SAR Chapter 5

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SAR
Chapter 5 Towing
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TOWING
AN IMPORTANT SKILL
TOW ONLY WITH AUTHORIZATION
from 5-9
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FORCES INVOLVED IN A TOW
Acceleration Forces - Getting the tow underway
Steady Forces - Force necessary to pull boat
through smooth water
Shock Forces - Forces caused by sea state
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From 5-11
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TOWING VESSEL PERFORMANCE
Engine Load
75 Red Line RPM 75 Maximum Torque
Controlling Engine Load
Vacuum gauge
Twin Screw Vessels
Inboard/Outboard Drives
Measure Cruising Slip and use Table
from 5-13
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Deriving Towing RPM
Slip at Maximum Continuous Cruise Vt/Vc .15 .2
0 .25 .30 .35 .40 .45 .0 .39 .45 .50 .55 .59 .63
.67 .1 .43 .49 .54 .58 .63 .66 .70 .2 .48 .53 .5
8 .62 .66 .70 .73 .3 .54 .58 .63 .66 .70 .73 .76
.4 .59 .63 .67 .71 .74 .76 .79 .5 .65 .69 .72 .7
5 .78 .80 .82 .6 .72 .75 .77 .80 .82 .84 .86 .7
.79 .81 .83 .85 .86 .88 .89 .8 .86 .87 .88 .90 .9
1 .92 .93 .9 .93 .93 .94 .95 .95 .96 .96 1.0 1.0
1.0 1.0 1.0 1.0 1.0 1.0
Table 5-1 Allowable Fraction of Max Cruise RPM
- Towing
from SSG 5-15 Table 5-1
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OTHER TOWING PROBLEMS
Load Distribution
Use a bridle
Stability in smooth water
Stability bridle
Drogue
Along-side
Special Problems
Trailer Attachment
Personal Watercraft
Sailboats
from 5-15
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TOWING BRIDLES
D
0.58 D
Figure 5-5 Sailboat Bridle
Figure 5-2 Bridle Device
from SSG 5-6, 9 10 Figs 5-2, 5-3, 5-5
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TOWING BRIDLES
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TOWLINE
We need a line that stretches
Must be long enough
Gravity Drop VERY important
Must be able to cleat off
from 5-19
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CONCLUSION
3 - STRAND TWISTED NYLON
1/2 OR 5/8 INCH DIAMETER
200 FEET MINIMUM (off-shore)
100 FEET POLY LINE (inland lakes)
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TOWLINE
for SMALL facilities
We need a line that stretches
Must be long enough
Gravity Drop VERY important
Must be able to cleat off
from 5-21
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CONCLUSION
3 - STRAND TWISTED NYLON
3/8 INCH DIAMETER
POLY LINE OKAY
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TOWING FORCE vs VESSEL LENGTH
500
400
Power
300
STEADY FORCE - pounds
Sail
200
100
20 25 30 35 40 45
WATERLINE LENGTH (Lw) - feet
from 5-17
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Characteristics of 200 Towline - Large Auxiliary
Vessels
1/2
1/2 5/8
3/4 DB
Tw Tw
Braid Nylon
Nylon Nylon
Poly Strength, pounds 8,500 7,500 11,700 10,575
Characteristics at 1,500
pounds Working Load Stretch, feet 14 33 29 6.5 Sto
red Energy, ft-lbs 7,800 14,600 13,900 4,600 Gravi
ty Drop, feet 26 16 27 51 Safety
Factor 4.5 4.0 6.3
Characteristics at 4,000 pounds Load Stretch,
feet 21 45 40 14 Stored Energy,
ft-lbs 28,100 44,100 41,400 26,300 Tip Velocity,
ft/sec 295 350 270 260 Gravity Drop,
feet 7.1 5.1 9.0 9.5 Safety Factor 1.7 1.5 2.3
from SSG 5-14 Table 5-2
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Characteristics of 100 Towline - Small Auxiliary
Vessels

3/8 3/8

Twisted Braided
Nylon
Poly Strength, pounds 4,200 3,000
Characteristics at 850 pounds Working
Load Stretch, feet 17 5 Stored Energy,
foot-pounds 4,144 1,850 Gravity Drop,
feet 1.6 4.3 Safety Factor 4.0 3.5
Characteristics at 2,100 pounds
Breaking Load Stretch - feet 22 9.2 Stored
Energy, foot-pounds 11,486 7,900 Tip Velocity,
feet per second 532 390 Gravity Drop,
feet 0.6 1.1 Safety Factor 1.6 1.4
from SSG 5-27 Table 5-3
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DOUBLE BRAIDED
TWISTED
Notes Lines are of same diameter
areas under the curves (energy) are equal
from SSG 5-15 Fig. 5-15
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SEA STATE
SWELLS
characterized by non-breaking sinusoidal shape,
and long period
Requires a short towline
WIND WAVES
characterized by whitecaps wave shape peaked and
short period
Requires a long towline with a lot of stretch
from 5-23
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EQUATIONS RELATING WAVE LENGTH VELOCITY, and
PERIOD
V is in knots ? is in feet Wi is in seconds
V 3.03 Wi
Period (Wi) Wavelength (?) Speed (V)
7 250 21 8 327 24 9 414 27 10 511 30 11 618 3
3 12 735 36 13 863 39
from 5-25
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EQUATION FOR WAVE EFFECT
Wh
C 1.86
Wi
C is in knots Wh is the swell height, in
feet Wi is the wave interval, in seconds
For example, a 6-foot swell travelling at 30
knots has an instantaneous current of 1.1 knots
in the direction of the swell at the top, and a
current of 1.1 knots in the opposite direction at
the bottom. Thus if the towing boat is on the
top when the towed boat is on the bottom, they
experience a velocity difference of 2.2kt. For
this example the wave interval is 10 seconds and
the wave length is 500 feet a 200 ft towline
places the boats almost at the worst possible
relative position, a half wavelength apart. It is
obviously impossible to keep the boats in step
that would require a towline 500 feet long!
from SSG 5-17
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Shock Force for 35 Auxiliary Vessel
2500
50 ft
45 ft
2000
40 ft
35 ft
1500
Shock Force - pounds
30 ft
1000
500
0
0
5
6
8
4
7
Wave Period - seconds
5-27 SSG 5-18 Fig. 5-8
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Shock Force Correction Factor vs Wind Direction
1.0
0.8
0.6
Shock Force Corr. Factor
0.4
0.2
0
0 30 60 90
Tell Tail Angle (degrees between
wind and tow heading)
5-29 SSG 5-19 Fig. 5-9
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SeaScape FACILITY SPECIALIZATION ILLUSTRATION
Name SeaScape Waterline Length 28
feet Displacement 6.0 tons Power twin
gasoline Props 24 Diameter by 22 Pitch Gear
Reduction 21 Continuous Cruise 18 knots _at_ 3,000
RPM Towline 300 feet, 1/2 3-strand Nylon
SSG 5-21 22
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TOW PLANNING ILLUSTRATION SCENARIO
Distressed Vessel Type Cabin Cruiser Waterline
Length Lw 38 feet Sea State 58 crests pass in
exactly 5 minutes Safe Harbor 30º off downwind
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from SSG 5-23 Fig. 5-10
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