-1.0 0.0 1.0 2.0 3.0 4.0
Trim (deg)
DEMIHULL C932 S/L = 0.24 C932 S/L = 0.30 C932 S/L = 0.36 C932
for Fn > 0.45 the C932 catamaran coefficients are lower in spite of similar values of demihull curves.
Regarding the last observation, it is possible to note that,
as observed for running trim, also for CT over Fn = 0.45, catamarans of both models display an higher value than the demihull. Evidently, the connection between running trim and total resistance, typical for displacement fast hulls, is very critical for catamaran configuration.
0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 Fn
Figure 15: [Running trim angle C932 – L/1/3 = 4.66]
-1.0 0.0 1.0 2.0 3.0 4.0
Trim (deg)
DEMIHULL C925 S/L = 0.24 C925 S/L = 0.30 C925 S/L = 0.36 C925
For C932, in this speed range a greater AT value reduces running trim and improves performances.
100.0 120.0 140.0 160.0 180.0
20.0 40.0 60.0 80.0
0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 Fn
Figure 16: [Running trim angle C925 – L/1/3 = 4.66]
-1.0 0.0 1.0 2.0 3.0 4.0
Trim (deg)
L/Ñ1/3 = 4.66 S/L = 0.30 5.08 S/L = 0.36 5.74
L/1/3 = 4.66 L/1/3 = 5.08 L/1/3 = 5.74
CT x 103
DEMIHULL C932 DEMIHULL C925 S/L = 0.24 C932 S/L = 0.24 C925
S/L = 0.36 C932 S/L = 0.36 C925
S/L = 0.30 C925 S/L = 0.30 C932
0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 Fn
Figure 18: [CT comparison]
4.4 INTERFERENCE PHENOMENA In [1] and [2] the authors explain that
interference
phenomena are generated by variation of velocity field around demihulls, change of Form Factor value and superimposition of wave patterns.
0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 Fn
Figure 17 [Running trim angle C932 – S/L = 0.33] 4.3
MODEL COMPARISON
Catamarans and demihulls of both the models have been tested at L/1/3 = 4.66 and S/L = 0.24, 0.30 and 0.36. A comparison of the volumetric resistance coefficients
CTwas made to evaluate the different hydrodynamic behaviour of the models. From Figure 18 it is possible to note that the curves of all the configurations present similar trends:
for Fn < 0.40 demihull and catamaran configurations
of C925 present lower CT (probably due to the smaller transom area of C925);
for 0.40 < Fn < 0.45 the curves are superimposed;
Coherently to this breakdown, the interference factors have to be evaluated considering the viscous and the wave components separately. These procedures are of great scientific interest, but they need to measure directly the viscous and wave components. For catamarans these methodologies are really complex and the obtained data could be not
reliable (especially for the viscous
component). Moreover, errors in the evaluation of these components could amplify scale effects in model-ship correlation.
For this reason in this work to evaluate the interference phenomena, the ratio IT has been chosen:
C IT C
TDH TCAT
2R R
TDH TCAT
For the calculations, the values of the resistance have been read from the fair curves.
Since the IT factor is calculated with the total resistance, it depends on viscous resistance. So IT factor changes if different ship scales are considered. The reported values are referred to the model scale and to fresh water at 15°C.
©2007: Royal Institution of Naval Architects
B-27
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