The Art of Dredging

Dredging and shipping

Capsizing of "Bowsprite"

 

Intro

 

 

The “Bowsprite” was a small British flagged aggregates gravel dredger, supplying the British building industry with sand and gravel.  

She broke in two during a force 9 gale close to the Kwintebank, off the Belgian coast, in  December 1988, with the loss of 4 out of 10 crew.

 

Attached is the complete MAIB report.

 

 Report on capsizing Bowsprite (.pdf  1.8 Mb)

 

Fast facts

 

“Bowsprite” was underway from the Thames towards the sandwinning area off the Belgian coast.

Due to deteriorating weather “Bowsprite” wished to call at Nieuwpoort, but was not allowed in, the harbour was already closed.

Subsequently, the Bowsprite’s captain decided to return to the Thames (a 90 mile trip), steering away from the lee-shore (Belgian coast).

During the return trip wind turned to gale force 9. Bowsprite was steaming head in the waves. 

 

 

 

 

The ship broke in two halves, the forward part sunk, the aft part was salvaged, later scrapped.

 

 

Lessons Learnt:

 

Sand and gravel are heavy cargo (density 1.85 to 2 ton/m3).

 

On most hopperdredgers, the cargo is located in one midships hold, resulting in huge bending moments (sagging), especially when wavelengths are same as ship’s length.

 

 

Bending moments may well exceed design limits in heavy weather, as probably happened with “Bowsprite”.

 

“Bowsprite”  was not a new ship (23 years old).

 

 

 

Hull plate thickness was less than original, some wastage of plates –thus strength- is inevitable.  

In the report no mention is made of wastage of hopper plate thickness; this may also lead to smaller longitudinal strength.

 

 

 

Heavy weather,  hopperdredgers in ballast - extra notes

 

At the design stage of a hopperdredger, the classification society checks ship’s strength (especially in the foreship) on a minimum draught.

In most cases this minimum draught is the “ballast condition” (hopperlevel same as outboard waterlevel, and full forepeak tank).

 

Following these calculations, the foreship may be extra strengthened for “slamming”.

 

Sailing in heavy weather (with level in the hopper = outboard) and forepeak full is a safe procedure.

 

The draught should be sufficient to avoid slamming, or –if slamming occurs- the ship’s hull forward is sufficiently strengthened to handle these forces.

 

As a matter of good seamanship, slamming should be avoided anyway, by changing speed and / or course.

 

I’ve done my part of macho behaviour in the past, steaming away full ahead in short, steep waves at full power ahead, with some hull damage forward as result.  

 

Ships have a lot of propulsion power nowadays, and steaming full ahead in a seaway, does not give extra speed.

The excess power input is purely destructive, especially on the forward part of the ship.

 

Sloshing of water in the hopper is not taken in account during the design.

The hopper is build for a high-density load, and is sufficiently strengthened for sloshing of water.

 

 

Concerning extreme waves: the ship is designed to survive a (statistically) once-in-a-hundred-years-wave, loaded on international freeboard.

In ballast, the situation should even be better.

 

When sailing in bad weather, filling up the hopper with water is a better idea, minmising free surface effects in the hopper, thus making a longer roll period.

However: filling up the hopper results in a reduced freeboard. This –in turn- makes the sides of the ship more vulnerable to wave action.

 

 

Marc Van de Velde

 

Subscribe to our newsletter:

Write us:

reply.to.artofdredging (at) gmail.com



Manu's scripts

- a sailor's fifth column