In the Figure 5: b = half breadth of the cockpit
hDF θDF
Both b and hDF
m
= down flooding height at cockpit m = down flooding angle
degrees
by taking the width of the cockpit and dividing that by 2. θ is obtained by dividing hDF
that angle and then consulting a pocket calculator.
There are no rules covering privately owned narrowboats but, the nearest of the published rules are those for small commercial motor vessels, EU Category C waters it is recommended that they should be used as a guideline.
They are: Minimum metacentric height
Angle of maximum GZ value at least Maximum GZ value should be at least
Area under GZ curve to 30° Area under GZ curve to 40°
Area under GZ curve between 30° 40° Minimum down flooding angle
350 mm 25°
200 mm at 30° or greater 0.055 m.r 0.090 m.r 0.030 m.r 40°
A range of positive stability of at least (AVS) 60°
Most narrowboats cannot reach any of those criteria, except possibly the first and the third, hence the reason why they are confined to use on EU Category D waters.
The usual recommendation for down flooding angle is for the minimum to be 40o for a sea going boat. Again, a narrow boat is rarely able to reach that recommendation, but the author considers it to be good practice, when on a survey, for the marine surveyor to measure the down flooding heights and angles and report them. He should also measure the freeboard both sides and report both.
Strictly, the vessel may also down flood over a forward cockpit but, as the narrowboats usually trim by the stern, the after cockpit has a small down flooding height and angle.
It is good practice to measure and report both
forward and aft. Because of the trim, the measurements should be taken at the after end of both cockpits.
If possible, the marine surveyor should also carry out a simple rolling test on the boat and report the results. See Appendix 2.
N.B.3. The marine surveyor should also note that narrowboats are single compartment vessels. That is, they have a single bulkhead at each end of the main cabin neither of which are strictly water tight. It takes, therefore, only the single cabin between those bulkheads to be breached open to the sea for her to plunge sink.
The dangers of overplating an area of the hull have been discussed elsewhere and will not be considered further here except to state that, bearing in mind the Law of Unconsidered Consequences which is so often overlooked, one of the side effects of overplating is the changes that the practice have on the boat’s transverse metacentric stability. The extra plating adds approximately 7 kg per square metre of area per mm of thickness to the hull and that has a deleterious effect on the height of the centre of gravity, it also alters the metacentric radius, the height of the centre of buoyancy, reduces the area under the statical stability curve, the maximum GZ value and the angle at which it occurs, reduces the AVS and also reduces both the down flooding height and angle.
If carried out, it should,
therefore, only be done with careful thought and with such effects taken into account.
are easily measured. B can be measured by b to obtain the tangent of
There are no rules applying to narrowboats that state what freeboard should be obtained but, again it is good practice, to compare the measured values to those proposed by the IIMS in the Unit 27 of their Surveying Diploma and repeated here verbatim.
• The freeboard is to be measured at mid hull length from the top of the side deck to the still waterline and should be at least 530 mm for a vessel 12 metres hull length and 685 mm for a vessel 22 m hull length. Freeboards for intermediate lengths to be by direct interpolation.
Again, many boats will not reach that standard, and the freeboards should be simply reported without comment.
The Report • March 2019 • Issue 87 | 39
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