NVIC 10-86: Bow height equivalence for load line assignment

U.S. Coast Guard (G-MTH-5) Washington, D.C. 20593 Phone: (202) 267-2899 NVIC 10-86 5 Aug 1986 NAVIGATION AND VESSEL INSPECTION CIRCULAR NO. 10-86 Electronic Version for Distribution Via the World Wide Web Subj: Equivalence to Minimum Bow Height Requirements for Load Line Assignment

Equivalence to Minimum Bow Height Requirements for Load Line Assignment — PURPOSE. The purpose of this Navigation and Vessel Inspection Circular (NVIC) is to

clarify bow height penalty reduction calculations and procedures and to delegate the review of the calculations for all vessel types to all classification societies recognized by the Coast Guard for the purpose of assigning load lines. This NVIC will provide an equivalent standard to the minimum bow height requirements in the load line regulations (46 CFR 42.20-70 (Regulation 39 of the International Convention on Load Lines, 1966 (ICLL66))). Provision for such an equivalency is cited as "special consideration" in 46 CPR 42.20-70(c). Regulations 46 CFR 42.07-35 and 42.07.40 permit recognized classification societies to review load line calculations on behalf of the Commandant (G-MTH-5). This NVIC reflects current Coast Guard policy and allows recognized classification societies to review bow height equivalency calculations.

Equivalence to Minimum Bow Height Requirements for Load Line Assignment — DIRECTIVE AFFECTED. NVIC 15-82 is canceled.

DIRECTIVE AFFECTED. NVIC 15-82 is canceled.

Equivalence to Minimum Bow Height Requirements for Load Line Assignment — BACKGROUND. Minimum freeboard requirements were originally based upon having a certain

percentage of hull volume above the load line to provide sufficient intact reserve buoyancy to make the vessel seaworthy in heavy weather. Calculation of hull volume at that time was difficult to perform due to lack of electronic calculating aids. Therefore, when an international agreement concerning freeboards was achieved at the International Convention on Load Lines, 1930, certain assumptions were made which provided for freeboard determination based upon linear deviation from the geometry of a standard vessel. The same freeboard derivation methods were used again in ICLL66. A tabular freeboard is obtained from tables based on the length of the vessel. Corrections (both additive and subtractive) are then made to the tabular freeboard for various vessel particulars such as height and length of superstructure, block coefficient, length-to-depth ratio, and the sheer of the vessel. This corrected tabular freeboard is known as the geometric freeboard (Figure 1). The minimum bow height requirement is to ensure that a vessel’s bow configuration develops sufficient longitudinal righting energy (or trim resisting moment) to recover from a bow immersion in a seaway. It was introduced at ICLL66 after the previous requirement for forecastles on tankers was eliminated. However, the minimum bow height requirement was developed without considering the actual amount of resisting moment provided by a standardized bow configuration. Some current vessel designs are quite different from those conventional vessels used to develop the international load line requirements. A new vessel could be deficient in bow height according to the present regulations, but might have a bow configuration which develops a righting moment equivalent to that which would be developed by the standard bow geometry.

Equivalence to Minimum Bow Height Requirements for Load Line Assignment — DISCUSSION. Vessels which do not meet the minimum bow height requirements are given a bow

height penalty (Figure 1). Enclosure (1) discusses requirements for calculating a reduced bow height penalty. The calculations are based upon providing an equivalent bow volume to that of a standard vessel so that sufficient longitudinal righting energy is developed. Enclosure (2) provides a sample calculation to illustrate the method.

Equivalence to Minimum Bow Height Requirements for Load Line Assignment — ACTION.

a. The owner or owner's agent of a vessel which cannot meet the minimum bow height requirements of 46 CFR 42.20-70 may request special consideration from a recognized load line assigning and issuing authority to allow the vessel to meet the reduced penalty discussed in this NVIC. The owner or owner’s agent must prove to the assigning authority that the vessel has a bow configuration which yields a righting moment equivalent to that developed by the standard bow geometry. b. The request for penalty reduction should include the calculations based upon the principles in Enclosure (1) and, if not already submitted for the preliminary freeboard calculations, a lines plan and general arrangement plan. c. The load line assigning and issuing authority will advise the vessel owner or owner's agent of the results of the review. d. The Commandant (G-MTH-5) will provide oversight as discussed in NVIC 10-85 End: (1) Bow Height Penalty Reduction Calculations (2) Sample Calculation Non-standard Distribution: C:e Baltimore (45); Alameda (40); Port Arthur, Honolulu, Puget Sound (35); Miami, Mobile, Long Beach (25); Norfolk, Jacksonville, Portland OR (20); Boston, Portland ME, Charleston, Anchorage, Galveston (15); Cleveland (12); Cincinnati, Louisville, Memphis, Nashville, Paducah, Pittsburgh, St. Louis, Savannah, San Juan, Tampa, Buffalo, Chicago, Detroit, Duluth, Milwaukee, San Diego, Juneau, Valdez (10); Providence, Huntington, Wilmington, Corpus Christi, Toledo (5). C:m New Orleans (140); New York (70); Philadelphia (35); Houston (25); St. Ignace (5); Sturgeon Bay (4). D:l CG Liaison Officer MILSEALIFTCOMD M-65 STRAT MOB, CG Liaison Officer JUSMAGPHIL (1).