27 Problems of design in broad shallowdraught ships

Ships with high B/T ratios have two problems:

1. The propeller slipstream area is small in relation to the midship section area. This reduces propulsion efficiency.

2. The waterline entrance angles increase in comparison with other ships with the same fineness L/V1/3. This leads to relatively high resistance.

Ways of increasing slipstream area

1. Multi-screw propulsion can increase propulsion efficiency. However, it reduces hull efficiency, increases resistance and costs more to buy and maintain.

2. Tunnels to accommodate a greater propeller diameter are applied less to ocean-going ships than to inland vessels. The attainable propeller diameter can be increased to 90% of the draught and more. However, this increases resistance and suction resulting from the tunnel.

3. Raising the counter shortens the length of the waterline. This can increase the resistance. Relatively high counters are found on most banana carriers, which nearly always have limited draughts and relatively high power outputs.

4. Extending the propeller below the keel line is sometimes employed on destroyers and other warships, but rarely on cargo ships since the risk of damaging the propeller is too great.

5. Increasing the draught to accommodate a greater propeller diameter is often to be recommended, but not always possible. This decreases CB and the resistance. The draught can also be increased by a 'submarine keel'. Submarine keels, bar keels and box keels are found on trawlers, tugs and submarines.

6. Kort nozzles are only used reluctantly on ocean-going ships due to the danger of floating objects becoming jammed between the propeller and the inside of the nozzle. 'Safety nozzles' have been developed to prevent this. Kort nozzles also increase the risk of cavitation.

7. Surface-piercing propellers have been found in experiments to have good efficiency (Strunk, 1986; Miller and Szantyr, 1998), and are advocated for inland vessels, but no such installation is yet known to be operational.

Sterns for broad, shallow ships

High B/T ratios lead to large waterline run angles. The high resistance associated with a broad stern can be reduced by:

1. Small CB and a small CWP. Thus a greater proportion of the ship's length can be employed to taper the stern lines.

2. Where a local broadening of the stern is required, the resistance can be minimized by orientating the flowlines mainly along the buttock lines; i.e. the buttocks can be made shallow, thus limiting the extent of separated flow.

3. Where the stern is broad, a 'catamaran stern' (Fig. 2.33) with two propellers can be more effective, in terms of resistance and hull efficiency, than the normal stern form. At the outer surfaces of the catamaran stern the water is drawn into the propeller through small (if possible) waterline angles. The water between the propellers is led largely along the buttock lines. Hence it is important to have a flat buttock in the midship plane. Power requirements of catamaran sterns differ greatly according to design.

On broad ships, the normal rudder area is no longer sufficient in relation to the lateral plane area. This is particularly noticeable in the response to helm. It is advisable to relate the rudder area to the midship section area AM. The rudder area should be at least 12% of AM (instead of 1.6% of the lateral plane area). This method of relating to AM can also be applied to fine ships.

In many cases it is advisable to arrange propeller shafts and bossings converging in the aft direction instead of a parallel arrangement.

Figure 2.33 Catamaran stern. Waterplane at height of propeller shafts
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