IS12 and TP32
Printed From: myHanse.com
Category: General
Forum Name: Chit Chat
Forum Description: Talk about anything to do with your boat
URL: https://www.myhanse.com/forum_posts.asp?TID=1778
Printed Date: 27 March 2026 at 03:29
Software Version: Web Wiz Forums 12.06 - https://www.webwizforums.com
Topic: IS12 and TP32
Posted By: Johan Hackman
Subject: IS12 and TP32
Date Posted: 16 January 2008 at 19:38
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I am sure there are a few of you out there with the combination IS12 and TP32. I recently replaced my IS15 with IS20 and became aware that the tiller pilot is allowed to feed compass data into the system, which is something it did already with the IS12.
I now wonder if anyone has reacted to this, and in what way? It seems strange to me that the tiller pilot is used as a compass when its position in reference to the boat is depending on whether it's engaged or not. Johan -------------  http://www.johanhackman.se" rel="nofollow - http://www.johanhackman.se |
Replies:
Posted By: Johan Hackman
Date Posted: 10 February 2008 at 21:09
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I think the silence of this thread speaks for itself. The standard IS12 setup doesn't seem to give you the information I am talking about, so no one could probably have detected a problem.
However, I am changing my initial question to a little contest. The numbers in the photo below don't add up correctly. I will send the first one who can tell how they don't add up a bottle of red wine, regardless of where in the world they live. If you don't drink red wine I'll change the prize to whatever is your poison. Johan  ------------- http://www.johanhackman.se" rel="nofollow - http://www.johanhackman.se |
Posted By: Alain & Anne
Date Posted: 10 February 2008 at 22:05
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Hi Johan, I think that you should have 330° instead of 269° because you get the wind from port side. ------------- UHAMBO 430e#004 White hull-teck deck-Yanmar 55hp-long range cruising OCC Our blog: www.uhambo.fr |
Posted By: Scorpio
Date Posted: 10 February 2008 at 23:05
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Boat Heading is about 115, Wind is about 155 off the port bow, therefore True Wind direction is about 320 (360-(155-115)).
Instruments say True Wind is 269 - a difference of 51 degrees
Poses a few questions...
1. Has the steering compass been swung, if so what's the deviation (not 51 degrees I hope!)
2. Has the fluxgate compass in the TillerPilot been swung and what's the deviation
3. Is the TillerPilot installed in it's correct position or is just lying on the cockpit seat, turned about 50 degrees from its mounted position?
Ed ------------- Ed Essery Scorpio GBR 9824T Hanse 342#465 - Solent Based |
Posted By: Junker
Date Posted: 11 February 2008 at 07:17
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Johan,
What is displayed on the IS 20?
I assume the compass data is coming from a tilller pilot.
I think this is mounted on the "wrong" side. I.e starbord, and the pilot still thinks that it is mounted on the port side. I guess this is a setting/ calibration issue.
regds
Rune
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Posted By: Johan Hackman
Date Posted: 11 February 2008 at 08:42
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Wow! I didn't expect you guys to be so quick. Both the Uhambo crew and Ed are correct, the Wind Direction should be somewhere between 320 and 330 degrees. Alain and Anne were first so they won! Congratulations!
Ed asks three interesting questions and the third one is the one I am trying to get at. However, I don't think there is a "correct" position for the autopilot. It's either engaged and hooked on to the tiller, or it's disengaged and lying on the cockpit seat. In this case it's lying on the cockpit seat and that's why the 269 degrees are so erratic. IS20 is also capable of showing Heading based on the compass data fed into the system. If the TP32 is the source the information will be just as wrong. It's not showing the boat's Heading, but the Heading of the autopilot whatever the position of the autopilot is. And you can't turn that off! I just wanted to see if anyone else could spot a thing like this and I thank you all for your input! Johan  ------------- http://www.johanhackman.se" rel="nofollow - http://www.johanhackman.se |
Posted By: colincooper
Date Posted: 11 February 2008 at 08:43
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Johan,
As you say it makes little sense using the compass in the tillerpilot. My guess is you have disconnected it and swung lain it on the bench to get the above wrong readings.
