In the case of a partial loss of a ship or its equipments, the old materials are to be applied towards payment for the new. Whether the ship is new or old, a marine insurer is liable for only two-thirds of the remaining cost of the repairs after such deduction, except that anchors and cannon must be paid for in full, and sheathing metal at a depreciation of only two and one-half per cent for each month that it has been fastened to the ship.