Page 26: of Maritime Reporter Magazine (April 15, 1973)

Sea Trials— (Continued from page 26) the determination of the ship's lo- cation.

Turning Circles

Figure 1 is a typical turning cir- cle plotted from data taken from a chart produced by the radiometric equipment. With the accuracy af- forded by this method, the actual position of the ship can 'be shown at any desired spot, and a circle can be developed from the locus of points corresponding to any lo- cation on the ship. In this case, the circle has been drawn through the ship's center of gravity.

Similar charts can be produced for the quick-reversal trial which record accurately the stopping dis- tance and time, the course, heading and deceleration of the ship during the entire maneuver.

Operating Conditions

Steady operating conditions are essential if meaningful trial results are to be achieved. Plant conditions should be stabilized and preferably at the design values during all pow- er runs. Special attention should be given to superheater outlet tem- perature and pressure and condens- er vacuum, and adjustments should be made prior to the runs to bring these values as close as practicable to design conditions.

No changes should be made to the plant during the runs that would affect power output, and variances in auxiliary load should be kept to a minimum. The use of make-up steam and the possible dumping of auxiliary exhaust to the condenser should be carefully monitored and eliminated.

The most frequent source of torque variation, but also the easi- est to correct, is that due to ex- cessive use of the 'helm. This gen- erally results from : 1. The captain's insistance on maintaining a straight course, and 2. the use of improper- ly adjusted automatic steering equipment which produces exces- sive rudder angles. Small rudder movements induce disproportion- ately large changes in shaft torque, particularly at higher powers. Since course keeping is not essential ex- cept during standardization, rudder angles should never exceed two degrees.

Course changes should never be made during power runs without the bridge first notifying the com- puting room. Rudder angles should be kept to a practical minimum during such changes, but should not exceed five degrees.

If these precautions are followed, the deviations in power and fuel consumption from one time period to another should be well within one percent.

Fuel Analysis

Experience has proven to the

MarAd Trial Board that the aver- age commercial laboratory cannot be relied upon to provide the con- sistent accuracy required in the de- termination of the higher heating value (HHV) of the fuel.

For this reason, this value used in calculating the official fuel rate should be determined by the Na- tional Bureau of Standards, if at all possible. Unfortunately, the quality service provided by NBS is limited to other Government agencies and is presently available only to those contractors building ships under

MarAd or Navy programs. Unless these services are available, it is the author's opinion that a guar- anteed fuel rate is meaningless and should not be specified.

Fuel-Rate Calculations

If the fuel-rate calculations are being made for the purpose of de- termining a specified guaranteed operational rate, it is my opinion that no correction should be made for the deviation from design con- ditions of steam pressure and, per- haps, of propeller rpm. Further- more, the propriety of correcting for steam temperature deviations might also be questioned, particu- larly if the cause is not correct- able or will not be corrected. This is particularly applicable to the bonus - penalty type of contract where the owner has attempted to ensure that he will obtain an ef- ficient propulsion system by agree- ing to reward the contractor if the fuel rate is 'below a specified amount.

One of the most significant fuel- rate correction factors is due to deviation from design conditions of the condenser vacuum. This cor- rection is justified since the vacu- um will obviously vary with the temperature of the seawater. Oth- er corrections which are proper since they are variables beyond the control of the designer and contrac- tor are for items such as generator load, distiller and ship - service steam.

Steam-Rate Corrections

As in the case of fuel-rate de- terminations, every effort should be made to have steam and vacuum conditions at design values. How- ever, if this is impractical, correc- tions should be applied to all values.

This apparently inconsistent opin- ion is logical when it is realized that in the case of the fuel rate, the objective should be to deter- mine the true capability of the complete propulsion system as constructed and not as designed.

In the case of the steam rate, what is desired is to verify that the turbine manufacturer provided a unit meeting the design expecta- tions, and he should neither be penalized nor given an advantage for reason of off-design operating conditions. Similarly, a correction factor should be provided for any deviation of rpm since this will certainly have an adverse effect on the steam rate, and it is a factor over which the turbine manufac- turer has no control.

Spiral-Maneuver Test

The T&R Bulletin C2 states that this test "should be conducted only in relatively calm seas and winds of less than five knots." It is the author's opinion that the test should not be attempted except in a flat calm and zero wind, and these conditions must remain throughout the entire test which usually re- quires three or four hours. The odds against finding such condi- tions coincident with the trials are astronomical.

Since the validity of this test is so dependent on extremely improb- able environmental conditions, the author questions whether it should ever be specified or even included in the Code for Sea Trials.

Standardization Trials

The use of radiometric equip- ment permits great latitude in the conduct of standardization trials with reference to direction, distance from shore, time of day, weather conditions, etc. Daylight free of haze and fog is no longer essential, and in fact, many of the recent standardizations have been con- ducted at night. But equally im- portant is the flexibility in select- ing the course, thus permitting a heading to be chosen that will minimize the effect of wind and sea.

Since distance from shore is no longer a constraint, it is recom- mended all such trials be conducted beyond the 60-fathoms curve to eliminate any possibility of shal- low-water effect.

Trial Reports

The trials cannot be considered complete until they have been docu- mented by a comprehensive but concise report. Assuming that all instruments were accurately cali- brated, all tests successfully com- pleted, and all data properly re- corded, there will still remain the large task of summarizing this mass of material and presenting it in a form that will be meaningful and useful.

After reviewing the formats used by the various shipyards in prepar- ing their preliminary reports, some good, but many of a hodgepodge nature, the MarAd Trial Board de- veloped a standard reporting form.

If this form is followed, all perti- nent data will be presented in a logical sequence so that the re- port can serve not only as a docu- mentation of the trials but as a ready reference as well. This book- let also includes instructions for calculating fuel rates and steam rates, as well as some recommen- dations in regard to instrumenta- tion and test procedures. Copies may be obtained from the MarAd

Trial Board.

SNAME T&R Bulletin C2

The Society of Naval Architects and Marine Engineers will soon publish Technical and Research

Bulletin C2, Code for Sea Trials.

This document consolidates, am- plifies and updates the previously published SNAME codes. It has been exceptionally well done and covers many of the recommenda- tions the author has made. How- ever, after a careful review of the final draft, there are still many "do's" and "don't's" that are not properly a part of this document but none the less must be observed for good trial results. 28 Maritime Reporter/Engineering News