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Chapter 11
Vessel Stability – Warning Signs, Precautions
Of all accident types, founderings and capsizes caused by a loss of stability
are the most likely to lead to a fatality on the water. Many of these
accidents could have been avoided if operators took the necessary precautions
and observed the warning signs.
A well-designed vessel will resist capsizing or foundering in severe
conditions if it is operated properly. To reduce the likelihood of these
incidents, keep these rules in mind:
- Be aware of external forces – wind, waves, and water depth. Always
check the weather forecast before departure. Avoid rough weather conditions.
- Don’t overload your vessel. Be aware of the amount of weight
added to your vessel and available freeboard. Distribute the passengers
and cargo evenly.
- Make sure that all cargo is well secured and remains secure during
the voyage. Secure cargo below deck if possible.
- Partially filled water ballast and fuel tanks contribute to instability.
Free surface liquids must be contained so their influence will not upset
the balance of your vessel.
- Prevent water from entering the interior of your vessel by keeping
hatches, doors, and windows closed, as practicable, when underway. Regular
maintenance of gaskets and fastening devices will help to ensure watertightness.
- Any water shipped on board must be removed as quickly as possible.
Scuppers and drains must meet design criteria and be kept in good working
order.
- Open vessels and those with large well decks may be prone to swamping,
which may lead to sinking or capsizing.
- Adjust course, speed, or both as practicable to minimize vessel motion,
rolling in particular.
- Avoid sharp turns or turns at high speed when loss of stability is
possible.
- Salt water is denser than fresh water. Your draft will increase and
your freeboard will be reduced when leaving the sea and entering fresh
water.
Stability Warning Signs
- Observe the stability and roll of your boat. Make sure the vessel’s
movement and reaction to sea conditions is normal, steady, and safe.
- Check to make sure your boat is visibly stable. It should not be listing
to port or starboard or trimmed excessively by the bow or stern.
- Observe freeboard and check for flooding. A flooded vessel may appear
stable when it is in fact not.
- Has the cargo shifted? Make sure the load is well secured and remains
secure during the voyage.
- Make sure that bilge level alarms are operational. Unusual operation
of bilge pumps may indicate an excessive amount of water is entering
the interior of the vessel.
A combination of prevention efforts and awareness of the warning signs
of instability, along with operator knowledge, can accomplish a great
deal in reducing the number of boating fatalities caused by instability
and capsizing.
ABOUT STABILITY ^
Figure 11-1 – Stability
 Stability
refers to the ability of a vessel to withstand high winds and seas and
resist capsizing by returning to an upright position after being heeled
over. Many forces influence the stability of a vessel in the water and
each type of vessel reacts differently to heeling forces. Operators should
be aware of how the design and loading of their boat interact with external
forces of nature and affect their boat’s stability. A properly designed
and loaded boat should resist heeling forces when operated within its
environmental limits (see Figures 11-1 and 11-2).
Figure 11-2 – Heeling
Figure 11-3 – Added Load
 Adding
weight above a boat’s centre of gravity will change its stability.
If the centre of gravity is raised too much, the boat will become unstable.
As a result, less tilt is required to capsize the vessel (see Figure 11-3,
Added Load). Removal of weight from below the centre of gravity
also decreases stability.
Figure 11-4 – Capsizing
The most important factors in preventing a boat from capsizing are a
well-designed, maintained, and loaded vessel and an experienced operator
and crew. Preventing an unstable vessel condition and being able to recognize
the warning signs when such a condition does occur can save lives. You
should be on constant watch for loss of stability (see Stability Warning
Signs above).
FREEBOARD ^
Adequate freeboard is essential, so take care not to overload your vessel.
Freeboard is the distance between the water and the working deck of the
vessel. If the deck edge goes under water when the vessel heels, the danger
of capsizing is increased. An overloaded vessel will have too low a freeboard,
and the deck may submerge with even a light heel caused by wind or water
conditions.
Figure 11-5 – Freeboard
FREE SURFACE EFFECT ^
"Free surface effect" influences the stability of a vessel.
When a vessel with full tanks heels over, the tank’s centre of gravity
does not change, so it does not affect the vessel’s stability. However,
water on deck, liquids in holds, bilge water, and partially filled tanks
will cause a shift of the liquid with the movement of the boat. When this
happens, the centre of gravity also shifts, making the vessel less stable.
This "free surface effect" reduces stability and increases the
danger of capsizing.
A good operational practice is to minimize free surface effect by dividing
tanks with baffles and fluid cargo holds with bulkheads and by keeping
the number of partially filled tanks and holds to an absolute minimum.
Figure 11-6 – Free Surface Effect Cross-Section
MORE INFORMATION ^
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