Fishing Craft Technology

Unit 5 - Marine Engine and propulsion system

5.4.Estimation of engine power for fishing
Power estimation for small fishing vessels are determined by empirical means and also tank tests is utilized for some vessels’ power calculation. Free running speed is generally estimated from Shaft Horse Power of Brake Horse Power curves drawn for a number of displacements on a common length. The power of trawlers in a certain fleet of fishing vessels does not normally vary much, although there is a distinct tendency to increase the power in relation to the size and to an 'inflation' in the rating of engines. The power is expressed in 'HP' which, according to the metric system, denotes the work of lifting 75kg/m/s. Although this definition is a simple one, many different HP are used to determine the size of propulsion of vessels. The most important different horse powers are given below

Power:
Rate of doing work is called power.

Horse power:
It is a unit of measurement of power. It is given as
1 HP = 75 Kg-m/s or 745 watts
1 HP = 4500 Kg-m/min
1 Watt = 1 J/s = 745.5 Nm/s
1 J = 1 Nm = (745.5/9.8) Kgm/s
N = (1/9.8) Kg

1. Thermal Horse Power (THP):
It is a horsepower produced by the combustion of fuel
THP = [Fuel consumption x calorific value of fuel (Kcal/min) x J ] / 4500
Where, J = Mechanical equivalent of heat.

2. Indicated Horse Power (IHP):
It is the power generated in the engine cylinder and available at the top of the piston.
IHP = [P L A N_p] x n / 4500
Where, P = Mean effective pressure( Kg/cm²)
L = Length of the piston stroke ( m)
A = Cross section area of piston ( cm²)
n = number of cylinder
N_p = number of power stroke (for two stroke engine the rpm of engines is N and for four stroke engine N_p is N/2)

3.Brake Horse Power (BHP)
It is the horsepower delivered by the engine at the end of the crankshaft. It is measured by dynamometer fitted at a suitable position, just near the end of the crankshaft.
BHP = (2 π NT/4500)
Where,
N = rpm of engine; T- Torque (kg-m) = (r x f), r is the radius of flywheel and f is force exerted on the periphery of flywheel.

4. Friction Horse Power (FHP):
It is the power required to run the engine at a speed with out producing any useful work. It represents the friction and pumping losses of the engine.
FHP = IHP – BHP

5. Shaft Horse Power (SHP):
It is the power delivered to the propulsion shafting. It is sum of the power required to overcome the resistance of the vessel, losses at the propeller and the losses in the shaft bearings and stuffing boxes
SHP =0.94 x BHP

6. Effective Horse Power (EHP):
In addition to these main definitions, the naval architectures also work with the effective horse power (EHP), which is the power that would be required to tow the vessel (thrust movement of vessel). It is calculated by the tank test through model vessel
EHP = 0.23 X SHP
By considering 20% sea margin for fishing vessel
EHP = 0.23 x0.80 x BHP ≡ 0.25BHP

7. Break Mean Effective Pressure (BMEP):
The pressure, which is developed in side the cylinder to work the system effectively.
For four stroke, BMEP = [BHp x 4500 / L x A x N x n/2].
For two stroke, BMEP = [BHp x 4500 / L x A x N x n]

8. Thermal efficiency:
It is the ratio of the horsepower out put of an engine to the fuel horse power.
Brake thermal efficiency (ηB)
ηB= BHP x4500/ (fuel consumption per hr x calorific value of the fuel)
or
ηB = (BHP/THP) x100

9. Mechanical efficiency (ηM)
It is the ratio of the BHP to IHP of the engine and is expressed as
ηM = (BHP/IHP) x 100