MARINE DESIGN (NA31002)

Ritwik Ghoshal

Spring 2023

Contact:

ritwik@naval.iitkgp.ac.in

Lecture-7

Hull form design

Dwt/Disp vs Dwt

Resistance

group wave-making resistance, form resistance, eddy resistance and frictional form resistance into one force termed residuary resistance

Resistance

Wave system generated for a double wedged model according to Wigley Hull

Prismatic Coefficient vs Parallel middle body

HULL FORM – 1 Ship Length

• Longer ship performs better in a seaway

​

• Longer ship at a given speed will operate at lower Froude number (Fn), which in turn will reduce wave making resistance (Rw).

​

• Longer ship will have increased wetted surface area resulting in increased viscous resistance (Rv).

​

• As Rw/Rv increase with speed, a high-speed ship should be longer than a low-speed ship at a given displacement

​

• Length of the speed should be chosen such that a hollow in the wave making resistance (Rw) curve occurs at design speed.

​

HULL FORM – 2 Ratio B/T

• Minimum B/T for stability is chosen

​

• Increase in B/T will increase the wave making (Rw) as volume is moved closer to the surface

​

• For low-speed ships, a small reduction in L and corresponding increase in B is likely to result in little or no increase in resistance as because of decrease in wetted surface area (w.s.a.).

​

• Larger T will allow a larger propeller dia and thus better propulsive efficiency

​

HULL FORM – 3 Fullness & Displ distribution

• Fullness of the ship and its longitudinal distribution is obtained from the sectional area curve. The critical regions in the sect area curve are:
• Fore shoulder
• Aft shoulder

​

• A too large normal curvature in the fore and aft shoulders will cause unnecessarily deep water troughs

​

• Two main parameters of the sect. area curve are:
• Prismatic coeff Cp
• Longitudinal centre of buoyancy LCB

​

HULL FORM – 4 (Optimum Cp vs Fn)

Planing hull