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help me understand wind speeds.

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help me understand wind speeds.

Postby kevinnem » Fri Aug 31, 2012 10:33 am

Hello all. I am very poor as understanding wind speeds.

I did a wind finder report on were I am going this weekend, (sylvan lake), and there areavergae of 14 knot range. peak near twice that.

I was wondering some things, firstly, I fly a genoa. At what point does the boat "round up" - assumeing full out genoa, and full main (and heading upwind).

At what point do you reef sails, and pull in head sails, ect.


I know you "do these things when you need to", but I just am wondering if right form the get go, it is going to be too rough for me out there.

I know there is some indications (sometimes unreliable) about how wind speed and sea state relate, but I guess I am looking for a boat centric view, of wind speeds.

Kev.
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Re: help me understand wind speeds.

Postby WHRoeder » Fri Aug 31, 2012 11:28 am

THE BEERFORT SCALE: A MEASURE OF WIND AND SEA CONDITIONS

  1. Force 0: Sails hanging limp. Tiller tends itself.
  2. Force 1: Beginning pressure on sails. If sheet is eased out, the tiller still tends itself.
  3. Force 2: Sails flapping in the breeze, and boat drifting sideways to leeward. Sheets must be tightened and one hand put on the tiller. As the wind fills the sails, the boat heels. Case of beer must be placed on cockpit floor.
  4. Force 3: The beer may be knocked over and must be supported or held in hand.
  5. Force 4: Empty bottles rolling against each other on cockpit floor. Must be thrown over side.
  6. Force 5: All beer streaming behind boat must be hauled in.
  7. Force 6: Nobody can hold onto more than one beer at a time.
  8. Force 7: The case of beer slides back and forth on cockpit floor. One person must be appointed to sit on it.
  9. Force 8: Bottles can still be opened by one person. Beginning of difficulties pouring into the mouth without spilling.
  10. Force 9: Bottle must be held with two hands. Only experts can get the cap off by themselves.
  11. Force 10: Two people required to open bottles. Empties must be thrown to leeward only. Very difficult to find mouth. Some teeth may be knocked loose.
  12. Force 11: The beer tends to foam out of bottle. Very difficult to drink. Lips split and teeth fall out.
  13. Force 12: All open bottles foam. Impossible to drink. Temporary abstinence may be required.
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Re: help me understand wind speeds.

Postby Judy B » Fri Aug 31, 2012 1:04 pm

If you know it's going to be blowing 15+ kts, I recommend that you start out as cautiously as you can: with a deeply reefed genoa sheeted to the cabintop tracks, and deeply reefed mainsail. You can always increase the sail area if you feel confident.

Most daysailers and trailerables are designed to be sailed with a 100% jib and full mainsail in winds up to about 15 kts of true wind, assuming the boat is in the hands of a knowledgeable skipper who know how to shape the sails to depower them. If the sails are baggy or the skipper is not skilled, it's prudent to use even less sail area than that.

15-20 kts = Small Craft Advisory. Caution is always advised. Around here, San Francisco Bay, the winds are 15-25kts of wind every day, with much higher gusts. We have lots of experience teaching high wind skills around here.

In winds expected to be over 15 kts, inexperienced skippers are advised to reef their mainsails before leaving the dock, and use an 80 to 100% jib. They are taught to practice reefing and unreefing in light winds before being allowed to go out in high winds.

I would advise you, if you've never sailed in high winds before, to plan to be able to get off the water (or drop sails on the water) when the winds are over 15 kts, just in case it's overwhelming. It would be good if you could find a sheltered location and practice there before going out into stronger winds.
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Hints on depowering the sails:

Mainsail Draft postion: Be sure to get the foot of the reefed mainsail flat, no draft pocket there. Pull the mainhalyard tight enough to keep the deepest part of the draft in the bottom section in front of the middle of the boom. If your mainsail is too baggy to do that, it's going to be very difficult to keep the boat from heeling and rounding up.