However, using the compass in a fixed autopilot setup (we use the AP25) makes perfect sense. The sensors probably don't know the difference. Is there a NMEA message saying "this compass direction is on a moveable compass"? Probably not. I guess you can delve in and tell the setup to ignore your compass - or ignore the reading yourself. ------------- Colin (owner of Hilde - a 370) |
Posted By: colincooper
Date Posted: 11 February 2008 at 08:45
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Hey you posted the picture just as I wrote that..... confirmed. ------------- Colin (owner of Hilde - a 370) |
Posted By: Junker
Date Posted: 11 February 2008 at 14:25
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Hi
It should be possible to use data from the GPS instead of the compass from the TP
Just an suggestion
regd
Rune
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Posted By: sulutime
Date Posted: 12 February 2008 at 13:54
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Sorry Johan if I'm being stupid but what about pulling the plug of the tiller pilot when disconnected? This should take the pilot built-in compass signal out of the IS20 system and assure reliable readings. You can even go exoteric and find out which pin in the plug carries the heading signal and remove it altogheter. Wind system will work with its heading and the tiller pilot will still have its internal heading signal to steer. I don't think they ever work togheter, instead there must be somewhere in the software a priority program to choose wich signal is master when two or more are present. Fabrizio |
Posted By: Johan Hackman
Date Posted: 12 February 2008 at 15:09
There are many solutions to the problem. One is to unplug the SimNet cable, but then the autopilot will not steer to wind, only course. Another obvious solution is to paint the displays with a thick layer of black paint... What I am saying is that this is a bug that should be rectified.
The reason for my initial question is that I know that this was already an issue with IS12 and it puzzled me how no one could have reported this issue before. On the other hand, I now understand that the standard IS12 instruments don't show Heading or Wind Direction. You need to have the Mega instruments, and still have a tiller pilot, which might be a very rare combination. The new Hanses fitted with IS20 will all be wheel-steered so I will probably be the only one to discover this problem. As has been discussed in another IS20-thread, the new system is built around a number of components including compass transducer, Simrad's new autopilot, drive unit and of course - GPS! I don't think that should mean that the system runs havoc if you don't buy all components. Johan ------------- http://www.johanhackman.se" rel="nofollow - http://www.johanhackman.se |
Posted By: Junker
Date Posted: 12 February 2008 at 20:28
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Johan do you mean that it is not possible to choose to take the heading from the GPS instead of the Compass? (and use this for calculating the true wind direction?) I know for sure that the Nexus system I had on my previous boat could do this, by changing the setup. I currently have Raymarine, but I'm not sure how it is there. But I assume it is possible. Maybe you can choose this as a option when setup ask for the compass source, or set up a separate screen on the instrument showing GPS data (including heading) |
Posted By: Johan Hackman
Date Posted: 12 February 2008 at 21:12
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Rune, the GPS and the compass will give you two different things - Course Over Ground and Course Through Water. Depending on leeway and current they will differ. True Wind is calculated from wind angle, wind speed and boat speed. It will be different if you let the GPS supply the information.
Are you happy with Raymarine, by the way? Very few Hanse have that fitted. Johan ------------- http://www.johanhackman.se" rel="nofollow - http://www.johanhackman.se |
Posted By: Junker
Date Posted: 12 February 2008 at 22:32
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Johan
I agree that it not exactly the same, but it will (hopefully) not be 50 degr wrong!
When I was talking about true wind, I was meaning true wind direction (magnetic) Not relative to the boat. This can be a good indication to monitor wind changes instead of comparing TWA and compass Heading
This means that your only option is to install a separate compass if you would like compass data when stearing manually?
I have the IS12 speed/depth instr, but the AP and wind is Ramarine.
The main reason for this choise was the AP . I have a wheel pilot, and the only alternaive was Simrad, and this is not good compared to Raymarine.
(Hanse was not able to deliver the boat without the IS12 standard instr.)
I'm in general happy with raymarine, and have used this on different boats.
I speacially like the new ST70 as this is probably the most flexible system existing at the moment. (and colour dispays) On my next boat I will most likly fit this ( when that time comes)
The best thing is that there is only 1 type of display and you set it up as you like
Rune
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Posted By: Scorpio
Date Posted: 12 February 2008 at 22:56
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I'm confused as to what the problem is functionally?
