Manoeuvring

Question 1

What are Yv’, Nv’, Yr’, and Nr’?

Answer 1

' non dimensional
Y is sway force
N is yaw moment
r is yaw rate
v is sway velocity
So Yr' is the non dimensional rate of change of sway force wrt yaw rate

Question 2

Give the equation for stability

Answer 2

(Nr’-m’Xg’)/(Yr’-m’)>Nv’/Yv’

N on top y on bottom

Question 3

If the stability equation gives an unstable condition, give the new stability equation (with an autopilot)

Answer 3

(Nr’-m’Xg’+bNδ’)/(Yr’-m’+bYδ’)>Nv’/Yv’

Question 4

Why is manoeuvring important?

Answer 4

For the protection of persons, property, and the environment. From contacts, groundings, collisions.

Question 5

What are the six degrees of freedom and give their numbers? Which three are used in manoeuvring?

Answer 5

1) surge. 2) sway. 3) heave. 4) roll. 5) pitch 6) yaw. Surge sway and yaw are used in manoeuvring

Question 6

Name the different types of rudders.

Answer 6

Balanced, spade, full skeg, semi balanced skeg,

Question 7

What 3 things are carried on ships for manoeuvring criteria?

Answer 7

Pilot card, wheel house poster and manoeuvring booklet

Question 8

Give the different types of manoeuvring trials.

Answer 8

Turning ability test, initial turning test, course keeping ability (zigzag), stopping tests, pull out test and spiral test. The last two are only for when dynamic instability is indicated in the results of the standard tests

Question 9

How do you find the rate of change of sway force wrt sway velocity and the rate of change of yaw moment wrt sway velocity?

Answer 9

Static drift test. Measure Y and N for given β. β is proportional to sway velocity so can plot Y and N against sway velocity get straight line graphs with negative gradient. Angle is dY/dv and dN/dv

Question 10

Describe a static drift test.

Answer 10

In a towing tank. Set model at a drift angle β and measure the resulting sway force and yaw moment. For a given carriage speed sway velocity is directly proportional to drift angle. Repeat for various drift angles. Plot a graph of Y against v and N against v

Question 11

How do you find the rate of change of sway force (and yaw moment) wrt yaw rate?

Answer 11

Rotating arm test. For a range of arm radii (from which carriage rate and yaw rate can be found) measure sway force and yaw moment. Plot these against yaw rate. Get graphs with negative gradient straight line. The angle is dY/dr and dN/dr.

Question 12

Describe a rotating arm test.

Answer 12

Set the rotating arm radius and measure the forces and moments. Yaw rate is proportional to radius for a given carriage rate. Test for a range of arm radii so that a plot of yaw moment and sway force against yaw rate can be plotted.

Question 13

How do you fined the rat of change of yaw moment (and sway force) wrt rudder angle?

Answer 13

Using rudder tests where there is no sway velocity but various rudder angles the yaw moment and sway forces can be measured. These are plotted on graphs where the gradient is dY/dδ and dN/dΔ

Question 14

What are hydrodynamic forces predominantly dependent upon?

Answer 14

Density, velocity, gravity, length, viscosity (m2/s)

Question 15

How do you non dimensionalise a force (what do you divide by)? And therefore how do you non dimensionalise a moment?

Answer 15

Divide by 0.5ρv^2L^2

For a moment divide by 0.5ρv^2L^3

Question 16

How do you check for dynamic stability in the characteristic equation? What must T1’ and T2’ be?

Answer 16

All the roots should have negative real parts therefore T1’ and T2’ should be positive

Question 17

What does thestability equation tell us?

Answer 17

That the centre of pressure in pure yaw must be ahead of the centre of pressure in pure sway if the ship is to possess positive directional stability

Question 18

Give the equation for a simple autopilot.

Answer 18

δ=br’

Question 19

Giv wthe conditions of constants for an autopilot for stability.

Answer 19

C-Eb>0

Question 20

What is K’ and how do you calculate it?

Answer 20

The non dimensionalise gain constant.

K’=-((ψ1-ψ2)/(integral(δdt’)))
Integral between t1’ and t2’

Question 21

What is T’ and how do you calculate it?

Answer 21

Non dimensional time constant.

K’/T’=-(r3’-r4’)/integralδdt’

Integrate between t3’ and t4’