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 TB 55-1900-232-10
V = speed of tow in knots, relative to still water
f3 = a coefficient depending on the shape of vessel subjected to wind (from Table G-4)
G-2.4 TOTAL RESISTANCE . The total resistance is determined by solving formulas 1, 2 and 3 and adding the results.
This tow resistance then is used in selecting the tow ship and designing the tow connection as described in Chapter 6
G-2.5 EXAMPLE. The following example is given to show the use of the three resistance formulas already outlined.
The problem is to determine the extreme requirements for towing AFDL 1 Class floating drydock under all weather
conditions To Illustrate this design, the following known factors have been selected:
a.
Hurricane wind speed 69 knots
b.
Towing speed during hu ricane: 6 knots
r
c
Bottom condition of drydock-moderately rough and in need of cleaning use fl coefficient of 0 65
Substitute into resistance formulas, using above factors and coefficients from Table G-4
R = fl x S (V/6)2
= 0 65 x 13,000 x (6/6)2
= 8,450 lbs
G = 285xBxf2xV2xK
= 2 85 x 220 x 0.4 x 62 x 1.2
= 10,835 lbs
W = C x 0.004 x (Vw + V)2x f3
= 1,400 x 0.004 x (69 + 6)2 x 0.7
= 22,050 lbs
Total resistance
- R+G+W
= 8,450 + 10,835 + 22,050
= 41,335 lbs
G-3 TOWING RESISTANCE OF BARGES
The previous section on towing resistance of drydocks can be adapted for computing the total resistance of barges.
G-3.1 FRICTIONAL RESISTANCE . The method is identical to the one used in Section G-2.1. The wetted surface (S),
is simply the barge's length times width plus perimeter times draft
G-3.2 WAVE FORMING RESISTANCE The cross section area (B), is width times draft. For the typical rake-ended
barge, use f2 = 0.2. This also is applicable to the comparatively blunt ship-shaped bow of some barges such as YFBNs
and APLs For square-ended barges, use f2 = 0.5.
This treatment probably understates the added resistance due to waves m Sea States above 4 or 5 In these cases, the
tow direction may have to be changed to avoid excessive slamming of the bow structure of the tow, assuming adequate
stability of the barge Thus, added barge resistance in heavy seas tends to be limited by other factors. However, if the
barge has a ship-shaped bow or otherwise can withstand ahead wave heights over 8 to 10 feet, Curve 1, Figure G-2, can
be used to estimate the added resistance from waves
G-3.3 WIND RESISTANCE. Use the formula contained in Section G-2.3. The cross sectional area (C), is the freeboard
times beam plus height times width of the deck house or any deck cargo. Use f3 = 0 60 as an average barge figure m
the formula
G-19
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