Stability! You need to know this!
Stability is something we all know something about or at least we should but many out there don’t seem to place enough emphasis on it.
Failing to think about stability has resulted in the loss of many vessel and sadly many crew members and passengers!
So…what are some of the factors affecting stability?
- Loading of equipment, supplies and other items on upper cabin tops
- Suspended weights (loads on cranes, trawl nets, etc.)
- Loads on fishing gear
- Free surface effect (all liquids in partially filled tanks have free surface
- Water on the deck
- Structural changes
Any of these things, and many others can and do impact on the stability of your vessel and in turn increases the potential for disaster.
Let’s look closer at each of these potential disaster causing situations
Loading of equipment, supplies and other items on upper cabin tops
So many people see the upper cabin (usually the wheelhouse) top as an ideal place to store all sorts of things, all of which add weight high up. This changes the vessels Centre of Gravity which, if too high makes the vessel unstable and can result in capsize!
When a weight is lifted by a crane or derrick, the centre of gravity of the weight will be immediately transferred to the point the weight is suspended from (the head of the crane or the end of the derrick or boom). This occurs the instant the weight is lifted and from that point on the centre of gravity will not change further no matter how high the weight is lifted.
Loads on fishing gear
When towing trawls or other fishing gear, the force exerted by the tow will be felt at the point of suspension, as shown in diagram below. This is the equivalent of a weight acting at the point of suspension.
If the point is high above the deck, such as occurs when towing from a boom end, then the movement of G1 towards the point of suspension may be large. This can have a detrimental effect on stability.
The same situation applies when gear is being lowered or lifted on board, using booms or powerblocks. If a vessel has good stability these operations should present no problems but, if stability is poor, then steps should be made to improve stability.
If gear becomes foul when towing, there will be two effects:
- Dynamic effect – the vessel will heel over because it will be still trying to move ahead.
- Static effect – as long as there is any strain on the gear, the circumstances will be the same as described above, i.e., the vessel will heel. The angle of heel will be less than that caused by the dynamic effect.
All strain should be taken off the gear as quickly as possible by stopping the engines and if possible, slacking away on the trawl winches. If necessary, stability should be improved before action is taken to free the gear.
Free surface effect
All liquids in partially filled tanks have a free surface, which is free to slop backwards and forward with the motion of the ship. This free surface effect can cause a serious stability problem if the movement of the liquid is not contained.
The vessel will roll slightly to a small angle of heel as a result of the wave forces. The internal forces of the shifting water in slack tanks then increase the list further as the liquid flows to the low side. If this causes the vessel to list so that its deck edge is immersed below the waterline, it could well capsize.
Free surface effect is at a maximum in tanks which extend right across the breadth of the vessel.
Water on the deck
If water is shipped on board, then the effect is three-fold. Firstly, a weight is added high up in the vessel, thus reducing stability. Secondly, that water has a free surface effect, which will further reduce stability.
Thirdly, the added weight causes the vessel to sink further in the water, thereby reducing freeboard, and reducing seaworthiness. Freeing ports are provided on deck, so that the water shipped on board can be cleared rapidly.
If a vessel is changed structurally, for example if a new wheelhouse is added or if an extra mast or winch is installed, the effect on stability is exactly the same as though these items were added weights.
Because structural changes are usually complex and old material is often taken off the vessel as well as adding new material it is a survey requirement that all of the vessel’s stability is reworked after structural changes have taken place.
Here’s our 6 top recommendations:
- At all times minimise what you store on upper deck cabin tops;
- Maintain a watch our all of your tanks (fuel, water, ballast and brine) to minimise the free surface effect. Only a completely empty or completely full tank will have zero free surface effect;
- Maintain awareness when you have weights suspend (e.g., cod ends, crane loads, etc.);
- Always be alert when trawling in the event of a hook-up;
- At all times keep freeing ports (scuppers) clear. They should never be blocked;
- If you make structural changes ensure the vessels stability data is updated.
If you’re considering making any changes to your vessel and it’s not going to be “like for like” then get an Accredited Marine Surveyor involved to ensure you meet all the requirements. By simply making changes and hoping no one notices you risk AMSA placing a Prohibition Notice on your vessel stopping it working!
How do you record all your maintenance?
You can do it electronically, in your SMS or in a Maintenance Log Book. Why not use our maintenance Log book, check it out here