Mainsail twist off: If you have a vang, be sure to ease it when on a reach or going upwind, and pull the traveller car to windward (past the centerline) so the upper section can twist off noticably in the wind to de-power the top of the sail.

To gauge how much twist you have in the top of the sail relative to the bottom, look at the top batten and its position compared to the boom. The back of the top batten should be pointing a little more downwind than the back of the boom does in gusty or high winds. In contrast, in moderate conditions, the top batten should be parallel to the boom.

(Or, the 2nd batten from the top, dpending on the design of the mainsail)

Genoa shape: You won't be able to shape the deeply rolled genoa up very much. You really would prefer to use a 100% jib but the genoa will have to do.

Forestay tension:
If you have a backstay, keep it fairly tight so the forestay doesn't sag. A saggy forestay makes the draft in the front sail deeper, especially in a gust, which makes it handle badly. The headsail develops more power and the sailplan becomes unbalanced with every gust.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Hints on reading wind speed by looking at the water

When you start to see a few whitecaps on a waveless lake, the wind is blowing about 14-15 kts. If you're seeing frequent white caps, the wind is blowing about 15-16 kts. If there are white caps everywhere, it's blowing around 18+ kts. (If there's a current flowing against the wind, you will see white caps at starting lower wind speeds. If the current and wind are going in the same direction, you won't see white caps until the wind is blowing about 15 kts faster, relative to the current)

Don't wear polarized sunglasses. You won't see the white caps when they're just starting. You won't be able to see wind lines on the water when a gust is coming, either.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Winds feel stronger when you sail upwind.

Keep in mind that the wind feels stronger when you're sailing into the wind, and weaker when you're sailing downwind. When sailing upwind in 15 kts of true wind, the sails are experiencing 20 kts of "apparent wind".

If the sailplan's power is just right downwind, but you're overpowered upwind, you proably need to reduce sail are.

~~~~~~~~~~~~~~~~~~~~~~~~~~
When gusts hit, they suddenly seem to be coming from a different direction

Be aware that when a gust hits, the angle of the wind seems to change - it's no longer coming from so close to the bow. If you hold your course, it seems like the wind is now coming more on the beam. You should steer so you point a little higher just before or just as the gust hits, so you don't have to let your sails out because you're suddenly heeling. You can usually see the gust approaching, and feel it in the helm.

In like manner, if you suddenly slow the boat down by releasing the mainsail, the wind will seem to shift so it not so close to the bow (so it's more on the beam). So you will have to retrim your sails or adjust course until you bring the boat back up to speed.

In moderate winds, this is true too. But the effects are more dramatic in high winds, which can be a little scarey if you don't know what to do.

(on edit: Here's an alternative to steering higher when a gust hits: you could use the traveller to decrease the mainsail's angle of attack. But this doesn't take advantage of the momentary chance to point higher)

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~`
Ask the locals. They have "Local knowlege "
Sylvan lake near in Alberta Canada? I took a look at the windfinder forecast and it appears to be a high wind location beloved by windsurfers. Usually, in inland lakes, the winds cycle from weak to strong depending on the time of day and the amount of sun. Ask the windsurfers when winds are more moderate and pay attention to the building winds.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Hope this helps. You'll soon be sailing with confidence.if you start out cautiously.

Fair winds,
Judy B

PS. I don't have time to proof read this carefully, so if I made some mistakes, I apologise in advance. Please don't hesitate to point them out if you find them!!!
Last edited by Judy B on Sat Sep 01, 2012 10:26 am, edited 4 times in total.
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Re: help me understand wind speeds.

Postby Catigale » Fri Aug 31, 2012 2:04 pm

If your mainsail is too baggy to do that, it's going to be very difficult to keep the boat from heeling and rounding up


..and if you have a stock mainsail from the factory, your main is almost certainly in this "too baggy" condition for 15 kts of wind or more....they come this way as a feature...
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Re: help me understand wind speeds.

Postby Tomfoolery » Fri Aug 31, 2012 2:29 pm

Catigale wrote:....they come this way as a feature...

And then they get even baggier after 13 years . . . :(
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Re: help me understand wind speeds.

Postby Phil M » Fri Aug 31, 2012 5:59 pm

Beerfort Scale :D :D
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Re: help me understand wind speeds.

Postby JohnCFI » Fri Aug 31, 2012 6:13 pm

Phil M wrote:Beerfort Scale :D :D


was excellent :D :D
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Re: help me understand wind speeds.

Postby Catigale » Fri Aug 31, 2012 6:47 pm

...worth five reefs minimum...

:D :D :D :D :D
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Re: help me understand wind speeds.

Postby cruiser » Fri Aug 31, 2012 7:53 pm

Once again Judy, thanks for your very helpful input.
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Re: help me understand wind speeds.

Postby March » Fri Aug 31, 2012 8:40 pm

Great post. May be tested in the garage in absence of significant wind. By Fore 6, relief in the family bathroom is optional.
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Re: help me understand wind speeds.

Postby dustoff » Fri Aug 31, 2012 8:46 pm

I have one question for Judy B.
Shouldn't you want to decrease mainsail twist in heavy winds? Meaning you would move the traveler to LEEWARD to have the mainsheet pull more vertically on the boom.
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Re: help me understand wind speeds.

Postby Judy B » Sat Sep 01, 2012 11:35 am

dustoff wrote:I have one question for Judy B.
Shouldn't you want to decrease mainsail twist in heavy winds? Meaning you would move the traveler to LEEWARD to have the mainsheet pull more vertically on the boom.


That's a great question. I think the answer is" it depends"..... on whether you're underpower or overpowered at the time the gust hits.

