FOR TOW RESISTANCE
This appendix provides a means of calculating towing resistance for various ships and craft. Data for ships,
barges and drydocks have different sources and, therefore, are presented in different formats.
The calculation often is a trial-and-error process. Resistance is computed for several assumed towing speeds
and different wind and sea conditions. The resulting values are then compared to tow hawser sizes and towing
capabilities of available tow ships.
The towing connection-bridles, chain pendants, etc.- are then selected, towing hawser length determined,
catenary checked and towline hydrodynamic resistance estimated. This process may result in a towing ship
pull requirement or total hawser tension that will require an adjustment to the assumed tow speed.
G-1 SELF-PROPELLED SURFACE SHIPS
The following method combines hull, propeller, wind and sea state resistance into one calculation for
determination of the total tow resistance of a ship. Resistance of the towline is considered. Paragraph 6-3.2
and Table 6-1 provide a method for estimating towline resistance. Table G-1 is used to determine the
resistance of the tow itself by following the step-by- step procedure below.
Identify the ship class, or select a class as close as possible to the one under consideration from
Select displacement (A) in long tons from table G-2. If the ship is known to be lighter than full load,
adjust the full load figures from Table G-2 accordingly.
3. List frontal windage area (AT) in square feet from Table G-2. AT is estimated for ships not listed.
4. List wind drag coefficient (Cw) from Table G-2.
5. List projected area of all propellers (Ap) in Feet from Table G-2. This assumes that propellers are
locked. If propellers are trailing, reduce the Ap by one-half. If propellers have been removed, use Ap
6. Select curve number for hull resistance from Table G-2. See Section G-1.1
7. List curve number for sea state resistance (R ) from Table G-2 . See Section G-1.2.
8. Determine Beaufort wind force number based on expected or measured wind velocity or observation
of the sea state with the aid of Table G-3. The estimate should be conservative and should account
for anticipated changes of weather.
9. List relative wind speed (VR) in knots. (For worst condition, use tow speed plus true wind speed.)
10. Select a heading coefficient (K). If the relative wind is dead ahead, use 1 0. If th relative wind is 20
to 40 degrees off the bow, use K = 1 2. For 40 to 90 degrees relative wind, use K = 0 4. There is
higher wind resistance to ahead movement when the wind is broad on the bow than when directly
ahead, because of the larger ship area presented to the wind. As the wind veers farther aft, however,
the wind effect on the ahead direction falls off faster than the increase of the area presented to the
wind. See Section G-1.3 for additional guidance.