Wind sensor positioning

[Scott T-Bird]

I always see the sensor pointing forward - ahead of the mast - on just about all boats. I pointed it backwards because it was a better fit on the mast head (anchor light positioning made it awkward to find a space for the base) and because I don't use a windex, and the pointer is easier to see when I look up from the helm.

What say you, does it really affect the wind reading?

 

[Rick486]

There will be some disturbed air aft of the mast from the other devices at the masthead but this will be insignificant when compared with the effect of the motion of the boat multiplied by the length of the mast. The most significant effect will be on the wind direction measurement, but that should be constant and subject to some calibration. On all the boats I have owned I have a wind tell on the aft of the mast, otherwise I couldn't see it

 

[jssailem]

@Scott T-Bird
I believe it is all about "Your boat your rules"!
Purest will tell you that to race your boat you need every edge that you can control. Putting the transducer ahead of the mast head will help you with a little edge. How much would be dependent on your mast head and the stuff you have on top. If you could put the rig in a wind tunnel and had the benefits of sensors and expensive measuring equipment you could identify the difference and have an exact understanding of the factors and relevance of the positioning. Perhaps if you were outfitting an Americas Cup yacht you would have the funds and the willingness to solve such a problem.

What is the difference? .002 to 2.0%? I do not know. As a cruising sailor I put this at a lower priority than if I was attempting to race in an elite class.

Your boat your rules. After it is all said and done our boats are a series of compromises surrounded by water.

 

[LeslieTroyer]

My wind vane is always pointing to where I’m trying to get to. Oh.....your talking about the mount. Cleaner air out front as others have said, but I’ve also seen them on aft radar poles, so pointing aft has to be better than that. I don’t sail by the vane, I use the sails and trim to fit. I do use it to run the Autopilot if the wind isn’t on the nose and there it relative so go for it.

Les

 

[Don S/V ILLusion]

Anytime the wind is aft of the beam, it has “clean” air so that consideration makes any preference for placement as moot.

My opinion is that as long as it doesn’t increase the likelihood of spilling my beer, it’s irrelevant.

 

[DrJudyB]

I've never mounted a windex or anenometer instrument "backwards" like that so I don't have any observations to share. Try it and find out.

It's worth noting that an anenometer measures both speed and direction of the apparent wind.

I can tell you that there is always a trailing vortex behind the masthead, Its generated by the heads of both the jib and the mainsail, as well as the mast extrusion and masthead hardware. If the sensor is above the turbulence, the anenometer will give good readings. If it's in the trailing vortex, you will get erratic, unreliable readings for both speed and direction. Try it and see, after you calibrate the instrument.

Also, due to the "upwash" effect of the jib on the airstream over the mainsail, the direction reading might indicate you are pointing closer to the apparent wind than you really are. That may not matter a whit to you in practical terms, but keep it in mind when you're bragging in the bar about high high your boat points

 

[Charlie Jones s/v Tehani]

My wind vane is always pointing to where I’m trying to get to. Oh.....your talking about the mount. Cleaner air out front as others have said, but I’ve also seen them on aft radar poles, so pointing aft has to be better than that. I don’t sail by the vane, I use the sails and trim to fit. I do use it to run the Autopilot if the wind isn’t on the nose and there it relative so go for it.

Les

lol in the B Bahamas and Keys, they are called "destination finders"

 

[dlochner]

What @DrJudyB said. Any inaccuracies will be more pronounced going to windward, which is when knowing the apparent wind and wind direction are more critical for sail trim and heading.

I don't use a windex, and the pointer is easier to see when I look up from the helm.

The whole point of the very expensive masthead wind instrument is to not have to look at the mast head to see the wind angle. You have that fancy B&G instrument to show the apparent wind.

Judging from the photograph, I think you are going to get turbulence coming off the sails and masthead when going to weather.

 

[Scott T-Bird]

Interesting comments … Thanks all! Dave, you are right, I don't really look at it nearly as often as I read the instrument. The issue is that I have had it this way for over 10 or 11 years. Honestly, I wouldn't know the difference until I switch it to pointing forward. Obviously, I have calibrated the instrument for the mounting direction.

It certainly does look flatter (lower relative to the mast head) in that photo. When I look at the mounting angle of the sensor (in my hand before it is mounted), though, I wonder how it could be affected by a vortex. My suspicion is that it is. It often seems that the apparent wind speed is less than it should be when I am sailing upwind. I have no basis for comparison, but there are times when I'm sailing upwind in gusts that would seem to be higher velocity than indicated by the apparent wind speed reading (based on readings that I can see when anchored). This year will be my first opportunity to read true wind, so I'll perhaps have a better sense of it. However, I have no idea how to really tell if it is accurate or not.

I do sense that the wind direction is often more erratic than I would guess. For years, sailing on Lake Hopatcong, I figured that the erratic direction was mainly due to wind shifts on the lake. However, now with winds in an open bay, I still sense that the direction can be more erratic than I would expect. Perhaps it (the erratic readings) is impacted a lot by angle of heel? Perhaps moreso than it would if it was mounted forward.

