# 46 CFR § 170.170 - Weather criteria.

§ 170.170 Weather criteria.

(a) Each vessel must be shown by design calculations to have a metacentric height (GM) that is equal to or greater than the following in each condition of loading and operation:


$GM\phantom{\rule{.2em}{0ex}}\ge \phantom{\rule{.2em}{0ex}}\frac{PAH}{W\phantom{\rule{.2em}{0ex}}\mathrm{tan}\phantom{\rule{.2em}{0ex}}\left(T\right)}$
Where -
P = .005 + (L/14,200) 2 tons/ft 2 . . . for ocean service, Great Lakes winter service, or service on exposed waters.
P = .055 + (L/1309) 2 metric tons/m 2 . . . for ocean service, Great Lakes winter service, or service on exposed waters.
P = .0033 + (L/14,200) 2 tons/ft 2 . . . for Great Lakes summer service or service on partially protected waters.
P = .036 + (L/1309) 2 metric tons/m 2 . . . for Great lakes summer service or service on partially protected waters.
P = .0025 + (L/14,200) 2 tons/ft 2 . . . for service on protected waters.
P = .028 + (L/1309) 2 metric tons/m 2 . . . for service on protected waters.
L = LBP in feet (meters).
A = projected lateral area in square feet (square meters) of the portion of the vessel and deck cargo above the waterline.
H = the vertical distance in feet (meters) from the center of A to the center of the underwater lateral area or approximately to the one-half draft point.
W = displacement in long (metric) tons.
T = either:
(1) the lesser of either 14 degrees heel or the angle of heel in degrees at which one-half the freeboard to the deck edge is immersed; or
(2) for a sailing vessel, T = the lesser of either 14 degrees or the angle of heel in degrees to the deck edge.

The deck edge is to be taken as the intersection of the sideshell and the uppermost continuous deck below which the sideshell is weathertight.

(b) If approved by the Coast Guard Marine Safety Center or the ABS, a larger value of T may be used for a vessel with a discontinuous weather deck or abnormal sheer.

(c) When doing the calculations required by paragraph (a) of this section for a sailing vessel or auxiliary sailing vessel, the vessel must be assumed -

(1) To be under bare poles; or

(2) If the vessel has no auxiliary propulsion, to have storm sails set and trimmed flat.

(d) The criterion specified in this section is generally limited in application to the conditions of loading and operation of flush deck, mechanically powered vessels of ordinary proportions and form for which the righting arm (GZ) at the angle (T), calculated after the vessel is permitted to trim free until the trimming moment is zero, is not less than the minimum metacentric height (GM) calculated in paragraph (a) of this section multiplied by sin(T). On other types of vessels, the Coast Guard Marine Safety Center requires calculations in addition to those in paragraph (a) of this section. On a mechanically powered vessel under 328 feet (100 meters) in length, other than a tugboat or a towboat, the requirements in § 170.173 are applied.

[CGD 79-023, 48 FR 51010, Nov. 4, 1983; 49 FR 37384, Sept. 24, 1984, as amended by CGD 88-070, 53 FR 34537, Sept. 7, 1988; CGD 85-080, 61 FR 944, Jan. 10, 1996; 61 FR 20556, May 7, 1996; CGD 95-028, 62 FR 51217, Sept. 30, 1997; USCG-2007-0030, 75 FR 78084, Dec. 14, 2010]