Ground effect pressure wave

[Bidgood]

Today an interesting thing happend while cruising. We were running our Albin 30 FC in a well marked channel in Estero bay. (SW Florida where the water is still liquid and the air is 80 deg F.) The channel depth is very consistant at around 4.5 ft. I turned around a mark at cruising speed (18 mph) and found that I had little rudder control. The boat seemed to want to track straight and true despite the helm being turned hard starboard. I completed the turn and noticed that at the same time the depth finder blinked off and then reported that there was 22 ft of water. I slowed down to idle speed and watched as the depth finder wound down to the correct 4.5 ft. Since I knew the waters I tried successfuly to repeat the event.

I am suspecting that in the relatively shallow water the pressure wave between the boat and the bottom was the culprit. Any one have any thoughts on the matter?

[joe.baar]

Here's a set of photos taken in the Larkspur Channel leading to San Francisco Bay next to San Quentin. The vessel is a 200-foot high-speed catamaran with twin waterjet propulsion. You'll see how the vessel drags a lot of water as it accelerates down the channel, and the wave effects in the shallower parts, almost right alongside, are spectacular. Steering under these conditions is very difficult as you noted.