Log impeller magnet

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NorthUp

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Hi all,
I have an old through hull impeller log fitted to my 30-odd year old 26' yacht. There were no electrical pulses from the log on testing, and it appears that the magnet has disppeared from the impeller- I am left with a 2mm dia hole, and about 20mm long throught the impeller. The log display is badged Sowester, now long gone, and looks to be a different physical size , although similar design to the NASA version.
Any suggestions for a new magnet before I give up and buy a new one? Will it be a magnet or just a steel or iron rod thats gone- I would expect a magnet is needed to provide a sufficient pulse for the electronics, but have never pulled one apart before.
thanks
northUp
 
Had a similar problem with my Nasa log a couple of years ago - the impeller on these have two magnets - One had completely corroded and disappeared - the other was still there so the instrument read half speed - had me confused for a while -

It was definitely a magnet - why not try butchering a fridge magnet to see if you can "repair" your impeller - they're pretty cheap and you really have little to lose if the alternative is a new sender?

JuSW
 
A very easy way to wreck a log impellor is to hit it with the high pressure jet when cleaning off - Revs it to about 200kts at which point the magnets fly out!

Should be repairable if you can finbd a magnet the right size to fit the hole.
 
I doubt the material used ina fridge magnet can be shaped to a cylinder to fit in the hole. Perhaps a better bet is a suitable sized drill bit shank cut to length and pressed in. You will then need to magnetise it by stroking it with a good magnet or by puting a coil around it and giving a burst of high DC current.
The mmagnet is required usually to operate a reed magnetic switch to produce the pulses. This will be embedded in the body of the transducer.

I got more life out of mine by replacing the reed switch with a hall effect transistor and amplifying circuit. It all needs a bit of extra circuitry.

good luck olewill
 
Most impellers are manufactured by Airmar. If you go onto the Airmar website there is a look-up matching transducers to instruments.
From that you should be able to find the right service hit which includes wheel, O-rings etc.
I managed to get a kit for a very old transducer by referencing the label attached to the cable.
 
Update-
The fridge magnet was a soft plastic which can be cut to shape with a blade, but it didn't work.
I tried a moving a powerful magnet over the coil (4 Hz/ knot!) with no effect, although there is 300 ohms across the coil, so its probably a duff coil.
I've sent for a new assembly from NASA, only £35 with P&P.
Given the age of the hull fitting plastic, its probably not a bad move.

Thanks for all the suggestions.
 
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