barryg
Active Member
Could you please advise I know the difference between true and apparant but how do I use them when sailing
True and apparant wind
- Thread starter barryg
- Start date
bbg
Well-Known Member
Personally I come from a racing background, and we always had the instruments on True rather than Apparent. What did we use them for?
TWS - sail selection - so you know which headsail or spinnaker to hoist on the next leg, and when to change headsails
TWD - to identify wind shifts
TWA - upwind to help assess with pointing/footing mode, and to hit our "targets". Downwind to sail to the optimum polars. Downwind at night to prevent sailing by the lee/accidental gybe.
Given that I grew up using true, I suspect I will always be more comfortable with it, but I know a lot of people prefer apparent, esp when cruising. Don't know why - I am sure someone will enlighten me.
TWS - sail selection - so you know which headsail or spinnaker to hoist on the next leg, and when to change headsails
TWD - to identify wind shifts
TWA - upwind to help assess with pointing/footing mode, and to hit our "targets". Downwind to sail to the optimum polars. Downwind at night to prevent sailing by the lee/accidental gybe.
Given that I grew up using true, I suspect I will always be more comfortable with it, but I know a lot of people prefer apparent, esp when cruising. Don't know why - I am sure someone will enlighten me.
gandy
Well-Known Member
True wind is what you'd feel or measure if you were standing still. Say 10kn Northerly.
Apparent wind is the air speed and direction across the boat, which will depend on both the True wind and the boat's speed and direction. Apparent wind is what the sails will be feeling.
Apparent wind is the air speed and direction across the boat, which will depend on both the True wind and the boat's speed and direction. Apparent wind is what the sails will be feeling.
William_H
Well-Known Member
In practice forget the true wind. It doesn't make a lot of difference unless you are in a catamaran going fast in light winds.
On a lead mine (keel boat) you are proably limited to around 7 knots which means that in a 30 knot wind there is little efect.
ie when running square the apparent wind becomes 30 knots minus 7 or 23 knots while when beating the boat speed is added to the wind speed (almost) to give 36 knots or so apparent wind. Rather than the true 30 knots.
Likewise at higher wind speeds the angle of apparent wind gets closer and closer to true wind as the wind speed increases.
In any case it is the apparent wind you have to deal with. So forget true wind it is only there if you stop.
One effect which is noticeable is that I race most summer Sunday afternoons on Swan river. The coursae is tyically to a buoy up river which is down wind (usual 20 knot sea breeze) we beetle along at 7 knots plus with spinnacker up thinking the wind has dropped or not so bad and maybe the tiny jib we had on is really too small. The 20 knot true wind has become 13 knots apparent. 20 is a blow 13 is pleasant.
So we turn around at the buoy harden up to beat back and the 13 knots has suddenly become about 25 knots apparent on the nose. A definite blow. Suddenly it is cold wet and miserable. We really have to discipline ourselves to expect the balmy to become a real blow just by changing direction. Put on the spray jackets and sweaters.
Just sail with what you get across the deck and make the most of it. olewill
On a lead mine (keel boat) you are proably limited to around 7 knots which means that in a 30 knot wind there is little efect.
ie when running square the apparent wind becomes 30 knots minus 7 or 23 knots while when beating the boat speed is added to the wind speed (almost) to give 36 knots or so apparent wind. Rather than the true 30 knots.
Likewise at higher wind speeds the angle of apparent wind gets closer and closer to true wind as the wind speed increases.
In any case it is the apparent wind you have to deal with. So forget true wind it is only there if you stop.
One effect which is noticeable is that I race most summer Sunday afternoons on Swan river. The coursae is tyically to a buoy up river which is down wind (usual 20 knot sea breeze) we beetle along at 7 knots plus with spinnacker up thinking the wind has dropped or not so bad and maybe the tiny jib we had on is really too small. The 20 knot true wind has become 13 knots apparent. 20 is a blow 13 is pleasant.
