Interceptor amidships without step The promising results of the interceptor at the step edge led to the question whether the step was really necessary? Was it possible to create two separate planing surfaces just by introducing an interceptor blade below a smooth hull? And if it was, how effective would this be compared to the stepped hull and the stepped hull with interceptor? An interceptor blade was therefore introduced


at the same longitudinal position as the step. Fig 8 shows the effect on resistance of an interceptor amidships, compared to the smooth hull and the hull with both transverse step and interceptor. As we see from Fig 10 the interceptor blade


below a smooth hull has basically the same positive effect as the hull with both step and interceptor. Above a certain speed the interceptor blade alone creates a significant ventilation length, thereby developing two separate planing surfaces without using a step. It also seems that with a proper tuning of the


interceptor depth with speed, the reduction of the resistance is about the same as for the hull with both step and interceptor. At


Resistance of planing catamaran with mid interceptor


100000 120000


20000 40000 60000 80000


0 30 35 40 Speed, Vs [kn]


step but with mid interceptor compared to the smooth hull and the 0.25m stepped hull with 16.7mm interceptor.


45 50


Smooth hull Dss=0.25m Dis=16.7mm Dss=0m Dis=25.0mm Dss=0m Dis=16.7mm Dss=0m Dis=33.3mm


the craft can be run to gain the most optimum resistance, as a function of speed and sea state. For low speeds, the craft can be run as a


traditional smooth planing hull. When the speed exceeds the speed where the interceptor gives a sufficient ventilation length to reduce the total resistance relative to the smooth hull, the interceptor can be lowered. As the speed further increases, the interceptor depth can be reduced in order to optimise the sum of the


Keel Ventilation length in Full Scale 10.00


0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00


30 35 40 Speed, Vs [kn]


Fig 11. Keel ventilation length, smooth hull with mid interceptor. Fig 11. Keel ventilation length, smooth hull


with mid interceptor. The base drag method gives us the possibility


increased resistance due to the interceptor and the reduced resistance due to the ventilation behind the interceptor. In the same way, the craft can be run without


interceptor if the sea state is so harsh that the ventilation behind the interceptor cannot be kept stable. These are advantages that are impossible with a traditional stepped hull.


Fig 10. Total resistance for the hulls without step but with mid inte Fig 10. Total resistance for the hulls without


approximately the same speed as both the stepped hull with and without interceptor at the step edge (somewhere between 30knots and 35knots in our design), the resistance for the hull with interceptor amidships becomes lower than for the smooth hull. These effects proves that the step is not really


required in order to get the advantages of two separate planing surfaces, and even more remarkable, the interceptor alone seems to be more effective than the traditional design of the step alone. The results further indicate that at the speed where the hull with interceptor gets a favourable resistance over the smooth hull, the interceptor depth needs to be relatively larger than for higher speeds. Fig 11 shows how the ventilation length for


the smooth hull with mid interceptor develops compared to the stepped hull with interceptor. As we see the ventilation length is not as large as for the hull with interceptor at the step edge, although the reduction in the resistance is about the same. The ventilation length also increases with increasing interceptor depth, as seen for the speed of 40knots in Fig 11. The advantages found for the interceptor


blade below the smooth hull gives opportunities to think design of high speed planing hulls in a whole new way. By applying a dynamically controlled interceptor below a smooth hull,


64


Scaling Resistance is scaled to full scale using the ITTC’57 correlation line without form factor to extrapolate the frictional resistance. Running wetted surface is used to calculate the frictional resistance. The wetted surface was observed from photographs and visual observations. In the case of ventilation, the frictional


resistance is calculated separately for the fore and aft part of the hulls, using the local Reynolds number calculated from the observed wetted length. A roughness allowance was included using the formula:


DCF = [110.31x (HxVs )0.21


- 403.33] x CF 2


Where H is the roughness height in mm. A


value of H=70 was used. Also, the roughness allowance was calculated using the local frictional coefficient. The step heights were chosen based on the


base-drag method. The base drag is used in ship resistance predictions to account for the pressure difference at the transom. Here the base drag coefficient was used to predict how large the step area had to be in order to get sufficiently low pressure behind the step. The base drag coefficient is expressed as:


CBD = 0.029 x (SB = FBD 1/2 rV 2 / SF) 1.5/ CF 0.5 The base drag is due to the pressure force at


the transom, and can be expressed as: CBD


SF =DrBD From this it follows that: SB /(1 /2 rV2 SF )


Propulsion The original smooth hull that we tested was arranged for waterjet propulsion. Waterjets with conventional flush inlets is however not practical in our modified design, because so large amount of the afterbody is ventilated,


SHIP & BOAT INTERNATIONAL MAY/JUNE 2007


to estimate the necessary step height to get ventilation. As we see, the necessary step height is very dependent of speed. For low speeds, it is impossible to get ventilation, while for high speeds, very small step heights will be sufficient to get proper ventilation. The model experiment showed that in model


scale, the base-drag method estimated the onset of ventilation quite good for the combinations of speed and step height that were tested. If the base-drag method is used in full scale, the step heights become relatively smaller than with the direct scaling, since the base drag coefficient is larger in full scale than in model scale. The wetted surface and wetted lengths,


including the effect of ventilation, was scaled geometrically. When step height and interceptor height is optimised, it is really an optimisation of ventilation length and lift related to the step or interceptor that is done. It is assumed that the step and interceptor heights will be optimised in full scale to obtain roughly the same ventilation length, so that the question of scale effect is mainly a question of how to scale optimum step and interceptor heights, not a question of scaling the resistance or resistance benefit of using a step or interceptor. In this paper the scaling of the step height


and interceptor depth has been done directly (by the scale-factor). The interceptor depths in model scale were chosen based on the boundary layer momentum thickness. The momentum thickness is relatively smaller in full scale than with direct scaling. One might argue that the interceptor height should be scaled using the same ratio relative to the momentum thickness rather than by direct scaling. However, that has not been done here.


DrBD = rgh This gives the following expression for the


step height: Ds


Dss=0.250m Dis=16.7mm Dss=0 Dis=25mm Dss=0 Dis=16.7mm Dss=0 Dis=33.3mm


45 50 = 4756 X [(Dr2 CF ) /r2 BD V 4 ) X SF X (cosb/B) The ventilation of the step is by air with


atmospheric pressure, the base pressure is then given by:


0.029 (SB


/SF


) 1.5


/CF


0.5 = DrBDSB


/(1


/2


rV2


base area to wetted area in front ratio: SB


/ SF= 4756 X(DrBDCF ) / (r2 v4 )


For a V-shaped hard chine planing bottom with deadrise angle b, the base-, or step-,area is expressed as:


SB = (B /cos b) Ds


SF


) We then get the following expression for the


Rts [N]


Ventilation Length [m]


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