Advice on designing the stern underwater form

Attention should be paid to the following:

1. Minimizing flow separation.

2. Minimizing the suction effect of the propeller.

3. Sufficient propeller clearance.

Separation at the stern

Separation at the stern is a function of ship form and propeller influence. The suction effect of the single-screw propeller causes the flowlines to converge.

Figure 2.25 Transom stern with flared profiles

Distorted form

Straight-knuckled form

Distorted form

Straight-knuckled form

This diminishes or even prevents separation. The effect of the propeller on twin-screw ships leads to separation. Separation is influenced by the radius of curvature of the outer shell in the direction of flow, and by the inclination of flow relative to the ship's forward motion. To limit separation, sharp shoulders at the stern and lines exceeding a critical angle of flow relative to the direction of motion should be avoided. If the flow follows the waterlines rather than the buttocks, a diagonal angle or a clearly definable waterline angle is usually the criterion instead of the direction of flow, which is still unknown at the design stage. The critical separation angles between waterline and longitudinal axis for cruiser sterns and similar forms are:

iR = 20° according to Baker—above this, separation is virtually inevitable.

iR = 15° according to Kempf—separation beginning.

An angle of less than 20° to the longitudinal axis is also desirable for diagonal lines. Adherence to these two angles is often impossible, particularly for full hull forms. Most critical is the lower area of the counter, the area between the counter and the propeller post (Fig. 2.26). In areas where the flow mainly follows the buttocks, no separation will occur, regardless of the waterline angle. This happens, for example, below a flat, transom stern and in the lower area of the stern bulb. If a plane tangential to the ship's form is assumed, the angle between longitudinal axis of the ship and this tangential plane should be as small as possible.

Figure 2.26 Position of greatest waterline angle

Figure 2.27 Separation zone with stern waterlines, above the propeller

The stern waterlines above the propeller should be straight, and hollows avoided, to keep waterline angles as small as possible. Where adherence to the critical waterline angle is impossible, greatly exceeding the angle over a short distance is usually preferred to marginally exceeding it over a longer distance. This restricts the unavoidable separation zone (Fig. 2.27) to a small area.

The waterline endings between counter and propeller shaft should be kept as sharp as possible (Fig. 2.28). The outer shell should run straight, or at most be lightly curved, into the stern. This has the following advantages:

Frequently used, Moulded edge of plating joins not recommended stern line with (virtually)

Figure 2.28 Plating—stern post connections

1. Reduced power requirements. Reduced resistance and thrust deduction fraction.

2. Quieter propeller operation.

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