So we turn around at the buoy harden up to beat back and the 13 knots has suddenly become about 25 knots apparent on the nose. A definite blow. Suddenly it is cold wet and miserable. We really have to discipline ourselves to expect the balmy to become a real blow just by changing direction. Put on the spray jackets and sweaters.
Just sail with what you get across the deck and make the most of it. olewill
TiggerToo
Well-Known Member
Olewill,
for once I must disagree with some of what you say. As you clearly explain in the last paragraph, knowledge/awareness of true wind is essential, especially when a change in course (or wind) is likely even in cruising. How many of us have been caught out with down-wind sailing with too much canvas out when we need to veer into the wind? If you have open water it may not be a problem, but if you are in a tight spot it can soon become catastrophic. As you very aptly explain, the difference in apparent wind between 13kn and 25 kn is huge, and needs to be predicted by knowing the true wind: where it is coming form and where it is going to.
for once I must disagree with some of what you say. As you clearly explain in the last paragraph, knowledge/awareness of true wind is essential, especially when a change in course (or wind) is likely even in cruising. How many of us have been caught out with down-wind sailing with too much canvas out when we need to veer into the wind? If you have open water it may not be a problem, but if you are in a tight spot it can soon become catastrophic. As you very aptly explain, the difference in apparent wind between 13kn and 25 kn is huge, and needs to be predicted by knowing the true wind: where it is coming form and where it is going to.
Danny
Well-Known Member
[ QUOTE ]
So we turn around at the buoy harden up to beat back and the 13 knots has suddenly become about 25 knots apparent on the nose. A definite blow. Suddenly it is cold wet and miserable. We really have to discipline ourselves to expect the balmy to become a real blow just by changing direction. Put on the spray jackets and sweaters.
Just sail with what you get across the deck and make the most of it. olewill
[/ QUOTE ]Agree with what you say, olewill, but it seems to me you make a good argument against your last statement!
How about: Use 'apparent' for sail trimming and steering and use 'true' for navigation, prediction and course changes, etc.
So we turn around at the buoy harden up to beat back and the 13 knots has suddenly become about 25 knots apparent on the nose. A definite blow. Suddenly it is cold wet and miserable. We really have to discipline ourselves to expect the balmy to become a real blow just by changing direction. Put on the spray jackets and sweaters.
Just sail with what you get across the deck and make the most of it. olewill
[/ QUOTE ]Agree with what you say, olewill, but it seems to me you make a good argument against your last statement!
How about: Use 'apparent' for sail trimming and steering and use 'true' for navigation, prediction and course changes, etc.
TiggerToo
Well-Known Member
[ QUOTE ]
How about: Use 'apparent' for sail trimming and steering and use 'true' for navigation, prediction and course changes, etc.
[/ QUOTE ]
yep, this sums it up admirably!
How about: Use 'apparent' for sail trimming and steering and use 'true' for navigation, prediction and course changes, etc.
[/ QUOTE ]
yep, this sums it up admirably!
fireball
Well-Known Member
You do have to be careful of instruments calculating TRUE wind from your water speed - if you are in a strong current/stream then your water speed could be significantly different to your SOG - making your instruments calculation of TRUE speed wrong.
john_morris_uk
Well-Known Member
To answer your question...
Apparent wind is what you trim your sails to. Its what you feel on you boat as you sail and its the wind that hits your sails. (Its direction is always f'wd of the true wind by the way, whatever point of sailing you are on.)
If true wind is available on your instruments it is useful for checking wind direction and speed when racing, and perhaps checking when sailing downwind when cruising as its easy to not notice the wind building when it is well aft.
Some sailmakers specify maximum true wind speed for some of their sails.
Hope this helps.
Apparent wind is what you trim your sails to. Its what you feel on you boat as you sail and its the wind that hits your sails. (Its direction is always f'wd of the true wind by the way, whatever point of sailing you are on.)
