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Puget Sound Pounder
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Discussion Starter #1
Hello,

I have a Simrad IS20 analog wind instrument. About a month back my paddle wheel for boat speed got hung up and would not provide boat speed. This caused my wind instrument to not provide True Wind Speed. And Simrad verified that it uses boat speed for the calculation (as opposed to SOG).

I was always under the impression that you need SOG in order to calculate true wind speed. I sail in an area of mid/high current, I would think that without using SOG how can you accurately calculate true wind speed?

Am I all wet on this?
 

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Puget Sound Pounder
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Discussion Starter #3
I think I am also.

Example:
Boat tied at dock (ie not moving)
Wind coming straight over the bow 10kts
Current pushing my paddle wheel at 2kts, coming from same direction as wind

AWS and TWS should both be at 10kts (as SOG would be 0)
but...
My instrument would show AWS 10kts and TWS 8kts

That just seems broken to me.
 

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Puget Sound Pounder
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Discussion Starter #5 (Edited)
Thanks for your clarification, I thought I was just simply an idiot:)

I had nearly the same response from Simrad. What is this, an industry wide conspiracy? For a $500 instrument connected to an expensive nav network, at the very least they should provide the option of using SOG.

harrumph!!!

I suppose I can break out the old TWS tables, and use SOG rather than Boat Speed for the calculation:) Anyone have an old TI calculator that they wouldn't mind me programing:)

http://www.fortunearchive.com/Boating/Boating Physics/True Wind.pdf (tables at end of pdf)
 

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Hey,

As you wrote, you do need SOG to accurately calculate TWS. However, the difference between STW and SOG usually isn't that great, and IMHO, AWS (and direction) is more important than TWS. Does knowing the wind is 12 kts and not 10 make that big a difference? I think the instrument vendors use STW because it's easy to get from the paddlewheel and likely to be available.


Barry
 

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Slight typo there, Barry (SOG vs STW)

In a related 'WTF' item.. my Standard Horizon plotter seems to use SOG rather than VMG to calculate time to destination.. in other words on an unfavoured tack, the calculated time to destination remains appropriate to an SOG calculation even with a negative VMG?????
 

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Puget Sound Pounder
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Discussion Starter #9
Sometimes it can be huge for us up here in the PNW, it can be 5-6kts, Admiralty Inlet for instance. Like when my wife asked, "why is our rail in the water with only 16kts of wind?". It just seems to me that there should be an option for those of us who deal with it on nearly every outing. COG rather than heading might also be a nice option for TWD.

Hey,

As you wrote, you do need SOG to accurately calculate TWS. However, the difference between STW and SOG usually isn't that great, and IMHO, AWS (and direction) is more important than TWS. Does knowing the wind is 12 kts and not 10 make that big a difference? I think the instrument vendors use SOG because it's easy to get from the paddlewheel and likely to be available.


Barry
 

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Puget Sound Pounder
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Discussion Starter #10
Wow. It just goes to prove the best instrument is the one located between the ears.

Slight typo there, Barry (SOG vs STW)

In a related 'WTF' item.. my Standard Horizon plotter seems to use SOG rather than VMG to calculate time to destination.. in other words on an unfavoured tack, the calculated time to destination remains appropriate to an SOG calculation even with a negative VMG?????
 

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The Raymarine forum thread also showed how to get the Raymarine plotter to show it based on SOG. There is a "Ground Wind" computed from SOG instead of "True Wind" that is computed from paddle wheel. I'll try it and report back.
 

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In a related 'WTF' item.. my Standard Horizon plotter seems to use SOG rather than VMG to calculate time to destination.. in other words on an unfavoured tack, the calculated time to destination remains appropriate to an SOG calculation even with a negative VMG?????
I'm still new at this, but my understanding is that most plotters use that same (faulty) logic, and are just not capable of giving VMG (nor ETA) because the program just isn't written for sailboats. The only one that I've seen that really IS designed for sailing is the Zeus by B&G.

Of course, I bought a Raymarine e7 about 3 months before I heard of the Zeus. :(

If you have a tablet, the Sailtime App combnied with their bluetooth wind vane might be a good option to get that desired info without replacing the (much more expensive) fixed plotter or MFD that a sailor already has.

OTOH, by the time I retire and do any serious cruising, either Raymarine and the others will have advanced firmware to accomplish this, or I just plain WGAF, or both! :)
 

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Just in case anyone is confused, by definition, True Wind speed/direction = the motion of the air relative to the earth.

The problem with measuring both true and apparent wind speed/direction on a yacht using a typical masthead anemometer is that wind speed/direction changes with both air temperature and the motion of the top of the mast (particularly in the rough). For this reason, some wind instruments are fitted with temperature sensors, 3D accelerometers and rate gyros referenced to the instantaneous position of the top of the mast - not the position of the boat - via in-built GPS.

http://www.airmartechnology.com/uploads/brochures/pb200.pdf

The result is wind speed accurate to 0.1kt and direction to 0.1 degrees so it depends how serious you are I suppose. Perhaps something for the guy who has everything. :D
 

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Puget Sound Pounder
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Discussion Starter #15 (Edited)
The result is wind speed accurate to 0.1kt and direction to 0.1 degrees so it depends how serious you are I suppose. Perhaps something for the guy who has everything. :D
For me it is more about the 2.0 to 5.0 kt difference from Boat Speed to SOG used in the calculation of TWS. (for those of us lucky enough to deal with tidal waters) :)
 

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Did you check to see if your Garmin electronics offer the "Ground Wind" feature that Raymarine does?

It wasn't obvious to me, but this provides exactly what you are looking for. It is "true wind" derived from SOG/COG instead of paddle/heading.
 

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Puget Sound Pounder
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Discussion Starter #17
That is EXACTLY what I am looking for!

I have Simrad, and I did check the plotter manual, no dice, damn. I will have a chat with the Simrad rep at the upcoming boat show.
 
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