Use of interceptors and stepped hull to improve performance of high-speed planing


In this paper, first presented at The Royal Institution of Naval Architects’ High Speed Craft conference*, T Hansvik and S Steen, from the Norwegian University of Science and Technology, in Trondheim, Norway, shown how calm water performance of high-speed planing catamarans can be improved.


H


IGH-speed catamarans have, since their introduction in the 1970s, become


the dominating ship type for fast passenger transport. They have favourable resistance characteristics and larger deck area than monohulls, and are cheaper to build and more robust than advanced ship concepts like hydrofoils or SES. The first commercial catamarans, built


by Westermoen, in Norway, had planing, asymmetric demihulls. It was soon discovered that for the moderate Froude numbers at which these vessels operated it was better to use slender, symmetrical demihulls with round- bilge shape and little dynamic lift. Since then, the trend has been to increase the slenderness of the demihulls when higher Froude numbers shall be obtained. With a few exceptions, like the Buquebus


55knot planing catamaran design by Nigel Gee & Associates, all fast catamarans of more than around 30m, have been built with slender, symmetrical, demihulls. However, at high Froude numbers it has been found that planing catamarans offer better resistance characteristics. Using a planing hull form allows lower overall L/B-ratios without compromising resistance, and the simpler geometrical shape allows for cheaper construction. It is shown by means of model test results


how the introduction of a transverse step close to amidships might reduce the total resistance considerably, by ventilating a large part of the aft ship so that the wetted surface is reduced. It is further shown how the introduction of an interceptor blade at the step will increase the ventilation length and decrease the resistance. Also, the application of an interceptor at


the position of the step, but without any step height, is found to be a very simple and quite effective means of decreasing the resistance since it provides approximately the same type of ventilation as a step. When ventilating the aft part of the


hull, propulsion by means of waterjets or conventional propellers become difficult. Alternative propulsion systems, suitable for this kind of hull concept are discussed.


Model and model tests The planing catamaran hull form was designed by Marintek on behalf of the shipyard Brødrene Aa, in Norway. The original demihulls had symmetrical V-shaped cross sections and hard


*High Speed Craft - ACVs, WIGs & Hydrofoils, was held in London from 31 October to 1 November 2006.


62 SHIP Length OA , LOA Length, bp LPP Breaded moulded, B Draught at LPP /2, T MODEL 26.400m 3.168m 24.000m 2.880m 8.000m 0.750m Volume displacement, 51.6m3


Longitudinal CB From AP, LCB


Wetted surface, S


Wetted surf of Transom stern, AT


9.936m 140.67m3 2.60m2 0.960m 0.090m 0.089m3 1.192m 2.026m3 0.037m2 Table 1. Specifications for the model.


Specific Resistance = (1000 xRTS)/(rxgx) where RTS is in N.


The hump in the resistance curve between


10knots and 20knots is the primary resistance hump, for this hull located around Fn=0.4


Transverse step The hull lines were first improved by introducing one transverse step close to amidships of each demihull. The step was placed with an LCG to Ls ratio of 0.85, based on earlier experience with similar designs. The step heights that were tested was Dss=0.125m, Dss=0.250m and Dss=0.375m. The results for the resistance are shown in Fig 2. For the lowest speeds, the resistance increases


chines. Specification for the model is given in Table 1. The model was made in lightweight polyurethane foam by CNC milling, followed by sanding, pasting, sanding again, and spray painting. The resistance tests were performed in the


large towing tank (260m x 10m x 5m) at the Marine Technology Centre in Trondheim, Norway. During tests the model was fixed in sway and yaw, while free to heave, trim and roll. The tow force was measured in the horizontal direction. Sinkage was measured at the fore and aft using potentiometers connected to the trim posts. The model had windows installed in the bottom of each side hull in order to observe the wetted length, which is of interest in the case of step or interceptor. The transverse step was made by cutting the


model transversely close to amidships. The vertical position of the two halves could then be adjusted so that the step height could be varied from zero to about 5cm (model scale). This way of making the step means that the


step is perfectly normal to the longitudinal direction. This is in contrast to steps used on most commercial high-speed boats, where the step is usually swept backwards, so that the step is located further forward at the keel than at the chine – the step is approximately parallel with the intersection between the hull and the water in planing condition. An aluminium plate was inserted in the cut


between the two halves of the hull. This plate could be lowered to act as an interceptor. Just behind the step (at the front end of the rear part of the hull) two 4mm holes were made in each demihull. The holes were connected to flexible tubes that went above the waterline. The holes were made in order to ensure ventilation of the step (or interceptor). The tubes could be closed in order to see the effect of the ‘artificial’ ventilation.


with increasing step height. The speed at which the resistance for the stepped hull becomes favourable over the unstepped hull seems to move somewhat up as the step height increases. This is probably related to the base drag of the step, which is larger for a larger step height. For the highest speeds however, the largest step heights has the lowest resistance. The ventilation lengths behind the step are


shown in Fig 3. Comparing Fig 2 and Fig 3, we see that the ventilation has to be of some amount before the resistance decreases, since the reduction in wetted surface must first compensate for the base drag of the step before a net drag reduction can be found. The ventilation length increases with increasing step height, and also the onset of ventilation starts at lower speeds as the step height increases. The


Smooth hull


100.0 150.0 200.0 250.0


50.0 0.0 0 10 20 30 Speed, Vs [kn]


Fig 1. Specific resistance of smooth hull Fig 1. Specific resistance of smooth hull.


Fig 1. Specific resistance of smooth hull. Total resistance stepped hulls compared to smooth hull


100000 120000


20000 40000 60000 80000


0 15 20 25 30 35 Speed, Vs [kn]


Fig 2. Total resistance for the hulls with step compared to the smooth hull.


SHIP & BOAT INTERNATIONAL MAY/JUNE 2007 40 45 50


Smooth hull Dss=0.125m Dss=0.250m Dss=0.375m


40 50


Performance of smooth hull The resistance characteristics for the smooth hull are shown in Fig 1, in terms of specific resistance as function of ship speed. Specific resistance is defined as:


Rts [N]


Specific resistance, [-]


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