If true wind is available on your instruments it is useful for checking wind direction and speed when racing, and perhaps checking when sailing downwind when cruising as its easy to not notice the wind building when it is well aft.
Some sailmakers specify maximum true wind speed for some of their sails.
Hope this helps.
AngusMcDoon
Well-Known Member
If you assume true wind speed is the speed of wind over the water, not over the ground, then true wind speed will be correct when using boatspeed through the water in the calculation when in a tidal flow.
In my opinion wind speed over the water is a more valuable figure of true wind speed than wind speed over the ground because as far as sailing is concerned (not navigation of course) the water is your frame of reference, whether it is moving or not.
In my opinion wind speed over the water is a more valuable figure of true wind speed than wind speed over the ground because as far as sailing is concerned (not navigation of course) the water is your frame of reference, whether it is moving or not.
DaveS
Well-Known Member
I suppose it depends what you want to use it for. For helping to see where you are in a weather system I would have thought that wind speed over the ground is more use.
Salty John
Well-Known Member
It is only when you have wind speed and direction information available that you have to make any distinction - you are sailing in apparant wind, like it or not. You set course by it, trim to it and make sail changes based on it
As others have noted, you have to be aware of the increases and decreases in apparant wind that occur when you change direction, even though the true wind is steady.
If you add electronic wind speed and direction information to the mix you can use the availability of true wind speed and direction to plan your moves, possibly helpful if you are racing, but always be aware that it is apparant wind in which you are sailing.
As others have noted, you have to be aware of the increases and decreases in apparant wind that occur when you change direction, even though the true wind is steady.
If you add electronic wind speed and direction information to the mix you can use the availability of true wind speed and direction to plan your moves, possibly helpful if you are racing, but always be aware that it is apparant wind in which you are sailing.
MarkGrubb
Well-Known Member
Confused? You should be.
It may have been pointed out, but apparent wind direction is also a function of boat speed. So as your boat speeds up, for example as you accelerate out of a tack, the apparent wind direction will move forward, as you slow down it will move aft.
Though it is probably only the racers that worry about this level of detail and trim their sails accordingly. The same applies going upwind as well as downwind.
And of course boat speed through the air is affected by tide, have we mentioned tidal wind yet?
So if you sit in the channel on a spring ebb with your sails down and no engine then you will feel an apparent wind which is the true wind speed and direction modified by the wind you feel because you are drifting through the air on the tide. If you start to sail the apparent wind will be further modified by your boat speed and direction. If your boat speed and true wind speed remains constant but the tide slowly weakens and turns then the instruments, unless they are very sophisticated and know about the tide, will incorrectly interpret the resulting change in apparent wind as a change in true wind.
Apparent wind is also a function of tidal stream strength and direction. Hence the Lee Bow effect and also why you can be headed or lifted by sailing into a stronger or weaker tide.
It may have been pointed out, but apparent wind direction is also a function of boat speed. So as your boat speeds up, for example as you accelerate out of a tack, the apparent wind direction will move forward, as you slow down it will move aft.
Though it is probably only the racers that worry about this level of detail and trim their sails accordingly. The same applies going upwind as well as downwind.
And of course boat speed through the air is affected by tide, have we mentioned tidal wind yet?
So if you sit in the channel on a spring ebb with your sails down and no engine then you will feel an apparent wind which is the true wind speed and direction modified by the wind you feel because you are drifting through the air on the tide. If you start to sail the apparent wind will be further modified by your boat speed and direction. If your boat speed and true wind speed remains constant but the tide slowly weakens and turns then the instruments, unless they are very sophisticated and know about the tide, will incorrectly interpret the resulting change in apparent wind as a change in true wind.
Apparent wind is also a function of tidal stream strength and direction. Hence the Lee Bow effect and also why you can be headed or lifted by sailing into a stronger or weaker tide.
William_H
Well-Known Member
High Tigertoo and Danny yes you are quite right. Perhaps it is all a question of how much you worry about apparent and true wind.
