Dodgy log - ideas?

[Twister_Ken]

First sail last weekend after being released from doctor's order. Impeller has been withdrawn all winter. Checked it spun freely, popped it down its hole, set sail. When moving it showed zero speed until it leapt immediately to 1.3kts (which I estimate was more like 3kts in real life). Thereafter, whenever the indicated speed dropped below 1.3kts it went straight to zero, despite the fact that we were very obviously moving through the water. Withdrew the impeller and checked - spins freely, no growth, no dead shrimps. Airmar impeller, TackTick log. Any ideas?

 

[Iain C]

My NASA one was doing exactly the same. Apparently the water can get into the plastic and the metal parts go rusty. I put a new paddlewheel on and it's fine now...

 

[malcp]

You could check the speed reading whilst you are spinning the impeller by hand to check it reads more than 1.3kts. If it does then the problem is not likely to be electronic, but to do with external fouling around it on the hull once inserted in the tube. This assumes the boat was in the water over the winter?

 

[GrahamM376]

Does your impeller have one or two magnets? If two, then it sounds as if one is missing.

 

[Twister_Ken]

This assumes the boat was in the water over the winter?

It was.

 

[LadyInBed]

First sail last weekend after being released from doctor's order. Impeller has been withdrawn all winter.

Is that what the Doc was sorting for you?

 

[malcp]

Another possibility is that if the paddlewheel has some lateral float on it's axle then it might be free running when held in one orientation (paddlewheel at one end of it's float), but not so free when held the other way up (paddlewheel at the other end)? I've successfully used kettle descaler to dissolve any hidden calcium deposits if it's a bit sticky on it's axle.

 

[ITH]

Ken, had similar problem; log not working but would register when spinning impeller by hand, but no joy when back in the water. Raymarine suggested replacing the paddle wheel, saying: "it is also possible that the Paddle wheel is failing and may not be producing a strong enough magnetic field for it to work in the water."

Replaced paddle wheel and problem sorted. Mark you it was a hefty (£30'ish) amount for the replacement.

 

[macnorton]

I covered mine in Castrol underwater grease (black and sticky) and it seems to work ok now.

 

[LadyInBed]

You may have a build up of weed and / or barnacles in front of the impeller as the boat has been in over winter.

 

[Blueboatman]

Surely that little paddle spinning round slows the boats performance anyway?

Seriously, mine under reads so , if inclined, I recslibrate it each year using....tadaah, drumroll, ....walker trailing log. Soak the paddle fitting in freshwater and a drop of bleach, then attack with old toothbrush, if it really matters?