Tell tails on a run. - Page 3 - SailNet Community
Old 09-09-2007
.

Join Date: Nov 2006
Posts: 10,851
Thanks: 0
Thanked 5 Times in 2 Posts
Rep Power: 15

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.
Giulietta is offline

Old 09-09-2007
.

Join Date: Nov 2006
Posts: 10,851
Thanks: 0
Thanked 5 Times in 2 Posts
Rep Power: 15

Then...off course, the famous Law and Principle of Alex wherby:

Any body submerged in a liquid becomes....wet. Not applicable to Mercury.
Giulietta is offline
Old 09-10-2007
Senior Member

Join Date: May 2006
Posts: 825
Thanks: 0
Thanked 1 Time in 1 Post
Rep Power: 11

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.
chris_gee is offline

Old 09-10-2007
Senior Member

Join Date: Sep 2006
Location: Scotland
Posts: 2,365
Thanks: 0
Thanked 11 Times in 11 Posts
Rep Power: 11

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.
Rockter is offline
Old 09-10-2007
Senior Member

Join Date: Sep 2006
Location: Scotland
Posts: 2,365
Thanks: 0
Thanked 11 Times in 11 Posts
Rep Power: 11

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.
Rockter is offline
Old 09-10-2007
Larus Marinus

Join Date: Jan 2007
Location: Brussels
Posts: 1,756
Thanks: 0
Thanked 2 Times in 2 Posts
Rep Power: 11

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.
Idiens is offline
I don't discuss my member

Join Date: Aug 2007
Location: Chesapeake Bay
Posts: 2,557
Thanks: 0
Thanked 33 Times in 33 Posts
Rep Power: 10

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?
zz4gta is offline
Old 09-10-2007
Telstar 28

Join Date: Mar 2006
Location: New England
Posts: 43,290
Thanks: 0
Thanked 18 Times in 14 Posts
Rep Power: 16

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)

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

Last edited by sailingdog; 09-10-2007 at 10:23 AM.
sailingdog is offline
Old 09-10-2007
Senior Member

Join Date: Jun 2007
Location: Wisconsin
Posts: 108
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 10

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.
Stryker72 is offline
Old 09-10-2007
Senior Member

Join Date: Sep 2006
Location: Scotland
Posts: 2,365
Thanks: 0
Thanked 11 Times in 11 Posts
Rep Power: 11

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.
Rockter is offline

Message:
Options

By choosing to post the reply above you agree to the rules you agreed to when joining Sailnet.

## Register Now

In order to be able to post messages on the SailNet Community forums, you must first register.
Please note: After entering 3 characters a list of Usernames already in use will appear and the list will disappear once a valid Username is entered.

User Name:
OR

## Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

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