Autohelm Seatalk Bidata depth/knotmeter displays 2.8 knots when GPS shows speed 6.3. Any advice one how to calibrate? Cleaned up a little wheel in the hull when I painted the bottom last Spring. No effect. Does this instrument use that wheel or a transducer? I'm lucky if I can set the alarm on my digital watch and don't have the directions for setting the depth alarm. Any advice welcome. Glenn WindWisp 26.5 Hunter
- Status
- Not open for further replies.
J
John Baumgartner
different speed
The GPS speed is speed over ground, the knot meter is speed thru the water. The difference could be current, in your example the current could be 3.5 knots going in the same direction as you are going and giving you a push of 3.5 kts your GPS would see the speed as 6.3 over the ground while the wheel in your knot meter thru the water is only turning 2.8 worth.That make any sence, cents..... $.02 worth
The GPS speed is speed over ground, the knot meter is speed thru the water. The difference could be current, in your example the current could be 3.5 knots going in the same direction as you are going and giving you a push of 3.5 kts your GPS would see the speed as 6.3 over the ground while the wheel in your knot meter thru the water is only turning 2.8 worth.That make any sence, cents..... $.02 worth
S
Steve
Raymarine Web Site for Manuals
Glenn, Check out the Raymarine web site and download the manual you need. It will give you the step-by-step instructions to calibrate the speed settings. You will pick a "best guess" from what is displayed from your GPS, but I'm sure you can get it closer. Steve
Glenn, Check out the Raymarine web site and download the manual you need. It will give you the step-by-step instructions to calibrate the speed settings. You will pick a "best guess" from what is displayed from your GPS, but I'm sure you can get it closer. Steve
L
Len Fagan
Speed Setting
You need to calibrate knotmeter. The manual gives several ways to do it. I used the manual method by using GPS in water with no current. Len Fagan
You need to calibrate knotmeter. The manual gives several ways to do it. I used the manual method by using GPS in water with no current. Len Fagan
J
joe phibbs
too fast or too slow?
OK,so there is a discrepancy, but you should know which reading is closer to the truth: course over ground or velocity made good, as the old LORAN folks used to say. So, do calibrate the little wheel. However, is it possible that a boat charging INTO a 3knot current actually making 2 knots would show 1 knot? The resistance of the water would slow the little wheel down, not speed it up. Of course, I could be mistaken. Let me know. Joe
OK,so there is a discrepancy, but you should know which reading is closer to the truth: course over ground or velocity made good, as the old LORAN folks used to say. So, do calibrate the little wheel. However, is it possible that a boat charging INTO a 3knot current actually making 2 knots would show 1 knot? The resistance of the water would slow the little wheel down, not speed it up. Of course, I could be mistaken. Let me know. Joe
Bad Transducer?
Glenn: I don't know about your Autohelm transducer, but we had a Navico unit that one of the pickups that went bad. This caused the knotmeter to read half speed. Before you start messing with the re-calibration check with Autohelm and test your transducer. I don't think that they are usually off quite this much.
Glenn: I don't know about your Autohelm transducer, but we had a Navico unit that one of the pickups that went bad. This caused the knotmeter to read half speed. Before you start messing with the re-calibration check with Autohelm and test your transducer. I don't think that they are usually off quite this much.
E
Ed
Joe answer to question
If you were going into a 3 knot current and the boat speed was an actually 2 knots, the speed device would show a speed of 2 knots. However you would be losing ground by 1 knot per hour and the GPS would read a speed of 1 knot, but would show you going in the direction you would be headed - which would be backward, to complicate this issue the way it would be displayed on the GPS would vary depending if you had it set as course up, north, etc. Make any sence?
If you were going into a 3 knot current and the boat speed was an actually 2 knots, the speed device would show a speed of 2 knots. However you would be losing ground by 1 knot per hour and the GPS would read a speed of 1 knot, but would show you going in the direction you would be headed - which would be backward, to complicate this issue the way it would be displayed on the GPS would vary depending if you had it set as course up, north, etc. Make any sence?
