Trans RINA, Vol 157, Part A3, Intl J Maritime Eng, Jul-Sep 2015
increases with increasing speed, however absolute values remain small and do not differ significantly between hull shapes. For Fr > 0.37 trim increases with a larger gradient for less slender models. Displacement appears to have little effect on the trim angle, but a small increase with increasing displacement was observed.
Figure 6: Shear force coefficient for light, medium and heavy displacement compared to ship-model correlation lines of Grigson and ITTC.
3.2 (d) Pressure Drag
The fraction of the pressure drag varied between 0.25 – 0.60 of the total drag at the light displacement. It ranged between 0.25 – 0.30 for the 190 m hull and between 0.45 – 0.60 for the 110 m hull, whereas the highest values occurred at the highest Froude number (Fr = 0.49) for the light displacement and at the lowest Froude number (Fr = 0.25) for the heavy displacement.
For the medium
displacement the relative pressure drag varied from 0.30 – 0.70 and for heavy displacement from 0.35 – 0.75. The lowest value was always observed at Fr = 0.37.
3.2 (e) Sinkage and Trim
Figure 7 shows the vessel sinkage for increasing Froude number normalised by vessel
length. The sinkage
increased for increasing Froude number and is more pronounced for the vessels of shorter length. A maximum is reached around Froude number of Fr = 0.45 after that the draft starts to decrease again.
Figure 8: Trim presented with respect to Froude number for medium displacement.
3.2 (f) Free Surface Elevation
Figure 9 shows the free surface elevation around the 130 m model at medium displacement for the full speed range under consideration, where dark grey indicates a wave trough and light grey a wave peak. Black and white corresponds to free surface deformations of 1% of the ship length. At Fr = 0.25, characteristic patterns can be seen such as a wave shedding off the bow towards port, humps and hollows between the demihulls and a rooster tail past the transom stern. With Froude numbers increasing to Fr < 0.33 this wave pattern becomes more noticeable. For speeds of Fr = 0.33 – 0.45 a distinctive wave pattern on the
outwards facing side can be
observed. One trough leading away from the bow and one around amidships of the demihull are getting more pronounced with increasing speed. A fine crest between these troughs results in a typical pattern for these types of vessels at medium speeds. The two troughs are hardly distinguishable at Fr = 0.49 when the boat enters the planing regime.
Figure 7: Heave normalised by draft presented with respect to Froude number for medium displacement.
Figure 8 presents the trim angle with respect to Froude number, negative values indicate bow up trim. Trim
Between the demihulls and especially at the centre line the formation of back-to-back hollows and peaks can be seen to occur in a more roundish shape rather than distinct crests and troughs. For Fr < 0.37 the first hollow is usually more strongly pronounced than the first peak, while at higher Froude numbers the peak appears to be larger than the trough. The first pair of hollow and peak resulting from the superposition of the bow wave move further downstream with increasing Froude number and finally disappear at Fr = 0.49 due to the main hollow that
forms around the
demihulls. The most notable characteristic is the rooster tail seen aft of the transom stern. The depression past the transom increases with increasing Froude number and the angle relative to the demihull centreline of the trailing wave
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©2015: The Royal Institution of Naval Architects
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