Page 3 of 5 FirstFirst 12345 LastLast
Results 21 to 30 of 46

Thread: True Wind

  1. #21
    Join Date
    Mar 2004
    Posts
    7,524

    Default Re: True Wind

    Quote Originally Posted by noelex View Post
    All sorts of reasons. Perhaps their home berth is difficult to get into with a strong wind, perhaps they don't fancy the proposed anchorage in stronger wind, or perhaps they want to know when they get out of the channel and out of the tidal influence, if there will still be enough wind to sail etc etc. Ground wind will answer all these questions.

    Ground wind is useful. In fact, this is what most sailors believe their instruments are displaying when set to true wind.

    True wind is useful as well, but it needs to be accurate information and often on a cruising boat the information is inaccurate and of little value. Many people do not realise that by changing the setting they could, as an alternative, get an accurate ground wind reading (although this not possible on some older instruments) and it will stay accurate so they can have some confidence in the information.
    I think ground wind will mislead far more often than it informs. And let's face it, anyone who actually understands the difference between Ground wind and true wind will know easily how to ascertain if their proposed anchorage will be untenable, or if there will be enough wind to sail.
    Another example of it misleading.
    Imagine you are doing the passage that is so beloved of Solent sailors going foreign. You're going to Cherbourg to stock up on cheap plonk and eat Moules.

    It's a lovely summer's day with a light Westerly wind blowing. Say about 6 knots as measured by the needles light house. You know that your boat really needs 8 knots of true wind to make meaningful progress under sail.
    Since you're a good skipper well used to planning your trip using the tides, you have been nicely shoved out of the needles channel, which you've motored down into the wind, by a West going tide and are heading South. Your ground wind instrument shows 6 knots of wind so you think, oh, I shan't bother trying to sail, I know that's not enough wind to sail in. So you motor. But of course you're being shoved into the wind at about 2 knots, so really you have 8 knots on the beam. Ideal stuff!
    Anyway, after a while you think "this looks windier than 6 knots, maybe my wind instrument is under reading, and you put some sail up and hey presto, lovely stuff! "Must be under reading after all, 6 knots is fine!
    Then of course the tide slacks and changes, and you gradually reduce the true wind, to 6 knots at slack and 4 knots once the tide is running the other way. Meanwhile your wind instrument stubbornly shows 6 knots. But you only have 4 knots, and progress is slowed to the point where the Moules are under threat... You keep trimming the sails trying to extract speed but eventually give up and turn the motor on, confused as to why the wind instrument has steadfastly shown 6 knots when clearly the wind is half the strength it was earlier. And book a raymarine service engineer to have a look at your whirlygig...

    Now of course that is all prevented by knowledge of the difference between true and ground winds. But thread after thread on these boards have proved that many don't...

    And then what was the use of ground wind display on that passage? None whatsoever, all you wanted the figure for was to know whether you should try and sail or not.
    You never know, I might be right!

  2. #22
    Join Date
    May 2001
    Location
    Daytona Beach, Florida
    Posts
    14,529

    Default Re: True Wind

    Slightly off topic, you really need speed through the water for navigation using dead reckoning, SOG is no good. However since proper nav techniques have faded with gps plotters et al, paddlewheels seem less required. Discussions arounfd this here in the USA get me going as they have a total blind spot and only see SOG as being needed, together with COG ( so why do they even have a compass by that logic?). To be fair so many are power boaters with speedy boats in low tide flows, leastways where we are unless crossing the gulf stream.

    I am reminded of meting a nice American couple in a Cherbourg restaurant many years ago. They had inadvertently parked their boat, that had brought them over the ocean safely on the rocks east of the eastern entrance, whilst trying to match CTS versus COG and effectively almost going in circles trying to achieve it. Trying to explain that over here, especial to motor boaters has been a waste of time with 'oh well it is just a very special case in strong tides' MY boats have always had paddlewheel transducers and kept clean by thinly applied antifoul and occasional removal for a tooth brush clean up in the berth.On the rare occasions the paddlewheel stopped ( critter in the works) I really missed not having true wind to sense if I really could turn off the donk and sail again or if it was only in my dreams. It is not always a simple add/subtract job unless going exactly up/down the stream.
    Sermons from my pulpit are with tongue firmly in cheek and without any warranty!

  3. #23
    Join Date
    May 2007
    Posts
    26,777

    Default Re: True Wind

    Quote Originally Posted by Robin View Post
    .....On the rare occasions the paddlewheel stopped ( critter in the works) I really missed not having true wind to sense if I really could turn off the donk and sail again or if it was only in my dreams. It is not always a simple add/subtract job unless going exactly up/down the stream.
    Really?
    Why do people find it so hard to think along the lines of 'I'm motoring at 4 or 5 knots upwind, therefore the true wind is about 4 knots less than the apparent?
    Or do people need an answer to several decimal places before unfurling the jib?

