Tell tails on a run. - Page 3 - SailNet Community

   Search Sailnet:

 forums  store  


Quick Menu
Forums           
Articles          
Galleries        
Boat Reviews  
Classifieds     
Search SailNet 
Boat Search (new)

Shop the
SailNet Store
Anchor Locker
Boatbuilding & Repair
Charts
Clothing
Electrical
Electronics
Engine
Hatches and Portlights
Interior And Galley
Maintenance
Marine Electronics
Navigation
Other Items
Plumbing and Pumps
Rigging
Safety
Sailing Hardware
Trailer & Watersports
Clearance Items

Advertise Here






Go Back   SailNet Community > General Interest Forums > Learning to Sail
 Not a Member? 


Reply
 
LinkBack Thread Tools
  #21  
Old 09-09-2007
.
 
Join Date: Nov 2006
Posts: 10,855
Thanks: 0
Thanked 3 Times in 2 Posts
Rep Power: 12
Giulietta is just really nice Giulietta is just really nice Giulietta is just really nice Giulietta is just really nice Giulietta is just really nice
Actually if we take into consideration the fact that the sail is acting as "wind catcher", the the principles that apply are the ones demostrated by Stevin, in the comunicating vessels theory.
Reply With Quote Share with Facebook
  #22  
Old 09-09-2007
.
 
Join Date: Nov 2006
Posts: 10,855
Thanks: 0
Thanked 3 Times in 2 Posts
Rep Power: 12
Giulietta is just really nice Giulietta is just really nice Giulietta is just really nice Giulietta is just really nice Giulietta is just really nice
Then...off course, the famous Law and Principle of Alex wherby:

Any body submerged in a liquid becomes....wet. Not applicable to Mercury.
Reply With Quote Share with Facebook
  #23  
Old 09-10-2007
Senior Member
 
Join Date: May 2006
Posts: 822
Thanks: 0
Thanked 1 Time in 1 Post
Rep Power: 8
chris_gee is on a distinguished road
Well SD any anenometer I have seen is mounted at the top of the mast, so your argument doesn't wash. If the wind speed at say 60' up is .3, then sure as hell at deck level or midpoint it is going to be less.
Of course one explanation might be that the gauges over-read.
Reply With Quote Share with Facebook
  #24  
Old 09-10-2007
Senior Member
 
Join Date: Sep 2006
Posts: 2,179
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 8
Rockter will become famous soon enough
Apparent wind is not betrayed by a tell tale wavering in the cup of a boomed out headsail. Apparent wind is a relative velocity concept... the velocity of the wind relative to the boat.

No matter how you word it, no matter what your experience, no matter what your physics qualifications, no matter how many times you run a video, YOU CAN NEVER, EVER, EVER GET A SAILBOAT TO SAIL AS FAST AS THE WIND SAILING DOWN WIND ON A DEAD RUN.

That is determined by Isaac Newton, not Bernoulli. The governing equation is...

F = d{m*u)/dt = m*du/dt

Now let u go to zero in that equation.... u is the change in velocity of the wind. When u is zero, F is zero and you have no drive.

If you listen carefully sailing downwind you will hear the water gurgling below the boat. That energy has to come from somewhere, does it not? From where? If you have zero apparent wind, where is your energy coming from?
You always lose arguments with Newton.

I am talking about downwind now. Not some beach yacht reaching to weather.
Reply With Quote Share with Facebook
  #25  
Old 09-10-2007
Senior Member
 
Join Date: Sep 2006
Posts: 2,179
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 8
Rockter will become famous soon enough
Let me correct that.... "du" is the change in velocity of the wind.... set du to zero, and F will be zero.

Also, on ANY point of sail you must have relative wind. If you dont, then the boat will see still air, by definition. If the air is still around the boat, it cannot get propulsion.... unless the motor is on or you are paddling or something.
Reply With Quote Share with Facebook
  #26  
Old 09-10-2007
Idiens's Avatar
Larus Marinus
 
Join Date: Jan 2007
Location: Brussels
Posts: 1,756
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 8
Idiens is on a distinguished road
Are you guys also accounting for the wind speed being rarely constant and the wind instruments integrating to get an average wind speed over a few seconds. Also the mast-head indicator is averaging its wind-speed, which changes with mast-head motion too.
The biggest averaging effect is from the mass and momentum of the boat itself, so it is quite possible that the boat is doing 5 knots fairly consistently and the wind is giving puffs above 5 knots to keep it going but the tell-tales and the instruments are not showing those puffs so well.
Also aerodramatics is a funny thing, so Bernoulli does play a part down wind, particularly with spinnakers but also with main and genoa. Witness how much faster a boat will go if you can trim the sails so they don't quite collapse in light airs. If they are not collapsing then there is some pressure differential holding them inflated.

