I have read in sailing books that knotmeters need to be recalibrated periodically. Can anyone explain how this is done? According to my GPS my knotmeter is off by 1.5 knots.
Thanks,
Warren Moore
CD 28 Crisscross
wmoore@peconic.net
Recalibrating a knotmeter
Moderator: Jim Walsh
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Warren Kaplan
Re: Recalibrating a knotmeter
Can't help you with how to recalibrate your knotmeter. I'd like to know that myself. But one thing to consider (and forgive me if you already know this) when comparing speed on the knotmeter vs speed on the gps is that the two are not measuring the same thing. The knotmeter is measuring speed thru the water while the gps is measuring speed over the ground. My gps and knotmeter almost never are the same because of currents either with or against me. So take that into account before you try and calibrate the knotmeter to the reading on your gps.Warren Moore wrote: I have read in sailing books that knotmeters need to be recalibrated periodically. Can anyone explain how this is done? According to my GPS my knotmeter is off by 1.5 knots.
Thanks,
Warren Moore
CD 28 Crisscross
Setsail728@aol.com
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Michael Stephano
Re: Recalibrating a knotmeter
I recently did mine. I motored in a calm area and used the GPS. In my case it was a different meter using the same paddles. There is an adjustment screw on the back of most knot meters and by steering a straight course back and forth I was able to calibrate the meter. It is important that you clean the paddles on your sender before doing anything. They may just be filthy with scum and growth. Check the meter after cleaning and before you change anything.
mundo@visi.net
mundo@visi.net
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D. Stump, Hanalei
Re: NOT calibrated to speed but to DISTANCE!
Warren,
Data Marine knotlogs are not calibrated to speed. They are calibrated to distance! If you do it this way, you don't have to worry about opposing currents. To do it, find two marks at sea (permanent bouys, or points of land between which you can read the true distance off of a chart. Turn the knot meter off, this resets all distance measurements to zero. Approach your first mark on a heading to the second mark. As the first mark is abeam, turn the meter on. As you pass the second mark abeam, turn the knot meter to calibrate. Turn around and retrace your first route. As you have the second mark abeam, turn the knotmeter back to run. As you make the first mark abeam, turn the meter back to calibrate and read the miles covered(both ways combined). Divide the result by two. The remainder should be the oneway distance between the two marks. If it isn't, adjust the calibrate screw appropriately. You're done, and you didn't have to even think about the boat speed through the water(that is all a knotmeter tells you, speed through the water, not speed over ground!)
DataMarine has the full procedure in the owners manual. If you don't have one, I can post the exact procedure after next weekend when I go back to the boat. Let me know if you want it, I may have stated a detail a little incorrect above, I'm not sure.
I re-cal'd my DataMarine knotmeter on Hanalei a few months ago, and now when I compare speed indicated to the GPS, the difference is verified in the Nautical Almanac as the speed of the current(tide)at that particular location in Fisher's Island Sound. It worked great!
Dave Stump
Captain Commanding
s/v Hanalei CD-30.
Data Marine knotlogs are not calibrated to speed. They are calibrated to distance! If you do it this way, you don't have to worry about opposing currents. To do it, find two marks at sea (permanent bouys, or points of land between which you can read the true distance off of a chart. Turn the knot meter off, this resets all distance measurements to zero. Approach your first mark on a heading to the second mark. As the first mark is abeam, turn the meter on. As you pass the second mark abeam, turn the knot meter to calibrate. Turn around and retrace your first route. As you have the second mark abeam, turn the knotmeter back to run. As you make the first mark abeam, turn the meter back to calibrate and read the miles covered(both ways combined). Divide the result by two. The remainder should be the oneway distance between the two marks. If it isn't, adjust the calibrate screw appropriately. You're done, and you didn't have to even think about the boat speed through the water(that is all a knotmeter tells you, speed through the water, not speed over ground!)
DataMarine has the full procedure in the owners manual. If you don't have one, I can post the exact procedure after next weekend when I go back to the boat. Let me know if you want it, I may have stated a detail a little incorrect above, I'm not sure.
