Trans RINA, Vol 157, Part A3, Intl J Maritime Eng, Jul-Sep 2015 LIGHT
At Fr = 0.29 (Figure 12 a), increasing displacement results in a larger dimensionless drag force. The heavy displacement case shows a decreasing resistance with increasing slenderness, but a minimum in the resistance curve cannot be observed. The demihull slenderness ratios under consideration did not include those of minimum resistance.
MEDIUM
At Fr = 0.37 (Figure 12 b) it was observed that the drag was generally lower than at Fr = 0.29, 0.45 and the differences between the three displacement cases was less pronounced than it was at Fr = 0.29.
HEAVY a) Fr = 0.29
Figure 11: Free surface elevation of 130 m medium- speed catamaran at Fr = 0.41 for light (top), medium (middle) and heavy displacement (bottom).
3.2 (g) Demihull Interaction
The demihull separation was varied by a half demihull beam which leads to +/- 10% change in overall beam at constant displacement. An insignificant variation in drag of below 3% for the 130 m and 170m hull at Fr = 0 .37, 0.45 for light and heavy displacement was observed and the influence of varying separation on the resistance using the parameters under consideration for such slender hulls was assumed to be small.
3.3 DESIGN SPACE EXPLORATION
In this section the resistance data was interpreted to derive guidelines for the design of large medium speed catamarans with low drag and high transport efficiency. Two approaches are presented: firstly fully non-dimensional representation of the resistance to determine appropriate demihull slenderness ratios at certain Froude numbers and displacements; secondly, the results are presented in terms of transport efficiency to derive the appropriate overall length for a certain operating speed range.
Figure 12 a-c shows the total drag non-
dimensionalised by buoyancy force and divided by Froude number square plotted over the slenderness ratio for constant Froude number (Fr = 0.29, 0.37, 0.45) at light, medium and heavy displacement. The difference in result for varying displacement is due to the change in transom immersion as the drag force was normalised by the displacement force.
c) Fr = 0.45 b) Fr = 0.37
Figure 12: Non-dimensional drag as a function of demihull slenderness ratio for a) Fr = 0.29, b) Fr = 0.37 and c) Fr = 0.45.
A-168
©2015: The Royal Institution of Naval Architects
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76