G
Glenn Bollin
Transducer vs Impeller, Thanks for the response
Now you guys really have me wondering. I can understand how an impeller will give you water speed readings as compared to distance over land. While at anchor in a strong tide the impeller should register some speed, mine doesnt. Also when motoring with or against the tide, no change in knots/hr. I'm thinking my impeller is the problem although it is clean, lubed and spins freely. Does a transducer have anything to do with measuring speed? I understand how they measure depth but am real foggy about how it would be used to measure speed. Again, I appreciate your responses. Glenn WindWisp 26.5 Hunter
Now you guys really have me wondering. I can understand how an impeller will give you water speed readings as compared to distance over land. While at anchor in a strong tide the impeller should register some speed, mine doesnt. Also when motoring with or against the tide, no change in knots/hr. I'm thinking my impeller is the problem although it is clean, lubed and spins freely. Does a transducer have anything to do with measuring speed? I understand how they measure depth but am real foggy about how it would be used to measure speed. Again, I appreciate your responses. Glenn WindWisp 26.5 Hunter
Knotmeter.
Glenn: Not sure all of these devices work exactly the same but the transducers have a couple of pickups in them. As the impeller spins around the pickups determine how fast the impeller is spinning. The faster it spins the 'faster' you are going. If one of the pickups were to go dead, it would indicate that you are going 1/2 of the actual speed that you are really going. If you place a transducer next to your keel if may indicate that you are going faster than you actually are because the water is accelerating around the keel. If you where to place the transducer behind the keel it may indicate that you are doing slower than you actually are because the water flow may be blocked by the keel. I suggest that you contact the mfg and have them give you a testing procedure for your instrument and transducer. They should be able to give you a range of voltage/resistance etc. that you should expect through this device.
Glenn: Not sure all of these devices work exactly the same but the transducers have a couple of pickups in them. As the impeller spins around the pickups determine how fast the impeller is spinning. The faster it spins the 'faster' you are going. If one of the pickups were to go dead, it would indicate that you are going 1/2 of the actual speed that you are really going. If you place a transducer next to your keel if may indicate that you are going faster than you actually are because the water is accelerating around the keel. If you where to place the transducer behind the keel it may indicate that you are doing slower than you actually are because the water flow may be blocked by the keel. I suggest that you contact the mfg and have them give you a testing procedure for your instrument and transducer. They should be able to give you a range of voltage/resistance etc. that you should expect through this device.
T
Tim
Here's the deal
Glenn, When you say your knotmeter does not read while anchored in a "strong tide", are you saying there is a definite flow of water past the boat? Is the boat aligning downstream of the mooring bouy in this tide? Can you see water streaming past the boat as if you were motoring or sailing? The "transducer" is the whole unit in which the "impeller" is mounted. The impeller travels at a speed proportional to the speed of the water passing it. The internal electronics of the transducer measure the speed of the impeller and transmit it to the display computer. Now, as far as motoring with or against a tide or current and the speed doesn't change, that is normal. As a flight instructor, I have found this whole concept is difficult to understand for a lot of folks. The concept applies to airplanes as well as boats. Let me see if I can explain... Think of the water not relative to stationary land, but as a giant moving mass. Now put a boat in that mass which is not motoring or sailing or anchored. How fast is the boat moving? Through the water its sitting in, zero. Past the sea bottom, however fast the mass of water is moving. What does the knotmeter say? Zero. The knotmeter reads the speed through the water. What does the GPS say? However fast the mass of water is moving. The GPS reads the speed of its receiver across the face of the earth. Lets say you are in a river with a 3 knot current in a boat motoring at 5 knots: Going upstream (against the current), the knotmeter will say 5. All it cares about is the speed of the boat relative to the mass of water its in. The GPS will say 2 knots. All it cares about is the speed of the receiver relative to the face of the earth. 5 knots within the mass of water, MINUS the 3 knots the mass of water is moving equals 2 knots across the bottom. Going downstream (with the current), the knotmeter will read 5. All it cares about is the speed of the boat relative to the mass of water its in. The GPS will read 8 knots. All it cares about is the speed of the receiver relative to the face of the earth. 5 knots within the mass of the water PLUS the 3 knots the mass of water is moving equals 8 knots. Hope that clears it up! Tim
