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post #1 of 17 Old 01-06-2014 Thread Starter
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TWS question

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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post #2 of 17 Old 01-06-2014
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Re: TWS question

Kelly:
I think you are correct. You need SOG to calculate TWS. Just boat speed won't do it even without current.

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post #3 of 17 Old 01-06-2014 Thread Starter
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Re: TWS question

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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post #4 of 17 Old 01-06-2014
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Re: TWS question

My (modern) Raymarine instruments also use speed through water instead of SOG to calculate true wind speed. I don't understand it either.

I'm curious, so I've asked on the Raymarine forum. I usually get a response on there from someone at Raymarine within a few days:
http://raymarine.ning.com/forum/topi...d-calculations

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Last edited by Alex W; 01-06-2014 at 12:37 PM.
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post #5 of 17 Old 01-06-2014 Thread Starter
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Re: TWS question

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/Boatin...rue%20Wind.pdf (tables at end of pdf)

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post #6 of 17 Old 01-06-2014
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Re: TWS question

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.


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post #7 of 17 Old 01-06-2014
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Re: TWS question

There have been a few useful replies in my thread on the Raymarine forums, including a link to this white paper:
http://www.ockam.com/docs/WhyPaddles.pdf

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post #8 of 17 Old 01-06-2014
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Re: TWS question

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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post #9 of 17 Old 01-06-2014 Thread Starter
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Re: TWS question

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.

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Originally Posted by BarryL View Post
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.


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post #10 of 17 Old 01-06-2014 Thread Starter
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Re: TWS question

Wow. It just goes to prove the best instrument is the one located between the ears.

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Originally Posted by Faster View Post
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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