An alternative to the IS20 Wind

Seeing that all Hanses nowadays come with Simrad IS20 instruments this topic might be interesting to other Hanse owners. I hope no one will be offended by the length of this post.

I am very happy with my IS20 instruments - except for the IS20 Wind where the needle frequently jumps out of position and points in the wrong direction. I have had the instrument replaced no less than four times and I have tried a number of software updates, but to no avail. I also know that there are other users that have experienced the same thing and I have therefore been looking at a replacement from another manufacturer.

I am now happy to announce the successful result of my search - the Furuno FI-501 Wind instrument. It is NMEA2000 compatible, it fits in the existing hole and it also fits in esthetically with the IS20 Graphic is sitting above it.



While compatibility is part of the NMEA2000 protocol and the idea is that it should be "plug-and-play" there is one thing that stops an easy swap of one brand of wind instrument with another - calibration. My view is that calibration was "forgotten" when the MNEA2000 protocol was designed, and to find out what wind instruments on the market that would work with my SimNet setup has been extremely difficult. Very few people in the industry have seemed to know what I am talking about when I have asked about it.

There are two things that need to be calibrated in order for the wind instrument to display the correct information. One is wind vane offset which needs to be set if the wind sensor is not exactly in line with the boat's centerline. The other is the boat speed, which is the same as "boat wind" (in the opposite direction) and which has to be subtracted from the apparent wind to get all sorts of "calculated wind" - True Wind Speed, True Wind Angle, True Wind Direction, Velocity Made Good To Wind etc. The paddle wheel will most often not give the exact boat speed due to growth, its position on the hull and the hull shape. The speed value is corrected by multiplying it with a calibration factor.

The question is where the calibration is done and where it is stored. Simrad uses the IS20 Graphic (or Combi, which I don't have) instrument to calibrate the boat speed. It cannot be done with the Wind instrument. The wind vane offset, however, can be set in both the Graphic and the Wind instrument. The boat speed calibration factor is stored in the Simrad instruments while the wind vane offset is stored in the wind sensor itself. Only the corrected value (in this case the apparent wind angle) is broadcast on the network. If you connect an instrument from another manufacturer it will not read the boat speed calibration factor but it will read the corrected apparent wind angle. For comparison, the depth offset which will compensate for the location of the depth sensor, is also stored in the sensor but the depth is not broadcast as a corrected value, it is broadcast as two values, depth and sensor off-set. If you add the two values you will get either the depth from the water surface or the keel, depending on your preferences.

I have asked a question about the choice of wind instrument in the forum of the excellent marine electronics site www.panbo.com. I have learnt a lot of things about NMEA2000 and I recommend anyone who is intrested in the subject to take a look at the thread, that can be found here.

Now, unlike the Simrad IS20 Wind, Furuno's FI-501 does have the capability to set the boat speed calibration factor. If the calibration is already done in a Simrad instrument you only have copy the value. The factor is stored in the Furuno instrument and would be read by other Furuno instruments if there were any other in the network. This is called "synchronization" and is exactly the same that Simrad does within their instruments. The FI-501 will read the corrected wind angle from the Simrad wind sensor but it is also capable of setting the offset within the instrument. If so, the offset will not be stored in the wind vane so it will not be read by any Simrad instrument. For example, if I set the offset to five degrees to starboard on a Simrad instrument, all instruments including the FI-501 will show wind angles corrected five degrees to starboard. If I then set the offset five degrees to starboard in the FI-501, the Simrad instruments will still display five degrees to starboard while the FI-501 will show ten.

In order to visualize what I mean I made a little experiment. Firstly, my speed sensor is a Maretron DST100 triducer and the calibration factor is set to 1,03. This low factor obviously means that calibrating the boat speed is not of such big importance for for me, but at other times the factor has been 1,44 when it would matter. I wanted the FI-501 to use a boat speed double the boat speed used by IS20 so I set the calibration factor in the FI-501 to 2,06.



I began motoring head into the wind (which you can see on the wind instrument needle). The wind in my hair then consisted of the wind blowing across the water surface and the wind created by the boat. This is called the apparent, or relative, wind and as you can see on the IS20 Graphic the wind speed was 10,1 knots. If you subtract the boat speed, 4,1 knots, from that you will get that the true wind was 6,0 knots. As you can see, this calculation was correctly done by the IS20 Graphic. The FI-501 however, used the double boat speed, 8,2 knots, and came to the conclusion that the wind across the water surface was 2,0 knots. 10,1 minus 8,2 is 1,9 so this is fairly close.



Then I turned the boat to motor away from the wind. This time I had to add the boat wind to the apparent wind to get true wind. 4,1 knots of boat wind plus 3,7 knots of apparent wind means a true wind of 7,8 knots. The IS20 Graphic said 7,6 knots. The FI-501 “thought” the boat wind was 8,2 knots, and with an apparent wind of 3,7 knots it should have displayed a true wind of 11,9. It displayed 11,6 knots which is close enough to prove that it used the “fake” calibration factor.



I hope this information is of any use to anyone. Even if not, I have certainly found use for it myself although it has cost me some sweat to gather it.

Johan




Edited by Johan Hackman - 26 April 2011 at 12:31

Thank you for such an interesting post, Johan. I look forward to hearing how you get on with the Furuno.
You mentioned about the paddle wheel proving speed info for the various wind calculations. I assume the accuracy of the paddle will be different a few weeks after calibrating, so would you advocate regularly calibrating against the gps?

Why can't speed be obtained from the gps for use by the wind instrument?

Charles

Thank you Johan, very interesting.

May i call you Dr. Johan now?








Edited by Peter-Blake - 12 April 2011 at 08:23

One more word about my little experiment.

The Simrad Graphic in my pictures above shows clearly what happens if you change course in a fairly steady wind and the boat speed is calibrated - the true wind speed remains the same. The difference between 6 and 7,6 knots is due to a small variation in the wind at the time.

The Furuno Wind instrument is not calibrated for boat speed (in this case an exaggerated non-correct calibration) and the result is that there is a drastic change in True Wind Speed when I turn the boat - from 2 to 11,6 knots. That change does not reflect the variation in the wind at the time of the experiment.

I think this points out the importance of calibration of the boat speed. It is not just to show how fast the boat goes. It is also to make the wind information being correct.

Johan

I agree, it seems obvious, but I still struggle with the concept.
I think of a boat travelling against a fast 6 knot stream into the wind under maximum revs of the engine. Speed over ground zero; speed through the water 6kn; if the wind was 6kn, the true wind after allowing for correction from the paddle wheel would show as zero, (or would it?) which does not seem correct. But it seems to me that if sog was used in this case, wind would read more correctly. Where is my thought going wrong in this case?

Charles