High Performance Marine Vehicles Flashcards
what is hydrofoil?
a foil that reduces drag and makes the hull loft out of the water increasingly with wind
What is
leading edge
trailing edge
chord line
angle of attack
leading edge: foremost edge
trailing edge: rear edge
chord line: connects leading and trailing edges
angle of attack: the angle between chord line and inflow velocity seen by hydrofoil
What are the fluid forces on hydrofoil?
they result from a combination of tangential force (friction) and normal force (pressure)
they vary in magnitude, but the forces can be integrated over the whole of the structure for a resultant force
Describe fluid friction
- related to energy dissipation in the boundary layer
- the speed of the flow is relative to the structure and changes rapidly as we approach the surface
- KE of the fluid is reduced snd eventually turned into heat, leading to friction
describe fluid pressure
- pressure variants over the surface result from several ends
-Bernoulli (high speed = lower pressure, lower speed = higher pressure)
what are the two main categories of fluid forces
- drag - forces parallel to the flow
- lift - forces normal to the flow
How is friction drag created?
in a symmetrical body, the pressure effects on opposite sides cancel out due to symmetry, friction effect add to create friction drag
How is lift created?
Lift results from asymmetry either of structure of the flow
What is the importance of the reference area?
- will affect the actual numerical value of lift/drag
- have to specify it before reporting lift/drag coefficient values
What does the lift coefficient vary with?
angle of attack while flow is sttached
Describe static stall and when it happens
at large attack the flow separates and the foil stalls. This results in reduced lift and large increases in drag.
Static stall only happens at large attack angles
When does the dynamic stall happen?
- due to rapid change of attack angle
- may happen even at low attack angle
Describe flow seperatin
if the body isnt streamlined then the flow will seperate and the pressure st the back will be lower than at the from resulting in additional drag
Describe wave drag
Its most relevant to high-speed ships. Its related to the dissipation of energy in waves.
What causes a wave to be made?
disturbance between 2 fluids (water, air)
What are the characteristics of a wave induced by ship?
- at low speed: transverse wave dominates
- at high speed: divergent wave dominates
- wave length increases with ship speed
Describe wave making resistance
- some energy is lost by the waves traveling away from the ship
- drag is induced due to dissipation of energy from the ship (wave creation)
what are the key component of resistance for ships?
friction, form drag, wave-making resistance
How to reduce friction drag, drag, wave-making drag?
friction drag- reducing wetted area,
drag- designing streamlined hulls,
wave-making drag- reducing beam and displacement
What is a design of a high speed craft dominated by?
- reduce drag
- maintain deck area
-hydrostatic stability
How to reduce the beam, how does it work?
You can make two or more hulls to still maintain stability. It will give a low wave making resistance, more deck area, larger wetted area.
How to reduce displacement?
- additional vertical force must be added
- reduced displacement = reduced beam
-you can make a hovercraft
what is apparent wind?
Wind measured by stationary observer
what is true wind?
sum of true wind plus wind due to the speed of the yacht. it acts on the sails