When the TP32 is Nav mode it uses XTE (Cross Track Error) and DTW (Distance to Waypoint) to calculate the course. This is why it won't go into Nav mode if the XTE is Large. All the autopilot does is steer the boat to keep XTE at zero - the GPS does the rest.
When the TP32 is in Steer to Wind mode it uses AWA (Apparent Wind Angle) which it gets from the Wind instrument to steer. AWA is the raw data from the Wind Instrument. TWA (True Wind Angle) is the True Wind Direction relative to the boat taking into account Log Speed although it would be more accurate if it used SOG (Speed over the Ground). TWD (True Wind Direction) is TWA in absolute terms and is calculated from TWA and HDG (Heading) which requires a reliable fluxgate compass.
When the TP32 is in Auto (Compass) mode it steers relative to the course the boat was on when the Auto button is pressed and it doesn't really matter what the actual number is as it steers relative to whatever the number was when the button was pressed.
As I understand it the TP32 worked with your old IS12 instruments in all three modes.
The difference now is that with the IS20 instruments you get the opportunity to see the actual number (the bearing) the fluxgate compass in the TP32 is on. And it's wrong! That's not an IS20 problem and functionally the TP32 is working as it did before.
In the TackTick network (as I have) the only data items on the Network which require the presence of a Fluxgate compass are Heading, True Wind Direction, Off Course Alarm and Tack Heading (what your heading would be on the other tack)
Or am I missing something?
Ed ------------- Ed Essery Scorpio GBR 9824T Hanse 342#465 - Solent Based |
Posted By: Junker
Date Posted: 13 February 2008 at 08:13
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Ed,
I agree to you.
My point is that IF you need/want the Heading data it should be an option to get this from the GPS if no compass are available
When sailing an use Heading, True Wind Direction, Off Course Alarm and Tack Heading I can't see that it is very imortant with the difference of COG or Mag Heading. It will be relative to the direction of sailing anyway.
I understand your point reg. Nav. mode
But if your XTE is is high, why don't you just calculate an new course to the same WP, and you will get XTE=0
You will off course loose the "history.
Regds
Rune
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Posted By: Johan Hackman
Date Posted: 13 February 2008 at 08:17
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Rune, I think it would be very handy to get True Wind Direction and a separate compass transducer will certainly solve the problem. A GPS will not give you Heading before you are moving, so that solution might not be the best.
Ed, you are right in most of what you say although I disagree with you when you say TWS and TWA will be more accurate if you use the GPS to get your speed. It will be a different thing just as COG/SOG and CTW/STW are two different things. "Different" doesn't mean "more accurate", does it? I was blabbering on about this in http://www.myhanse.com/forum_posts.asp?TID=1523&PN=5 - this thread, and will try not to repeat myself here. My previous IS15 system didn't have SimNet, only NMEA0183. I needed to see if people who have IS12 on their boats had discovered things like this. I take it that no one has. Johan ------------- http://www.johanhackman.se" rel="nofollow - http://www.johanhackman.se |
Posted By: sulutime
Date Posted: 13 February 2008 at 08:52
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Hi again Johan, I'm following your reports but still the problem seems to me quite clear. If you use autopilot then it is the TP32 compass feeding magnetic heading into the system, bein the master instrument of the moment; if TP32 is not used, keeping it connected only fools the system into believing it is being used,so it should be unplugged. As for wind , relative (raw) data is turned into true(calculated ) data with the water speed and heading only if nothing more accurate is available. True wind means geografic and only GPS data (COG/SOG) can achieve this. Anything else will be close but not on the money.
I have IS12 on my boat but the compass problem cannot occur since the steering is wheel and autopilot is obviously a fixed type.
If really we should find a fault in the system, then is the TP32 that should be programmed to discontinue sending heading data when is switched to stand-by to be disconnected.
