You kind of answered my planned response in the last sentence. The paddlewheel knotmeter is used to calculate true vs apparent wind speed on most systems, and of course does give you some indication of current. But whether you need to replace it, is something only you can answer. I know that the Signet units, like all instruments are expensive!

Measuring the speed through the water is very handy for sail trim.
It will let you know if your speed has increased or decreased regardles of the current.

In racing, we use GPS speed for route planning but the paddlewheel for everything else.

We find our paddlewheel seems to be inaccurate vs the chartplotter/GPS. We have one GPS driving the sailing instruments and we have a separate chartplotter, they always agree on speed/heading, etc. The paddlewheel always disagrees with them and it varies by tack. On one tack it reads higher than the GPSs do and on the other it reads lower. The mileage never agrees with the GPSs. Currents are essentially non-existent where we sail. I don't put much faith in the knotmeter.

Harold,
I agree with Gomez and Jason. The paddlewheel will tell you almost instantly how your speed has changed when you are tweaking sail trim. Being in fresh water, I doubt that your paddlewheel would get too fouled. I also have slightly different speed readings on different tacks as Bud does, but I don't think a GPS will give you as accurate and as instant feedback as a paddlewheel will. Then again, if you set your sails once and forget about them for the rest of the day, the GPS is more than adequate!! :) Your boat, your choice.....

Also, I was wondering about your pwheel being out so much in accuracy... - last summer my pwheel got fouled and I went down below and pulled it while underway (not the smartest idea, btw, as the water pressure was incredible... doh!) When I put it back in I must have got it offset in the forward/aft direction, which I didn't think was possible; and it read very erroneously. When we finally got to anchor I re-did it (lot less pressure!!) and got it straight in fwd/aft and it's been fine since.
Just something you might want to check....

It is easy to calibrate the Raymarine knot meter. For that, a couple GPS fixes would serve as a distance datum, but you'd have to make sure you were in an area with no current. I'll bet even you lake sailors have current to deal with, as current can be created by the wind.

FWIW, I can't imagine not having an accurate knotmeter. That is, not knowing how fast the boat is going through the water. This information is fundamental to good sailing, and to basic navigation as well.

One problem with paddlewheels and C36's is that our boats make quite a lot of leeway. Couple this with a hull that has a lot of deadrise (Vee-shape to the bottom) and the paddle wheel is in the lee of the 'crease' on one tack. This definitely makes for an error that can't be corrected out with calibration.

If one was seriously concerned, I would mount the wheel bang on centerline, or maybe further aft than most of use have and try to get it into the straighter water flow beside the keel.

Jason, what you said is exactly what I have always felt the problem is. We really do have diddly for current, though on the Great Lakes we do get a sort of tidal effect from the wind. I've seen that drop the water on the north shore of Lake Ontario by 6" overnight with a strong northerly. But the currents would never top .1 knots unless you were in a river, which we're not. Link below has some interesting graphs and data about seiches.

[url]http://www.geo.msu.edu/geogmich/seiches.htm[/url]

And if I can get away with it, I really do like to set the sails once a day and then leave them alone. But that rarely happens.

Couple reasons I like to have both GPS and knot meter -

a) redundancy, if one fails you have a backup
b) I can judge just how much I'm losing or gaining to tidal current by comparing the GPS speed to the knotmeter. Tidal current in the northern Chesapeake is usually < 2 knots, but sure makes a difference if you're trying to tack against it. I often base departure times around this.
c) I use the knotmeter to adjust my speed coming into the dock and my slip. I've had a procedure in place for years that works well.

I must admit to having a GPS that came over on the Mayflower (Garmin GPS 12XL), and I might change my preferences if I had a newer unit.

I remove the paddle wheel every time I leave the boat as it will foul with muddy Bay critters in no time, no matter what I coat it with.