33 CFR §401.70

All explosive vessels requiring a Seaway Explosives Permission Letter in accordance with § 401.68 and all tankers carrying cargo with a flashpoint of up to 61 °C, except those carrying such cargo in center tanks with gas free wing tanks, shall be equipped with a sufficient number of non-metallic fenders on each side to prevent any metallic part of the vessel from touching the side of a dock or lock wall.