True v Apparent wind.

Fann Linn

Hi folks, I just cannot get my head around this. I've read the explanations, have the apps, but just finding it so confusing. I don't know if I'm mixing it up with tacking/wearing or drift, but could someone point me in in the right direction. Thanks.

noby

Fann Linn wrote: »

Hi folks, I just cannot get my head around this. I've read the explanations, have the apps, but just finding it so confusing. I don't know if I'm mixing it up with tacking/wearing or drift, but could someone point me in in the right direction. Thanks.

Are you having trouble with the formula, or the general concept of it?

Fann Linn

noby wrote: »

Are you having trouble with the formula, or the general concept of it?

Its actually both to be honest, although if I got my head around the concept I'm sure the formula would fall into place.

I can grasp it when the vessel is going full ahead or astern however I seem to be at a loss when the wind, both true and apparent is coming from either side.
Thanks.

noby

Fann Linn wrote: »

Its actually both to be honest, although if I got my head around the concept I'm sure the formula would fall into place.

I can grasp it when the vessel is going full ahead or astern however I seem to be at a loss when the wind, both true and apparent is coming from either side.
Thanks.

Sorry, got sidetracked earlier.

Apparent wind is basically the wind you feel when on a moving vessel.

As an extreme example, if you stick your head out of the car window while diving 100kph down the road on a calm day, the wind you feel coming straight at you is apparent wind.

With sailing, it's a little more subtle, as we tend not to get to those speeds.
If you're sailing north, and the true wind is coming from the west, then the combination of the wind and your forward movement makes it feel like the wind is north westerly, which is the apparent wind.

HeidiHeidi

As a rule of thumb, if the wind is directly from in front or behind you, you can add or subtract the true wind speed from your own speed to get the apparent wind speed. It's proportionately less in between (don't have an exact formula for that!)


The true wind direction will be dragged forward by your own speed (higher boat speed means more apparent wind effect), so your apparent wind direction will always be forward of the true wind direction, except when the true wind is directly behind you. The effect of your speed on the true wind direction is less and less as the wind goes from forward to your beam to your quarter to behind you (when it should be effectively nil).


That all makes sense in my head (took me years to wrap my mind around it!), hope it helps you a little bit.

gaffer falls

Did ye not do vectors in maths at school
Vector.....ayeeeeee

Fann Linn

gaffer falls wrote: »

Did ye not do vectors in maths at school
Vector.....ayeeeeee

Thanks for the help smartarse.

pedroeibar1

I can understand why you are confused by ‘drift’ because similar maths are at play. Forget drift and leeway for the moment.

True wind is that which blows across the top of your pint when standing on the club balcony; apparent wind is what blows across the top of your pint when sailing along. Apparent wind is more important than true wind because it’s what acts on the sails and results in performance.

Let’s assume that you are on a sailing keelboat with a 25 foot waterline. That means your maximum hull speed is 6 knots. (Rule of thumb for max hull speed = 1.2 X sq.root of LWL).
The wind speed is 20 knots so when sailing dead downwind the wind you feel (the apparent wind) is the 20 knots less the boat speed of 6 so it’s 14 knots.

At the end of your downwind leg you have to round a mark and sail upwind into the same 20 knots of wind. IF the boat could sail directly into the wind, the apparent wind would be the boat speed (6) plus the actual wind (20) total 26. However, as you have to sail diagonally into the wind two factors come into play (i) the angle of the boat to the wind and (ii) the boat speed. Both of these influence the apparent wind spped and direction. So the apparent wind will be between 20 and 26 knots, nearer the 26 than the 20. The faster the boat is traveling the direction of the apparent wind also moves forward.

From a sailing perspective it is important to take note of apparent wind speed as a change of direction often might necessitate a change of headsail – e.g. it might be ok to sail downwind under spinnaker but if the wind has increased during that leg it could be more appropriate to change down to a smaller genoa for the upwind leg. (And do up the oilies!)

Fann Linn

pedroeibar1 wrote: »

I can understand why you are confused by ‘drift’ because similar maths are at play. Forget drift and leeway for the moment.

True wind is that which blows across the top of your pint when standing on the club balcony; apparent wind is what blows across the top of your pint when sailing along. Apparent wind is more important than true wind because it’s what acts on the sails and results in performance.

Let’s assume that you are on a sailing keelboat with a 25 foot waterline. That means your maximum hull speed is 6 knots. (Rule of thumb for max hull speed = 1.2 X sq.root of LWL).
The wind speed is 20 knots so when sailing dead downwind the wind you feel (the apparent wind) is the 20 knots less the boat speed of 6 so it’s 14 knots.

At the end of your downwind leg you have to round a mark and sail upwind into the same 20 knots of wind. IF the boat could sail directly into the wind, the apparent wind would be the boat speed (6) plus the actual wind (20) total 26. However, as you have to sail diagonally into the wind two factors come into play (i) the angle of the boat to the wind and (ii) the boat speed. Both of these influence the apparent wind spped and direction. So the apparent wind will be between 20 and 26 knots, nearer the 26 than the 20. The faster the boat is traveling the direction of the apparent wind also moves forward.

From a sailing perspective it is important to take note of apparent wind speed as a change of direction often might necessitate a change of headsail – e.g. it might be ok to sail downwind under spinnaker but if the wind has increased during that leg it could be more appropriate to change down to a smaller genoa for the upwind leg. (And do up the oilies!)

Thanks very much for that indepth explanation and also to Heidi & Noby.