I re-cal'd my DataMarine knotmeter on Hanalei a few months ago, and now when I compare speed indicated to the GPS, the difference is verified in the Nautical Almanac as the speed of the current(tide)at that particular location in Fisher's Island Sound. It worked great!
Dave Stump
Captain Commanding
s/v Hanalei CD-30.
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Neil Gordon
Re: NOT calibrated to speed but to DISTANCE!
>>Data Marine knotlogs are not calibrated to speed.<<
What's the practical difference? GPS now gives accurate speed over ground... if you know how fast you are going, you know how long it will take to go whatever distance you want. Is the same formula. So is just as easy to motor until the knotmeter reads 3 knots or whatever, and see what the GPS indicates as speed. Reverse course and the different GPS speed would indicate the reversed current. Average the two speeds per GPS and compare to the constant boat speed. Adjust accordingly.
For any number of reasons, it's easier to calibrate while motoring. Keep in mind, also, that the position of the thru-hull can cause readings to differ when sailing depending on the tack.
Oh... and I agree with the need to clean the sucker... there's all sorts of little critters that increase friction and slow the paddlewheel.
Regards, Neil
s/v LIQUIDITY
Cape Dory 28 #167
neil@nrgordon.com
What's the practical difference? GPS now gives accurate speed over ground... if you know how fast you are going, you know how long it will take to go whatever distance you want. Is the same formula. So is just as easy to motor until the knotmeter reads 3 knots or whatever, and see what the GPS indicates as speed. Reverse course and the different GPS speed would indicate the reversed current. Average the two speeds per GPS and compare to the constant boat speed. Adjust accordingly.
For any number of reasons, it's easier to calibrate while motoring. Keep in mind, also, that the position of the thru-hull can cause readings to differ when sailing depending on the tack.
Oh... and I agree with the need to clean the sucker... there's all sorts of little critters that increase friction and slow the paddlewheel.
Regards, Neil
s/v LIQUIDITY
Cape Dory 28 #167
neil@nrgordon.com
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D. Stump, Hanalei
Re: Speed through the water....
Neil,
The difference is that a knotmeter tells you speed through the water. The GPS tells you speed over the ground. As far as calibrating the knotmeter, what I related is how Datamarine says to calibrated the meter. Their procedure is designed to eliminate concerns about cross currents.
D. Stump
The difference is that a knotmeter tells you speed through the water. The GPS tells you speed over the ground. As far as calibrating the knotmeter, what I related is how Datamarine says to calibrated the meter. Their procedure is designed to eliminate concerns about cross currents.
D. Stump
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Neil Gordon
Re: Speed through the water....
>>Their procedure is designed to eliminate concerns about cross currents.<<
I'm missing how it does that.
The idea of running a fixed distance in two directions is to eliminate (by averaging the time) a head current in one direction and a following current the other way. In effect, you run a 2x course and see how long it takes, but eliminate the inefficiency and distance of the turn. But if there is a cross current, you'd have to travel a greater distance through the water in both directions. The course would be longer through the water than on the chart and the calibration would be off as a result. Is no different with speed over ground per GPS or speed over ground as calculated by covering a course in both directions.
Either way, you have to measure the speed or distance where there's no appreciable cross current.
Regards, Neil
s/v LIQUIDITY
Cape Dory 28 #167
neil@nrgordon.com
I'm missing how it does that.
The idea of running a fixed distance in two directions is to eliminate (by averaging the time) a head current in one direction and a following current the other way. In effect, you run a 2x course and see how long it takes, but eliminate the inefficiency and distance of the turn. But if there is a cross current, you'd have to travel a greater distance through the water in both directions. The course would be longer through the water than on the chart and the calibration would be off as a result. Is no different with speed over ground per GPS or speed over ground as calculated by covering a course in both directions.
Either way, you have to measure the speed or distance where there's no appreciable cross current.
Regards, Neil
s/v LIQUIDITY
Cape Dory 28 #167
neil@nrgordon.com
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D. Stump, Hanalei
Re: Speed through the water....
Neil,
I'm only relating the procedure that DataMarine publishes in the operator's manual. If you think you have a better way of doing it, go for it! I only know that DataMarine's procedure worked fine on Hanalei, and I now trust what the knotmeter says.
Dave S.
