Dangerous Icing and Instability
by Ann Backus, MS

In the icing conditions shown here the life raft is frozen shut. If this were to happen at sea, it is very possible that should the vessel capsize or any accident happen that requires abandonment of the vessel, the life raft would not deploy in time to be of use. The EPIRB in these conditions would undoubtedly be welded to the pilot house by ice. Photo: Kevin Plowman, USCG

As those who fish in winter conditions know, it is difficult and dangerous to try to chip away ice whether from rigging or deck. Mallets and other blunt tools are recommended above sharp tools such as pick axes and shovels in order to avoid damage to the boat and the instruments such as radar. In addition, it is often recommended that those who climb to de-ice, tie-off or wear a harness to protect from falls. The US Coast Guard and naval architects recommend that vessels in this condition seek warmer locations such as shelter from the wind, steam more slowly and/or proceed downwind rather than into the wind to minimize the amount of ice accretion.

Icing And Fishing Vessel Stability
According the Fishing Vessel Stability Training Program (December 6, 2003), a CD produced by the Society of Naval Architects and Marine Engineers Ad Hoc Panel #12, “stability is the ability of a fishing vessel to return to its upright position after being heeled over by any combination of wind, waves, or forces from fishing operations” The stability of a vessel is determined by the interaction of two forces, the center of gravity (COG) and the center of buoyancy (COB). The center of gravity is defined as the center of the weight of the vessel; this force pushes down. The center of buoyancy is the geometric center of the underwater portion of the vessel and its contents; this force buoys or pushes up.

A technical discussion of these forces includes the description of the righting arm which is the horizontal distance between the COG and the COB. In an upright vessel the COB is directly under the COG. As the boat heels, the geometric center of the underwater portion of the hull and its contents, the COB, shifts to the outboard side and the COG does not change unless water comes aboard to change the weight of the vessel. The length of the righting arm (distance between the COG and COB) increases, and the buoyant force/righting arm counteracts the heeling forces and attempts to right the vessel. However, at some degree of heel that is specific for each vessel, the COB moves back toward the COG, passes directly under the Center of Gravity, and continues to the inboard side of the COG, creating a negative righting arm value at which point the boat capsizes. The movement of the COG back toward the COG is rapid and the vessel quickly becomes unstable.

The running lights of the vessel are nearly 100 percent occluded by ice. So whether underway or stationary, this vessel is a hazard to other traffic and a hazard to the crew. Several weeks ago Kevin Plowman, Commercial Fishing Vessel Safety Examiner, at the US Coast Guard Marine Safety Office in Portland, Maine, provided these photos which he took during a dockside exam. Photo: Kevin Plowman, USCG

Freeboard is “the vertical distance between the waterline and the highest watertight deck” (p. 8). For purposes of discussion, let’s assume that the change in freeboard during heeling is an indication of the movement of the COB during heeling. As freeboard decreases the COB moves to the outboard side – except, as noted above — at some point the COB moves quickly back toward the COG. When that happens, the potential for capsize increases dramatically as the length of the righting arm decreases and eventually becomes negative. Maintaining sufficient freeboard is critical to avoiding capsize. Any condition which decreases freeboard has the potential to increase the likelihood of capsize.

In conditions of icing the center of gravity is raised, because the weight is carried higher on the vessel. This creates a less stable situation. Picture a pole vaulter: The higher she is on the pole the more momentum she has for swinging over the bar. Not only is the COG higher under icing conditions, but the freeboard is reduced even in the upright position because of the increased weight of the ice. The two conditions: high COG and decreased freeboard create conditions in which the vessel can rapidly become unstable during heeling. The high COG increases the tendency to heel, and the decreased freeboard results in the flooding of the deck and possibly of watertight compartments above the waterline, thus causing the COG to move to a more outboard position (where the bulk of the onboard water is). The righting arm runs out to its maximum outboard position and back again, across the newly moved COG, very rapidly resulting in capsize at heeling angles that would not be a problem for the boat were it not loaded with ice.

The situation is further complicated by the fact that icing is often accompanied by other de-stabilizing conditions. The table below lists what happens to the center of gravity and the freeboard in various situations. This table is a greatly simplified abstraction of information in the Fishing Vessel Stability Training Program CD. The importance of the table to this discussion is that icing, in combination with any condition that raises COG and/or decreases freeboard, presents an extremely dangerous condition.

Stability must be preserved throughout the trip of the vessel. If tons of product are expected to be on board, then the weight of that cargo and whether it is stored below decks in small compartments to minimize the free surface effect or above decks where it raises the COG must be taken into consideration in order to avoid having the loss of freeboard and/or rise in COG contribute negatively to various wind, water, weather and fishing conditions that may arise during the trip.

The Fishing Vessel Stability Training Program CD offers the following 10 suggestions (p. 50).

Vessel Operations And Seamanship
1. Ensure all freeing ports in the bulwarks are kept clear for rapid draining of water on deck.
2. Keep bilges pumped to minimum levels to prevent free surface effects and minimize excess weight.
3. When heavy seas are encountered,
-Suspend all fishing operations.
-Secure all fishing gear and cargo to prevent shifting.
-Head the vessel into the seas to minimize water on deck and the vessel’s motion.
4. Avoid operating in icing conditions. If icing cannot be controlled, leave the area immediately for shelter.
5. Avoid operating in following or quartering seas. They can cause heavy rolling or difficulty in steering especially when riding on the wave’s face leading to broaching.
6. When towing fishing gear, always use extreme caution. Tow the gear directly off the stern to minimize capsizing forces on the vessel.
7. When lifting, always use extreme caution. Whenever possible, secure the load to prevent shifting during the lift.
8. Minimize the time lifting heavy fishing gear over the side, such as retrieving purse seines. Perform these lifts only in sheltered areas.
9. Maintain effective means for quickly releasing any towed or lifted fishing gear in the event the gear snags an obstruction or the fish dive to prevent capsizing the vessel.
10. Carry an effective sea anchor to keep the vessel’s bow into the seas in the event of propulsion failure or loss of the ability to control the vessel’s heading.