ill-considered modifications before she even leaves harbour. While at sea, stability may be lost through water ingress and ice growth on the superstructure may impose additional top weight.
In many
cases, it will be a combination of factors which will lead to the onset of unstable conditions and, possibly, the loss of the vessel. When working out the maximum load allowed, the marine surveyor must consider weight creep – a problem which is also known as stability creep. Owners also should be warned to guard against such unaccountable gain in weight of a vessel with time; a gain which is due perhaps to rust formation, timbers becoming saturated, coats of paint being added, extra spare parts, old spare parts kept just in case, sludge in ballast and oil tanks etc.
It has
been estimated that a vessel can gain as much as one per cent in weight and an equal reduction in KG for each year of its life with a consequent reduction of freeboard and loss of transverse metacentric stability.
It is good practice,
therefore, after a rolling test also to carry out a light weight check.
Light Weight Check
In order to carry out a light weight check it is necessary to measure the vessel’s freeboard accurately when she is floating at rest in still water. To be accurate, the measurement has to be carried out directly on at least two clearly identifiable places along each side of the boat making a minimum of four measurements in all. The boat has to be in light condition i.e., with no fuel, stores, fresh water or personnel on board. Direct measurement with a steel tape will be found difficult as the slightest wind will cause the tape to flap about making the process next to impossible.
It is recommended,
therefore, that the marine surveyor make up a stave such as that illustrated in the Figure 1 below. The angle bracket is screwed to one side and the metre long steel rule screwed to the adjacent side. The bracket can then be rested on the vessel’s deck or rail at the point of zero sheer and the height of the
deck at side line at that point above the water read off directly. A large section of a translucent plastic bottle cut off and placed round the stave at water level will reduce the effect of any wavelets or ripples to nothing. To use the stave correctly it should be when the vessel is moored but afloat with the marine surveyor standing on the wharf. The offside measurement should be taken with the marine surveyor working from a small boat. Using the stave while standing on the deck and leaning over the side will give a false reading. The freeboard should be read on both sides of the vessel.
If the vessel has a load line
mark, then the freeboard to that line on both sides should be taken and recorded as well as the actual freeboard at the time of the survey.
It is further recommended that a rolling test and a lightweight check be carried out at a pre-purchase survey and a light weight test every five years thereafter and the results recorded in a formal report handed to the owner. A typical such outline report is shown below.
50 mm x 50 mm Hard Wood Stave
Steel Shelf Support Bracket
Deck at side line Steel Rule 1.50 m
Readings, though usually taken in metric units, may also be taken in Imperial units or, better, both.
Figure 1. Freeboard Measurement Stave
46 | The Report • March 2018 • Issue 83
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