Water never compressed well
I think the extra drag on those vessels isn’t properly reflected in your curves because the hull is completely submerged. How much extra drag does the topsides, which are intended to never be underwater, add?
Earlier I remarked that the curve “lacks accuracy” but that I thought that the general idea that the effect of less cargo doesnot translate into a large difference in speed is still valid. The extra drag is the result of the extra weight of cargo taken on and as such is (partly?) incorporated in the curve.
The curve gives not more then a rough idea about the effect of the weight on the speed. It cannot be accurate as all parameters are unknown such as L, B, D, Cb, Pe, V, (extra) wetted surface, form drag, skin drag and more.
I like the “plug” concept. I think that is most of what’s going on here.
Deeper draft in a shallow river also involves increased bottom effect.
The increase in wetted surface and submerged cross section isn’t that great when you increase barge draft from 10’ to 11’, but it often makes a big difference in speed going up a shallow river. It can make a big difference in steering too.
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Absolutely, tugsailor