110,000 long tons
Compute speed length ratio as 0.51. Using the same units (speed/length and feet), inspect Table G-2 for a
similar hull. With the help of one of the many publications providing data on Navy ships, note the T-AGM 20
(14 kts/595 ft) has a speed/length ratio of 0.57. This is close enough for the purpose intended. Enter curve 5
for the T-AGM 20 (Figure G-1) at the assumed tow speed and read resistance per ton. For instance, at 6 knots,
read 0.70 lb/LT . If the disabled ship is loaded fully, its hull resistance is 1.25 x 0.75 x 110,000 = 103,125 lbs.
Even without estimating propeller, wind or sea state resistance, by inspecting Figure 6-1, it is apparent that
towing this ship at this speed is impractical for all single Navy tow ships except the T-ATF and ATS Classes.
Vessels of the latter class can accomplish such tows only under very favorable wind and sea conditions. While
working on curve 5, it will save time to also compute hull resistance for 5, 4, and 3 knots as well, for future use.
The resulting hull resistance values are 66,000, 42,600 and 20,600 pounds, respectively.
Note 3: Ships of comparable size and speed have comparable propeller size . For ships not listed in Table G-
2, select a comparable listed ship for propeller projected area . In this case, there is no comparable ship listed
in Table G-2 . For a ship of comparable speed-the T-AGM-20, propeller size projected area is 150 ft . The
ship in question is 4.4 times the size of the sample, so it will require roughly 4.4 times the power for the same
speed. Therefore, estimate the propeller projected area as 4.4 x 150 ft = 660 ft .
Note 4: At higher wind strengths, Force 5 to 7 (the latter with winds of 28 to 33 knots), the tow will be forced to
head into the wind because of the sea conditions. Therefore, the most severe weather expected should be
checked for head wind and seas to confirm tug and especially tow hawser selection.
G-1.1 HULL RESISTANCE CURVES . Those familiar with previous editions of this towing manual will recall
that a large number of hull resistance curves formerly were displayed. Figure G-1 has only five curves. This
simplification is the result of the following factors:
The hull resistance curve for ships tends to become unique at higher speeds where wavemaking
resistance predominates. At low speeds usually associated with towing, there is not a wide variance
m the curves when plotted on a per-ton basis. Five curves were found to cover the ships of interest.
It was found that some divergence from the simplified curves used here commenced at seven-or
eight-knot speeds. Even at 10 to 12 knots, the error introduced is modest.
Additional accuracy is not justified in view of the rather gross assumptions made elsewhere, e.g, 25
percent fouling allowance.
The methods for predicting additional resistance due to waves are not well developed for the low
speeds associated with towing. The degree of accuracy of estimating the added resistance due to
waves, see Figure G-2, does not justify relatively higher accuracy in the other components of total
Most towing ships have the capability of measuring towline tension. These observed resistance
figures will supersede any computed values in the Captain's operational decision-making process.
Nevertheless, the ability to predict tow resistance is extremely important in tow planning, and should continue
despite the comparative lack of precision in the process.
(Text continued on page G-17.)