Knot Meter Question

[Bad Obsession]

When I purchased my boat three years ago the knot meter was not working properly. It was showing slower speeds than the boat was going (quite a bit slower). I verified this by a few other people's knot meters that were sailing along side of me and also by my GPS (I know that current etc. has a effect on this but not this big). It was usually off by 1-2 knots but sometimes more.A short time after I had bought the boat it stopped working all together. The display wasn't showing a thing. I didn't seem to mind because it didn't mean that much to me (I wasn't racing the boat).Last weekend I was messing around and I just decided to check the voltage going into the knot meter. Well, there was none. I found a short in the wire that was at the connection to the knot meter and another short right before the inline fuse. I repaired these shorts and the display came on. I never left the slip so I am not sure if the rest of it is working but I know that the paddle wheel is working fine because when I hauled the boat, less than a month ago, I checked it and cleaned it really well.All of that being said, if my knotmeter had a short going into it, originially, would it cause the low speed readings? If I get the knot meter working and the speed seems low is there an acurate way to adjust it? (I know that there is an adjustment screw on the instrument because I have seen it).Any thoughts or comments are appreciated. I will find out more when I finally get to leave my slip... I am heading for Hampton starting Sunday

 

[sailaway78]

A short could certainly

cause the meter to read wrong, and probably did. Low voltage and then no voltage as the short worsened and lost it's connection. Don't know what system you have and whether or not there is any adjustment.

 

[GuyT]

Port and Starboard tacks..

can have different speeds if the paddle wheel is not located in the middle of the boat.What would also cause low speeds is the paddlewheel not turning properly (gunked up).My speed differs between port and starboard tacks and it usually is an all or nothing affair. It seems once it is turning, its ok - its just getting the thing unjammed to start turning.There are at least 4 wires going into a speed sensor - there are +/- DC for powering up the unit and there are two wires from the sensor - this is usually co-ax cable or twisted pair. If the short was on the sensor - it would just read no speed, probably no damage if it was wire to wire short. If the +/- DC wires were shorted together, it should have blown a fuse or tripped a breaker.Dont know about adjustments - this would vary by manufacturers. I would think not since the knot-meter is a simple counter that counts electrical pulses from the paddle wheel. The adjustment you saw may be for the display intensity.

 

[Bob]

adjustment

If the knotmeter is a raymarine product, there is an adjustment in the control head to correct the speed readout. It is best to go on line and download the manual from Raymarine. It will explain what buttons to push in the calibration mode in order to increase or decrease the calculation.

 

[Warren Milberg]

Virtually every name brand

knotmeter that I know of has a way of calibrating it. If you don't have the manual for your model, you may want to hunt for it online. Most are there and explain how to calibrate the unit. Or they may have a phone contact to their tech support unit. Having said this, I had an Alberg Sea Sprite years ago that had the same (read ancient) Datamarine Knot and Depth meters I currently have on my H28.5. The knotmeter read low and slow. After a lot of checking of electrical connections and calibration, I finally traced the reason to being a slightly bent axle for the impeller. I probably hit something submerged in the water without realizing it and it bent the axle just enough to keep it from spinning true. I was able to get a new one from Datamarine for a few bucks, drift out the old axle and insert the new one. Bingo.

 

[Bill Roosa]

knotmeters work off pulses not voltage

With that said the receiver does have to sense the voltage pulses to count them. If the voltage was low it may have been missing some of the pulses and reading slow.