You can alter the input used for the "true" wind calculation to SOG on some instruments. It is easy with B&G equipment, but impossible on some brands.

When altered in this way, the calculated wind should be called "ground wind". This is different to true wind, but is arguably more useful for a cruising sailor as it always accurate even if the paddlewheel is not properly calibrated or is fouled.

 

if the paddlewheel is not properly calibrated or is fouled.

That‘s another great point. I’m due for a bottom cleaning next week and my STW reads a full knot below even the slower speed I expect. I think I may have gotten some transducer paint on the axle too, which will wear off.

Does anyone know how to make this input change on Raymarine? Axiom plotters, i70s MFDs and tri-ducer. I tried to look it up and so far only found an old manifesto, from a Raymarine tech, that everyone does it the STW way. I still have to try to pour through the online manual to see if that’s outdated.

 

Unfortunately, Raymarine is one of the companies that has traditionally not offered a ground wind option. Whether it is possible on some of their newer equipment I am not sure, but I think it is unlikely.

Why they do not offer this option is a mystery. Before GPS was available the only speed input was via the paddlewheel and ground wind could not be calculated, but GPS is hardly new technology.

Even if you prefer true wind over ground wind, having a simple software switch between the two is a useful option if your paddlewheel becomes fouled or defective during a passage.

 

In 99% of circumstances, I don’t think there is a meaningful difference in the result. I’m going to have to do the math out of curiosity.

Mostly, I’d just like it to work, when I forget to put the speed wheel in or it’s fouled, which is a meaningful error.

 

Tradition. They linked with STW because it was invented before GPS was widespread. Also, GPS is prone to large instantaneous errors unless a smoothing algorithm is applied; it's based on locations, which are not that accurate. If true wind is based on calculations of instantaneous speed and bearing, expect considerable variation in the speed and direction.

I never really cared. If you know the AW and the boat course and speed, you should be able to calculate a rough solution for TW in your head as fast as you could reach for the buttons that change mode. Isn't this an instinctive part of dinghy sailing? All sailing? As for docking, you look at the tide on the pilings and the wind indicators on the boats in the marine. More accurate information about the micro-conditions.

 

Knowing TWA accurately is important when sailing deep downwind on autopilot, particularly when boat speed (thus AWA) is changing a lot on surfs and wallows. Our AP will actively prevent the boat from jibing through the TWA.

For some it is the opposite in that they care about TW because AW is so obvious and easy to know. Most racers will have their instruments set to show true wind, because this is what they need to sail to for their polars.

Mark

 

B&G is great at letting one easily set the STW or SOG as the speed measurement, but I wish they would allow the user to pick where that is used instead of applying it to everything. For example, I would like the wind to use SOG, but the boat speed to use STW. Without STW, the instruments no longer detect or report current or leeway.

Mark

 

Even on a run, isn’t the jibe risk still based on apparent wind? When I’m that deep, I don’t usually have the main up anyway. If yawing on the waves or wind is shifting dramatically, I’d sail far enough from a jibe that I don’t think TWA would fine tune my approach. Makes sense that racers need true wind relative to current for polars. I don’t cut my toothbrush in half to reduce weight either.

Turns out the i70s has a Ground Speed display option, which uses SOG and COG. Oddly, the few display options don’t include the combined wind needle and speed in knots in the center, the way true and apparent screens allow. The closest has a wind needle and speed on the Beaufort scale? WTF. Are they just screwing with us?

 

AWA will never be greater than the TWA. The risk when running by AWA is that the TWA will go to the other side of the boat while the AWA doesn't change, and then the boat slows down in a wave bringing the AWA to the TWA - jibe. In addition, by sailing deep to TWA, the AP isn't chasing the AWA all around as boat speed changes.

All of this is useful even without the main, and even with a symmetrical spinnaker, although it is less critical then.

The usefulness of true wind isn't just for racing, and the need can be more than just a rough estimation in your head.

Most current instrumentation show TW and AW data simultaneously, so no need for pushing buttons or going into menus.

Mark

 

The risk when running by AWA is that the TWA will go to the other side of the boat while the AWA doesn't change, and then the boat slows down in a wave bringing the AWA to the TWA - jibe.

I set my autopilot on wind mode to apparent. It allows me to chose either apparent or true. Apparent is what is being read real time, by the transducer at the masthead. True is being calculated. Sort of a head scratcher how it would calculate true to cross 180.

 

Interestingly, Olympic dinghies don't have wind instruments (keel boats only IIRC). How ever do they manage?

We'll have to watch in Paris.

 

Dinghies and short courses are the answer to that. Entirely different type of racing, with the dinghies not sailing to polars, but rather sailing to tactics determined by their immediate surrounding boats. Let alone that a dinghy with wind and speed instruments is competitively disadvantaged, if they were even allowed by the rules.

Watch in Barcelona for a different thinking in this. Or any offshore race of any type.

I bet you know this.

As for cruising boats, sailing deep winds in offshore conditions is safer with an AP that can use TW data (or ground wind, I'm not making that fine of differentiation here).

Mark

 

I would suggest cruising sailors try both ground wind and true wind and make up their own mind. The advantage of ground wind is that the speed input is always perfectly calibrated so it is data you can trust.

 

For clarity, I haven't been making a distinction between true and ground wind in discussions. Only between apparent and "actual", however one wants to describe that. Also, boat instrumentation will present/label ground wind as true wind, because they also don't make that distinction.

But the point about needing calibrated speed input is correct - including keeping the bottom of the boat clean in the flow path, and correcting for healing and tack. As well as calibrated wind speed and direction for the same. It is very difficult to truly achieve good calibrated data.

Mark

 

I'm an engineer and I understand the math perfectly well. I just prefer the ethic of sailing by the seat of you pants and the wind in your face. Maybe it's the difference between riding a bicycle and and riding and e-bike, which is really just an electric moped.