The mechanics of what happens when mass moves
Stability is not fixed once a yacht leaves the berth — every movement of mass onboard changes it. Understanding why each effect occurs is what the examiner is testing; the figures alone are not enough.
Weight shifts
When a weight is moved onboard, the vessel's centre of gravity (G) moves directly toward the new position of that mass. This is the governing principle for all weight-shift problems.
Transverse shift — moving a weight to port shifts G to port, reducing the righting lever (GZ) on that side. The vessel takes on a list. Critically, this is a static change: the righting moment is reduced at every angle of heel, not just at small angles. A vessel that is already tender becomes dangerous when a large portable mass (a tender, fuel drum, or even a group of guests) moves to one side.
Vertical shift upward — moving a weight higher raises G. A rise in G reduces GM (metacentric height) directly: GM = KM − KG, so as KG rises, GM shrinks. A reduced GM means a sluggish, tender vessel with reduced initial stability. If G rises above M, the vessel has negative GM and will loll. Understanding this is why you stow heavy items low and central.
Vertical shift downward — lowers G, increases GM. The vessel becomes stiffer. Very high GM produces a short, violent roll period that is uncomfortable and potentially dangerous for crew and rig.
Freeboard and its effect on stability
Freeboard is the distance from the waterline to the freeboard deck. It matters to stability because it determines the range of positive stability — the angle at which deck-edge immersion begins and the downflooding angle are both directly linked to freeboard. A vessel loaded down with stores or fuel will have reduced freeboard, meaning deck-edge immersion occurs at a smaller angle of heel. This compresses the GZ curve: maximum righting lever and the angle of vanishing stability both reduce. The examiner will want you to connect low freeboard to a shortened range, not just a vague sense of danger.
Slack tanks
A partially filled (slack) tank introduces a free surface. When the vessel heels, the liquid shifts toward the low side, moving the centre of gravity of that liquid transversely. The effect on stability is represented as a virtual rise in G — denoted as GG¹ or the free surface correction — which reduces effective GM. The formula is: free surface moment = ρl × i (second moment of area of the free surface about its centreline), divided by vessel displacement. The key examiner points are:
- The correction depends on the dimensions of the free surface, not the volume of liquid — a nearly empty tank can be as dangerous as a half-full one.
- Multiple slack tanks produce additive losses to GM — each one independently reduces effective GM.
- The remedy is to either press tanks full or empty them completely, eliminating the free surface. Running tanks at 90–95% removes the free surface without allowing air compression issues.
- Subdividing a tank longitudinally with a centreline wash bulkhead reduces the free surface moment significantly (i is proportional to the cube of the breadth).