I'm only relating the procedure that DataMarine publishes in the operator's manual. If you think you have a better way of doing it, go for it! I only know that DataMarine's procedure worked fine on Hanalei, and I now trust what the knotmeter says.
Dave S.
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Matt Cawthorne
Don't use this procedure in a strong current.
Dave,
You may wish to wait for slow moving currents to use this technique. Consider the case of a 2 knot current where you were truely going 5 knots through the water between two marks 1 mile apart. The up current time between marks would be 1/3 of an hour and the down current time would be 1/7 of an hour. The total time would be .476 hours. In that time you would have gone 5 x .476 or 2.38 miles through the water which is 19% different than the 2 miles you would be using to calibrate the instrument. It is still much better than the 66% error that you would have introduced in the distance if you had not averaged the up and down current distances, but it is still best to calibrate when the current is small.
I hate to admit that the modern way might be better, but the GPS method (getting instantaneous measurements in both directions) should work out better in the case of a strong current.
Matt
mcawthor@bellatlantic.net
You may wish to wait for slow moving currents to use this technique. Consider the case of a 2 knot current where you were truely going 5 knots through the water between two marks 1 mile apart. The up current time between marks would be 1/3 of an hour and the down current time would be 1/7 of an hour. The total time would be .476 hours. In that time you would have gone 5 x .476 or 2.38 miles through the water which is 19% different than the 2 miles you would be using to calibrate the instrument. It is still much better than the 66% error that you would have introduced in the distance if you had not averaged the up and down current distances, but it is still best to calibrate when the current is small.
I hate to admit that the modern way might be better, but the GPS method (getting instantaneous measurements in both directions) should work out better in the case of a strong current.
Matt
mcawthor@bellatlantic.net
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D. Stump, Hanalei
Re: Distance = Speed / Time
Matt,
As I replied to another, this procedure is what DATAMARINE recommends. It is in the owner's manual! If you calibrate simply to distance, the time it takes you to get there doesn't make any difference. The knotmeter will take care of the upcurrent/downcurrent time difference. If it didn't, how could it ever tell you the DISTANCE you have traveled?
Re-read your first paragraph, you don't average upcurrent and downcurrent distance. The DISTANCE is fixed, between the two bouys or whatever! The difference(in distance) and error in calibration is shown on the knotmeter readout, i.e. if it says 2.5 miles, the error is 0.5 miles! Adjust the knotmeter calibration screw to indicate 2 miles, and you are done. Speed will then be indicated correctly by definition!
Hey, guys, believe me! I sat at my desk trying to figure all this out. I even drew pictures! Finally I read the owner's manual and it all made perfect sense.
D. Stump
As I replied to another, this procedure is what DATAMARINE recommends. It is in the owner's manual! If you calibrate simply to distance, the time it takes you to get there doesn't make any difference. The knotmeter will take care of the upcurrent/downcurrent time difference. If it didn't, how could it ever tell you the DISTANCE you have traveled?
Re-read your first paragraph, you don't average upcurrent and downcurrent distance. The DISTANCE is fixed, between the two bouys or whatever! The difference(in distance) and error in calibration is shown on the knotmeter readout, i.e. if it says 2.5 miles, the error is 0.5 miles! Adjust the knotmeter calibration screw to indicate 2 miles, and you are done. Speed will then be indicated correctly by definition!
Hey, guys, believe me! I sat at my desk trying to figure all this out. I even drew pictures! Finally I read the owner's manual and it all made perfect sense.
D. Stump
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don sargeant
How do you keep the crud off...
except by flooding the bilge, bashing your knuckles and scaring everyone in sight?
Don Sargeant
~~COQUINA~~
CD25D #189
No speed read in Greenwich Cove
don@cliggott.com
Don Sargeant
~~COQUINA~~
CD25D #189
No speed read in Greenwich Cove
Michael Stephano wrote: I recently did mine. I motored in a calm area and used the GPS. In my case it was a different meter using the same paddles. There is an adjustment screw on the back of most knot meters and by steering a straight course back and forth I was able to calibrate the meter. It is important that you clean the paddles on your sender before doing anything. They may just be filthy with scum and growth. Check the meter after cleaning and before you change anything.
don@cliggott.com
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Michael Stephano
Re: How do you keep the crud off...
