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4.2 Effective Pod Area
Considering the pod strut to act as a lifting surface and the pod body to act in the same way as an aircraft wing-tank, there are three steps to consider.
・The geometric increment of span and aspect ratio when adding the pod body
・The effective reduction due to the flow around the pod body
・The increment due to the end-plate effect
Fig. 6 Pod Effective Area
Firstly, the geometric increment should be taken into account by including the added profile area as a part of the lifting surface. Secondly, it is assumed that the flow gets around the pod body so that the span, S is effectively reduced by ΔS = -r, where r is the maximum body radius [17]. Taking these first two points into account the effective pod area should be represented as shown in Fig. 6. The mean chord is given by Eq.(14) and the double body effect is accounted for in Eq.(15), taking k as 2 if the strut is flush with the hull and 1 if significant clearance exists. Next, the end plate effect is accounted for by taking the ratio of the projected strut and pod body cross-section as shown in Fig. 7. This end plate effect is then included as a correction to the effective aspect ratio as shown in Eq.(15). Finally, the lift curve slope is for the pod unit is given by Eq.(16).
Fig. 7 Pod Body End Plate Effect
4.3 Fin Controls
Many new pod designs include a controllable flaps attached to the back of the strut or the underside of the pod body. This flap can be used to control the ship without rotating the pod. A method of estimating the control and stabilising effect of flapped pods is now proposed.
The stabilising effect of the pod should be calculated as before, but now including the movable fin areas with the profile area. Further, if the flap is attached to the back of the strut the lift generated can also be considered using a lift coefficient base on the entire profile area [18].
For a movable flap the flow contribution from the propeller race is parallel to the advance and must be taken into account. Then, assuming axial momentum theory the propeller race contribution can be approximated as in Eq.(17). Where, KM is equal to 0.5 at the propeller and 1 infinitely down stream, then assuming 0.5 for pod calculations. If the required parameters are not yet available or where spurious accuracy would result, UR can be taken as 1.2 times the ship speed. The proportion of the pod which is in the propeller race must be taken into account as shown in Fig. 8. Thus, if AR is the lateral pod area within the race the total lift force FL is given by Eq.(18).
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