YC 1633 Maximum Towing Speed for an Ocean Tow.pdf

YC 1633 Tow Analysis (Maximum Speed - Ocean Tow)

PMS 325T

Prepared by: Peter W. Witherell/ MSCorp

21 October 2020

Maximum Speed for an Ocean Tow of YC 1633 The main reference used for U.S. Navy Towing procedures in general and applicable to YC 1633 in particular is the “U.S. Navy Towing Manual” (NAVSEA Technical Manual SL740-AA-MAN-010).

As a “Bottom Line Up Front” response to the question of maximum towing speed for YC 1633, the Tech Manual states in Section 6-4.2:

“Barges generally should not be towed faster than about 8 knots under mild sea conditions. Small service craft and some dry docks should be limited to about 6 knots. Deterioration of weather conditions requires appropriate speed reduction to ensure continued safe towline loading.”

Relative to other Navy barges such as the YON and YFNB, the YC 1633 might well be considered a “small service craft” and hence the recommendation for a 6-knot maximum speed would apply.

However, the first statement that “barges generally should not be towed faster than about 8 knots under mild sea conditions” suggests that maximum towing speed for a barge depends on many variables. Setting aside powering limitations of the towing vessel, sea conditions are the main consideration, but not the only one (see Towing Manual Section 6-4.2). Three YC 1633 specific considerations on maximum towing speed are discussed in the following paragraphs.

YC 1633 Towing Provisions

NAVSEA drawing 70001257 provides the design for towing padeyes at the bow (port/starboard) of YC 1633 and also provides a flounder plate design to be used for towing (see Figure 1). The flounder plate Safe Working Load (SWL) is indicated to be 82,000 pounds, suggesting that the SWL for each padeye is at least 41,000 pounds. The 1-7/16” bolt holes provided in the flounder plate are consistent

Figure 1. - YC flounder plate detail with the use of 1-3/8” Crosby alloy bolt type G-2140 anchor shackles, having a working load limit of 21 metric tons (44,000 pounds). In calm water, the drag of an unloaded YC 1633 at 8 knots will be less than 10,000 pounds so, as long as the legs of the towing bridle are each 30’ long to be compatible with the design arrangement, YC structural provisions for towing would not preclude an 8-knot towing speed in mild sea conditions or even 9 knots in calm seas.

YC 1633 Trim and List

Per Section 5-7.2 of the Towing Manual, the YC 1633 should have about 1’ of aft trim while being towed. It must be kept in mind that the towline has the effect of adding weight forward and the catenary of the towline will tend to pull the bow down as the barge is pulled through the water. An emergency anchor setup or deck cargo at the stern, well secured, can be used to achieve 1’ – 2’ of trim aft to offset the towline downward pull forward. As mentioned in the same section of the Towing Manual, it is also important that the YC be towed with zero list, since list would increase the barge’s tendency for yaw and sheer while being towed.

YC 1633 Yaw and Sheering Tendencies

With raked ends, a relatively flat bottom, shallow draft and no skegs, YC 1633 will be very prone to yaw and sheering while being towed. The only way to minimize these tendencies without some towing speed adjustment will be maintain a slight aft trim and zero list, as described above. Both bow-down trim and list will increase yaw and sheering tendencies. Should the prescribed aft trim and zero list fail to produce a stable tow situation while being towed, the only other options for achieving tow stability (described in Towing Manual Paragraphs 6-3.3.4 and 6-3.3.9) will be to adjust towing speed and/or dragging lengths of line (drogues) off the stern of YC 1633. Aside from environmental factors, the maximum speed at which YC 1633 produces acceptable tow stability (i.e., acceptable yaw and sheering while being towed) will define the maximum towing speed at any given time.

Conclusions and Recommendations

Assuming that YC 1633 is in good condition and that “U.S. Navy Towing Manual” (NAVSEA Technical Manual SL740-AA-MAN-010) guidelines are followed, the maximum potential towing speed for the barge should be about 9 knots in calm water or 8 knots under “mild sea conditions”.

Whether or not YC 1633 can actually be towed at those speeds will depend upon yaw and sheering tendencies. Under “normal” sea conditions with some allowance for bad weather, a 6-7 knot average towing speed would be more appropriate to use for voyage planning purposes.