Position of Nasa Log transducer.

[JAYEL]

The Nasa Log paddlewheel transducer on my Trapper300 Fin keeler is just 4" to starboard and 4" back from the leading edge of the keel. It rarely works for any length of time after the lift in. Last Sunday out on the Thames estuary well heeled to port it suddenly burst into life, but did notice that on a port tack the log would read 3 knots, it would read 6 when heeled to starboard. Obviously the transducer is not in the optimum position, where should I be considering moving the paddlewheel to to get the best results. I could swop the skin fitting for the transducer with the skinfitting for the heads outlet which is about 1ft ahead and to starboard of the keels leading edge. Thoughts please. <font class="small">Code:</font><hr /><pre> </pre><hr />

 

[earlybird]

I don't think your transducer is in a good position. Forward of the keel LE would certainly more accurate, and reduce reading differences between tacks. Your heads outlet position sounds reasonable, except that it might have an inlet directly forward. The turbulence would affect the reading to some extent, but perhaps not too severely. However, it seems unlikely that the transducer position explains the intermittent operation. I would think that the paddle-wheel might be sticking.

 

[Stemar]

I'm inclined to agree with Earlybird. Swapping is well worth a try.

It's the nature of log transducers to be temperamental and they do need to be pulled and cleaned from time to time - shrimps seem to find the paddlewheel housing an extremely des res! For that reason, they need to be easily accessible so you're confident of pulling the transducer out and getting the cap on, then getting the cap off and the transducer back in quickly enough that the amount of water that comes in won't be a problem. If it's too awkward, you won't dare touch it between liftouts and you'll only have a working log for the few weeks of your season!

BTW, I thought I might be able to avoid all the water by pulling mine while dried out on the Portsmouth Harbour mud. Not a good idea. /forums/images/graemlins/tongue.gif

 

[bobgoode]

An of-centre impeller just in front of the keel will always under-read on one tack and be over on the other, your figures are about right! On one tack it's on the low pressure side of the keel and the high pressure side on the other. In a n ideal world the log should be on the centre line and well ahead of the keel but that can lead to other problems.

 

[JAYEL]

Thanks guys for that, as it happens I keep a number of clean spares close by, and replace as needed, however I do have to rotate them in the holder to kick start the paddlewheel,and more often than not they seem to perform better with them. positioned back to front and facing the keel by about 10 degrees. I think the next step will have to be an ultrasound unit, More expense!

 

[chewi]

The scrapyard would be my suggestion. I know of two yachts in my small immediate circle of acquaintance where the NASA skin fitting has snapped off at the hull line, mine needed softwood bungs to get to shore, the other thankfully fell off whilst ashore. Apparently Sikaflex embrittles the plastic skin fitting, but lots of boats have these and many will hacve sealed them that way.

Never again for me.