True wind from GPS?

[GarryD]

Simple if you consider (a)the boat speed through the water & heading, (b)the tide speed & direction and (c)the boat actual speed across the ground & direction i.e. COGSOG as 3 vectors (they have magnitude & direction for the purists)


Add the boat vector to the tide vector gives COGSOG


To work out the tidal element of the situation, subtract the boat speed & direction through the water from COGSOG. ie. (b) = (c) - (a) in vector terms



Best done as a scale drawing.


Great in theory - but do you really know your boat speed? Really?? Same with your heading. Most compasses are graduated at 5deg intervals.


Your tidal estimate will only be as accurate as your knowledge of your boat.


Oh, I hadn't forgotten about sideways drift!! Gets really complicated.


Best open a nice bottle of something!!

[Dockhead]

Originally Posted by GarryD

Simple if you consider (a)the boat speed through the water & heading, (b)the tide speed & direction and (c)the boat actual speed across the ground & direction i.e. COGSOG as 3 vectors (they have magnitude & direction for the purists)

Add the boat vector to the tide vector gives COGSOG

To work out the tidal element of the situation, subtract the boat speed & direction through the water from COGSOG. ie. (b) = (c) - (a) in vector terms

Best done as a scale drawing.

Great in theory - but do you really know your boat speed? Really?? Same with your heading. Most compasses are graduated at 5deg intervals.

Modern stabilized compasses are good to a degree or two; satellite compasses (like the Furuno one I'm using) about 0.5 degrees.


But that's not all that important -- you care more about repeatability than absolute accuracy, since you will be thinking about True Wind in relation to what your compass reads.


Boat speed is harder, and racers go to great lengths to get well-calibrated boat speed. One of the posts above explains how it's done. It's a bit of work but not impossible for a reasonably keen cruiser.

Originally Posted by GarryD

. . .Your tidal estimate will only be as accurate as your knowledge of your boat.

Why? Your tidal calculation (set and drift) will be as good as your STW and HDG data. That plus COG and SOG from your GPS is all you need. "Knowledge of the boat" doesn't really figure. Most instrument systems will display set and drift directly calculated from STW, HDG, COG and SOG.

Originally Posted by GarryD

Oh, I hadn't forgotten about sideways drift!! Gets really complicated.

We call it "leeway". Indeed, you need to take leeway into account when you're sailing upwind. Degrees of heel is a good proxy for leeway, and that's how True Wind is compensated for leeway by the H5000. I believe Expedition does it the same way.



We now have an instrument which will measure leeway directly -- the Airmar DX900. Unfortunately no instrument system and no software package I know of can use this data. Probably Expedition will be the first.

Originally Posted by GarryD

. . . Best open a nice bottle of something!!

A nice bottle of something is always a good idea!

[OneBoatman]

Actually, they are different things. GPS is the position system which superseded and replaced LORAN so mariners today use GPSs many with a chartplotter (if equipped) to assist navigation in place of dead reckoning. True wind is the direction (and most agree, its velocity) from which the wind is coming. It doesn't mean anything different on land, on water, or in the air. Relative wind is the angle between the True wind and your boat or airplanes heading. Ex: You might be sailing close hauled at 34 Deg relative. Sailors mainly care about relative wind, that is how they trim their sails. Any vessel can now borrow and marry certain GPS displays using NMEA protocol to share wind information if they also have a wind instrument. This allows them to read both the True wind and also the Relative wind by comparing the True wind sent from the vanes transducer and selectively calling up its difference from the vessel heading using the electronic compass in the GPS. Not hard, but simple and without having to plot vectors or use a compass and anemometer. However, you still need to render Set if you don't have a chartplotter.

[wingssail]

Originally Posted by valhalla360

...Since most folks don't have the mythical ability to keep a paddle wheel operating accurately for any length of time and GPS does it by default, you are trading off two error sources For most, that's makes using GPS data the better option.

If you are racing and an extra 0.005kts makes a difference, it may be worth fiddling with the paddle wheel.

That's a red herring. No racer I know is actually trying for that kind of accuracy.

And I think the ability to keep a paddle wheel operating is NOT a mythical ability. It is simple and quick: Keep your paddle wheel pulled out when you are not going to use it for several days or longer, pull it and clean it frequently, and keep the bottom of the boat clean. That's all you have to do. I doubt if it takes me more than 10 minutes in a month. People who say this is impossible are just lazy.

Exact accuracy is more difficult, however it is not essential.

To be truthful we don't really care if the knotmeter is off by a tenth. For speed purposes we're mostly interested in changes: are we gaining speed or loosing?, are we faster now than 5 minutes ago? Did that trim change speed us up or slow us down? It's a relative thing. We don't try to compare today's speed to yesterday, it's apples and oranges, there are always too many other factors.

As an input to the true wind calculation the same thing applies. If the water speed is slightly off everything else follows, and when you try to calculate exact leeway or tide, you're going to be wrong, but again, it is the relative that matters. Who the heck cares if you have 15.7 knots of true wind or 15.4? And whatever your instruments say, you'll be checking the chart plotter and making a correction based on what your track is showing.

Let's stop studying our navels.

[Dockhead]

Originally Posted by wingssail

That's a red herring. No racer I know is actually trying for that kind of accuracy.

And I think the ability to keep a paddle wheel operating is NOT a mythical ability. It is simple and quick: Keep your paddle wheel pulled out when you are not going to use it for several days or longer, pull it and clean it frequently, and keep the bottom of the boat clean. That's all you have to do. I doubt if it takes me more than 10 minutes in a month. People who say this is impossible are just lazy.

Exact accuracy is more difficult, however it is not essential.

To be truthful we don't really care if the knotmeter is off by a tenth. For speed purposes we're mostly interested in changes: are we gaining speed or loosing?, are we faster now than 5 minutes ago? Did that trim change speed us up or slow us down? It's a relative thing. We don't try to compare today's speed to yesterday, it's apples and oranges, there are always too many other factors.

As an input to the true wind calculation the same thing applies. If the water speed is slightly off everything else follows, and when you try to calculate exact leeway or tide, you're going to be wrong, but again, it is the relative that matters. Who the heck cares if you have 15.7 knots of true wind or 15.4? And whatever your instruments say, you'll be checking the chart plotter and making a correction based on what your track is showing.

Let's stop studying our navels.

What this guy said!