It took a long time for us to realize that running our boat at normal cruising speed, seven to nine knots, in shallow water, (under nine feet) not only caused a distinct rumbling sound, serving as an excellent depth sounder, but caused the back of the boat to sink, so our swim platform submerged. We found it was common knowledge among tug boat captains, but it was a slow learning process for us. This slows the boat noticeably, another clue something is happening.