Fabrizio
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Posted By: Scorpio
Date Posted: 13 February 2008 at 09:45
Johan,
TWA (True Wind Angle) and TWS (True Wind Speed) are relative to the planet not the water which is why SOG (Speed Over Ground) would be better combined with AWA (Apparent Wind Angle) than BSP (Boat Log Speed). In situations where there is no current or tide it makes no difference but in areas of strong current or tide it does.
For example if you are headed down the Alderney race between the Cherbourg peninsula and the Channel Island of Alderney the tidal rate can hit double figures on springs, more than doubling SOG. This will bring the apparent wind forward (decrease AWA) although the TWD (True Wind Direction) and therefore TWA are unchanged. If the instruments are using BSP to calculate TWA and TWS (as all instrument systems I know of do), the values will be wrong: TWA will be too low and TWS will be increased by the Cosine of TWA times the Tidal Rate which would be a significant percentage if you are sailing in anything less than a gale. If however TWA and TWS were calculated using SOG, the figures would be correct.
It's a difference between sailing in tidal and non tidal waters.
Ed
PS The Alderney Race is a very uncomfortable/dangerous place to be in any sort of wind over tide conditions...! ------------- Ed Essery Scorpio GBR 9824T Hanse 342#465 - Solent Based |
Johan Hackman wrote:Posted By: colincooper
Date Posted: 13 February 2008 at 10:46
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True wind is, as you say, relative to the planet. There is a third wind reading (Water Wind Angle) which is what actually hits the boat if not moving under any power.
Which is more useful to the yachtsman? True wind or water wind? If running downwind then knowing if Water Wind is getting close to 180 is pretty important.
Maybe the instruments should show all three?
But this is slightly off the track. The problem here is that the compass in the TP continues to broadcast a course even though it has no idea what the course actually is. IMHO it is a fault of the TP. In standby mode it should stop broadcasting. Blaming the instruments for assuming the data is correct is a little unfair.
------------- Colin (owner of Hilde - a 370) |
Posted By: Johan Hackman
Date Posted: 13 February 2008 at 19:18
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Ed, in your example from the Alderney race you say "will bring the apparent wind forward (decrease AWA) although the TWD (True Wind Direction) and therefore TWA are unchanged".
By TWA are you referring to the angle that is calculated from boat speed, AWS and AWA, i.e. what you read from you Wind Instrument aboard if it is set to show True Wind? Johan ------------- http://www.johanhackman.se" rel="nofollow - http://www.johanhackman.se |
Posted By: Scorpio
Date Posted: 13 February 2008 at 20:36
Johan,
In tidal waters if the direction of the boat remains unchanged and the real wind speed and direction remains unchanged when the boat speeds up (due to getting into the tidal race), the apparent wind will appear to come from closer to the bow - the true wind direction and the boat's direction have not changed therefore the correct value for the true wind's direction relative to the boat (TWA) will not change either. The instruments, however, will display TWA wrong if they calculate it from the Boat's Log/Water Speed (BSP) as opposed to the Boat's Ground Speed (SOG).
More correctly my text above should read... "will bring the apparent wind forward (decrease AWA) although the TWD (True Wind Direction) will remain unchanged and therefore the TWA should remain unchanged although it won't where TWA is calculated from BSP."
Ed
------------- Ed Essery Scorpio GBR 9824T Hanse 342#465 - Solent Based |
Posted By: Johan Hackman
Date Posted: 13 February 2008 at 21:21
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It's getting clearer to me that we mean different things. I see nothing wrong with calling the wind blowing over land True Wind, but I am used to the other definition - the wind that would blow over the deck if the boat is not in motion, i.e. the wind the instruments call True Wind.
I feel sorry if anyone has felt that I have tried to contradict them. I am just so childishly fond of these kinds of questions. My own tidal experience is limited to tacking down the Elbe in 11 knots, with a True Wind (my definition) of 7 to 9 m/s. Johan  ------------- http://www.johanhackman.se" rel="nofollow - http://www.johanhackman.se |
Posted By: Junker
Date Posted: 13 February 2008 at 22:08
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Hi
it might be that different sytems are calculating this in different ways.
They can also be set up using different input to calculate the values
But what I'm used to is:
TWS= the wind over land ( calculated from AWS and boat speed in water)
AWS= the wind blowing over deck/ sail
AWA= the wind angel over the sails
TWA= the directions from where the wind is coming relative to bow of the boat
TWD= this is the geo. wind direction in degr.
