Effect of weather on trial speed results

The acceptance of a new ship depends in many cases large-on the fact whether the ship attained the clarge-ontract speed m the acceptance trials. The trial conditions are described, is far as wind and sea are concerned, by means of the Beau-fort scale. In general a wind force equal to or below three Beaufort is specified in the contract. This condition may be a zource of difficulties, since the weather and sea conditions in he areas where the speed trials are often carried out, either n a measured mile or in a special Decca area, are unsteady during the whole year.

The date for the speed trials having to be fixed well in ad--ance, the prevailing weather and sea conditions on that date have mostly to be accepted, even when they are considerably worse than those specified in the contract.

It is clear that these worse conditions cause higher allo-wances on DHP tank than indicated in the tank prediction, which is based on the specified conditions.

The results of a large number of trial measurements, carried mt by the N.S.M.B. on a measured mile or in a special Decca area, enable the determination of the effect of weather and sea :onditions on the trial allowances.

This information enables the conversion of the actual allo-wance figure to that belonging to the Beaufort scale specified

n the contract.

For this purpose the trial results of 212 ships, built after World War II. w ere analysed.

The displacements of the ships concerned varied from

70 tons to a maximum of 110,000 tons.

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NoI19117 FORCE ( loso,dwoort ooC.0.LE)

Effect of Weather on Trial Speed Results

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Prof. Dr. Jr. W. P. A. van Lam mer en

Besides the speed, the shaft horsepower was determined in almost all cases by means of one or more Maihak torsion meters. In some cases a meter of another manufacture was applied.

The speed and power measurements were mainly carried out by technical experts of the N.S.M.B., anyhow one of them attended the trials.

The 212 ships consist of :

89 cargo liners, 70 tankers. 23 passenger and cargo liners, 10 bulk carriers, 10 coast guards and 10 ships of various types. The loading conditions were:

103 full load, 25 half load and 84 in ballast.

167 ships were run on a measured mile, for the other 45 ships the covered distances were determined by means of a special Decca Navigator.

Based on the displacements the ships can be divided into 7 groups viz.

Group Displacement between

1 0 and 2,000 tons 2 2,000 and 5.000 tons 3 5,000 and 7,500 tons 4 7,500 and 10,000 tons 5 10,000 and 30,000 tons 6 30,000 and 50.000 tons 7 50,000 and 110,000 tons

The allowances found for each group were plotted to a base of wind force (Fig. 1, diagrams 1 through 7).

The allowances may be poriitive as well as negative.

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Average trial allowances for 7 groups of ships with trial displacements between:

collected results from the

diagrams 1 through 7 Schiffstechnik Bd. 12 1965 Heft 64 1 0 and 2 000 tons 2 2 000 and 5 000 tons 3 _{5 000 and 7 500 tons} 4 7 500 and 10 000 tons 5 10 000 and 30 000 tons 6 30 000 and 50 000 tons 7 _{50 000 and 110 000 tons} 4 01 9 1 4 7 v .

Straight lines, representing the average allowances for each group. were computed by the method of the least squares.

Diagram V of Fig. 1 presents the seven average lines, ob-tained for the various groups.

Fig. 2 was derived from diagram V. The Figure shows the allowances, plotted to a base of displacement with the wind force as parameter. Above 50.000 tons displacement the lines are dashed, since only little information of these ships was available. Fig. 2 permits the determination of the average allowances for ships having displacements between 900 and 50,000 tons for wind forces between 0 and 8 Beaufort scale. It should be noted that the displacement is the actual trial dis-placement.

A critical examination of the diagrams 1 through 7 of Fig. 1 learns that in some cases the scatter of the points is rather large. This scatter may be due to various effects.

The model results have a great accuracy, the finish of the model hull being very constant and the water smooth.

The possibility of variations in the finish of the ship's hull and the appendages is considerably greater. A low standard finish may cause an increment of the ship resistance in the order of 10 °/o in comparison with a high standard finish. This difference implies an important effect on the speed. Long and high superstructures have more wind resistance than short and low ones. Running a ship in an unfavourable wave pat-tern may lead to a significant speed loss.

The precision of the torsion meter, whidi is normally 1 or 2 per cent may be affected unfavourably by sources from out-side. The wind force is only visually observed. A discrepancy of plus or minus 1/2 Beaufort number in regard to the correct value can easily occur.

Schintechnik Bd. 12 1965 Heft 64 142

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Fig. 3 Frequency of the occurrence of a certain wind force in decca areas and on measured miles, each based on 42 obser-vations in the same period

Fig. 2 (left) Relation between wind force an allowances on D.H.P. for ships with a tria displacement varying from 1000 to 60 000 ton based on the Froude extrapolation meth.

One or more of these discrepancies may have affected the tria speeds and thus the allowance figures obtained on many of thi ships from the results of which Fig. 2 was derived. So it shout( be kept in mind that the allowances, given in this Figure an average values.

It was tried to find out whether the results obtained fron measured mile trials scattered more than those obtained fron Decca records. For this purpose the allowances of 42 ships fo each procedure were compared. These ships made their trial in approximately the same period of time. No essential differen ces were established.

For both series of ships it was investigated whether ther, was any particular difference in wind force. It appeared tha the arithmetical mean wind force was 3.52 Beaufort for th, Decca measurements and 2.70 for the measured mile trials Fig. 3 gives the frequencies of the occurrence of wind forces It appears that the chance to conduct trials in wind force 0 to Beaufort is on the measured mile 11/2 times as large as in di, Decca areas. This can easily be explained by the fact that th, measured miles are purposely traced in sheltered areas, where as the Decca areas are located in open sea.

The analyses were made, taking the tank results to th, Froude extrapolation method as a basis for comparison. 0 course the curves can easily be re-arranged for other extra polation methods, but it is felt that the mutual differences be tween the curves for various wind forces will not chang, materially.

Acknowledgements

The trial measurements were mainly conducted by th N.S.M.B.'s technicians H. J. de Jong, H. Nijding and S. J Groothuis. The analyses were made by Mr. De Jong who als took care of the publication of these data in the Dutch lar guage.

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OIS99_4, 9.91EHT 114 91E.TIO1C TONS x 1.000

400 700

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