    The 'true' wind speed and direction tends to vary significantly both minute to minute and from place to place.
    In the Western Solent, the tide might well give a 3 knot difference between true wind and ground wind at some place in the deep water, but half a mile away or in ten minutes time, the ground wind could be 20degrees and a few knots difference anyway. It's not even the same in the middle of the rig as it is at the masthead.

    In my dinghy, True Wind is about 40 degrees above the close hauled heading.
    That's all I need to know, which is lucky, because it's all I can know.

    Here a little puzzle for the racers:
    A start line with a bit of tide running across it at an angle.
    Is there any merit in using ground wind (instead of true) to pick the windward end of the start line?

  4. #24
    Join Date
    Jun 2004
    Location
    Fareham
    Posts
    6,009

    Default Re: True Wind

    I always remove the paddle wheel before leaving the boat on the mooring and I tend to re-callibrate the log later in the season to adjust for the increase in fouling around the transducer. Each of these jobs takes a matter of minutes and I really feel lost without a display of STW.
    ۞

  5. #25
    Join Date
    May 2007
    Location
    Cambridge
    Posts
    899

    Default Re: True Wind

    I'd never be without my paddlewheel.

    I'm always trying to improve my sailing, experimenting with different sail techniques and configurations.
    Trying to measure small differences (maybe just 0.1 kt) with only SOG and variable currents to around 2kt (round my way) is, frankly, a waste of time. Even if it's inaccurate in an absolute sense, the log shows me whether I'm doing better or worse with my tweaks.
    Save someone's life ... https://www.mind.org.uk/donate

  6. #26
    Join Date
    May 2001
    Location
    Daytona Beach, Florida
    Posts
    14,529

    Default Re: True Wind

    Quote Originally Posted by lw395 View Post
    Really?
    Why do people find it so hard to think along the lines of 'I'm motoring at 4 or 5 knots upwind, therefore the true wind is about 4 knots less than the apparent?
    Or do people need an answer to several decimal places before unfurling the jib?

    The 'true' wind speed and direction tends to vary significantly both minute to minute and from place to place.
    In the Western Solent, the tide might well give a 3 knot difference between true wind and ground wind at some place in the deep water, but half a mile away or in ten minutes time, the ground wind could be 20degrees and a few knots difference anyway. It's not even the same in the middle of the rig as it is at the masthead.

    In my dinghy, True Wind is about 40 degrees above the close hauled heading.
    That's all I need to know, which is lucky, because it's all I can know.

    Here a little puzzle for the racers:
    A start line with a bit of tide running across it at an angle.
    Is there any merit in using ground wind (instead of true) to pick the windward end of the start line?
    A cherry picked example relating to only a part of what I posted, but correct of course for that cherry.
    Sermons from my pulpit are with tongue firmly in cheek and without any warranty!

  7. #27
    Join Date
    May 2001
    Location
    UK East Coast
    Posts
    29,156

    Default Re: True Wind

    Quote Originally Posted by mainsail1 View Post
    I am amazed that a manufacturer has not found a way to design out the endless paddlewheel cleaning problems but there we go.
    They have; they recommend antifouling the transducer housing and the paddlewheel. I do it about twice a year, it works.

  8. #28
    Join Date
    Jul 2005
    Location
    Living on dirt waiting for our new yacht to be built.
    Posts
    2,532

    Default Re: True Wind

    Quote Originally Posted by flaming View Post
    It's a lovely summer's day with a light Westerly wind blowing. Say about 6 knots as measured by the needles light house. You know that your boat really needs 8 knots of true wind to make meaningful progress under sail.
    Since you're a good skipper well used to planning your trip using the tides, you have been nicely shoved out of the needles channel, which you've motored down into the wind, by a West going tide and are heading South. Your ground wind instrument shows 6 knots of wind so you think, oh, I shan't bother trying to sail, I know that's not enough wind to sail in. So you motor. But of course you're being shoved into the wind at about 2 knots, so really you have 8 knots on the beam. Ideal stuff!
    Anyway, after a while you think "this looks windier than 6 knots, maybe my wind instrument is under reading, and you put some sail up and hey presto, lovely stuff! "Must be under reading after all, 6 knots is fine!
    Then of course the tide slacks and changes, and you gradually reduce the true wind, to 6 knots at slack and 4 knots once the tide is running the other way. Meanwhile your wind instrument stubbornly shows 6 knots. But you only have 4 knots, and progress is slowed to the point where the Moules are under threat... You keep trimming the sails trying to extract speed but eventually give up and turn the motor on, confused as to why the wind instrument has steadfastly shown 6 knots when clearly the wind is half the strength it was earlier. And book a raymarine service engineer to have a look at your whirlygig...

