Trial tests of the built ship are an important prerequisite for the acceptance of the ship by the shipowner and are always specified in the contract between shipowner and shipyard. The problem is that the trial conditions differ from both model test conditions and design conditions.

The contract usually specifies a contract speed at design load at a given percentage of the maximum continuous rating of the engine, this at calm sea without wind and current on deep water. Trial conditions are usually in ballast load, natural seaways, in the presence of currents and often shallow water.

Only on rare occasions is it possible to perform trial tests under ideal conditions as specified in the contract. However, upper limits for the wind and sea conditions are usually defined in the contract and test trials are performed only at times or places were the actual conditions are within the specified limits.

The difference between contract and trial conditions requires various corrections to correlate trial results to contract conditions. Apart from the difficulties and margins of uncertainties in the trial measurements, the correlation procedure is plagued by many doubts.

The traditional methods are partly empirical, involving curves with manual interpolation etc. It was not uncommon that the results of various consultants, e.g. towing tank experts, differed by several tenths of a knot for the obtainable speed under contract conditions. This margin may make a difference between paying and not paying considerable penalties! Subsequently, trial evaluation was susceptible to disputes between representatives of shipowners and shipyards.

(Continued)

 

 

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