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Commercial Fisheries News
Volume 34 Number 8
April 2007
Power of water: Effects on stability
In this, my third article on flooding, I want to talk about factors that can influence a vessel’s stability. (See previous articles at <www.fish-news.com/cfn/editorial/FV_Safety.html>.)
By being aware of how actions affect stability, an operator can take precautionary measures to improve or maintain his vessel’s stability.
Operators also should be aware that these effects can be cumulative, meaning that when added together, they can reduce a vessel’s stability to dangerous levels.
A vessel can be designed with a substantial amount of righting energy and a broad range of stability. Yet, these design features may be all for naught if the operator doesn’t maintain the watertight integrity of the vessel and allows premature downflooding to occur.
Downflooding is defined as the entry of water into the hull that results in progressive flooding and the loss of stability. Potential downflooding points include: doors into engine rooms and crew quarters; hatches into fish holds or lazarettes; portholes; and any other openings through which water can gain entry.
Although the hull has significant righting energy available, it cannot be fully used if the vessel takes on water. The importance of closing off openings, particularly in heavy weather, can’t be overemphasized.
If operators make it a practice to maintain this watertight integrity, it really does become a habit, and that can significantly improve your chances of avoiding a catastrophic flooding situation.
Water on deck
It’s best not to have water on deck during day-to-day operations. However, in heavy weather it’s hard to avoid. Excessive water on deck can degrade the hull’s ability to develop righting energy.
Water on deck has four detrimental effects on the stability of the vessel:
It causes a rise in the center of gravity, similar to moving the load up on deck;
It increases displacement, increasing draft and probably causing trim;
It contributes to free surface effect (shifting fluid); and
It can amplify the rolling motion of the vessel, which may result in capsizing.
Do what you can to keep water off the deck. Possible actions include: decreasing trim; increasing freeboard; installing freeports and scuppers; and changing course and/or speed.
If you take water on deck, ensure that freeing ports and scuppers are unobstructed to maximize drainage. Crewmen must be aware of the function and location of these freeing ports and scuppers and keep them clear of debris.
It is dangerous to lock scuppers in a closed position at sea.
Free surfacing
The term free surfacing refers to the detrimental effect that a partially filled tank or compartment can have on the stability of a vessel. This effect is due to the shifting of liquids as the vessel heels and results in the rise in the center of gravity.
A half-full tank or fish hold seriously reduces the righting energy developed by your vessel. This reduction can be significant in the case of a large compartment, but the effects of several small compartments are cumulative and must be considered in total.
Operators should be aware that the free surface effect is controlled by the width of the tank or hold much more than any other factor, including length, depth, or volume of the tank or hold. The free surface effect is also greater for a half-filled tank or compartment than for other fill levels.
In an operational sense, free surface can be avoided or reduced by working with a minimum number of partially filled (slack) tanks – fuel, fresh water, refrigerated seawater – at any one time.
As much as you can, maintain your fish tanks completely empty or filled to overflowing at all times. The act of filling or emptying any large tank at sea causes a loss of stability during the transition period until the tank is either filled or emptied.
Catch that is free to move about can cause a similar effect. Penboards, which divide a fish hold up into smaller compartments, are necessary to reduce this effect.
Always remember to never expose a crewmember to free surfacing to prevent injury and/or drowning. I always recall this example, cited by an instructor to why this is so important.
If a crewman is standing in a tank 18' wide with 3' of water in it and the boat experiences a 12° roll, the water’s force is equivalent to a 300-pound man running 10' and hitting that crewman at the knees.
It’s an example of the amazing power of water and the need to respect it.
Fred Mattera
NESTCo
A qualified Coast Guard-approved marine drill instructor, Fred Mattera is the owner/president of North East Safety Training Co. (NESTCo), which conducts fishing vessel drills and inspections and basic safety training workshops.
Mattera has been a commercial fisherman since 1972 and the owner of the Point Judith, RI-based 84' freezer trawler Travis & Natalie since 1984.
He also has been the president of the Point Club, a fishing vessel mutual insurance group, since 1998 and has served on the board of directors for Sunderland Marine Mutual Insurance Co., the principal underwriter for the Point Club and more than 2,000 US fishing vessels, since 1998.
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