HEHEHEHE. It is certainly a sobering experience for anyone that has not done this before.
mundo@visi.net
mundo@visi.net
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Matt Cawthorne
Re: Distance = Speed X Time
Dave,
I think that you mean that the formula is D = s X t. The issue here is that the distance through the water is not the same as distance over the ground. The current messes that up. Your knot meter can only measure distance/speed through the water. The bouys indicate the distance over the ground.
Here is a little better explanation of the calculations described earlier. Take the case of a knotmeter which is well calibrated and reads the true speed of the boat through the water. Ther current is two knots and the boat is moving 5 knots through the water which is what the knot meter is reading. Again, the bouys are 1 mile apart and the current is going in line with the two bouys.
Upstream: Upstream the boat speed over the ground would be only 3 knots since the speed of the current is subtracted from the boat speed (through the water). At 3 knots over the grounthe time for the upcurrent leg would be
T= D/S = 1/3 =.333 hours
The distance through the water would be
D= S x T = 5 x .3333 = 1.667 miles
Downstream: When going downstream the current would add to the boat's speed through the water so would be 7 knots over the ground. The time to go the one mile leg would be
t= d/s = 1/7 = .1428 hours.
The distance through the water would be
D = T x S = .1428 * 5. =.714 miles
The total distance through the water that should have been measured by the correctly working knot log would be 1.667 + .714 = 2.38 miles. If you adjusted it to read only two miles your knotmeter would always be reading low.
The process is not bad when the current is low. For instance when the boat is moving 5 knots a two knot current would introduce a 16% error in speed or distance. That is not very acceptable. If, however you wait until the current is one knot, the error introduced would only be 4%. If you wait until the curent is 1/2 knot the error would be only 1%. The process is far better than just doing one direction. For instance a one knot current would introduce a 20% error instead of the 4% that you get doing both directions.
I have no doubt about it being Datamarine's recommended process, but I stick by my recommendation to do it when the current is a low as possible. I have used it to check my knotmeter/speed log prior to owning a GPS, but did it when the current was very low. I am a little surprised that Datamarine did not make the same caution in their manual.
Matt
Mcawthor@bellatlantic.net
I think that you mean that the formula is D = s X t. The issue here is that the distance through the water is not the same as distance over the ground. The current messes that up. Your knot meter can only measure distance/speed through the water. The bouys indicate the distance over the ground.
Here is a little better explanation of the calculations described earlier. Take the case of a knotmeter which is well calibrated and reads the true speed of the boat through the water. Ther current is two knots and the boat is moving 5 knots through the water which is what the knot meter is reading. Again, the bouys are 1 mile apart and the current is going in line with the two bouys.
Upstream: Upstream the boat speed over the ground would be only 3 knots since the speed of the current is subtracted from the boat speed (through the water). At 3 knots over the grounthe time for the upcurrent leg would be
T= D/S = 1/3 =.333 hours
The distance through the water would be
D= S x T = 5 x .3333 = 1.667 miles
Downstream: When going downstream the current would add to the boat's speed through the water so would be 7 knots over the ground. The time to go the one mile leg would be
t= d/s = 1/7 = .1428 hours.
The distance through the water would be
D = T x S = .1428 * 5. =.714 miles
The total distance through the water that should have been measured by the correctly working knot log would be 1.667 + .714 = 2.38 miles. If you adjusted it to read only two miles your knotmeter would always be reading low.
The process is not bad when the current is low. For instance when the boat is moving 5 knots a two knot current would introduce a 16% error in speed or distance. That is not very acceptable. If, however you wait until the current is one knot, the error introduced would only be 4%. If you wait until the curent is 1/2 knot the error would be only 1%. The process is far better than just doing one direction. For instance a one knot current would introduce a 20% error instead of the 4% that you get doing both directions.
I have no doubt about it being Datamarine's recommended process, but I stick by my recommendation to do it when the current is a low as possible. I have used it to check my knotmeter/speed log prior to owning a GPS, but did it when the current was very low. I am a little surprised that Datamarine did not make the same caution in their manual.
Matt
Mcawthor@bellatlantic.net