[brak]

Regardless of whether it would be less accurate, that is how most of the companies do it. I'm willing to bet B&G and the other manufacturers are no different.

Most boats will have both a knotmeter and a wind instrument, usually both by the same manufacturer.... but not all will have a GPS connected to the wind instrument. Without having the GPS and wind instrument connected, the only way to approximate TRUE wind speeds is to use STW. Many areas don't have noticeable currents, so the difference between using STW and SOG is minimal.

Using STW for true wind calculation is more of a "tradition" - i.e. when all you have available is instruments, that's what they do. "To a hammer everything looks like a nail".

I think I referenced this entry from my blog a while ago, but it seems slightly pertinent: PolarCOM "True" Wind Calculation « Polar Navy Co

 

[JiffyLube]

I don't use our knot meter either. I was wondering if there would be another useful way of using the hole in the hull beside the knot meter?

 

[Noelex]

I'm willing to bet B&G and the other manufacturers are no different.

B&G (at least the H3000) will calculate true wind from the GPS display (in fact it can use either STW or SOG). It is very useful, stays rock steady through tacks and gibes etc
Like a lot of cruising boats I have given up on STW. I agree its useful to have if accurate, but on many boats its inaccurate reads differently on the different tacks etc.

 

[rhumbunctious]

My new (old) boat's paddlewheel was dead when I bought her, and for the delivery trip home, I got a Knotstick

which worked a treat, and I'm considering not bothering with a new paddlewheel log, but just glassing over the hole and using GPS most of the time and the Knotstick on those occasions when I want/need speed through water.

 

[Noelex]

I don't use our knot meter either. I was wondering if there would be another useful way of using the hole in the hull beside the knot meter?

A second depth unit is useful to essential depending on boat construction and cruising ground.

 

[brak]

A second depth unit is useful to essential depending on boat construction and cruising ground.

If you do that, might as well install a combo depth-speed and have STW available anyway.

One big advantage of STW is that it will be available even if something messes up GPS reception - unlikely though it may be in our times, it happens.

 

[speciald]

What has always bothered me is that the Raymarine instruments calculate true wind based on the paddle wheel boat speed rather than the SOG from the GPS. I asked the technical guru at a boat show once and he gave me a lame excuse.

 

[Noelex]

If you do that, might as well install a combo depth-speed and have STW available anyway.

One big advantage of STW is that it will be available even if something messes up GPS reception - unlikely though it may be in our times, it happens.

Yes good point.
I went instead with a fishfinder, which gives a second backup depth, as well as providing an idea of the bottom type. It will also read deeper, (the B&G still reads 200m+ which is OK most of the time). The fishfinder I wanted was not available with a combined transducer.

You need to find one with a different frequency if you want to use both together.

 

[sailingdog]

Yes, but I'm betting that the B&G H3000 is considerably more expensive than the ST70 level gear from Raymarine.

B&G (at least the H3000) will calculate true wind from the GPS display (in fact it can use either STW or SOG). It is very useful, stays rock steady through tacks and gibes etc
Like a lot of cruising boats I have given up on STW. I agree its useful to have if accurate, but on many boats its inaccurate reads differently on the different tacks etc.

 

[Noelex]

Yes, but I'm betting that the B&G H3000 is considerably more expensive than the ST70 level gear from Raymarine.

Yes you are right You will pay about double for the B&G equipment. There is a big difference in quality, however, B&G use things like glass bonded displays that don't scratch or fog up.
My wife summed it up she said "The Raymarine stuff looks like toys. The B&G like a piece of military hardware".
If you can afford it (or are prepared to go with a simpler system) a least give these premium products (B&G and NKE) some consideration.

 

[AE28]

Yes you are right You will pay about double for the B&G equipment. There is a big difference in quality, however, B&G use things like glass bonded displays that don't scratch or fog up.
My wife summed it up she said "The Raymarine stuff looks like toys. The B&G like a piece of military hardware".
If you can afford it (or are prepared to go with a simpler system) a least give these premium products (B&G and NKE) some consideration.

Price vs. value!

 

[mitiempo]

speciald

If the true wind speed was calculated from the gps any current present would cause an error. It has to be determined from speed through the water.

 

[sailingdog]

speciald

If the true wind speed was calculated from the gps any current present would cause an error. It has to be determined from speed through the water.

Me thinks you're a bit confused.... True wind is that calculated by removing the boat's actual SOG vector from the apparent wind vector. A GPS SOG would give you a far more accurate True Wind than using Speed through the water, since a current will affect the boat's speed through water as determined by the paddlewheel.

For example. Say the wind is out of the north at 10 knots. You're sailing dead downwind (due south) at 5 knots SOG... with an apparent wind of 5 knots from the north. If the wind instrument was calculating True wind according to SOG (5 knots due south) and apparent wind (5 knots from the north), it would come up with a true wind of 10 knots from the north, which is the case.