This means that AWS and AWA is the best to use for trimming of sails, as this is the values whom affecting them.
I found an article on this here:
http://www.soverel33.com/Articles/PDF/UsingTargets.pdf - http://www.soverel33.com/Articles/PDF/UsingTargets.pdf
This document says:
True Wind Speed, TWS, and True Wind Angle, TWA, are the speed of the wind relative to thewater and the angle of the wind relative to the bow of the boat, respectively.
Apparent WindSpeed, AWS, and Apparent Wind Angle, AWA, are the vector sums of the True Wind Speed and angel and boat speed. This is the wind you feel as the boat moves through the water and the wind the sails see.
Interesting discussion!
 regds
Rune |
Posted By: Scorpio
Date Posted: 13 February 2008 at 23:06
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Interesting stuff on Target Boat Speed.
If you feel so inclined, and have a PC on board, you can replace that duct tape with electronic instruments.
For instance if you have a TackTick system with the NMEA transmitter you can hook up a PC and use http://www.tactictool.com - this software . It will read data off the TackTick network and compute Target Boat Speed, Performance, Time to Lay Line and Distance to Lay Line amongst other things and pump them back into the TackTick Network for display on the instruments using some of the six configurable data items that TackTick make available for such use. It uses data entered into the computer from the Boat's polars to calculate this, or alternatively, data it has logged itself in the past.
Other packages do the same thing, some including a built in chart plotter. When you start drilling into this it's a horribly complex subject where we discover that even AWA is wrong on the instruments unless you apply "Upwash Correction"
Taken from the 2.3MB Ockam instruments http://www.ockam.com/docs/sysman.pdf - system manual at http://www.ockam.com - http://www.ockam.com , Upwash is explained "The wind is bent by the sails as it approaches them. In fact, the drive created by the sails is caused by this bending. However, if the masthead is inside this disturbed flow, it is not reading the proper angle and speed. This effect is termed upwash and is a complicated concept to understand.
This calibration attempts to correct for upwash by modifying the apparent wind angle only when beating, and and not when running. In heavy air, upwash tends to disappear because the sails are trimmed to spill air aloft, or are reefed. In both cases, the bending in the vicinity of the masthead is eliminated." I am now at (actually beyond) my level of understanding! Whilst I do plan to put a laptop on Scorpio at some stage and pump Target Boat Speed on deck, I'm going to draw the line at correcting for Upwash. Guess I'll never make it on an Americas Cup campaign!
Ed ------------- Ed Essery Scorpio GBR 9824T Hanse 342#465 - Solent Based |
Posted By: Gregor
Date Posted: 14 February 2008 at 07:00
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Goodmorning,
This glossary I found in the Geonav Wind manual (I'm considering of buying this program). Sorry for the mess of words, it came from a table. ARPA target Radar signal identified by an apparatus equipped with Automatic Radar Plotting Aids (ARPA). The radar apparatus can transmit, by the NMEA 0183 protocol (TTM and TLL sentences), the position, speed and route of the targets identified. AWA Apparent Wind Angle AWS Apparent Wind Speed Bearing Route angle (expressed in degrees) calculated referring to the geographical North. COG (Course Over Ground) Ship course expressed in degrees referring to the North, and calculated with respect to the movement of the boat over the ground. This data is supplied by the GPS device. The COG value can be either "Magnetic" or "True", which equals to the magnetic value after the correction with the Magnetic Variation has been applied. The True COG can be supplied by the GPS or calculated by the program. If the COG value is missing, the program will use the HDG value instead (if available). Cursor Symbol representing the mouse pointer on the chart. The cursor can take different shapes (triangle, flag, etc.) depending on the operation that is currently being carried on (Route editing, Marker insertion, etc.), or depending on the position of the cursor itself on the chart. ETA (Estimated Time of Arrival) Estimated time and date of arrival to a certain Waypoint. GPS Global Positioning System Hardware Key Hardware device to be connected to the USB port of the PC (parallel port version available on request), to protect software and chart data from illegal copy. A Serial Number that make it unique to the program for which it was customized identify each hardware key. HDG (Heading) Direction in degrees referring to the North the ship is heading to. The HDG value can be either "Magnetic" or "True", which equals to the magnetic value after the correction with the Magnetic Variation has been applied. The True HDG can be supplied by the GPS or calculated by the program. If the HDG value is missing, the program will use the COG value instead (if available). MOB Man Over Board Overzoom Representation of the chart data at a zoom level greater than the maximum detail provided by the vector chart used. When in overzoom mode, the chart image is calculated by the program by interpolating the greatest available zoom level. Be warned that the more the chart is overzoomed, the more the chart is distorted. Path Path to reach a given file within the PC hard disk. For instance, "C:\Programs\Geonav Power32\GeoPower32.exe" is the path to reach the Power32 program (GeoPower32.exe), that is stored in the Geonav Power32 subfolder inside the "Programs" folder on the hard disk (C:). Pixel The smallest graphic unit that can be represented on screen. The resolution of the screen is expressed by two numbers (e.g.: 1024 x 768) representing the number of pixels along the horizontal and vertical sides of the screen. The GEONAV software series require a minimum resolution of 800 x 600 pixels. Ship's Position Position of the boat on the chart. The position is represented by a symbol chosen when customizing the program (see "Settings/Display Options" function, "Ship" tab). The Ship's Position is set by the data received from the GPS or when activating the Simulator. SOG (Speed Over Ground) Speed of the boat calculated referring to the ground. Target Waypoint Next position to reach within a predefined route. The Target Waypoint is defined by the “Mode/Marker Mode” function. TWA True wind angle based on the ship prow TWD True wind direction TWS True Wind Speed UTC (Universal Time Coordinates) International standard to express the time at any position worldwide, referring to the time at the Greenwich (UK) meridian (GMT, Greenwich Mean Time). Wav File Audio file: wav. files contain sounds that can be reproduced by the PC internal loudspeaker, which can be used by the program, for instance to signal the activation of an alarm. Waypoint Position to reach within a predefined route. Gregor |
Posted By: colincooper
Date Posted: 14 February 2008 at 07:08
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As well as upwash altering the wind direction there is also simple heel angle. If you think of the goemetry a large heel will alter the angle the sensor at the top of the mast makes to the wind - by quite a lot when beating hard. You could have a heel sensor onboard to compute and remove that too. ------------- Colin (owner of Hilde - a 370) |
Posted By: samuel
Date Posted: 14 February 2008 at 07:21
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I have a flag at the top of my mast & it works beautifully !! ------------- Daydream Believer- Hanse 311- No GBR9917T- Bradwell Essex |
Posted By: colincooper
Date Posted: 14 February 2008 at 07:26
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A very interesting item on racing (and the concept that sailing too fast in a race is bad).
For racing and sail handling we need to use True (Water) Wind. If navigating (on a chart) then True (Land) Wind. However, if racing in a non-linear wind or curent field (near a headland for example) then True (Land) Speed needs to be considered too for optimal track.
------------- Colin (owner of Hilde - a 370) |
Posted By: panos
Date Posted: 22 March 2008 at 23:14
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My comment commes a little bit late but might be usefull.
The apparent wind direction- AWD as measured by the wind instrument is the angle between the (apparent) wind and the centerline of the boat. To calculate the true wind direction -TWD we need :
the apparent wind direction - AWD
the direction of the boat centerline (true heading- HDG),
the direction of boat movement (cource over ground-COG),
the apparent wind speed -AWS and
the speed over ground- SOG.
The calculation is done in the wind instrument.
The equation to calculate the true wind direction and speed is using all the above variables. If the true wind instrument replaces the COG with HDG or SOG with BSP (Boat speed) because COG and/or SOG are not avalable then in case of tide or high drift ( leeway is the correct term) the calculation could be highly inaccurate.
Even if all data are available, in a lot of situations even small errors in the measured values (AWD,AWS,HDG ) can lead to huge errors in TWD. This is the reason that sometimes after a tack the true wind direction TWD as we read it on our instrument changes.
