I fully disagree to Raymarine ..... - SailNet Community

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  #1  
Old 04-04-2008
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I fully disagree to Raymarine .....

I contacted Raymarine support many times, but couldn't get a good answer. So, although I'm still a good Ray's customer, I'd like to share this. Raymarine's wind instrument shows TRUE wind only if you have speed over water information (i.e from the "paddle thing" installed). I refuse to make another hole to get wrong measurement anyway, and argue that TRUE wind could be calculated correctly only using SOG (speed over ground). Raymarine gave me several explanations (foolish ones) and I believe the only reason for that is to PUSH the market to purchase that "paddle thing". In critical situations (downwind with a spi, for instance), the wrong information could lead to wrong decisions and threaten your safety. Have you ever considered that ?
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Your ship will sail according to relative wind anyway, not true. But then you know that already.
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Then get one of the paddle thingy's that mount on the transom.

And, if you want to derive true wind, you're going to have to make a determination of vessel speed somehow; by estimate or empirical input. But then, as the Rock says, who cares what true wind is?
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Negrini-

You can get a triducer, which gives you the speed, depth and temperature in a single unit. It requires a slightly larger through-hull than your current depth transducer, but would solve the problem.
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negrini

Welcome to sailnet. I'm going to disagree with you on this. Current and set will affect the truewind reading which is why you need SOW. Of course if your sailing on a lake or non-tidal area, that's different.
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Speed through the water is an unnecessary and potentially misleading input when calculating true wind speed, just as Negrini is arguing.

Take this example: You're entering the San Francisco Bay with a true following wind of 20 knots. Your boat speed though the water, as derived by a paddle-wheel, is 5 knots. You have a flood current of 5 knots behind you. The apparent wind reading is 10 knots (20 knots minus 5 knots boat speed minus 5 knots current speed).

If you're just reading boat speed through the water, you'd conclude -- incorrectly -- that the true wind speed was 15 knots (10 knots apparent wind plus 5 knots boat speed).

The only way to calculate true wind speed is to know the speed of the boat over ground (SOG) and the true course over ground (COG), and to reconcile this with the apparent wind speed and direction. SOG could be derived from, e.g., a GPS input but certainly not from a paddle-wheel device.

And, yes, in a lake or low-current situation it wouldn't matter much.

And, yes, if the helmsman had both GPS speed info and apparent wind info he/she could calculate true wind speed easily enough. I was in just such a situation last week, when singlehanding with a following wind of up to 28 knots true and a following current of just over a knot. Example: with an apparent wind speed of 18 knots behind me, and a GPS-derived speed over ground of 8.5 knots (including an estimated following current of 1 knot), I could easily estimate the true wind: 18 knots apparent plus 8.5 knots COG = 26.5 knots true.

Why did I care about true wind speed? Easy. Because I was alone, sailing in relatively restricted waters, having to make significant course changes which involved gybing. Jibing a full main on a 40+ foot vessel in 26 knots of wind is no fun, and must be done very carefully if you're not going to break something. So it was very important to me to know that although my apparent wind was, e.g., only 18 knots, the true wind speed was over 26 knots.

Bill

Last edited by btrayfors; 04-04-2008 at 10:29 AM.
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Old 04-04-2008
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Breezing, let me beter clarify

Regardless the current, TRUE wind is a measure of speed over ground, not over water !! Suppose you're sailing dead downwind at 6knots against a counter-current of 6knots: your wind instrument will show you ZERO true wind, but reality is, since your are still relative to the ground (SOG is zero in this example), the reading should be the real 6knots of wind. So, your Raymarine instrument will fool you exactly by the current. And for the others not caring about this, I always read the instrument to decide for a spi, and as a single handed, I push my engine along with wind to create a 1-2 knots apparent. This makes setting the spi a breeze, and once I'm done, I just kill the engine and the spi fills; very safe right ? Unless you use Raymarine instrument. Since GPS is a commodity for boats nowadays, SOG is plenty available over the cables. Why simply Raymarine doesn't take SOG instead of SOW for true wind calculation ?
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Gosh bill ! We posted at the same time !!! I have this EXACT problem, and thanks for a so good clarification ............
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Good points from both of you. I guess I was meaning to say Raymarine uses both in order to get an instrument reading of accurate truewind. That SOG won't do it independently which is how I read the original Post. I'm not disagreeing with you. I'm just saying that for the Instrument to display an accurate Truewind, it does the math so you don't have to.
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I find it easier just to have a masthead wind speed indicator and to do the math in my head. The sea state tells me in most cases the true wind speed if I'm on a run holding a candle in my hand (grin). Frequently, I have been on a broach reach in rolly stuff doing six or seven knots cool as a cucumber...and then I see some similar boat rail down, bone in her teeth, beating toward us with the occasional bucket of green chucked back to the cockpit... That's when I think..."ah, best roll in the jib and lose a knot before we have to make that turn...dear, would you fetch me my foulies?"
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