~~~~~~~~~~~~~~~~~~~~``
When overpowered....

In this thread we're focused on how to sail when you're overpowered. When you've already reefed, and you're still overpowered, you can depowering the top of the sail by luffing it. (twisting it off excessively to decrease the angle of attack at the top of the sail)

When a gust hits, the apparent wind shifts aft and increases in velocity, and powers up the upper section. That may sound like the wrong thing to do, but.... if you hadn't twisted it off to luff it, it would have a too-sharp angle of attack and be overtrimmed. This causes increases drag vs lift, and heeling rather than driving the boat forward.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When Underpowered....

However, if you were underpowered, with normal twist in the sail (2nd batten parallel to the boom) at the time the gust hit, you could profitably harness the extra power of the increased wind velocity. As the apparent wind angle moved aft with the gust, iIt would be appropriate to ease the traveller to adjust the angle of attack while maintaing the same amount of twist (due to constant leech tension )

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Twist is controlled by changing the tension on the leech. If you increase leech tension by pulling down on the clew more, you reduce twist

The traveller controlls the angle of attack on the mainsail, without changing the twist. Steering is also a way to control angle of attack without changing the twist.
The easing the mainsheet changes both angle of attack and twist.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Here's another point to consider, which I oversimplified in my previous post:
Jib and mainsail: two foils or one big foil?

IF THE MAINSAIL *IS* IN THE DOWNWIND AIRFLOW OF THE JIB, THE TWO SAILS ACTS AS ONE FOIL
When you're pointing close hauled, the two foils act as one big foil . The jib acts as the leading edge and the aft part of the mainsail acts as back part of the foil.

In this case, easing the mainsail via traveller has the effect of reducing camber in the whole foil, and changing the angle of attack of the whole foil, from the leading edge of the jib to the leech on the mainsail. If you ease the traveller just a little in a gust (I don't mean dumping it!), the airflow stays attached and the boat harnesses the increase in wind velocity and turns it into lift.

