The Basic Theory of Added Mass

Visual Explanation of Loop Keel Added Mass

Definition

Added mass is the inertia added to a system due to the fact that an accelerating or decelerating body must move some volume of surrounding fluid as it moves through it, since the object and fluidcannot occupy the same physical space simultaneously. For simplicity this can be modelled as some volume of fluid moving with the object. The added mass force opposes forces, and acts similarlyto drag, although the two are only superficially similar

Normal Application of Added Mass in Sailing Yacht Design

Designers always allow an added mass component when they calculate the roll inertia of a yacht. This figure depends upon the mass and shape of the vessel. If you designed a yacht that had atotally circular cross section then it would have NO added mass component when rolling apart from that added by the rudder and keel. Old pilot vessels have large roll inertias as they combine aheavy displacement with a large lateral area of hull underbody, which entrains large amounts of water (added mass). This is one of the main reasons that these older vessels are seen as having akinder motion and being more seaworthy than more modern light displacement fin keel designs.

Additional Added Mass from the Loop Keel

The Loop Keel makes a yacht more stable by creating a bound vortex around the keel. All of the mass of the water in the bound vortex can be treated as if it had been added to the mass of the yachtand this in turn will allow you to change the behaviour of your yacht in different conditions. The important point is that the bound vortex around a loop keel artificially adds many times more addedmass than a standard hull and keel can naturally produce. In extreme conditions, with a normal configuration, you can add up to one third of the mass of the yacht again

The effect of added mass is easily proved with your proverbial 'pint of beer'. As the example below shows, if it was not for added mass, then a bubble of gas in a beer glass would startto accelerate upwards at 830 times the force of gravity. In reality the added mass of the beer around the bubble reduces this to about twice the force of gravity.