1. Formerly all these vessels were grouped according to the horsepower of their machinery, and were simply referred to as 20, 40 or 60 horsepower gunboats. In the Admiralty draughts for these vessels now held by the National Maritime Museum, it is revealed that for each type there is a named draught which also carries as marginal notes the names of the remainder of the class. It is this named draught which has been used to assign class names
2. In the text the actual diameter of the cylinder is given for the Penn trunk engines. As the internal area of the cylindrical trunk which passes through the cylinder forms no part of the actual steam bearing surface, frequently the ‘effective diameter’ is quoted, and is of a circle whose area is the same as that of the annular ring attached to the trunk. In the case of the 60 hp engine, while the actual diameter of the cylinder was 21 in, its effective diameter was 17·9 in, and it would be this latter figure which would be used for the calculating of horsepower and other measurements dealing with the efficiency of the engine