Wednesday, November 6, 2013

Sucking Bottom

I never thought of my PDQ as a freighter, but they have a few things in common. I keep expecting--hoping--her to behave like a beach cat, but she's not.

Something we all keep any eye on, at least occationally, is maximum speed under power. It 's an indicator of whether the engine is tuned well and the bottom is clean. If she reaches top speed, we know those things are fine.

Yesterday as I was motoring back in I pushed the throttle wide open and she went... 6.5 knots. I've been seeing 7.4-7.5 knots since I extended the transoms, so something had to be amiss. I looked over the stern at the transoms; they were 6 inches under water unde conditions that should have them just above! Am I taking on water?

And then it occurred to me that I was sprinting across 4.5 feet of water and the hull was feeling the bottom, both in terms of excessive stern wave and reduced pressure under the deepest part of the hull. In fact, I was much closer to the bottom than my nominal 3.5 feet of draft would suggest. I suppose I might have touched, though I didn't feel it. Perhaps sprinting across shallows is not so smart.

As soon as I moved into the channel--8 feet--the speed jumped to 7.5 knots and the transoms came up. I think that's fooled me a few times over the years, wondering what I'm dragging.


Pilots bring freighters into port often reduce speed in shallow areas to avoid what is called squat. It is more than simple caution and is calculated before entry.

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