Read Seaways online at http://www.nautinst.org/seaways May 2018 | Seaways | Feature: DUKCM – A note of caution
a number of reasons, including ionospheric scintillation, solar storms,
and GPS signal/data link interference, whether natural or manmade.
Most port authorities therefore use linear accelerometers, and opt
for less expensive, lower accuracy models. As with most sensors, they
require regular laboratory calibration to ensure the manufacturers’
quoted accuracies are being met. Again, this does not come cheap.PITCHING
Pitching is usually more pronounced when travelling into a head sea
and becomes significantly more pronounced when the speed of the
vessel is such that the lifting of the bow coincides with the arrival of the
next wave crest, ie synchronous pitching^4.HEELING
Vessels heel over when executing a turn, rolling or when there is a
sudden change in wind direction/strength.
When executing a turn, the degree of heel (q) is governed by:
l The speed (V) of the vessel (q proportional to V2)
l The tightness (radius) of the turn (q inversely proportional to R)
l The metacentric height, (q inversely proportional to GM)
l the speed/direction of the prevailing wind and current/tidal stream
relative to the ship’s heading (most ships turn more rapidly when
turning into the wind/current, thereby tightening the turn).
When rolling, vessels have a natural roll period (T), which is a
function of the dynamic rolling radius of gyration and the metacentric
height (GM).
The degree of roll will increase significantly when the period and
direction of the wave/swell coincides with that of the vessel’s natural
roll, ie synchronous rolling^4.
An additional factor that needs to be taken into consideration,
especially at the end of a voyage when fuel and water tanks are partially
empty, is the ‘free surface’ effect, which has the potential to increase
the degree of roll significantly.
If there is a sudden change in wind direction and/or strength,
the potential for error is high because the angle of heel is directly
proportional to the square of the wind speed^5 , and the maximum wind
speed is very difficult to predict accurately. For high-sided, broad-
beamed vessels such as fully laden container ships, even a 5 knot
increase in wind speed can cause a significant reduction in draught.
The impact of wind on the degree of heel can be substantial when
making a large alteration of course such that the change in relative
wind direction can generate windage which accentuates the heel
caused by the turn. For example, if a vessel is negotiating the Thorn
Channel, in-bound for the Port of Southampton, during an easterly
gale, a strong gust of wind at a critical moment during the 112 ̊ turn to
starboard would increase the angle of heel.SEAWATER DENSITY VARIATION
There can be significant density variation within estuaries, especially
where there is a pronounced ‘salt wedge’ and/or a large tidal range
and strong river outflow. To measure this variation, a vertical array
of conductivity, temperature and depth (CTD) sensors should be
deployed at one (preferably more) locations adjoining the channel.The ‘error’ risk component
It follows that the gross under-keel clearance should be increased
to include an ‘error’ risk component. Bearing in mind that the
estimated SE values used to compile the error budget shown above are
considered conservative values, the error allowance should never be less
than 0.7m. Moreover, since the RMS error value could well be higher,
it is vital that the gross under-keel clearance also include a safety
margin, as shown right:References
- MacPherson, D M, 2002. “Squat effects: A practical guide to its
nature, measurement and prediction”, presented to the Society of
Naval Architects and Naval Engineers (SNAME) - Serban, P & Panaitescu, V, 2016. “Comparison between formulas of
maximum ship squat”, Naval Academy Scientific Bulletin, Volume
XIX, Issue 1 - Vantorre, M, 2003. “Review of Practical Methods of Assessing
Shallow and Restricted Water Effects”, International Conference on
Marine Simulation and Ship Manoeuvrability (MARSIM) workshop - Clark, I C, 2005. Ship Dynamics for Mariners, published by The
Nautical Institute - McKinnon, A, 1990. “Underkeel Clearance in Pilotage” (pages
277–279 of The Nautical Institute on Pilotage and Shiphandling)
Under-keel clearance components
Ship’s draughtGross dynamic
under-keel
clearanceSquatHeelPitchingEnvironmental parameters
(tide, swell, seawater density,
atmospheric pressure, wind and current/tidal
stream speed and direction)Root mean square (RMS) error value
(not < 0.7m)Safety margin (suggest not < 0.5m)DUKCM_SGS, author.indd 11 17/04/2018 14: