Measuring Leeway

[thinwater]

Obviously a critical factor upwind. We all have out seat-of-the-pants gauge. A big part of the art of sailing is maximizing VMG by balancing power, leeway, and drag. We all know that footing a little to keep the flow up helps when the waves are large in comparison to the wind (or we are reefed). But how can we actually measure leeway (vs water, not overground) in real-time?

• Cross track using GPS. Given that we don't know the instantaneous tide, current, or surface current (wind effect) we can't calculate it from this information. The water is moving too. If we were motoring with zero free board there would be cross-track.
• Observe the wake. Pretty good. But I always wonder to what extent it could be blown to leeward.
• Compare speed and VMG (calculated) to targets. Helpful, but obvious limitations.

Lots of internet posts that contain errors.



There are gadgets that measure the flow angle across the hull. In significant waves that flow has got to be all over the place.


Ribbons. I've read of ribbons a few feet long hanging off rudders/transom. I'm not seeing how that could be accurate, and how would you measure it?


Yes, I understand that track over ground is probably the most important value when navigating, but when playing with sail trim, leeway through the water is what you are working with.



---


Thoughts? They most obvious method I have seen is what I observe when trolling lines >150 feet back. I could measure the angle. You would even get a fair bit of averaging. But the AC guys must have a cool method.

[Tupaia]

Maybe missing your point here but isn't leeway the difference between the compass heading and the GPS course. Obviously this will be affected by any current or tide but do it often enough and a mean value should be achievable.

Surely optimising VMG is more important than knowing the leeway.

[wholybee]

Some of the fancier ultrasonic speed sensors can measure it directly. That would be the easiest and probably most accurate way. The sensor would need to be placed on the centerline in clean water flow, so maybe not _that_ easy.

https://www.airmar.com/Catalog/Marin...re/DX900P-2000

There is a complicated way to calculate it. You have the COG, and the Heading. You can also calculate the true wind and the ground wind. The difference between true wind and ground wind will give you the current speed and direction of the water. Take the difference between COG and heading, and subtract the current, and that should equal your leeway. You also need to account for your SOG when subtracting the current, since current is a vector not an angle.

Something like that anyway. It would be a good math problem for someone that enjoys such things.

[thinwater]

Originally Posted by Tupaia

Maybe missing your point here but isn't leeway the difference between the compass heading and the GPS course. Obviously this will be affected by any current or tide but do it often enough and a mean value should be achievable.

Surely optimising VMG is more important than knowing the leeway.

These are two different questions. Are you making VMG towards the destination? Are you sailing the boat as well as possible? Not at all the same thing. For example, a tide could be either boosting or hindering. (an over simplification)



A different question than the one that interests you.

[thinwater]

Originally Posted by wholybee

Some of the fancier ultrasonic speed sensors can measure it directly. That would be the easiest and probably most accurate way. The sensor would need to be placed on the centerline in clean water flow, so maybe not _that_ easy.

https://www.airmar.com/Catalog/Marin...re/DX900P-2000

There is a complicated way to calculate it. You have the COG, and the Heading. You can also calculate the true wind and the ground wind. The difference between true wind and ground wind will give you the current speed and direction of the water. Take the difference between COG and heading, and subtract the current, and that should equal your leeway. You also need to account for your SOG when subtracting the current, since current is a vector not an angle.

Something like that anyway. It would be a good math problem for someone that enjoys such things.

a. I doubt it is possible to measure based on water flow near the boat when the waves are big. This is certainly even more true of mutihulls. For example, a hull can come clear out of the water and is certainly up in the waves more than down in solid water. Same with many sport boats. I might as well just look at the wake or take back bearings. I'm sure that is as accurate.


b. Not unless you know the current and tide and wind induced surface flow with great accuracy. The calculation will give you leeway vs. ground but not leeway vs. water. In a strong outgoing tide your calculated "leeway" could be several knots to windward! From a navigation perspective, fine, but not for seeing how well you are sailing.

[thinwater]

I should change the title.


Measuring Leeway through the Water (Not Over Ground)