Glenn, When you say your knotmeter does not read while anchored in a "strong tide", are you saying there is a definite flow of water past the boat? Is the boat aligning downstream of the mooring bouy in this tide? Can you see water streaming past the boat as if you were motoring or sailing? The "transducer" is the whole unit in which the "impeller" is mounted. The impeller travels at a speed proportional to the speed of the water passing it. The internal electronics of the transducer measure the speed of the impeller and transmit it to the display computer. Now, as far as motoring with or against a tide or current and the speed doesn't change, that is normal. As a flight instructor, I have found this whole concept is difficult to understand for a lot of folks. The concept applies to airplanes as well as boats. Let me see if I can explain... Think of the water not relative to stationary land, but as a giant moving mass. Now put a boat in that mass which is not motoring or sailing or anchored. How fast is the boat moving? Through the water its sitting in, zero. Past the sea bottom, however fast the mass of water is moving. What does the knotmeter say? Zero. The knotmeter reads the speed through the water. What does the GPS say? However fast the mass of water is moving. The GPS reads the speed of its receiver across the face of the earth. Lets say you are in a river with a 3 knot current in a boat motoring at 5 knots: Going upstream (against the current), the knotmeter will say 5. All it cares about is the speed of the boat relative to the mass of water its in. The GPS will say 2 knots. All it cares about is the speed of the receiver relative to the face of the earth. 5 knots within the mass of water, MINUS the 3 knots the mass of water is moving equals 2 knots across the bottom. Going downstream (with the current), the knotmeter will read 5. All it cares about is the speed of the boat relative to the mass of water its in. The GPS will read 8 knots. All it cares about is the speed of the receiver relative to the face of the earth. 5 knots within the mass of the water PLUS the 3 knots the mass of water is moving equals 8 knots. Hope that clears it up! Tim
B
Bryce Grefe
knot meter speed
Glenn, I have Raymarine ST-50's on my 410. The speed transducer serves two functions, water temp and speed thru water. the speed is determined by a Hall effect device that counts the magnetic impulse of the paddle wheel. The paddle wheel on mine had one arm broken off during a hall out a couple of years ago and my speed read around 75% of actual. The other thing I've noticed is that even tho the paddle wheel turns freely by hand, it sometimes has enough friction to read erroneously or not at all when in the water. I've found I need to remove the paddle wheel, thoroughly clean the axle and inside the paddle wheel and then I spray some McLube on the parts before reassembly. One other possibility is that the paddle wheels magnetic flux has deteriorated over time and the hall effect device can't pick it up reliably. Raymarine sells a rebuilt kit for my ST-50 transducer and it includes a paddle wheel. Hope this helps. Bryce S/V Spellbinder H 410
Glenn, I have Raymarine ST-50's on my 410. The speed transducer serves two functions, water temp and speed thru water. the speed is determined by a Hall effect device that counts the magnetic impulse of the paddle wheel. The paddle wheel on mine had one arm broken off during a hall out a couple of years ago and my speed read around 75% of actual. The other thing I've noticed is that even tho the paddle wheel turns freely by hand, it sometimes has enough friction to read erroneously or not at all when in the water. I've found I need to remove the paddle wheel, thoroughly clean the axle and inside the paddle wheel and then I spray some McLube on the parts before reassembly. One other possibility is that the paddle wheels magnetic flux has deteriorated over time and the hall effect device can't pick it up reliably. Raymarine sells a rebuilt kit for my ST-50 transducer and it includes a paddle wheel. Hope this helps. Bryce S/V Spellbinder H 410
G
Glenn Bollin
Thanks Tim: Eureka!
Finally it makes sense. Your speed is relative to the water mass or the Earth. Nice explanation of the principles. Thanks again. Glenn WindWisp Hunter 26.5
Finally it makes sense. Your speed is relative to the water mass or the Earth. Nice explanation of the principles. Thanks again. Glenn WindWisp Hunter 26.5
G
Glenn Bollin
Good Idea Bryce
I haven't removed the wheel to clean and lube but now will while I'm on the hard. Thanks for the advice. Glenn WindWisp 26.5 Hunter
I haven't removed the wheel to clean and lube but now will while I'm on the hard. Thanks for the advice. Glenn WindWisp 26.5 Hunter
- Status
- Not open for further replies.