I don't have wind speed or direction indicators for racing and so just sail to the wind across the deck. olewill
I don't have wind speed or direction indicators for racing and so just sail to the wind across the deck. olewill
hlb
RIP
The difference in wind speed, and direction,or apparent wind speed. is easily explained by a steam engine setting off from a station. At rest the smoke goes east west what ever. AS it sets off, the smoke becomes more and more behind it, as it picks up speed. This is the wind you are sailing by. So you sail off apparent wind not actual. So as you go faster you need to sheet in sail.
john_morris_uk
Well-Known Member
Thread drift - punctuation!!
[ QUOTE ]
Could you please advise I know the difference between true and apparant but how do I use them when sailing
[/ QUOTE ]
From all the resposes to Barry's post it seems that people are interpreting his question in two different ways.
I read it as him meaning to type:
Could you please advise.
I know the difference between true and apparant, but how do I use them when sailing.
Others seem to have read it as:
Could you please advise (how) I know the difference between true and apparant (and) how do I use them when sailing.
I suppose my post is my further moves in the directing of outraged of tunbridge on the decline of written English, I have spent most of the time reading the thread and peopl's responses thinking 'but he didn't ask that'. But I also suppose that if you post in text-speak - who knows what you really want to know anyway.
With the greatest of respects to Barry.
[ QUOTE ]
Could you please advise I know the difference between true and apparant but how do I use them when sailing
[/ QUOTE ]
From all the resposes to Barry's post it seems that people are interpreting his question in two different ways.
I read it as him meaning to type:
Could you please advise.
I know the difference between true and apparant, but how do I use them when sailing.
Others seem to have read it as:
Could you please advise (how) I know the difference between true and apparant (and) how do I use them when sailing.
I suppose my post is my further moves in the directing of outraged of tunbridge on the decline of written English, I have spent most of the time reading the thread and peopl's responses thinking 'but he didn't ask that'. But I also suppose that if you post in text-speak - who knows what you really want to know anyway.
With the greatest of respects to Barry.
fireball
Well-Known Member
[ QUOTE ]
the water is your frame of reference, whether it is moving or not.
[/ QUOTE ]
I agree - but on a coastal trip (especially around the solent) the water flow, speed and direction will change significantly in a short period of time - so True wind speed over the ground is more helpful
the water is your frame of reference, whether it is moving or not.
[/ QUOTE ]
I agree - but on a coastal trip (especially around the solent) the water flow, speed and direction will change significantly in a short period of time - so True wind speed over the ground is more helpful
bbg
Well-Known Member
Re: Thread drift - punctuation!!
Exactly what I was thinking - he knows what these two concepts are and the difference, but what does he use the instruments for in each mode?
I think putting the instruments on True is more useful in the sense that you are sailing in the apparent wind, so you can feel how strong it is across the deck, and trim the sails to it. What I think is useful to know is what the wind is doing in absolute terms. Changes in speed and direction can sneak up on you, but if you have (properly calibrated) instruments, the changes become more obvious when using True.
This isn't to say that you should need instruments to sail the boat. I have sailed and raced on a lot of boats without instruments - some where the class rules even prohibited speedos - but if there is another source of information why not use it? And for me, True is a more important source of info than Apparent.
Exactly what I was thinking - he knows what these two concepts are and the difference, but what does he use the instruments for in each mode?
I think putting the instruments on True is more useful in the sense that you are sailing in the apparent wind, so you can feel how strong it is across the deck, and trim the sails to it. What I think is useful to know is what the wind is doing in absolute terms. Changes in speed and direction can sneak up on you, but if you have (properly calibrated) instruments, the changes become more obvious when using True.
This isn't to say that you should need instruments to sail the boat. I have sailed and raced on a lot of boats without instruments - some where the class rules even prohibited speedos - but if there is another source of information why not use it? And for me, True is a more important source of info than Apparent.
