Maximize stability to keep boat upright
Wide hull& low CG
Movable ballast to windward
Wide hull
Keel to windward
To maximize engine size (sail area)
GZ - The rightening arm
B
G
B
GGZ
Rightening arm
Rightening moment = displacement*rightening arm = mg*GZ
Influence of Hull Shape
B
GZ
Equal displacementEqual heeling moment
11 deg22 deg
G
B
GZ G
Same GZ at different heels since B moves differently depending on hull shape
Stability characteristics set by beam and centre of gravity
Form stability
Righting Arm - GZ curve
-0.5
-0.2
0
0.2
0.5
0.7
0.9
0 45 90 135 180
0.8
0.6
0.4
0.2
0
-0.2
-0.4
Text
GZ (m)
0° 45° 90° 135° 180°
Angle of Heel (degrees)
Stability range
Max GZ:0.8m @60°
The moment equilibrium eq.
CE
CLR
G B
GZ
CEz-CGz
Moment equilibrium: MH + MR = 0
Heeling moment: MH = FAY /cos(heel)*(CEz+CGz) Rightening moment: MR = GZ*mg
FHY /cos(heel)
FAY /cos(heel)
The ability to effectively build rightening moment dictates the performance
At low drag cost
mg
Maximize GZ to keep boat upright
Beautiful picture of wide hull of TP52 boat RAN
Note position of movable ballast
Differences between boat types
Monohull
CatamaranTrimaran
Same rig => same heeling moment
Small GZ requires large forces, i.e. monohull must be heavier than cats/trimarans !
GZ
90° 180°
Catamaran TrimaranKeelboat
0°
heel heel heel
90° 180°0° 90° 180°0°
GZ GZ
GZ - for different hulls
Different Hulls Narrow hulllow G
Wide hulllow G
Wide hullhigh G
Effects on GZ
GZ ok
GZ ok GZ large
GZ ok
GZ okGZ ok
GZ<0
GZ<0
GZ<0
GZ≈0
Wide hulls superior at low heel angles
GZ large
Multihulls (high G) small stability range
Wide hull looses GZ...
Narrow hull large stability range
Water Ballast
-1.200
-0.960
-0.720
-0.480
-0.240
0
0.240
0.480
0.720
0.960
1.200
0 36 72 108 144 180
Rig
htin
g ar
m, G
Z (m
)
Angle of Heel (degrees)
no water ballastwater ballast added to windward side
Homework 2- Hull and Stability
Digitize a hull from a lines drawing and examine the flotation and stability
msy_hydrostatics Numerical tool for basic calculations
- Reads geometry files from text files (britfair)
- Hull discretized in sections (like loaf of bread)
- Hull loaded with mass and KG, LCG, TCG
- Calculates draft, equilibrium flotation, wet area, CB, GZ-curve etc
- Functionality for hull modification
Download from course web site
Homework 2- Hull and Stabilityand hull shapes
- Digitize the hull sections view into numerical model via Britfair file.
- How to find the scale? :-)
- Calculate GZ-curves
- Modify the hull and investigate effects
- Design waterline: Kwl
- Z-waterline position: Zwl
sections view
Stern Bow
Kwl=Zwl
B
D