Last edited by Idiens; 09-10-2007 at 05:43 AM.
Reply With Quote Share with Facebook
  #27  
Old 09-10-2007
zz4gta's Avatar
I don't discuss my member
 
Join Date: Aug 2007
Location: Chesapeake Bay
Posts: 2,394
Thanks: 0
Thanked 15 Times in 15 Posts
Rep Power: 7
zz4gta is on a distinguished road
So on a dead down wind run the tell tales really don't matter, but just off a DDW they should be streaming aft correct?
Reply With Quote Share with Facebook
  #28  
Old 09-10-2007
sailingdog's Avatar
Telstar 28
 
Join Date: Mar 2006
Location: New England
Posts: 43,291
Thanks: 0
Thanked 8 Times in 8 Posts
Rep Power: 13
sailingdog is just really nice sailingdog is just really nice sailingdog is just really nice sailingdog is just really nice sailingdog is just really nice
Rockter-

You're speaking theoretical physics, in an ideal world... however, in the real world, the wind speed at deck level isn't the same as the wind speed at the higher levels... and it could very well have a deck-level wind speed of zero, yet still have enough wind to move the boat at deck-level wind speed.

Stop using the rose-colored glasses and look at the real world.

I'm not saying the boat is going as fast as the wind at mast top level, or even 20' up... just that at the deck it is possible for the boat to be travelling at the same speed as the wind at DECK LEVEL.

The energy for making the water gurgle is coming from the wind higher up—not the wind at DECK LEVEL. You seem to want to treat the entire boat and wind as a single thing... it's not. The wind speed varies with height.

I've been using cassette tape tell tales, and they're generally light enough to at least indicate if we've got any apparent wind...AT DECK LEVEL. There are plenty of times when we've been sailing DDW or close to it, and there's been nothing at DECK LEVEL.
__________________
Sailingdog

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

Telstar 28
New England

You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

If you're new to the Sailnet Forums... please read this
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
.

Still—DON'T READ THAT POST AGAIN.

Last edited by sailingdog; 09-10-2007 at 10:23 AM.
Reply With Quote Share with Facebook
  #29  
Old 09-10-2007
Stryker72's Avatar
Senior Member
 
Join Date: Jun 2007
Location: Wisconsin
Posts: 108
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 7
Stryker72 is on a distinguished road
The apparent wind can be zero when sailing ddw. Think about it. when the boat is stopped, the apparent wind speed (appws) = actual wind speed(actws). If the boat reaches actual wind speed it will no long accelarate but it will constantly be trying to decel. (due to friction). As the boat decels the appws grow, thus pushing the boat along and until appws becomes zero. This is a a steady state for the boat.

this is all if the wind speed is within range of the boat speed. A boat that has more drag may not be able to reach zero appws due to the wind not being able to overcome the drag.
Reply With Quote Share with Facebook
  #30  
Old 09-10-2007
Senior Member
 
Join Date: Sep 2006
Posts: 2,179
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 8
Rockter will become famous soon enough
Well, my boat is real. I think so. It was the last time I looked.

No-one mentioned deck level, and the tell-tale wasn't at deck level, they just said no relative wind.
I did know that there is a wind velocity gradient with height. That's why gaff riggers stick a big boom up there.

The real world is that a boat cannot make progress without a relative wind... not downwind, not upwind... no progress without relative wind. Without relative wind it's in still air, and it'll stop.

It's not theoretical physics. It's not a theory. It's Newton.

You must deflect wind to have drive. You either slow it when it's dead behind you, or you must accelerate it with an aftward component to go to weather.

You cannot do either unless you have the wind attacking the ship... relative wind.
Reply With Quote Share with Facebook
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

 
Posting Rules
You may post new threads
You may post replies
You may post attachments
You may edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Skipper wanted to run Beneteau in Carribean allanweiss Crew Wanted/Available 3 08-23-2007 04:22 PM
boat battery run down chuckg5 Gear & Maintenance 5 07-02-2007 12:01 AM
Sailboat run aground in Pearl Harbor - KHON2 NewsReader News Feeds 0 05-12-2006 12:15 AM
Refrigeration—Part II Selecting a System Tom Wood Gear and Maintenance Articles 0 05-19-2003 08:00 PM
What to Do When You Run Aground Sue & Larry Her Sailnet Articles 0 12-18-2002 07:00 PM


All times are GMT -4. The time now is 01:02 PM.

Add to My Yahoo!         
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) Marine.com LLC 2000-2012