Interesting stuff, gents, and more than theoretical in my case, as I am dropping my 100 gallon x 2 SS water tanks from their position hanging beneath the deck to HDPE tanks, 4 x 50 gallon, sitting in welded, boltable frames atop the stringers either side of the engine.
This decision, which is largely intuitive, comes from noticing a certain tenderness at dock and a lateral roll I attribute to the elevation of the current tankage, plus a desire to break up the tanks into more easily maintained and accessed (from inspection ports on top) units. Also, the boat sailed obviously differently with near-empty water tanks than with full, and if one was draining faster than the other, you could detect a slight list.
I will also lower the hot water tank while I'm at it.
I lose "gravity feed" to the taps, which doesn't matter as a Flojet pressurized the water anyway, and I am going to dual feeds with foot
pumps for "at sea" use. I gain useful space above the tanks I can use to hang
line, chain, secured
anchors, and so on...but the main reason for this is that I feel lowering approximately one ton of water this close to the CE of the boat will stiffen her up a fair bit, and long, low, trapezoidal tanks (still with a couple of feet gap either side of the engine) should get the weight where I want it.
The addition of 4 8Ds just after of the mast above the middle keel
fuel tank completes the picture of apportioning "ballast" in aid of "comfort motion".
So these fairly academic and theory-based discussions are of great interest because they alert me to the wider scope of keeping a boat safe and seakindly, given the absolute design elements that can't be changed.
Sailing frigates used to regularly shift cargoes, sometimes to a significant degree, in order to attain an instinctual "ideal trim", which would manifest as speed and quickness in wearing and tacking. While modern boats are largely calculated to a higher degree of awareness of such details, they still affect performance in sometimes unpredictable ways.