Free surface effect

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Free Surface Effect

Free surface effect (FSE) is the virtual decrease in GM caused by the shifting of liquids (see free surface moment). Free surface effect is measured in length units (meters or feet), and can be subtracted from GM to correct the vessels righting lever for the effect of the fluids. The effect is caused by fluids moving towards (into) the direction of a vessels heel or trim.



FSE can be calculated by:

FSE = (i ÷ Δ) x (ρ)

Where i is the second moment of inertia of the tanks waterplane (about the ships centreline), in m4, Δ is the ships displacement in tonnes, and ρ is the fluid's density.

Dangers of FSE

Free surface effect can quickly compromise a vessels stability (particularly RORO vessels with long, unbroken vehicle decks; see MS Herald of Free Enterprise). The result is a rapid capsize.


Some collections of solids can exhibit a free surface effect, particularly grain and small fish like herring. These aggregates can be quite dangerous, as the effective ρ can be quite high and static friction can delay the recovery of cargo levels in rolling ship.

Minimizing FSE

Free surface effect can be eliminated by pressing (filling to 100% capacity) a tank, or emptying it (0%), making it's effective i = 0. As i is particularly breadth dependent, the subdivision of tanks longitudinally decreases the FSE by a factor of 1/(n2); that is, dividing a tank in half results in a FSE of 1/4 the size (the culmative i's are smaller than with one large tank). NOTE; Wash bulkheads do not provide a reduced FSE, as though they slow sloshing, they are not a barrier to the terminal settling of fluids in inclined tanks. Fluids need not be in a tank to produce a FSE, i.e. green water on decks. To this end, breakwaters may be fitted to decrease water shipped from the bow, and freeing ports or scuppers are mandatory in bulwarks. During firefighting operations, a burning ship may have holes drilled through decks to allow incoming foam or water to drain into the bilges (reducing both FSE and VCG, and thus the likelyhood of capsize. Downflooding ducts may be fitted for the same purpose).

Using FSE

The free surface effect can be used to reduce a vessels tendency to roll, if the induced FSE is out-of-phase to the vessels roll period. Tanks designed for this purpose are known as anti-roll tanks, however they present additional dangers to the operation of the vessel if not carefully designed and monitored.

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