As mv Apostolos P (1988–2007) flying the Greek flag and still looking good. It is difficult to imagine how it will look like with 2470 persons on board!
I have been tinkering around a bit with the hydrostatics and other statistics of the wrecked mv Spice Islander I. That exercise suits no other purpose except to satisfy my personal curiosity of what happens to a ship’s vital parameters when it is obviously so grossly overloaded with 2470 passengers and crew instead of the 690 that were allowed according the COI.
It is beyond my understanding that a captain, if you may call him that, thinks he can get away with such an overload, although I suspect that this was probably not the first time that such overloads occurred. A ship needs only a nudge of a wave or increasing wind force to tip over, it is a balancing act on a knife’s edge.
Here are some data which are publicly available.
- mv Marianna (1967–1988)
- mv Apostolos P (1988–2007)
- m Spice Islander I (2007–2011)
Length 60 m (196.85 ft)
Beam 11.4 m (37.4 ft)
Speed 11,5 knots
Propulsion: Two Poyaud 12VUD25 diesel engines of 1,560 horsepower (1,160 kW).
Capacity according to COI:
Crew 45 Passengers 645
Actual total number of passengers and crew on board 2470
First of all I will try to calculate the displacement. For that we have to find the block coefficient Cb.
To accomplish this I used Ventura’s Estimation Methods and calculated Cb with five different formulas. I dropped the lowest and highest value and averaged the remaining three, which were closely grouped. The resulting figure was Cb = 0.61.
Next we need to know the draft of the ship. With a ruler we can measure on the above picture the freeboard. First I measured the distance from the deck to the loadline marking and divide that by the length of the ship, again with the ruler. Multiply the quotient with 60 m and the outcome of the freeboard is about 3.12 m, not very accurate but sufficient for this purpose. With known ratios of more or less the same type of ships, such as B/D and T/D, we can calculate estimates of D and T. For the ratio B/D for instance the value 1.63 is taken and for T/D 0.49, then D = 6.17 m and T = 3.05 m. The freeboard D - T = 3.12 m.
The displacement or Δ = L x B x D x Cb = 60 * 11.4 * 3.05 * 0.61 = 1273 ton.
Next we calculate the GM with the USCG Weather Criterion formula: GM = PAH/ W * tan θ, where θ = 14°.
P = 0.055+(L/1309)^2 = 0.055 + (60/1309)^2 = 0.057101. PAH being Pressure x Area x Height - W is displacement and θ is the angle of heel (1/2 Deck edge immersion or 14 degrees). - The pressure being approx the equivalent of about 55 kts.
It is somewhat special that the angle with the point of deck immersion is cotan of freeboard divided by 1/2 B = 3.12/5.7 = 0.547368, then θ = 28.69°, the half value of that angle is a just a little bit over the other value of 14°.
From the picture I found:
A ≈ 450 m^2
H ≈ 5.49 m
PAH ≈ 141
GM = PAH/ W * tan θ = 141/1273 * tan θ ≈ 0.44 m.
For small passengers ships with a length of 50 - 120 m the ratio DWT/Δ (%) = 15 - 25. mv Spice Island I has a ratio of 17, which checks out for this type of ship.
The DWT can be divided in CDW = Cargo DW and SDW Ship’s DW.
Rule of thumb: DWT= 1.05 x CDW 225 = 1.05 x CDW -----> CDW = 225/1.05 = 214 ton. Ship’s DWT of SDW= 11 ton.
In modern times we work with GT instead of GRT. Just for the fun I will calculate the GT with the help of a simplified set of gross tonnage formulas set forth by the U.S. Coast Guard Marine Safety Center, which are based on three measurements: Length (L), breadth (D), and depth (D), thus not d or T as for draft but D for Depth!
For freighters the simplified formula is: Gross Tonnage = (.67 x L x B x D) divided by 100, with LBD in feet.
I further simplified the formulas for the GT for LBD both in feet and meters:
GT = 0.0067*(L * B * D) ≈ 493 - LBD in feet.
GT = 0.236427*(L * B * D) ≈ 493 - LBD in meters!
To be continued…