Estimating true wind speed from apparent

[DaveC426913]

I want to get better at estimating wind speed. Right now I could not tell you 10 knots from 20 knots.

I suppose it would be expedient to simply measure apparent speed, since that's always what's experienced on a boat, but since that's highly dependent on point of sail I've decided that I'd like to know true wind speed. (going from a broad reach to a beam reach can give you quite a surprise if you only knew apparent wind speed).

So I've got me a handheld wind speed meter. Now I'm trying to figure out how SOW (speed over water) affects apparent wind speed on different points of sail. I know there are precise calculations that will produce this exactly - I'm really interested in rough calculations I can do in a split second in my head. Also, I know I an get instruments to tell me this - but that won't help me learn to estimate by feel.

I only need it within a range of error of a few knots. i.e. know 10 from 15 from 20 from 25.

This is what I think so far:
When on a broad reach or run: apparent wind + SOW = true wind speed (If I'm moving at 5kn and I feel a wind of 10kn, then true wind is 15kn)
When on a close haul or run: apparent wind - SOW = true wind speed (If I'm moving at 5kn and I feel a wind of 10kn, then true wind is 5kn)
When on a beam reach: apparent wind will be a little higher than true wind (since the boat is moving forward).

Does anyone have any practical working rules for this that don't require pencil or calculator?

[DaveB]

Other than what you have stated I look at the waves and what they are doing. This takes pratice and you should download a force wind chart that shows in color what each force of wind looks like on water. Current and wind against tides also has to be taken into consideration. You always start off what is the true wind and than workout the angle wind to boat. I have Sailed for so long it just becomes natural even telling the gusts and the amout of wind it packs well before it hits the sails.I never liked the wind instruments as they only give apperant wind and this may help newer sailors that can practise reading the water while looking at the wind gauge while anchored.
Maybe the wind guage would come handy after 40 knots of wind as I can never come close to telling above that as I only been out in that type of weather a few times and I know I probably exagerate the speed as it feels a lot more than it probably is.(All I can say is it's HUGHaH)
Dave

I want to get better at estimating wind speed. Right now I could not tell you 10 knots from 20 knots.

I suppose it would be expedient to simply measure apparent speed, since that's always what's experienced on a boat, but since that's highly dependent on point of sail I've decided that I'd like to know true wind speed. (going from a broad reach to a beam reach can give you quite a surprise if you only knew apparent wind speed).

So I've got me a handheld wind speed meter. Now I'm trying to figure out how SOW (speed over water) affects apparent wind speed on different points of sail. I know there are precise calculations that will produce this exactly - I'm really interested in rough calculations I can do in a split second in my head. Also, I know I an get instruments to tell me this - but that won't help me learn to estimate by feel.

I only need it within a range of error of a few knots. i.e. know 10 from 15 from 20 from 25.

This is what I think so far:
When on a broad reach or run: apparent wind + SOW = true wind speed (If I'm moving at 5kn and I feel a wind of 10kn, then true wind is 15kn)
When on a close haul or run: apparent wind - SOW = true wind speed (If I'm moving at 5kn and I feel a wind of 10kn, then true wind is 5kn)
When on a beam reach: apparent wind will be a little higher than true wind (since the boat is moving forward).

Does anyone have any practical working rules for this that don't require pencil or calculator?

[mastreb]

Here's a PDF on how to estimate wind speed by sea state, directly from NOAA:

http://www.wrh.noaa.gov/pqr/info/pdf/beaufort.pdf

This is the best method I know of and what we were taught in the Navy.

I actually don't worry a whole lot about wind speed--I measure it by heel. I just reef to keep the heel down to 20 degrees and let it be whatever it's going to be If I can't get reefed down low enough, I bring the sails in and drop the motor, but that's only happened once

It's true wind direction that I am most concerned with. Wish I had a better trick for estimating true wind from apparent--I'm so bad at it that I'm installing a Maretron this afternoon to do it for me.

[Crikey]

Matt - would be nice to hear of your experiences with the Maretron after a bit.

Here's how I calculate true/apparent wind:
First, send the admiral below to set up some snacks and grog....

5 kts = Sweetie, you doin ok without me? I'll only be a few minutes longer.
10 Kts = Sure you're ok? I'll be done in a minute.
15 kts = Why are you doing that! I'm trying to work down here!
20 kts = It's impossible for me to do anything! Look at the mess you've caused!
25 kts = Are you crazy! What are we doing out here!!
30 kts = If we don't go in right now I'm staying inside!
35 kts = #$%!&*^@!!

Small variations in the tonality and timing can be used to ascertain increases of 1 to 2 kts between these divisions....

Cool runnings all!
Ross

[Phil M]

Good one.

Phil M

[markh1f]

Or you could try one of these:
http://www.westmarine.com/webapp/wcs/st ... sNum=10211

Mark

[DaveB]

The Beaufort measure doesn't include the Force winds and pictures showing actual wind effects on the seas in color.(there are pictures)
I have the 13th editions of Dutton's Navigation & Piloting by Maloney and Volumes 1 and 2 of 1977 of the American Pratical Navigator, by Bowditch. These were my tools to Navigate back in early 80's by Celetial Navigation on my 35 ft. Sailboat. Wind is something you learn thru trial and error but can learn much faster if learning how to measure the True wind speed from the start.
A lot of Sailboat races have been won due to a person on board that can smell and feel wind, baromic pressures,humidity and a damn good nose to sniff out a high/low pressure wave.
I will say this, there is no weather man/woman that can give a true picture of what can happen in a day in your area, It's Gods will. Iphone4 with a radar Noa is probably best bet unless you have a Sat. phone.
Dave

Here's a PDF on how to estimate wind speed by sea state, directly from NOAA:

http://www.wrh.noaa.gov/pqr/info/pdf/beaufort.pdf

This is the best method I know of and what we were taught in the Navy.

I actually don't worry a whole lot about wind speed--I measure it by heel. I just reef to keep the heel down to 20 degrees and let it be whatever it's going to be If I can't get reefed down low enough, I bring the sails in and drop the motor, but that's only happened once

It's true wind direction that I am most concerned with. Wish I had a better trick for estimating true wind from apparent--I'm so bad at it that I'm installing a Maretron this afternoon to do it for me.

[DaveC426913]

I actually don't worry a whole lot about wind speed--I measure it by heel.

I want to be able to judge the wind before I'm in it. Like, say, at the dock, when I decide whether to reef.

[dennisneal]

Dave,

If you want to estimate the wind speed out on the water while you're at the dock, you may want to look around for flags to see what they are doing. Also, check the trees. It usually takes a pretty stiff wind to make a flag stretch out horizontally.

I am usually more concerned with how rough the waves are, than I am with wind speed itself. Here in Southern California, if there's a stiff breeze from the West, which is typical in the afternoon, its likely that there are rough waters as well.

[Catigale]

When on a broad reach or run: apparent wind + SOW = true wind speed (If I'm moving at 5kn and I feel a wind of 10kn, then true wind is 15kn)
When on a close haul or run: apparent wind - SOW = true wind speed (If I'm moving at 5kn and I feel a wind of 10kn, then true wind is 5kn)
When on a beam reach: apparent wind will be a little higher than true wind (since the boat is moving forward).

Okay let's examine 5 knots of boat speed and 15 knots of Wind speed on the three main points of sail

Easiest one I s running dead down wind. YOu will feel 10 knots apparent wind astern since the 15 knots true wind is reduced by your 5 knot boat speed.

So Running will be 15 knots true = 10 knots apparent, with the direction of the wind unchanged. Be careful when you come off the wind from a dead run as the sails wil power up in the true wind the moment the helm is turned.

Beam reach next. The 15 knots shifts forward due to the progress of the boat in an amount given by arctan( 5/15) = 18 degrees. So your apparent wind will be felt at about a bearing of 72 degrees from the front of the boat, with an increased apparent velocity of about 15.8 knots

...and lastly close hauled with the true wind 45 degrees of the bow (hard to do in our boats)

The true wind shifts even further forward to the bow and, using the law of cosines I get 18.8 knots of apparent wind and an angle on the bow of 34 degrees




The mathematics here are provided as rough guides only, the errors and mistakes are to be considered natural variations in the beauty of this post.

[c130king]

When on a broad reach or run: apparent wind + SOW = true wind speed (If I'm moving at 5kn and I feel a wind of 10kn, then true wind is 15kn)
When on a close haul or run: apparent wind - SOW = true wind speed (If I'm moving at 5kn and I feel a wind of 10kn, then true wind is 5kn)
When on a beam reach: apparent wind will be a little higher than true wind (since the boat is moving forward).

Okay let's examine 5 knots of boat speed and 15 knots of Wind speed on the three main points of sail

Easiest one I s running dead down wind. YOu will feel 10 knots apparent wind astern since the 15 knots true wind is reduced by your 5 knot boat speed.

So Running will be 15 knots true = 10 knots apparent, with the direction of the wind unchanged. Be careful when you come off the wind from a dead run as the sails wil power up in the true wind the moment the helm is turned.

Beam reach next. The 15 knots shifts forward due to the progress of the boat in an amount given by arctan( 5/15) = 18 degrees. So your apparent wind will be felt at about a bearing of 72 degrees from the front of the boat, with an increased apparent velocity of about 15.8 knots

...and lastly close hauled with the true wind 45 degrees of the bow (hard to do in our boats)

The true wind shifts even further forward to the bow and, using the law of cosines I get 18.8 knots of apparent wind and an angle on the bow of 34 degrees




The mathematics here are provided as rough guides only, the errors and mistakes are to be considered natural variations in the beauty of this post.

Professor, will this be on the final exam?

[Phil M]

I want to be able to judge the wind before I'm in it. Like, say, at the dock, when I decide whether to reef.

Some of the straits in the Pacific northwest, Haro Strait, Rosario Strait, and the Strait of Georgia, are so dam big that it is hard to tell how difficult the waves are going to be until you are out there. With my luck, the current often seems to be opposite the wind. And of course, once you are out there, you might as well cross the strait.

Weather radio and the tides and currents book all help, but you still need to feel the experience on your Mac.

Phil M