required is huge.

So, you have a 'wind' moving at the speed of light propelling a vehicle using a sail which is 2 microns thick!!! Which nicely demonstrates how wind velocity and density combine to make motive power via a sail.
__________________
muskoka is offline   Reply With Quote
Old 15-12-2009, 07:47   #112
Do… or do not
 
s/v Jedi's Avatar

Cruisers Forum Supporter

Join Date: Feb 2009
Location: in paradise
Boat: Sundeer 64
Posts: 9,198
Quote:
Originally Posted by Ex-Calif View Post
So let's run with this as air is a fluid much like water.

Imagine hypothetically that you have a garden hose with a nozzle that adjusts from mist (like misting plants) to steady stream. The water exits the nozzle at 30 mph. You point the "mist" mode at the paddlewheel and read 15 mph. You adjust to steady stream and read 30 mph. You have just demonstrated density. Now turn the velocity up to 60 mph and use mist and you read 30 mph. You've traded velocity for density and the wheel sees 30mph.
You keep bringing up new ways to try to prove me wrong but it's not me you are disputing, it's the laws of nature and you will fail ;-)

This time, you shoot a mist of water at the instrument and declare that this is the same as a high density form of air. However, how do you know that the air that is between the water droplets, moves at the same speed and direction as the water droplets? It does not. What you are doing is exactly the same as shooting the instrument with a shotgun loaded with bird shot. You hit the instrument with a hail of small pellets and expect the instrument to turn at the same speed as those pellets travel through the air. You are a smart guy so you must know that this approach is flawed. The instrument only works when a homogeneous gas flows over it's paddles and then only within the speed range that it is designed and calibrated for.

So, how can you measure wind energy? you can with a wind generator by looking at it's generated output power in Watts. The blades will turn slower than the air flowing over them because of the resistance that the "alternator" inside it creates. When you blow hot air at 20 knots over it, it will produce x Watts. When you blow cold air at 20 knots over it, it will produce more then x Watts.

Read Paul's message as it explains it in yet another way and I agree with him that the effect on a sailboat is only noticeable in extreme temperature differences. But 20 knots in the tropics or 20 knots in the polar regions will be very noticeable.

cheers,
Nick.
__________________
s/v Jedi is offline   Reply With Quote
Old 15-12-2009, 08:08   #113
Registered User

Join Date: Jun 2008
Posts: 64
Hey Jedi, if you like using the Beaufort scale so much, why are you going to spend money on new wind instruments ?

On how to make a "wind force" anemomter, check out the wiki page. It describes various types, split between velocity and pressure.

Anemometer - Wikipedia, the free encyclopedia
__________________
zydecotoad is offline   Reply With Quote
Old 15-12-2009, 09:55   #114
Do… or do not
 
s/v Jedi's Avatar

Cruisers Forum Supporter

Join Date: Feb 2009
Location: in paradise
Boat: Sundeer 64
Posts: 9,198
Quote:
Originally Posted by zydecotoad View Post
Hey Jedi, if you like using the Beaufort scale so much, why are you going to spend money on new wind instruments ?
Because Beaufort doesn't tell me the direction nor any apparent wind info.

I really don't see this matter as "liking Beaufort" or not. I just can't imagine that people go and sail across oceans without having this basic knowledge. They will arrive in area's in the world where all the marine forecasts are in B. I'm afraid there are many US sailors that think that windspeed in knots is a world standard or something, or that knots is better because it is what the USCG uses in their forecasts. That's all fine, but you should be able to adjust to other systems when you want to sail abroad. It doesn't help you to keep saying Beaufort is no good when it's all you're gonna get; it would be much wiser to learn how to use it before you need it.
Just my view on the matter. I've accepted knots and start to get some feeling with that and think it's not more difficult the other way around.

ciao!
Nick.
__________________
s/v Jedi is offline   Reply With Quote
Old 15-12-2009, 10:12   #115
Registered User

Join Date: Jun 2008
Posts: 64
Quote:
Originally Posted by s/v Jedi View Post
...
That's all fine, but you should be able to adjust to other systems when you want to sail abroad. It doesn't help you to keep saying Beaufort is no good when it's all you're gonna get;...
ciao!
Nick.
Nick,

I guess that my reaction has been to the implied idea that "Beaufort is better than knots" and if I am sailing without thinking in Beaufort scale, then I am less skilled.

When I am in an area that uses Beaufort, I will use it and get used to thinking in it.
__________________
zydecotoad is offline   Reply With Quote
Old 15-12-2009, 10:25   #116
Registered User
 
Stillraining's Avatar

Join Date: Oct 2008
Location: Puget Sound
Boat: Irwin 41 CC Ketch
Posts: 2,876
Quote:
Originally Posted by zydecotoad View Post
Nick,

I guess that my reaction has been to the implied idea that "Beaufort is better than knots" and if I am sailing without thinking in Beaufort scale, then I am less skilled.

When I am in an area that uses Beaufort, I will use it and get used to thinking in it.

LOL...Ya..I have already been told as much a few posts back.....sad but true in my Case however..
__________________
"Go simple, go large!".

