Nasa Target 2 Depth Issues

[Pitterpatter]

Hi,

On our new to us boat, there is a Nasa Target 2 depth sounder. All seemed OK with it and I have watched the readout as the boat has dried out and noted that depths are fairly accurate. However, I recently noticed that the depth readings fluctuate with the engine running in reverse. At the weekend, digging in the anchor with about 2m under the keel, the readings got up to 33m. When the engine was put into neutral, the readings gradually returned to normal (probably over 20-30 sec).

Obviously this is not a huge issue, but it would still be nice to know how deep the water is regardless of whether we are going forwards or backwards. Has anyone else seen this issue and/or have solutions that might resolve it?

Thanks,

Paul

 

[theoldsalt]

ptucker

You don't give any info on the type of boat so it's difficult to be sure.

My guess is that the prop is pushing very disturbed/turbulent water possibly contining a large quantity of air bubbles under the hull. This confuses the depth sounder until you p[ut the engine in neutral. The turbulence/air bublrs gradually subside and the depth readings return to normal.

This can happen especially in motor boats if the prop is near the surface when air can get sucked into the water when going astern. It happens less in a sailing boat with a prop tucked well under the hull.

A similar effect can happen in rough conditions when a slamming hull traps air underneath as the bow fall off a wave.

 

[Pitterpatter]

Thanks. The boat is a Westerly Fulmar, twin keel. I haven't checked the bubble situation in reverse but will look next time.

 

[ianat182]

Yes this is a fairly common result particularly when say, motoring up the Hamble at or about a flooding or ebbing tide where back eddies and outgoing/incoming streams meet stirring up the bottom mud, readins then reverting to normal when in clear waters


ianat182