However, there is a one knot current from due south, and your STW according to the paddlewheel is not 5 knots, but 6 knots-5 knots of actual boat speed over ground plus 1 knot due to the current. Calculating the True wind using STW of 6 knots and an apparent wind of 5 knots would result in a True wind of 11 knots and would be wrong.

 

[pdqaltair]

Me thinks you're a bit confused.... True wind is that calculated by removing the boat's actual SOG vector from the apparent wind vector. A GPS SOG would give you a far more accurate True Wind than using Speed through the water, since a current will affect the boat's speed through water as determined by the paddlewheel.

For example. Say the wind is out of the north at 10 knots. You're sailing dead downwind (due south) at 5 knots SOG... with an apparent wind of 5 knots from the north. If the wind instrument was calculating True wind according to SOG (5 knots due south) and apparent wind (5 knots from the north), it would come up with a true wind of 10 knots from the north, which is the case.

However, there is a one knot current from due south, and your STW according to the paddlewheel is not 5 knots, but 6 knots-5 knots of actual boat speed over ground plus 1 knot due to the current. Calculating the True wind using STW of 6 knots and an apparent wind of 5 knots would result in a True wind of 11 knots and would be wrong.

Highschool physics, and SD was paying attention in class!

Clearly, the wind calculation has nothing to due with the water motion, only course and speed over ground.

Though, in an odd way wind over water is relavant too. The boat doesn't feel where the ground is, does it? For example, in the above case the true wind is 10 knots, but it "feels" like 11 knots with the counter current. If he turned up wind into the wind, he would get a 1 knot boost, the waves would be steep, and it would "feel" like more. Something to think about. Not mathamatically relevant to the question exactly, but interesting and valuable, which is why this is, perhaps, not a big deal.

Oh, I'm a cruiser; if I can't see it our feel it... to heck with it. The ONLY reason I care to have an electronic wind vane at all is to run the auto pilot.

 

[mitiempo]

Dog
I'm not confused. If there is a one knot current your speed through the water is still 5 knots, but your speed over the ground is 6 knots.

If gps was used and you were sailing at 5 knots downwind in a 10 knot breeze against a 5 knot current you would in effect be standing still. In other words 5 knots stw but 0 knots sog. Using speed through the water to calculate would give apparent wind speed plus boat speed = 10 knots. Using gps would give apparent wind plus sog = 5 knots.

 

[mitiempo]

Makes no difference - any current in any direction will lead to error. I think the only way to be truly accurate is by measuring all 3, sog, stw, and apparent wind and compute it from that.

Sail in the PNW - we have currents on this coast up to 16 knots in some places. You can sail at a good clip while actually going backwards if you are in the right place at the wrong time.

 

[SecondWindNC]

If gps was used and you were sailing at 5 knots downwind in a 10 knot breeze against a 5 knot current you would in effect be standing still. In other words 5 knots stw but 0 knots sog. Using speed through the water to calculate would give apparent wind speed plus boat speed = 10 knots. Using gps would give apparent wind plus sog = 5 knots.

In this scenario, though, wouldn't the wind instrument "feel" the full 10 knots of breeze, since the boat is actually standing still relative to the wind?

So, 10 kts apparent plus 0 kts SOG = 10 kts = true wind speed

 

[mitiempo]

You are right. The apparent wind would be higher by the speed of the current.

But what if you were not going with the current or dead against it. Would using gps be correct for a current at varying angles?

I think my revised answer is because it is less expensive to use stw than sog.
Raymarine went the less expensive route.

 

[SecondWindNC]

It seems to me (though my wife assures me that I can be, and often am, wrong) that the true wind would vary based on the boat's actual movement over the ground, not through the water, so SOG (and COG) should be calculated into the true wind reading, rather than using the STW (paddlewheel) reading.

In other words, if your SOG is 10 kts on a particular course, that's what's going to impact the reading from your wind instrument, regardless of whether that course and speed are affected by current.

As to why it's generally the paddlewheel STW reading that's factored in by the instruments, the instrument manufacturers probably just haven't bothered to change it since GPS, especially integrated GPS, has only become common relatively recently. Before that it would have made sense to factor in the only speed reading available - the one from the paddlewheel.

 

[Noelex]

The physics is really very easy.
If you want "True wind" to be the same as an observer on land (or a boat at anchor) to be you need to use the GPS and SOG to calculate true wind.
This seems to me to be the best and most useful definition of true wind.

If you use STW the "true wind" will no longer reflect this if current is present.

If my instruments (which measure true wind from the GPS) show the true wind is 10K. I know if I dropped the anchor at that instant and stopped the boat the wind would be 10k.