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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #11  
Old 01-10-2003
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Paddle wheel transducer

The problem with using GPS as a knot meter is that you then have to filter out current and drift to really determine speed. GPS is a very useful tool for navigation, but with the scrambling of the signal, (still at 50 meters without WAAS and 10 meters with WAAS) there are significant errors in the speed read out that would prevent the speed shown from being accurate enough to be useful for sail trim or other kinds of boat speed adjustments.

Tied to my dock, watching my GPS, the speed numbers output varied between zero and .7 knots, even though the boat was not moving at all. These speeds were simply the result of the changes in selective distortion which is inherent in a non-military GPS. That kind of inacuracy makes a GPS useless for performance information purposes.

A knotmeter is measuring the speed through the water, which is far more critical for performance adjustments since variations in current strength that would be inbedded in the GPS number are already filtered out producing a pure speed number.

Jeff
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Old 01-10-2003
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Paddle wheel transducer

Very interesting comments from all. I''ve only had experience with SH junk so I may be biased, but as long as a paddle wheel transducer will have intermittant errors of ? % - I''ll stick with my gps. ( with all appologies to Jeff )
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  #13  
Old 01-10-2003
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Paddle wheel transducer


I remember one time, sailing along in my buddy Jackís C34 when the paddlewheel unit for the knot log became fowled. Now, Jack liked to trim and tweak like he was racing for the Americaís Cup. He was getting upset that the patent thing didnít work and wanted it fixed, now! So, I offered to go below and take care of it.

I pulled the bilge plate up, located the plug and started to remove the paddlewheel unit. Now this was the first time Iíd ever played with one of these and wasnít sure what to expect. I undid the thread ring and started working the unit out with one hand while holding the plug at the ready with the other. It was just about this time that the boat plowed into a party-boat wake!

The lurch of the boat hitting the wake forced me to drop the plug and let go of the unit to catch myself as I fell in a prone position, down into the bilge directly over the hole! The rising water pressure put the final grace on the unit and it popped out with a will! It clonked me right in the head, followed by the inrush of water from its 2 inch opening as the plug rolled way up into the bilge, out of reach into the next bay.

An old focísel song we use to sing on the olí Truant came to mind, the first verse went:

Dares a leek in da boat dear skibber, dear skibber!
Dares a leek in da boat, dear skibber, a leek!

I dragged myself out of the bilge and grabbed the paddlewheel unit. Keeping my upper body over the inrush of water to keep the bunks and settee from getting soaked, I cleared the seaweed off of it and stuffed it back in the hole!

It really broke Jack up as I came on deck looking like a drown rat! Through his laughter he informed me that at least the log was now working. It was still a standing joke years later for Jack to ask me if I would go down and clear the knot log!

Seriously, I can live without another hole in my boat. I agree with Jeff that real-time speed through the water can give useful information for sail trim and such, but that would be of interest more to the racing crowd then it is to me. When navigating between the buoys in a Down East fog, Iím more interested in my actual speed over the ground. The old Loran and now the GPS gives me information that I can plot directly to the chart.

There are many old ways to judge the speed of your boat both through the water and over the ground. I havenít used them for so long that Iíve forgotten, off hand the formula for timing a chip log for speed trough the water or a lobster pot buoy for speed over the ground. I use to be able to read the wake and judge the speed to within a tenth of a knot or so. I suppose I could pull out my old logbooks for a refresher course. Some day, when the oil has all dripped dry, I may need to do just that!
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Old 01-13-2003
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Paddle wheel transducer

Thanks for the debate! I think that''s what this forum is all about!
As a new sailor, the reason I wanted to get the speed/log up and running is to fine tune sail trim. I''m still at the stage that I can''t sit still. I''m trying to learn the affects of "dropping the traveler a bit", tighten up the backstay, put a "little more twist" into the sail, etc. I thought the speed/log would be a better tool than the GPS. Just another source of information.
However, at the end of the day, I guess no one knows of a source of supply for paddle wheel transducers to fit the old SL-1.

Thanks again for all your input

Mike in Noank
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Old 01-21-2003
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Paddle wheel transducer

On a side note I keep a large tooth brush they sell for dogs in the bilge next to my paddle wheel through hull fitting. Every time I pull it out I scrud that sucker down.
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