Providence is probably right, It can happen that the Y coordinate along the plane of symmetry may be very close to "0" but not exactly "0", and this creates a leaking point and the volume is not considered as closed.
It may happen, too, that the volume's top is not closed (by a deck), or the topping is not well closed (Note: if your ship has no deck, or if there's a hatchway, you must close it, even if it's just with a transparent tarpaulin).
When you're in WIREFRAME display, there's a button enabled to get an other kind of display: "LEAKING POINTS". It highlights the breaches in your model.
Ken ar c'hentañ!
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