There's no cause for me to brag about pointing angles on the apparent wind angle. One look at the GPS track is enough to tell me how low my pointing angles really are!

I can't say that I have ever seen another boat with it mounted backwards, like I did mine. The installation manual, when I read it over a decade ago, seemed to indicate that it was a matter of preference and not that significant an issue. The masthead had an old instrument already mounted when I bought the boat, which I took off because it was not working. It was simply easier to mount the new one (10 or 11 years ago) in the original location with the sensor pointed backwards so the bar wouldn't interfere with the anchor light. I'm sure I could find a way to re-configure it. When I bought the Triton sensor last year, it fit my (Simrad) mount so I didn't bother to change the direction.

 

[dlochner]

Scott, tip vortices are a big deal. That's why airplanes have those little winglets on the ends of the wings, to reduce the drag induced by the tip vortice.

On a sail boat, the vortices come off the top of the mainsail and in your case the jib as it is masthead rig. The vortices enlarges as it moves away from the masthead. The spinning air will affect velocity and direction readings.

Here's a brief piece from North Sails: https://www.northsails.com/sailing/en/2017/02/the-science-behind-your-sail-trim

This video is interesting, but not all that clear

When I upgraded my wind instruments last year I had to build a custom mounting bracket because the new bracket didn't fit. Of course, the mast was down so I didn't have to do it while sitting a bosun's chair.

 

[All U Get]

After the way an osprey mangled ours, I’m going to mount the next one in the cabin.

 

[dlochner]

After the way an osprey mangled ours, I’m going to mount the next one in the cabin.

Imagine the looks you'll get when you post a photo of the instruments, showing 0 knots apparent on the nose and a boat speed of 6 knots.

 

[jssailem]

I’m going to mount the next one in the cabin.

That will resolve the wind vortex off any sail. It might actually point you in the direction you intend to go. Only one issue left. Will it go in reverse on your next sail through the Bermuda Triangle.

 

[Jackdaw]

As others note it will affect both direction and speed due to up wash. You’ve probably never seen another one mounted like that, and there’s a reason. The error can’t be nulled out via calibration, because it will be different on opposite tacks.

 

[Rick486]

I have had a windex on my mast head with every boat I have ever owned (5). The windex on Maxine is about 2 ft ft above the mast head and about 18 inches aft of the mast head. It's all about calibration. After sailing for awhile your mind calibrates the direction it is pointing so that you know you are pointing high enough. And it helps you maintain sail trim off the wind. I never, ever look at the electronic direction instrument. Way too unstable. I do look at the wind speed data. I think if I were sailing a 100 footer I'd still need a (larger) windex.

 

[Scott T-Bird]

As others note it will affect both direction and speed due to up wash. You’ve probably never seen another one mounted like that, and there’s a reason. The error can’t be nulled out via calibration, because it will be different on opposite tacks.

No, the calibration is simply 180 degrees to tell the instrument that the sensor is pointing the opposite direction. I guess my question is how much error could there be? Is it significant? Minor? Troublesome? Misleading? Consistent? Erratic? There probably is no good answer. My sense is that the error is probably erratic enough to make fine-tuning difficult, if I were to rely on the readings to a significant extent. If I rely more on tell-tales, the error may not be meaningful.

 

[Jackdaw]

No, the calibration is simply 180 degrees to tell the instrument that the sensor is pointing the opposite direction. I guess my question is how much error could there be? Is it significant? Minor? Troublesome? Misleading? Consistent? Erratic? There probably is no good answer. My sense is that the error is probably erratic enough to make fine-tuning difficult, if I were to rely on the readings to a significant extent. If I rely more on tell-tales, the error may not be meaningful.

That part I get. My points is the the backwash error cannot be removed, because it is different +x, -x on each side.

 

[Scott T-Bird]

I guess the real answer is clear air is best! I should find a way to turn her around!

 

[Rick486]

Not to get overly technical, but the wind vane on the electronic sensor is only 6 inches long and has low rotary inertia. Thus it responds rapidly to turbulence with very little damping, which results in the display in the cockpit exhibiting rapid oscillatory motion which is difficult to "average" optically. My windex is 16 inches long and damps out optically the small turbulence induced oscillations due to the distance from the eye. So I see a much more stable direction cue than if I had glanced at the Raymrine display. YMMV.

 

[Davidasailor26]

Not to get overly technical, but the wind vane on the electronic sensor is only 6 inches long and has low rotary inertia. Thus it responds rapidly to turbulence with very little damping, which results in the display in the cockpit exhibiting rapid oscillatory motion which is difficult to "average" optically. My windex is 16 inches long and damps out optically the small turbulence induced oscillations due to the distance from the eye. So I see a much more stable direction cue than if I had glanced at the Raymrine display. YMMV.

A damping factor can often be applied in the software of the instrument to hide those small oscillations. I know there's an option for that in our B&G menus, so I presume Scott's has it too.