We've been relying on GPS alone for a few years and although a knot meter would be nice, we don't really miss it. That said, currents in our neck of the woods generally run 1 knot or less. I figure it would be more useful for racing, as you mention, or in areas with significant current. The only reason we don't use our knot meeter is that the travel lift broke one of the impeller fingers and becuase it is an old unit we've been unable to find a replacement. Thus far, I've found may other uses for the money and agrivation of installing a new meter.

 

The direct comparison of GPS SOG and the traditional boat speed indicator reflects current/tidal stream and this can be of significant value in areas where there are strong currents and in particular, counter currents. The difference can seriously influence trip times and of race results.

 

Also useful for DR navigation. We have strong currents and I'm always using the knotmeter to find the strongest counter currents.

 

+1 what John said

In integrated systems such as the Raymarine E series, the data from the paddle, GPS, wind instruments are valuable as the computer calculates set and drift, etc..

Also the paddle provides a faster indication of the effects of changes in sail trim. The GPS provides an average speed of a series of fixes.

 

Cool.. thanks for the info guys! Looks like both is the way to go..

 

Our log died a year or so back and I've not got round to replacing it. I find using the GPS is quite adequate but we are talking about plodding around offshore and not racing.

As Andre so correctly states, the GPS indicated speed is not as instantaly reliable as a log.

whoops...not Andre, Jack.

 

actually, how "instant" is your SOG as provided by GPS depends on the type of your GPS device, display system and settings. some will be a lot more instant than others.

at the same time standard paddlewheel STW (speed through water) devices are also not instant - rather they usually have dampening (often not configurable). You can test that with boat out of the water by rotating the speed wheel manually (or if water is warm enough and wheel is accessible - put a mask on and dive to it). Either way, you are likely to find that as you spin the wheel up quickly, speed as shown by the instrument ramps up quite slowly, and goes down just as slowly - over a course of a few seconds at least. This is likely done on purpose, because otherwise as your boat goes up and down waves speed may vary a lot - too much to be useful really.

That said, speed through water is very useful for set/drift calculation (of course only provided your speed log is well calibrated).

 

Brak - good points. I am used to knotmeters that respond quickly.

Most knotmeters can be pulled while the boat is in the water. They have a plastic plug that can be inserted. Newer ones have a baffle that slows the flow of water while inserting the plug.

 

A former racer, I no longer have any use for the paddle wheel knot meter. Too much maintenance. In fact, it works perfectly well but I have had the plug in for over 2 years.

It would be nice to see the true wind function, but not nearly nice enough to bother. I believe, except for racers and gear nuts, they are (nearly) as obsolete as a sextant; IF the difference between the GPS heading and the compass heading and the difference between the boats GPS speed and visual speed through the water aren't obvious to you, well, spend the money and cleaning time if you must.

 

True wind is not calculated with speed through water rather with SOG from the GPS. I also agree that the maintenance required in my neck of the woods is not worth having the knotmeter working. After the first month of cleaning it weekly I decided to just leave the plug in.

In the end, for navigation, I care how fast the boat is moving over the ground and in what direction it is moving not pointing.

 

This i show true wind should be calculated.

In fact Raymarine ST60 stupidily use speed info from paddle wheel and not frpom GPS (which is usually available). I think just to sell paddle wheels.

 

I have a magellan hand held GPS i set it on a shelf infront of the binical if i pick it up and move it the 2 feet towards myself i can see the fluxuation in speed within 2 secounds this speeks volumes about how accurate the waas enabled gps really is.

Mitch

 

Using speed through the water to calculate true wind is just wrong. I would verify this to make sure. It is just basic vector addition. The wind does not care how fast you move through the water since apparent wind is based on how the boat moves through the air.

For example, if your boat has a speed through water of 5 kts and you are headed directly into a 5 kt current true and apparent wind are identical as your SOG is 0 kts. If you are using the paddle wheel to calculate your speed the answer would be erroneous.

I know the Raymarine stuff uses the paddle wheel for calculating the tracks of radar contacts relative to itself, but I would hope the big guys would get this right.

 

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.

 

I use my paddle wheel meter when I'm anchoring. I like to know when the boat has stopped and it does this better than the gps. Of course, I don't normally anchor where there is a significant tide.

 

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.

 

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.

 

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.

 

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.

 

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.

 

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

 

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.

 

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.

 

I don't think that it was because it was less expensive to use STW, but more that it was easier to use STW since the wind instruments and knotmeter are usually sold together, where the GPS is not generally bought at the same time, and if they depended on the GPS for SOG, the wind instruments would fail, unless hooked up to the GPS properly. That would have made the wind instrument marginally less useful to most people and more complicated to install to get full function out of it.

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.

 

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.

 

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.

 

Isn't there any technology that would allow speed thru the water to be measured withoud punching a big hole in the hull? Taffrail log is one. But, it must be launched and retrieved, and may get gooped-up or chomped by fish. That is what I like about depth sensors. They just sit snug inside the hull doing their job.

 

You could use a Dutch log.

http://www.sailnet.com/forums/electronics/57426-knot-log-calibration.html#post516582

 

I do find it disappointing that many instruments cannot calculate true wind from the GPS SOG. It would seem that a small software change would allow this calculation. If the log could also display SOG instead of STW as an option this would be very useful.
If nothing else if log became jamed or broken during a long passage the instruments could still display meaningful data while trying to fix the problem
Are you listening manufacturers?