Fishing Craft Technology

Unit 2 - Basic geometric concept of the fishing vessel

2.3. Form co-efficient

The definition of hull geometry is also specified in the form of certain co-efficient. Which will later prove as the guide values to decide the fatness or slimness of the hull.
a) Length breadth ratio:
It is defined as the ratio of length between perpendiculars to the breadth mould of the vessel.

LBP
= -------
B

It can be taken as resistance and stability influencing factor.

b) Breadth depth ratio:
It is defined as the ratio of the breadth to the depth of the vessel. It can be taken as a stability influencing factor.

B
---
D

c) Breadth draft ratio:
It is defined as the ratio of the breadth to the draught of the vessel. Which influence the resistance and stability of the vessel.

B
----
d

d) Length depth ratio:
It is defined as the ratio of the length to the depth of the vessel.

LBP
----
D

e) Block co-efficient (CB)
The block co-efficient of a ship at any particular draft is defined as the ratio of the volume of displacement at that draft to the volume of rectangular block whose sides are equal to the breadth, the mean draught and the length between perpendiculars.

Where,
CB = Block co-efficient
LBP = Length between perpendiculars
B = Breadth moulded
d= Mean draught
 = Volume of displacement
Mean values of block co-efficient might be 0.88 for a large oil tanker, 0.60 for an air craft carrier and 0.5 for a yacht.
Volume of displacement =  = L x B x d x CB

Mid- ship section co-efficient (CM):
Mid ships co-efficient to any draft is defined as the ratio of the mid ship section area to the area of rectangle whose sides are equal to the draught and breadth of the ship.
Its value usually exceeds 0.85 for ships other than yacht.

Where,
CM = Mid ship section co-efficient
AM = Area of mid ship section
B = Breadth
d = Draught
Area of mid ship section = AM = CM x B x d

Co-efficient of fineness of water (Cwp):
It is defined as the ratio of the area of water plane to the area of its circumscribing rectangle having the same length and maximum breadth. It varies from 0.7 to 0.9 for ships.

Area of the water plane (AW) = L x B x Cwp
Where,
CWP = Co-efficient of water plane
AW = Area of water plane
LWL or LBP = Length of load water line
B = Breadth extreme or Breadth moulded

h) The longitudinal prismatic co-efficient or prismatic co-efficient (Cp):
The prismatic co-efficient of a ship at any draft is defined as the ratio of the volume of displacement to that draft to the volume of prism having a length equal to the length between perpendiculars and cross sectional area equal to the mid-ship sectional area.

Where,
Cp = Prismatic co-efficient
 = Volume of Displacement m3
AM = Area of mid ship section
LBP = Length between perpendiculars
CB= Block co- efficient
Cm = Mid ship sectional co-efficient
Prismatic co-efficient can also defined as the ratio of block co efficient to the mid ship section co- efficient

Vertical prismatic co-efficient (CVP):
It is defined as the ratio of the volume of displacement to the volume of a prism having depth equal to draught and cross–sectional area equal to the water plane area.

Where,
Cvp = Vertical prismatic co-efficient
 = Volume of displacement
Aw = Water plane area
D = Draught