Another complication comes from mast movement (ex. because of waves). The relative movement of the tip of the mast relative to the boat creates an "apparent" wind that is felt by the wind vane but not by the sailors on the deck and affects the accuracy of the AWD and AWS. The software in the wind instrument might be able to filter it out but...
------------- Panos Hanse 630e - selling her - |
Posted By: Bereklauw2
Date Posted: 25 March 2008 at 10:29
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Johan, A bit late maybe, but: I had the same configuration on my Hanse 291, but not from Simrad, but Raymarine. It worked in the same way. I believe the tiller pilots are designed this way. There are also instructions as how to mount them in order to avoid misreadings. The configurations is also ideal, because the tiller moves slightly back and forwards during operation, causing other misreadings. I solved it by adding a separate electronic compass. The software overruled the compass in the tiller. The good thing is: you will always have a spare on.  As to the discussion of True Wind: The True wind is the resultant vector of the apparant windspeed vector, its angle to the boat center line and the boat speed vector. Some good old high school math calculations could solve it. If the boat speed equals zero, apparant wind equals true wind and apparant wind angle equals true wind angle. No other variables needed. Boudewijn ------------- Arwen, Hanse 370, #401, former owner of Mirage, Hanse 291. "Real men don't use bowthrusters!" |
Posted By: panos
Date Posted: 25 March 2008 at 17:43
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Dear Boudewijn, In theory the true wind is as you explain. But on the boat how do you measure the speed vector. Using the compass and the log is not correct since the boat doesn't move exactly along its centerline because of drift (leeway is the correct word) and current. So more calculations are needed to evaluate the drift and the possible current.
 Also (at least in racing) one needs to know the true wind direction relative to the North( sometimes called magnetic wind) so that you know beforehand the boat direction and speed after a tack and to choose the new sail after you round a mark. It is complicated to imagine it but actually simple to calculate. Here you are right, not more than school math is needed.
------------- Panos Hanse 630e - selling her - |
Posted By: panos
Date Posted: 25 March 2008 at 19:39
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Hi,
Let me be a little more specific.
Imagine you are beating in a race to the East in a NE light wind. The finish line is 4 nm to the North. Also assume True North is also Magnetic North (to make matters simple).
On your (well calibrated) compass you read Heading=90 and on the log your speed reads 4 knots. On the GPS you read COG=100 and speed 4.2 knots. The apparent wind speed reads 8 knots and the (apparent) wind direction is 35 to port. The question is to find the boat direction after the tack and the exact spot to turn.
At home it is possible to calculate both the current (direction of flow and strength) and the true wind speed and direction - both over the water and over the ground (using school math).
It is obvious that this simple problem is on a boat not solvable either by vector algebra or vector drawings nor by previously prepared formulas . The only solution is to have good instruments with computer which calculates correctly the true wind (magnetic wind) in presence of current, leeway and waves. Some real math is needed (to extract the average values from the constantly changing readings from the instruments) and adapting the formulas to the present situation to avoid inaccurate calculations.
------------- Panos Hanse 630e - selling her - |
Posted By: Johan Hackman
Date Posted: 27 March 2008 at 23:23
I am glad to hear that I am not the only one to have experienced this. I think this pretty much answers my question. Thanks! Johan ------------- http://www.johanhackman.se" rel="nofollow - http://www.johanhackman.se |
Posted By: Johan Hackman
Date Posted: 27 May 2008 at 19:28
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I found another interesting discussion about True Wind, or "True True Wind" if you like, here: http://www.askjackrabbit.com/2008/04/is-true-wind-re.html - http://www.askjackrabbit.com/2008/04/is-true-wind-re.html . If anyone cares to dig any deeper into it, I mean.
Johan ------------- http://www.johanhackman.se" rel="nofollow - http://www.johanhackman.se |
Posted By: Gregor
Date Posted: 28 May 2008 at 05:11
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Hi Johan,
The link does not work. Could you post again? Thnkx, Gregor |
Posted By: Johan Hackman
Date Posted: 28 May 2008 at 06:54
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It should work now.
Johan ------------- http://www.johanhackman.se" rel="nofollow - http://www.johanhackman.se |