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Knotmeter Caught Fibbing

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reuben
Navigator

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USA
202 Posts

Initially Posted - 08/31/2004 : 15:10:37

All season I've wondered why our Raytheon wouldn't register higher than 5.0 even when it seemed we were flying along -- I took it for the discouraging truth, worked at perfecting sailshape, and fretted when reading this or that gladsome post citing sustained speeds of 6.5 or higher. "I am a rotten sailor," I thought, "or everyone else is embellishing," a suspicion immediately discarded as fearful and even morbid.

Then, yesterday, amid glorious conditions on Leech Lake, this discovery: the Raytheon has been underestimating our speed (& therefore distance covered) all summer long. O lovely knowledge! O beauteous new GPS! For we broadreached all day in NW winds of 12-15, hitting 6.2, 6.3, 6.4 for long stretches while the Raytheon muttered about 4.8 etc like a grizzled old senator atwist under oath.

I am told, now, that the Raytheon can be "calibrated," can be re-educated, can be taught to love the truth. Well, I don't know. Old habits and all that. What I'm sure of is exactly this: the sun is finally shining at the end of this tepid Minnesota summer, and we're sailing faster than we thought, and a gorgeous amber autumn is beginning with the Twins in first place by seven games. Technically I suppose a person could ask for more, but it sure wouldn't be polite.

Reuben
C-250 WK #499
Bayfield, WI

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Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 08/31/2004 : 15:30:26

nice

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JJM
Navigator

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USA
170 Posts

Response Posted - 08/31/2004 : 21:16:55

Sounds like a dream has come true (Twins in first - uh, just kidding!!)

I just wanted to say that this past weekend I noticed that my knotmeter (or is that a NOTmeter?) was registering in the 3's while my GPS was registering 5 and above. Is this something that is normal and to be expected? What could cause that? Could it be the water current as it "truly" hits the hull? (whether we are heading to the wind or on a run?)I believe that there is a rotating transducer in the thru hull, and it's rotation may be subject to water current activity.
Am I all wet here? (no pun intended!)

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Dkn420
Captain

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USA
298 Posts

Response Posted - 08/31/2004 : 23:52:34

I was out a couple of days ago and the Raymarine knot meter on my boat wouldn't register at all, after working fine the previous outing. I found that the "paddle wheel" on the transducer in the thru hull had become sludged up. Took it out and cleaned it up and made sure it was put back with the arrow on the tranducer pointing in the proper direction and it was working again...looks like something that should be checked, at least in my case as my boat is wet slipped year around...I'm sure sailing more often would also help...Dan #727

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sailgal
Captain

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USA
400 Posts

Response Posted - 09/01/2004 : 10:46:27

Hey Dan! Ever get the spider crack situation under control?

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Al
Captain

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USA
269 Posts

Response Posted - 09/01/2004 : 12:03:49

John, your knotmeter is measuring speed through the water, and if your GPS is measuring speed over the ground and you were sailing with a 2 knot current, that would explain the difference. One way to check would be to do a 180 and reverse course. Now when your knotmeter indicates 3 knots, your GPS should indicate 1 knot. I guess an even simpler way (to see if you're in a current) would be to sit dead in the water and see what the GPS says.

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reuben
Navigator

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USA
202 Posts

Response Posted - 09/01/2004 : 14:06:53

For what it's worth, the GPS expert at a local sporting-goods shop told me these GPS-paddle wheel discrepancies are common. He said the wheels are often wrong by up to 1.5 knots in either direction, while the GPS readings are accurate to a tenth of a knot.

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JJM
Navigator

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USA
170 Posts

Response Posted - 09/01/2004 : 17:00:52

Al, I like your ideas on how to validate and check the readings. And Reuben, if the discrepancies between GPS and paddle wheel are commonplace, is there any reason to try and re-calibrate the paddle to get a truer reading, or is it "the nature of the beast"?

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 09/01/2004 : 20:16:23

Rueben: Thanks for a smiler!

John: If you're concerned with maximizing your speed by experimenting with sail trim, the knotmeter is generally the better tool (speed through the water). It will tell you whether a trim change actually changed your speed, without regard to what direction you're going with, against, or sideways to any current. Just think of the speed as being relative--not absolute.

If you want to know how much progress you're making toward your destination (speed over the bottom), the GPS gives you the absolute answer, but in waters with currents, it doesn't tell you that much about your sail trim. (Currents can vary substantially based on the shape of the bottom and other geographic features.)

If you sail in an inland lake, the two should theoretically be the same, but I've never seen a meter that's within a quarter of a knot. As you know, in that case, the GPS is right. In salt water, many paddle-wheels are non-functional by the end of the season.