IF THE MAINSAIL **IS NOT** IN THE DOWN WIND AIRFLOW OF THE JIB, THE TWO SAILS BEHAVE INDEPENDENTLY

In this scenario, easing the traveller has the effect of reducing the angle of attack of the mainsail. And doesn't have a great effect on the action of the jib. On my Catalina, I can control both the traveller and the mainsheet from the helm. On a reach, I play the traveller in the gusts, not the mainsheet. On a point, I steer higher and ride the lift.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There are lots of combinations of ways to use your trim controls: some affect angle of attack and twist, and that will affects lift and drag. Others affect draft position and depth. We cruisers only have two hands so when we get overpowered, so we have to choose judiciously amongst all the available options. (or four hands if you have knowledgeable crew)

Racing crews tend to have lots of hands working together as single organism. The helmsman steers and reads the wind. somebody plays the traveller, somebody trims the mainsheet, somebody trims the jib (and adjusts the cars on the jib to affect twist in the jib).... etc.

I like to think of cruising trim being the same as racing trim, just with fewer hands available, more time to respond to the wind, and a whole less adrenalin...... the theory is same...

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
DISCLAIMER:
As always, I am writing this off the top of my head, and greatly over-simplifying theory (as far as I understand it) to make it "learnable". I've probably overlooked a few points, oversimplified a few, and maybe made an outright mistake. I'm sure if I've overlooked or mis-stated something, some one will notice. Everybody: PLEASE feel free to correct what I've written or add to it. I'm open to corrections!
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~`

Fair winds,
Judy B
Last edited by Judy B on Sat Sep 01, 2012 12:55 pm, edited 4 times in total.
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Re: help me understand wind speeds.

Postby Catigale » Sat Sep 01, 2012 11:53 am

...and if you are reading this while the gust has hit, hold your IPad up out of the water while you wait for your round up to end....
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Re: help me understand wind speeds.

Postby bscott » Sat Sep 01, 2012 12:19 pm

Judy, excellent advice. I always look forward to your posts--a great asset for this 4M 8)

Bob
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Re: help me understand wind speeds.

Postby Judy B » Sat Sep 01, 2012 1:32 pm

One more comment about reaching vs pointing, as it relates to apparent wind angle and twist:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
You need relatively less twist in a sail from top to bottom when pointing.

When pointing, there is less change in the angle of the apparent wind from the boom to the top of the mast. A little twist does a lot to align the top of the sail with the wind to produce maximum power up there. Anther way to look at it when you are over-powered: you don't need to twist it off so much to luff the top.

You need relatively more twist when reaching.

When reaching, there is more twist in the angle of the apparent wind from bottom to top of the mast. You need to put more twist when reaching than pointing to depower the top of the sail.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Again, the way to increase twist is to let the clew of the sail rise (reducing tension on the leech of the sail).
On a mainsheet, you ease the vang and/or the mainsheet ot increase the twist.
On a headsail, you move the car aft to increase the twist. This makes the sheet more horizontal and weakens the vertical pull.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Keep the boat moderately heeled (15-20 degrees?) and steady, and you'll be fast and be able to sail upwind better.

All in all, you want the boat to sail in a steady "attitude". If it's rolling erratically (heeling) and pitching (bow up and down) and yawing (zig-zagging), the apparent wind is moving all over the place and there's no way to keep the air flowing properly over all sections of the sails. The wind attaches to the sails in one place, yanks the boat around, then detaches there and the boat lurches back. The boat is impossible to control if that's happening. It's confusing :? frustrating :x and it's NO FUN. :(
(of course it's the boat that's moving around too much, but from a sail-centric point of view, it feels like the wind is moving all over the place).

Dumping the mainsheet should be your last resort. The boat stops and the "apparent wind" moves back to where the true wind is. so you trim your sails for true wind and get the boat moving again..... and then the apparent wind shifts in direction and magnitude....

If you need to dump the mainsail, your boat is telling you that you need to depower your sails and/or reef. When you're not overpowered, and you can keep the boat steady, the boat handles predictably and IT"S FUN AGAIN! :D :D :D

Fair winds,
Judy B
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