Vessel stability made simple
Lookout! Issue 34, December 2015
The basic forces that create or reduce stability are the same for any vessel. Stability is determined by the force of buoyancy provided by the underwater parts of a vessel, coupled with the combined weight of its hull, equipment, fuel, stores and load. These forces can also be adversely affected by the prevailing weather conditions and sea-state.
Understanding the factors that influence stability will assist skippers to make the right decisions and take the right actions to keep their vessels safe.
Planning for stability
The stability of a vessel changes throughout a trip. It is important to be aware of how stability can change and to plan ahead for the worst-case scenario.
Different stability situations include:
- leaving port with full fuel and stores, but no fish
- being at a fishing site with a full catch
- coming home with a full catch and not much fuel or stores
- coming home with a small catch and not much fuel and stores.
Factors such as full fuel tanks normally being low down in a vessel and having some catch loaded in the hull can help stability, while reduced fuel, fishing operations, and rough weather and sea-state can reduce it.
Any significant hazard needs to be formally identified in a vessel’s onboard safety system, and measures put in place to avoid or reduce the risk to the vessel and its crew.
MNZ strongly recommends vessels are fitted with an inclinometer. These are not expensive and help raise awareness of stability issues for all people on board.
How to maintain a vessel’s stability
1. Keep weight low
New equipment added higher up on a vessel, or replacing gear with heavier equipment raises the centre of gravity and reduces the boat’s stability. Less catch can be loaded safely and a smaller wave and/or lower angle of roll will cause the boat to capsize.
2. Avoid overloading
The heavier the load in the hold, the lower the vessel’s freeboard and buoyancy will be. Loading extra catch on deck lifts the centre of gravity, reduces the freeboard and makes the vessel more top heavy.
Overloading a vessel before motoring home is highly dangerous in a rough sea.
3. Keep excess water off
A wave on deck or downflooding can add tonnes of extra weight and produce a strong rolling force (known as ‘free-surface effect’). The extra weight drastically lowers freeboard, raises the centre of gravity and, when the water shifts, tries to roll the boat over.
Reduce the risk by:
- avoiding conditions where breaking waves or following seas could swamp the decks
- keeping freeing ports open and unobstructed while at sea
- ensuring doors and hatches are closed and secured when they need to be
- keeping bilges and melted ice to a minimum
- ensuring bilge alarms are working
- keeping fuel and ballast tanks either full or empty.
4. Secure the load
Stow a vessel’s load, gear or catch on the centerline, or balanced port and starboard. Secure it so that it can withstand wave impact, rolling and wind loading without shifting. Keep the stowed positions as low as possible.
Hazards for fishing vessels
Make provision in your vessel’s safety manual for each of these potential hazards when using stabilisers:
- lowering or swinging the arms into position, and recovering them afterwards
- deploying and recovering the plate, lifting surface or ‘bird’
- lifting the arms and plates
- having stabilisers deployed in a deteriorating sea-state.
Stow the plates, lifting surfaces or birds on deck when not in use.
Trawling, dredging and towing
In a rough sea-state or if the vessel is heavily loaded, the combined effect of these factors on a vessel’s stability may be dangerous:
- tow-line tension pulling the stern lower in the water, reducing the freeboard aft
- the load’s downward weight taking the vessel lower in the water
- tension on the lines, transferred through the towing point or blocks, dramatically lifting the vessel’s centre of gravity.
Fouling or snagging of fishing gear
With any gear stuck on the seabed, a vessel is extremely vulnerable to hazards such as reduced freeboard, steep angles and rolling forces from tension on the lines.
Pay close attention to the effects on the vessel’s stability when undertaking operations to free the gear, because of the extra loads needed to free the gear.
Lifting and pulling on board
If you are lifting a catch, its weight is not acting where the object is, but at the top of the lifting point, block or winch that is lifting it. A sudden jump in the vessel’s centre of gravity can be very dangerous.
This shift in the centre of gravity applies to stern lifters as well. Any winch, block, wheel or lifting point used on an A-frame at the stern of a vessel will transfer some or all of the weight of the catch to that point. Similarly, for lifting or winching at the side, the weight acts at the point of lifting.
Every lift is only as safe as the load lifted, the condition of the vessel and the sea-state, size and direction.