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9 years ago

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PhoenixStar

QLD

477 posts

PhoenixStar

QLD, 477 posts

8 Nov 2016 10:53am

It's the only legit way to asses polar plots.

Crusoe

QLD

1197 posts

Crusoe

QLD, 1197 posts

8 Nov 2016 8:14pm

QLDCruiser said..
It's all a matter of definitions. This version of True Wind, which has been around since pre GPS days, is "True" relative to the water surface, whether that is moving due to currents or not. It is calculated from Apparent speed and direction, with appropriate vector offset by speed through the water.

There's another parameter, which I believe is called "Ground wind", which as you might expect is relative to earth's surface. Calculating that would require SOG.

You could probably make a case for either option being the more useful under various circumstances. However, as currents rarely get over 2-3 knots, and wind is rarely of any relevance unless it's several times that, it's hairsplitting in my view.

Well said QLD Cruiser. Doesn't matter what the theory is, the ST60 Wind Instrument indication requires a paddle wheel to differentiate between True & Apparent.

Maybe there are some other wind instruments brands that use GPS data but the ST60 doesn't.

PhoenixStar

QLD

477 posts

PhoenixStar

QLD, 477 posts

10 Nov 2016 10:29am

The ST60 manual is unclear if it uses SOG or STW, the Raymarine touch screen manual clears it up in chapter 14.14

; "> Note:
• I f Speed Over Ground (SOG) or heading data is
n o t available, vectors cannot be displayed.
• Laylines are only shown when the Boat Type
setting is set to a sailing vessel.

They are referencing true wind and layline overlays.

FreeRadical

855 posts

FreeRadical

WA, 855 posts

10 Nov 2016 10:00am

PhoenixStar said..
It's the only legit way to asses polar plots.

I am having so much trouble understanding this?

Below is a polar chart for a 40.7. You are sailing dead downwind and your boat speed (STW) is 5kts. Your apparent wind indicates 4kts. If TWS is based on STW, you would have a TWS of 9kts. Looking at the polar, it's spot on, 9kts should give you 5kts boat speed.

You then look at your SOG, and it says 8kts. (Boat speed still 5kts, apparent wind still 4kts). If TWS is based on SOG, you would say you now have a TWS of 12kts. (And that is what a guy standing on the headland watching you go by feels). If Polars are based on SOG TWS, you would look at the polar and say heh, I should be getting 6kts boat speed. If your SOG was 11kts, your TWS would be 15kts and your boat speed should be almost 7kts According to the polar. But it never is, your boat speed remains 5kts.

PhoenixStar

QLD

477 posts

PhoenixStar

QLD, 477 posts

10 Nov 2016 2:47pm

Polars are based on apparent wind and work ok if there is no current. They indicate your sails ability to drive the hull in still water. When you introduce current things get difficult. Downwind at 6 knots SOG but in a current of 6 knots (0 STW) and zero apparent wind speed there must be a true wind of 6 knots - but zero apparent means no drive so the polar works out. But why you must work with true wind even when using polars is when the wind is actually driving the boat. Say you were getting 3 knots STW in the above current, 9 knots SOG, you would need 5 knots apparent to reach that hull speed and that would imply 14 knots true wind. 9 knots of true is squandered chasing the boat. If you have true wind and current you can interpret your polar correctly.

Do you like polars? They are the devils hard work to produce and difficult to interpret if conditions on the day are different to the data collecting conditions. And you need a competent quick crew to do an effective sail change mid course. Useful on longer ocean races I guess.

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