Speed Log out by around 3 knots - SailNet Community

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Go Back   SailNet Community > On Board > Gear & Maintenance > Electronics
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  #1  
Old 03-30-2012
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Speed Log out by around 3 knots

My Nasa Cruiser speed log is out by around 275%! So when the display shows 2 knots my handheld GPS will show 5! Now I do understand that there is some difference between the speed over water but this is way off! The speed log transducer is of the standard paddle wheel type and fitted through-hull positioned in a correct location and with no obstructions. And whilst there is a calibration facility it is limited to +/- 50%!
Any ideas of what the problem could be?
thanks
J.
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Old 03-30-2012
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Re: Speed Log out by around 3 knots

Motor at a steady rpm up current for a measured mile--recording your time and the speed displayed on your log, reverse course, without changing your rpm and motor down current over the same distance again recording your time and the speed displayed on your log. Take your average time to determine your actual speed and compare that with the average speed reported by your log which will give your log error which can be adjusted out. Ignor the GPS which is meaningless with respect to speed through the water--particularly if, for example, your making 3 knots through the water into a 5 knot stream. Your GPS will tell you you're making 2 knots in reverse.

Alternately, buy yourself a "KnotStick" with which you can actually measure and calibrate your paddle-wheel knot meter quite accurately.
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Old 03-30-2012
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Re: Speed Log out by around 3 knots

"when the display shows 2 knots my handheld GPS will show 5"
In other words, your waterwheel is VERY SLOW.

When is the last time you removed the impeller and cleaned it out? Sometimes you'll find enough growth in there to freeze it up in just a week or two. I'll pull the log whenever the boat is being left at the mooring, and if it has been left in, it gets cleaned (old toothbrush) picked (dental picks) and if there is barnacle crud on it, you may need to soak it in vinegar as well.

Before you decide something needs calibrating--pull it and see if it isn't just crud.
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Re: Speed Log out by around 3 knots

Quote:
Originally Posted by hellosailor View Post
"when the display shows 2 knots my handheld GPS will show 5"
In other words, your waterwheel is VERY SLOW.

When is the last time you removed the impeller and cleaned it out? Sometimes you'll find enough growth in there to freeze it up in just a week or two. I'll pull the log whenever the boat is being left at the mooring, and if it has been left in, it gets cleaned (old toothbrush) picked (dental picks) and if there is barnacle crud on it, you may need to soak it in vinegar as well.

Before you decide something needs calibrating--pull it and see if it isn't just crud.
Amen.

Tho' it may sound odd, I have found that applying Neosporin anti-bacterial ointment to the paddle-wheel and inner faces of the transducer housing significantly extends periods between cleanings.
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Re: Speed Log out by around 3 knots

Zinc oxide (diaper cream or sunblock) probably would accomplish the same thing, killing critters without breeding antibiotic resistance. FWIW.

Knowing that our antibiotics are going to be useless when we still need them, that's scarier than sea monsters.
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Re: Speed Log out by around 3 knots

Quote:
Originally Posted by hellosailor View Post
Zinc oxide (diaper cream or sunblock) probably would accomplish the same thing, killing critters without breeding antibiotic resistance. FWIW.

Knowing that our antibiotics are going to be useless when we still need them, that's scarier than sea monsters.
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Old 04-02-2012
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Re: Speed Log out by around 3 knots

Quote:
Originally Posted by hellosailor View Post
"when the display shows 2 knots my handheld GPS will show 5"
In other words, your waterwheel is VERY SLOW.

When is the last time you removed the impeller and cleaned it out? Sometimes you'll find enough growth in there to freeze it up in just a week or two. I'll pull the log whenever the boat is being left at the mooring, and if it has been left in, it gets cleaned (old toothbrush) picked (dental picks) and if there is barnacle crud on it, you may need to soak it in vinegar as well.

Before you decide something needs calibrating--pull it and see if it isn't just crud.
thanks for taking the time to respond. I have never removed the impeller and cleaned it out! But the speed log is easily accessible and I remove it after every trip. So I dont think the problem is dirt! But having said that, I will try and remove the impeller.
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Old 04-02-2012
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Re: Speed Log out by around 3 knots

If you can clean around the whole "paddlewheel" without temoving it, don't remove it from the "whole plug". But sometimes the wheel is just rotating on a drift pin that easily slides out, making cleaning easier. As long as you don't lose that pin!

No, its not lost, its safely stored in the bilge.
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Old 04-02-2012
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Re: Speed Log out by around 3 knots

I say pull the shaft too.

It doesn't take much growth of anything to slow it down.
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Old 04-03-2012
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Re: Speed Log out by around 3 knots

thanks for your replies.

please see here: Nasa Paddle Wheel and Spindle - Product Details
It seems that there is a spare part that exists (shaft + paddle wheel) so I guess it would be safe to remove the shaft if i can figure out how!
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