HOW TO MARK NO GO AREAS - Abhishek Sabharwal

Marking No Go Areas is a task , which over a period of time , I have realised is a job which most of the second officers are not clear about (no offences/disrespect to anyone).
This inefficiency leads to charts being incorrectly marked (Let it be on an ECDIS or Paper chart) and the other duty officers in turn assume that the markings are correct,which is not the case.
So today we will identify the rights and wrongs of No Go Areas thereby covering the common mistakes done by everyone and the right procedure to do it.

No Go Area:

A  no go area is an area where the vessel cannot /should not go, as that is dangerous because of a shallow nature or any other surface / sub surface obstruction identified as dangerous to navigation for seagoing ships or any other condition which can compromise the safety of the vessel.

CALCULATIONS:

We will take the example of a vessel at sea and then try to understand the concept,

M.V ABCD
Draft (static) - 12.5 mtrs
Speed - 10 kts
Assuming block coefficient : 0.7 (to simplify things)

Squat (for confined waters): Cb x V(Square) / 50
                                         = 0.7 x 10 x 10 / 50 = 1.40 mtrs.

Assuming Height of Tide: 1.0 mtrs

Ukc (required as per individual company policy) : 20 % of static draft (in coastal waters)
                                                                               = 20 x 12.5 / 100 = 2.5 mtrs.
Depth Required for the vessel to stay safe: (criteria for marking no-go areas)
   = Draft + Squat + Ukc - Ht. of Tide
   = 12.5 + 1.40 + 2.5 - 1.0 = 15.40 mtrs.

MYTH:

A typical myth followed by lot of people on ships is , that the shallow pattern coloured areas (light blue) near a coastline are the lines joining no go areas and on most of the ships it is observed that people mark the entire light blue shaded portion on charts as no go areas near the coast , which is wrong and is a PSC deficiency and these no go shade lines are even extended onto the coast itself.

WRONG METHOD :

RIGHT METHOD :

The right way to do it, is to calculate the safety depth as calculated above and  all depths below this depth are considered to be as no go depths i.e depths below which it is considered unsafe for the vessel to navigate.
As in our case the safety depth as calculated is 15.4 mtrs and the nearest rounded off depth is 16 mtrs.Thereby, on the chart join all depths of 16mtrs. Now shade off the entire area between the coast and the line as explained above. Mark this area as a no go area.

Note: 

Please make a note of the fact that the marking of a no go area is a dynamical calculation i.e it varies with the variation in various parameters such as draft, ukc,vessel speed, height of Tide prevailing at that time

Hope the above explanation serves the purpose......For any suggestions / queries / comments ur most invited.....Thanks for stopping by........cheers....