    Now of course that is all prevented by knowledge of the difference between true and ground winds. But thread after thread on these boards have proved that many don't...

    And then what was the use of ground wind display on that passage? None whatsoever, all you wanted the figure for was to know.
    I fully agree that in the example you have given true wind is useful to have, but I think you are too readily dismissing the information that ground would also provide.

    Using the example you provided, ground wind will tell the skipper that the "real wind" has stayed steady as 6 knots, exactly as forecast through the day. The yachtsman using a true wind display would likely believe, incorrectly, that the wind rose to 8 knots only to drop away later to 4 knots.

    The skipper relying on true wind would also likely think the real wind was 8 knots and therefore not realise that the real wind was only 6 knots and the extra 2 knots were courtisy of the current. He would not e apparant that it was only possible to sail because of the tide. The skipper with the ground wind display would realise the real wind was only 6 knots and easily recognise that as the tide decreased he would be left with the real windspeed of 6 knots which would conually be displayed on his ground wind instrument. He would therefore realise that sailing would not be possible as the tide decreased. This sort of information helps strategic planning. It is the sort of information that is very useful to the cruising sailor.

    Using ground wind also removes any nagging doubt that the readings have become inaccurate because the paddle wheel has become fouled half way through the crossing.

    It should be remembered that the choice of true wind or ground wind is combined in both systems with a display of apparent wind. Apparent wind is what the sails use. The apparent wind information already takes into account the movement of the boat and importantly also the movement of the water. Combined with this apparent wind display we can choose to display ground wind, which is the real wind removing any effects of boat movement or movement of the water, or true wind which removes the the effects of the boat movement but does not remove the effects of the water movement. The combined displays of apparant wind and ground wind compliment each other nicely and provide the skipper with a lot of information.

    Until recently ground wind was only available on very high end wind instruments such as the B&G H3000 system that was fitted to our previous boat. Personally, I prefer using ground wind while cruising and true wind while racing (providing the paddlewheel and bottom are clean), but recognise that others may not agree. The ideal, of course, is to display both, but this is often not practical.

    At least I hope the discussion has opened people's eyes to the difference between ground wind and true wind and those who have not used a ground wind display give it a try. It is new information that was posdible to calculate until reasonably recently and therefore many do not realise the impotortant information it provides. If you have a crusing boat you will be delighted to see how you can turn onto any point of sail and the ground wind reading will stay rock steady continually displaying the same real value of the windstrength.

    To the OP I would say that you will miss STW. SOG is not a substitute for STW, but I think you will find, at least cruising, that while ground wind provides different information to true wind the ground wind information is just as useful, especially as you will always have the confidence that it is accurate. Of course for others you can keep STW but still have ground wind rather than true wind (at least on many modern systems).
    Last edited by noelex; 17-07-17 at 17:02.

  9. #29
    Join Date
    Feb 2007
    Location
    Essex
    Posts
    15,498

    Default Re: True Wind

    Quote Originally Posted by pvb View Post
    They have; they recommend antifouling the transducer housing and the paddlewheel. I do it about twice a year, it works.
    I have never antifouled my Raymarine transducer. I usually give it a light coat of silicone grease, but that's all. It generally lasts several months without any attention, but it is only the work of a minute or two to clean it, even under way. Modern transducers let in very little water in the process, and even on my last boat, with no valve, it only admitted a few cupfuls, with practice. I'm in fresh water in the Netherlands at the moment, so I've got even less to worry about.
    Far away is near at hand in images of elsewhere

  10. #30
    Join Date
    May 2007
    Posts
    26,777

    Default Re: True Wind

    Quote Originally Posted by Robin View Post
    A cherry picked example relating to only a part of what I posted, but correct of course for that cherry.
    But if you have a basic grasp of vectors the up and downwind cases are easy, across the wind the differences are a lot smaller and surely not that hard to estimate 'near enough' for the purpose?
    If you are motoring along and the apparent is roughly amidships do really want the true wind calculated before knowing if your boat will sail?
    I know people do get caught out motoring close to dead downwind, moaning that there's too little breeze to sail, then we come shooting past with a gennaker up.

Page 3 of 5 FirstFirst 12345 LastLast

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

Find Boats For Sale

to
to