MARINE DESIGN (NA31002)

Ritwik Ghoshal

Spring 2023

Contact:

ritwik@naval.iitkgp.ac.in

Lecture-8

HULL FORM – 4 (Optimum Cp vs Fn)

Planing hull

Hydrofoil Craft

HULL FORM – 5 (Optimum LCB vs Fn)

HULL FORM – 6 (Resistance components)

HULL FORM – 7 (Low speed Fn <= 0.2)

• Rv is by far the largest resistance component in this Fn range.
• Thus emphasis on minimizing wetted surface area
• Stern should be fine enough for boundary layer to develop slowly

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HULL FORM – 7 (Low speed Fn <= 0.2)

• Separation, particularly of the bubble type, should be avoided
• Boundary layer should be as thin as possible to minimise momentum losses
• Fine stern is also necessary to obtain good wake field
• An elongated, pointed bow will have a relatively large w.s.a; so it is better to make the bow more bluff
• Helps in softer fwd shoulder (i.e. larger radius)
• Less deep shoulder wave trough

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• As stern has to be slender than bow, LCB is fwd
• Large fullness of bow causes Cp and Cb to be high

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HULL FORM – 8 (Medium Displ speed Fn 0.2 to 0.3)

• Rw is relatively more important about 17.5% of Rt
• Bow needs to be made finer to reduce the large peak pressure caused by a bluff bow
• Thus, LCB is moved aft; Cb and Cp are reduced

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HULL FORM – 9 (High speed Displ Fn 0.3 to 0.5)

HULL FORM – 10 (High speed Displ Fn 0.3 to 0.5)

• Speed range is typical of fishing vessels and the Rw is the largest component (Rw ~ 62.5% of Rt)

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• Reasons not to make the bow and aft to fine: With fine ends the pressure distribution is reduced, but the peak of the pressure is moved towards midship at both ends (.i.e. wave making length is reduced)

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• With fine ends, a large part of the volume is concentrated near midship, which may cause normal curvatures of the of the waterlines to be unnecessarily large, resulting in deep wave trough being generated – This is called the ‘diamond effect’ – causes large Rw

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HULL FORM – 11 (High speed Displ Fn 0.3 to 0.5)

• A fuller stern at the aft body for vessels in this Fn range has the following advantage:
• Pressure in the stern region is reduced because of the effect of the boundary layer, particularly if the flow separates. Though this gives rise to increased Rvp, but it also results in reduction of the stern waves
• Balance between reduction in Rw and increase in Rvp

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• A relatively full stern at these Fn range results in reduction of trim which can become very high as the vessel approaches the last hump (Fn = 0.45)

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• A thin bow and a relatively full stern will result in higher Cp and LCB continues to move aft

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Streamlined Body:

• A streamlined body is defined as that body whose surface is aligned with the streamlines, when the body is placed in the flow.

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• Thus, the body offers least resistance in terms of Pressure drag.

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• Most of the drag experienced by the body is due to Viscous/Frictional drag, which depends on the surface area exposed to fluid.

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• In this case, the separation usually takes place at the trailing edge.

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Bluff/blunt body:

• A bluff body is defined as that body whose surface is not aligned with the stream-lines, when placed in the flow.

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• Thus, the body offers lesser resistance in terms of Viscous/Frictional drag.

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• There is very a large pressure drag, due to eddy formation after the body leading to a large wake region.

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• Thus the pressure drag depends on the cross sectional area of the body rather than the surface area exposed.

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