Edited by - Dave Bristle on 09/01/2004 20:26:39

Dkn420
Captain

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USA
298 Posts

Response Posted - 09/01/2004 : 22:00:17

Suzie: Thanks for asking about the "spiders"

. The boat was taken back to the factory by the dealer and the repairs were made and the boat returned. It took a while, a lot of angst, and enough letters to kill a tree...but all ended well.

I also keep checking for leaks in the coaming compartments and the front hatch, but so far nothing. This will be the second winter my boat is wet slipped so I will be checking it often. I hope you get your leaks solved...I understand your frustration

Dan #727

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sailgal
Captain

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USA
400 Posts

Response Posted - 09/02/2004 : 11:05:34

Thanks Dan, glad to hear all is dry and now "crack" free for ya.

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kbq
1st Mate

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USA
99 Posts

Response Posted - 09/02/2004 : 11:56:11

I'm sure that our meter lies a bit also (at least our GPS says so!), but last weekend on the SF bay, while reaching in brisk winds (20+), the meter read 7.2 knots.

Now, I don't believe that was *really* true - but we were sure having a lot of fun!

Kevin
"wait for me" 250 wk

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glstout
Deckhand

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USA
23 Posts

Response Posted - 09/02/2004 : 16:56:29

Reuben,

We have an ST40 Raytheon Bidata. If you have this or similar and an owners manual look up "User calibration". After you read that determine your GPS speed and at the same time note and record your knotmeter reading over a few outings. Average out both and taking into account the GPS may be reading MPH and the KM reading Knots; come up with a factor for the whole mess. Mine was about 1.3 and both continue to be close after 3 seasons.

As far as the wheel each Winter layup I disassemble and give it a good coat of VC 17, has not been a problem since the 1st year when it was; as Dave B says saltwater could be more difficult.

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JJM
Navigator

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USA
170 Posts

Response Posted - 09/02/2004 : 17:53:31

Alas, I am in salt and am aware of the sludge that accumulates in the paddle-wheel. I have cleaned it twice since it's late April launching. Would coating it with VC 17 at it's next cleaning help in salt water?

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ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 09/05/2004 : 14:29:48

"...Would coating it with VC 17 at it's next cleaning... "

You should should use only special water-based anti-fouling paint on the transducers. The solvents in 'regular' bottom paints can/will attack the plastic and degrade it. (per Raytheon owners manual). The special 'transducer paint' needed is available from many online vendors in little bottles for about $7.

Yes, the little paddlewheels are VERY sensitive to fouling. Here in Humboldt Bay the summertime fouling is so bad that bi-weekly cleanings were necessary. I discovered that it was easier to simply keep the blanking plug installed until I'm ready to go sailing.

My transducer is installed in a location where the swap-out for the blanking plug only takes a minute, and nowdays I'm quick enough to only get a sponge-full or so of water (or so) in the bilge when doing it.

The speed function should be calibrated for each installation. The owners manual (available online) has instructions on how do do it. A good calibration method is to run back to back reciprocal courses (opposite directions to reduce the effect of currents) with the GPS to establish a correction factor. The Raytheon will give you average speed over each run... ditto most GPS units. I've found that knotmeter calibration is easier to do under engine power so you can make consistent straight-line runs and don't have to fiddle with the sails, account for speed variations due to tacking, hull leeway & etc.

If you don't have a GPS, doing reciprocal course timed runs over a known distance is the way to go.

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reuben
Navigator

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USA
202 Posts

Response Posted - 09/08/2004 : 09:49:32

John, I think it probably *is* the nature of the paddle-wheel beast to be wrong, so don't plan to monkey with calibration, but then I have not yet been able to re-set the clock in my Dakota and must add 32 minutes to whatever time it claims in order to know whether I'm actually late. If I were better with gadgets I would take the time, but what did Eastwood say? A man's got to know his limitations.

Incidentally, last Saturday we briefly hit 7 knots (gps reading) while reaching amongst a happy varied Labor Day Weekend fleet. The Raytheon read 5 at that time.

Edited by - reuben on 09/08/2004 09:52:25

JJM
Navigator

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USA
170 Posts

Response Posted - 09/08/2004 : 14:29:14

Reuben, Sounds like you had a great weekend! I know what you mean about gadgets - and just about when you think you "might" have them figured out, then a new and improved one gets released to further boggle your mind!

This past weeekend we drove to our slip and the wind was up, so we were prepared for an exhilerating day! Got the boat out into the ocean and then Mother Nature took charge - calmed the wind to near nothing. Many sailboats were out with a high majority of those motoring only with no sails up. A very strange sight, indeed. However, our temperature on the water was definately a lot cooler than the approximate 90 degrees on land!

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