Pontoon effect

The pontoon effect refers to the tendency of a vessel whose flotation depends on lateral pontoons to capsize without warning when a lateral force is applied. The effect can be sudden and dramatic because the vessel is stable and self-righting as greater lateral force is applied, up to the point that the pontoon(s) on one side of the vessel are completely submerged. At this point, stability suddenly disappears and the vessel rapidly capsizes.^[1]

(The same term can also arise when describing a design in which the attributes of a pontoon are created without using explicit pontoons—when a design effectively incorporates pontoons. This page describes the specific phenomenon described above.)

The pontoon effect is theoretically possible whenever the vessel's entire weight exceeds the buoyancy of the pontoon(s) on either side. However, the pontoon effect is much more likely in vessels with a high center of gravity and low or non-existent displacement other than the pontoons.

A pontoon vessel such as a catamaran floats in a level position when the center of gravity of the entire vessel (including its load) is above the center of buoyancy. This is the opposite of the case in a traditional or displacement hull vessel, which derives positive stability from having its center of buoyancy above the center of gravity. If the pontoon vessel tips, it will remain stable as long as the center of gravity does not move further to the side than the center of buoyancy is moved by the change in the depth (and displacement) of each pontoon. Under these conditions a "righting force" (a turning moment) acts on the vessel to push it back toward the level position.

However, if the center of gravity is high relative to the width of the vessel, and the pontoons on one side are unable to bear the vessel's complete weight, the lateral movement of the center of buoyancy will be restricted. Even a relatively small lateral force can move the center of gravity further to the side than the center of buoyancy can go. At this point, the righting force will disappear, replaced by a turning moment in the opposite direction. This can capsize the vessel at the point at which one pontoon is completely submerged.

When using twin lateral pontoons, each pontoon should have enough buoyancy to bear the load of the entire vessel on its own. If the vessel is so heavy that either pontoon is mostly submerged when no lateral force is applied, it will be vulnerable to the pontoon effect. If sufficient lateral force arises (such as wind or shifting load), the vessel can tip enough to submerge one pontoon. At this point, the sunken pontoon will provide no further buoyancy to right the vessel. As the center of buoyancy cannot move further to that side to match the center of gravity, that pontoon will continue sinking. The tipping angle will increase until the vessel capsizes. This can continue until the vessel inverts completely with the pontoons again floating on the surface but the rest of the vessel underwater. At this point, the upside-down vessel will be highly stable. If, on the other hand, the vessel is designed and loaded so that each pontoon can support the vessel's entire weight (plus any lateral forces that arise like wind), the center of gravity cannot move transversely beyond the center of buoyancy at the most extreme tipping angle, and the pontoon effect cannot occur.^[2]

Note, however, that this is not the sole effect to be taken into consideration when assessing the likelihood of capsize. The change in hull windage as the vessel heels is also important. In the case of a trimaran designed for cruising, with solid wing decks (as opposed to racing-type designs with mesh or open wings), those with wide-beam floats able to support the whole weight of the vessel are more likely to capsize than those with narrow-beam floats of lesser buoyancy that can be submerged as the vessel heels. As the wide-beam float comes to take the entire weight of the heeling vessel, the centre hull lifts out of the water; this exposes the entire area of the underside of both wings to the wind, and also increases the turning moment of the wind force on the weather wing. There is now a considerable overturning force due to the wind and the vessel is very likely to capsize. In contrast, a trimaran with narrow-beam floats will simply submerge the lee float, exposing only the weather wing and that with a lesser moment. In practical sea situations the windage effect is greater than the buoyancy effect and so cruising trimarans with highly buoyant floats are more, not less, likely to capsize than those with less buoyant floats.^[3]

A trimaran is best stabilised not by adding buoyancy on the lee float but adding weight to the weather float. This is the basis of the "cool-tubes" stability system invented by Tristan Jones and L. Surtees. A large-diameter pipe closed at the aft end is attached to the keel of each float and fills with water. While it remains submerged buoyancy forces cancel the weight of the trapped water and the weight of the tube is effectively zero. But if the boat heels enough to bring the weather float out of the water, this effect no longer operates and the water provides a heavy ballast weight on the float, creating a righting moment.^[4]

In the abstract sense, the principles at work govern the stability of all boats and ships including those without lateral pontoons. See angle of loll and metacentric height.

References

1. Kinetic Sculpture Racing Course (see "Water" chapter of Kinetic Textbook for explanation and case study, retrieved 2008-03-02
2. What is the Pontoon Effect?, retrieved 2008-03-03
3. Clarke, Derrick (June 1969). Trimarans. Coles. ISBN 0229638899.
4. Jones, Tristan (1998-10-25). Outward Leg. Sheridan House. ISBN 1574090615.