Figure 10. Trim-Trim

ERDC TN-DOER-I2

July 2000

and then this quotient is divided by the unit weight of fresh water. Three water tests conducted

during system installation on the McFarland gave an average difference of 1.23 percent between

these two specific gravities. This average percent difference is higher than the value reported by

Scott et al. (1995) (1.1 percent difference), but the decreased accuracy may be attributed to the error

introduced by the use of only one amidship draft sensor on the McFarland, as opposed to the two

draft sensors used in the study by Scott et al. (one at the forward perpendicular and one at the aft

perpendicular). Water tests show an average percent difference of 0.85 percent between sensor-

measured and hopper water samples for the dredge Columbus.

In an ideal water test, the TDS value should be zero due to the absence of solid particles. Using a

sand specific density of particles value of 2,650 kg/m3 (specific gravity of 2.65) and water density

of 1,025 kg/m3 (specific gravity of 1.025), the calculated TDS values of the McFarland water tests

range from approximately 50 LT to +28 LT, with an average value of 16 LT (a cubic yard of dry

sand weighs approximately 1.2 long tons).

Trim-Trim. Trim-Trim is based on the fact that the surface of a static fluid is always horizontal.

The measurement of the fluid surface in the hopper by the acoustic sensors and the difference in

displacement by the draft sensors allows two independent inclination angles to be calculated

(Figure 10). If the respective sensors are functioning correctly, subtraction of these two angles

should ideally equal zero. In Figure 10, the top illustration shows a vessel with trim on an even

keel fore and aft where both inclination angles are equal to zero. The bottom illustration shows the

same vessel trimmed down by the stern and identifies the two nonzero inclination angles. The

European TDSS specifies a maximum allowable Trim-Trim angle difference of 0.3 deg. The

Trim-Trim tests of the Columbus have successfully met that criterion.

a. Vessel with trim on an even keel

b. Vessel trimmed down by the stern

Figure 10. Trim-Trim