Ship Design

A vessel floating on the water surface: a stable system

“Balance of Stability”: Heeling moment = Righting moment

Since Archimedean Principle applies: Weight Force = Buoyancy Force

-> balance of ship stability: heeling lever = righting lever

Problem in model tests:

Similarity according to Froude and according to Reynolds must be kept between model and real ship.

This is not possible due to the same viscosity of the water around the model and the ship.

The first and rough determination of required porpulsion power of a ship has always been done by naval architects by lookng at comparable ship designs, i.e. similar size, similar shape, speed.

Prognosis according to “Admiralty Forumla”

The so called Admiralty Formula is also based on the known resistance (or the propulsion power) of an existing comparable shop, i.e. the ships hull should have a comparable shape

-> do not compare a “fat” bulk carrier with a “slim” frigate", but rather

-> compare a frigate with a frigate or a corvette

The formula was developped by the British Admiralty as a quick and handy prognosis method.

Propeller:

• Input: Rotating Power (Torque x Revolutions)

• Output: Advancing Power (Thrust x Ship Speed)

• Propeller in free flow: efficiency between approx. 0,5 and 0,72

• Propeller behind the ship: efficiency can increase

• The best propeller is the “large diameter, slow turning propeller”

• Restrictions:

  • space behind aft ship for propeller arrangement (draught, beam)

  • power / speed requirements (bulk carrier vs. frigate)

  • propeller load / cavitation

Heavily loaded (and often not purpose designed) propellers show -> cavitation

-> bubbles / “tubes” of pressure below steam pressure

-> can damage material when they collapse

-> create sharp UW noise when the collapse

• speed -> and respective power requirement

• turning circle

• stopping distance from max. speed

• behaviour in sea states / ship motions

• frigate / destroyer designs optimized for at least two speeds:

  • cruising speed (18-21 kts) and

  • maximum speed (27-31 kts)

    -> different in comparision with merchant vessels, optimzed for one cruising speed

• long range for naval ships today more important than high speed

  -> i.e. optimize hull shape for cruising speed

• mostly two-shaft- / two-propeller-vessels today

• submerged stern -> hydrodynamically positive (virtually “longer ship”) good for ships with F_N > 0,3

  -> since the 1980ies: broad stern also favourablle for spacy helo - deck at rear

Three criteria:

• aft ship geometry

• number of shafts

• selected max. speed and max. power

Today: gas turbines, high-speed diesel engines, controllable pitch propeller — reverse speed -> maneuvering, continously variable ~ 20 MW per shaft

• minimize shaft inclination (reduce eddies and cavitation)

• necessary gap between ship’s hull and propeller

• undisturbed flow to propeller has priority vs. exploitation of wake -> homogeneous wake field

  ETA_H=(1-t)/(1-w)

  • typical for frigates and corvettes: 0.995 - 1.080

  • typical for merchant ships: 1.04 - 1.25

Essential criteria for frigate / corvette / naval propellers:

• low noise / vibrations

• low cavitation overall

• late cavitation inception, i.e. at highest possible speed

Essential ship motion criteria for corvettes / frigates / destroyers

1. “green water” on deck

2. “slamming”, especially at the fore ship

3. emergence of the sonar dome