jumpy depth gauge

[pvancorenland]

My depth gauge seems accurate at low RPMs going forward, up until about 3000 RPM. Above that, it goes to 1.5ft (which is the minimum I believe). When it does that, I have to back off just a bit (100-200rpm) to make it read fine again. There are rpm values above 3000 where it seems to read out just fine.

My sounder is mounted in the engine compartment, about 1ft forward of the transom.
My outdrive has dual props that are in like new shape, and my skeg and anti-cavitation plates are in great shape as well, so I don't expect any unusual air near the sender from cavitation.

Is this expected behaviour?

Boat is 2002 Cobalt 226 with VP 5.7GSI and DP-S outdrive

 

[Fireman431]

Re: jumpy depth gauge

You are probably getting air under the transducer, but it would be from the water moving under the hull, not from your props. When up on plane, the water passing under the hull gets air trapped in it and if you're using a thru-hull transducer (no hole in the hull), your just getting erratic readings.

 

[pvancorenland]

Re: jumpy depth gauge

The boat has no fouling on it in the area where the transducer sits.
Is this behavioir supposed to be the case?