Introduction

            At the request of the Ship Design Manager at NavSea Code SEA05D5, a DTMB model test engineer spent one week observing the model test of the proposed Lockheed Martin AGOR 26 design, conducted at the Offshore Model Basin.  The following includes a summary of the work completed during the week, a discussion of the model test procedure used for the resistance test, and a brief analysis of the data collected for the resistance prediction of the design.

 

Model

            The AGOR 26 model is constructed of fiberglass to a scale ratio of 15 (l=15).  Removable appendages are fitted to the model such that the appendage angles are adjustable.  Several measurements of model and appendage particulars indicated that, in general, the model is built to the design specifications.  However, a detailed surface scan and comparison to the design was not performed. 

The model was tested at even keel drafts of 23ft and 25ft.  The model was ballasted to the waterlines painted on the model.  A series of runs was made to zero the side force before running any of the conditions in the test matrix.

The model is constrained in all degrees of freedom.  Instrumentation includes a six degree-of-freedom force block to measure all forces and moments acting on the model.  An inclinometer is also attached to the model to ensure that there are no undesired changes in pitch during the run.  The force block is attached, on centerline, to the model cross-structure (figure 1).  The six degree-of-freedom force block was calibrated on a calibration stand prior to being mounted in the model to obtain the six-by-six matrix of calibration factors.  The calibration was repeated twice to ensure that consistent values were obtained.  Each morning, and at several times during the day, electronic spans were collected to check the instrumentation.  A video camera mounted on the carriage gives a profile view of the model as it is towed.

There is no turbulence stimulation on the model or the appendages.  Standard practice at all major towing basins includes some sort of turbulence stimulation on the model to ensure that fully turbulent flow is achieved over the model surface.  The standard ITTC 1957 friction line assumes that the flow is fully turbulent, as does the Navy’s Ship-Model Correlation Database.  The absence of turbulence stimulation increases the uncertainty of the extrapolation procedure used herein, and may result in an underprediction of residuary resistance coefficient.

 

Resistance Test

            For each speed and test condition, a single run was made and the resulting forces and moments were collected.  A cursory analysis of the collected data spot was performed after each run, but the analysis did not include a prediction of full-scale performance.  No repeat runs were made to enable an experimental uncertainty analysis to be conducted. 

To simulate a more conventional resistance test, where the model is free to heave and trim, the matrix of appendage angles was selected to find the point of zero heave force and trimming moment at 12 and 15knots full scale.  With the appendages set at the angles required to eliminate the heave force and trimming moment, a series of runs was made over the entire speed range.

 

Discussion of Results

The analysis presented here was reduced using the standard DTMB SWATH resistance prediction method.  Since there was no immediate data reduction performed by the test personnel, a comparison of scaling methods and full-scale predictions cannot be made at this time.  The full-scale performance predictions for the indicated model conditions are presented in Table 1.  The predicted full-scale barehull Pe for the 23ft draft condition appears to be high at 15knots, most likely due to an erroneous data spot.

 A visual inspection of the surface profile around the hull shows no abnormal flow details (figure 2).  Although the wave height along the hull was not measured, from a visual inspection it appears that the wave height at the higher speeds is approaching six feet (6ft), which could result in slamming of the cross-structure in a seaway.  The lack of a wake survey, or other detailed flow measurement, does not allow an analysis of the subsurface flow patterns.

The proposed AGOR 26 shows similar performance as previous swath models.  Figure 3 shows the full scale predicted performance of the proposed design as well as the full-scale predicted performance of several other swath designs.  The performance of the other swath models was calculated for a full-scale vessel with the same length as the proposed AGOR 26 design.

 

Uncertainty Analysis

            Standard test procedure at DTMB includes an assessment of the uncertainty of the collected model data.  This assessment is based on collecting ten (10) or more repeated data spots and analyzing the deviation from the mean in accordance with the Student’s t distribution to determine the confidence level of the collected data.  Repeated runs were not made for the AGOR 26 resistance test, so a quantitative analysis of the uncertainty of the collected data is not possible.

 

Conclusions

Based on data collected at the Offshore Model Basin, and analyzed as shown in Appendix A, the barehull effective power of the proposed AGOR 26 design at the design speed of 15knots is 2745hp and 2647hp for 23ft and 25ft drafts respectively.  It appears that the predicted power at the 23ft draft is too high at this speed, possibly due to an erroneous data spot.  Over the speed range tested, the comparison of resistance per ton of displacement indicates that the performance of this design is very similar to other swath designs.  However, the lack of turbulence stimulation on the model casts some doubt on the validity of these results.