Question about Lowrance M68C GPS Receiver

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Depth finder problem

When i reach speed of 10-20 mph the depth indicator goes haywire and give depth ranges of 350 to 700 feet in water that is some where in the range of 5 to 20 ft. once you stop or slow back down the problem corrects itself. i have had the transducer moved twice as this is on a somewhat flat bottom 19ft skiff. my dealer says its a problem all flat bottom boats have, and i'm finding that hard to believe.........

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Re: depth finder problem

The problem is not so much the flat bottom, although they are prone to this problem. The way the sonic waves have to travel through the water is affected by turbulence, especially by air bubbles and cavitation, and by the transducer actually leaving the water, which happens frequently on flat bottomed boats. The best solution to this is to have the unit mounted through the hull so it is completely flush with the hull surface - often they are mounted with a surface flange on the outside of the hull which although thin still causes cavitation around the transducer at higher speeds. about 14 knots will usually interfere with the accuracy of the readings on most hull shapes - forward looking sonars perform usually better. Another thing to check is that the transducer is actually seating right in the housing, if it is not flush with the flange then you have problems - even half a millimetre is too much. check no muppet has painted the transducer with antifouling paint - that never helps, also check if there is a build up of marine growth on or near the transducer - a smooth hull will always keep your depth working until a higher boat speed. if the transducer is growing stuff itself then gently scrape it off - be really careful not to scratch the surface. There are now available non-sonic depth sounders, especially ulf radio signal devices, but I have heard no good reports about them, last i saw they were very expensive and maybe they experience other problems at high speed. there also have been towed devices available that may suit your needs better, although probably a lot of trouble in shallow waters. If you can borrow a waterproof video camera, you could mount it behind (not too close) the transducer, this would be a pain to do, but it may give you an insight into what part of the hull if any is causing the problem (tie the camera to the top of the boat securely in case the mount fails) Hope this helps, sorry but your dealer is not totally wrong :)

Posted on Mar 26, 2007

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Re: depth finder problem

Try adjusting the trim or get the boats nose up even to the point of porpoise (I think thats how you spell it) if the signal improves with the nose up you may need to lower the transducer more than the manufacturer recommends. the slight looking foward will also help. My current sounder works pretty well up to about 25kn

Posted on Jul 03, 2007

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