Relationships are everything to me...everything else in life is just a tool to enhance them.
Stillraining is offline   Reply With Quote
Old 15-12-2009, 15:49   #117
Senior Cruiser
 
Jim Cate's Avatar

Cruisers Forum Supporter

Join Date: May 2008
Location: cruising SW Pacific
Boat: Jon Sayer 1-off 46 ft fract rig sloop strip plank in W Red Cedar
Posts: 11,454
G'Day All,

Some interesting if highly imaginative arguments posted here. Frankly, I don't much care if the met office uses the B scale, knots, mph or m/s... it isn't hard to work it all out into whatever is meaningful for the individual skipper.

But all this talk about anemometers... seems to me that the traditional 3 cup anemometer is rather different than a paddlewheel waterspeed transducer. The paddlewheel has the lower half of the wheel exposed to the nearly laminar flow along the hull, while the upper half is hidden in the instruments body in water that while not really unmoving, is not speeding past. IN the three cup transducer we have both "sides" exposed to the same air flow, and I reckon that the rotation of the thing is related to the difference in drag between the convex and concave sides of the cups. Since the air density is the same on both sides of the instruments axis, changes in density should cancel out pretty much.

Incidentally, the anemometer is the one on board instrument that I have never been able to accurately calibrate. Anyone have any useful ideas about how to do so?

Cheers,

Jim and Ann s/v Insatiable II lying Burnett River, Qld Oz
__________________
Jim Cate is offline   Reply With Quote
Old 15-12-2009, 16:31   #118
Do… or do not
 
s/v Jedi's Avatar

Cruisers Forum Supporter

Join Date: Feb 2009
Location: in paradise
Boat: Sundeer 64
Posts: 9,198
Yes, sail close to an airport and call their meteo. You can calibrate wind speed and barometer with them. Ask for barometer at sea level and wind speed at 10 meters altitude.

ciao!
Nick
__________________
s/v Jedi is offline   Reply With Quote
Old 15-12-2009, 17:49   #119
Senior Cruiser

Cruisers Forum Supporter

Join Date: Aug 2009
Location: between the devil and the deep blue sea
Boat: a sailing boat
Posts: 17,314
But not so close that the landing planes may wipe away the top of your mast, together with your newly calibrated wind instrument. Which may happen in Bonaire ;-)))

Truly, this thread went nuts, but, frankly, the nutty part gave me some most valuable new (to me) knowledge.

That is exactly why I like this forum.

barnie, the padawan
__________________
barnakiel is offline   Reply With Quote
Old 15-12-2009, 19:07   #120
Moderator Emeritus
 
Ex-Calif's Avatar

Cruisers Forum Supporter

Join Date: May 2007
Location: Singapore
Boat: Maxi 77 - Relax Lah!
Posts: 11,514
Images: 4
Quote:
Originally Posted by muskoka View Post
Actually the physics are simply that less dense air applies less force than denser air even if winds are moving at the same speed. Sails translate wind energy (density & velocity) into boat speed (propulsion energy less drag). So 5 knots of wind at sea level will propel the same boat faster than 5 knots of wind at 10,000 feet. Though in fact, the difference would be pretty slight.
You are contradicting yourself. If less dense air applies less force then you would read a lower wind speed as well. If you are in fact reading the same windspeed (5 kts) the force is the same as at sea level.

Quote:
Originally Posted by Pblais View Post
The dense air will provide more acceleration than the less dense air. f = mA. <snip>


When air density is low you see it best in say Salt Lake City, UT on a summer afternoon at the airport. They start bumping passengers off the small commuter flights because the air density is insufficient for a safe take off / landing.

I hope we can all agree that f-mA - LOL - we aren't talking about A we are talking about m. For f(sl) to equal f(10,000') for a standard atmoshperic day A must be higher because we all agree air has less mass at higher temperature and altitude.

So let's work in force. If I read 5 kts on the gauge, density, altitude and temperature are irrelevant. I am reading force measured in knots. 5 kts is 5 kts.

The reason that passengers are offloaded is runway length.

The engine can only produce so much power. In less dense air it takes longer to accelerate. However when the pilot rotates the airplane he will be reading the same airspeed. Because 80kts is 80kts of force regardless of temperature & altitude.

The wings are sails - they need 80kts of force to produce enough lift to fly.

Quote:
Originally Posted by s/v Jedi View Post
You keep bringing up new ways to try to prove me wrong but it's not me you are disputing, it's the laws of nature and you will fail ;-)

<snip>


Read Paul's message as it explains it in yet another way and I agree with him that the effect on a sailboat is only noticeable in extreme temperature differences. But 20 knots in the tropics or 20 knots in the polar regions will be very noticeable.

cheers,
Nick.
Not trying to "prove' anyone wrong - just discussin'

I agree with you and Paul that density is an extremely minor issue for a sailboat.

We are talking about <10% change in energy due to density.

However change the A from 5 kts to 10 kts and we have doubled the energy.

Bottom line is that density is not a factor for sailing.

It is still true that if you are reading 5, 10 or 20 knots on a gauge you are reading the force (calibrated in knots, mph, kmh or whatever) not the m and not the A.
__________________

__________________
Relax Lah! is For Sale <--- Click
Click--> Custom CF Google Search or CF Rules
You're gonna need a bigger boat... - Martin Brody
Ex-Calif is offline   Reply With Quote
Reply

Tags
safety

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
your max. force you have experienced? Karletto The Sailor's Confessional 13 20-08-2008 11:12



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 23:54.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.