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Ordnance department internal minute, representing a late stage in the discussion of tenders to go out for new ironclads.


Requirement 1: Shallow river gunboat.

Draft to not exceed 5 feet. Width and length to not be excessive.
Armour to be sufficient to protect against a twenty-pounder rifle charged with three pounds of powder firing a wrought iron bolt at a range of 20 yards, without spalling or severe working of the shield. Armour to cover all areas of fighting value (engines, battery, conning station) and the waterline.
Capable of carrying at least one 8" rifle, and of providing fire in all aspects.
Minimum contract speed 6 knots.

Requirement 2: larger river ironclad

Draft to not exceed eleven feet.
Armour to be sufficient to protect against an 11" Dahlgren gun charged with sixteen pounds of powder firing a wrought iron bolt at a range of 20 yards, without spalling or severe working of the shield. Armour to cover all areas of fighting value (engines, battery, conning station) and the waterline.
Capable of carrying at least two 11" Dahlgren guns or equivalent weight, and of providing fire in all aspects.
Minimum contract speed 8 knots. Must also be able to tow 3,000 tons of barges at a minimum speed of 3 knots.

Requirement 3: Harbour defence ironclad

Draft to not exceed eighteen feet.
Armour to be sufficient to protect against the heaviest weapons in service at a range of over 100 yards, when charged with the highest normal load of powder and firing wrought iron penetrating bolts. Armour to cover all areas of fighting value and the waterline, or the whole hull if the hull is wooden.
Capable of directing the fire of at least two of the most powerful weapons possible upon a given orientation.
Minimum contract speed 2 knots.

Requirement 4: Coastal service ironclad

Draft to not exceed eleven feet.
Armour to be sufficient to protect against the 11" Dahlgren gun when charged with 20 lbs of powder and fired at a range of 100 yards, and to protect the whole hull as well as engines, conning station and battery.
Capable of carrying two heavy guns, including at least one 15" Dahlgren gun or equivalent, and of firing these weapons at ten degrees of elevation; also capable of employing this firepower on both broadsides.
Minimum contract speed 8 knots.


Requirement 5: Sea service ironclad

Draft to not exceed twenty-five feet.
Armour to be sufficient to protect the whole ship against the 11" Dahlgren gun when charged with 20 lbs of powder and fired at a range of 100 yards, and to protect the whole hull as well as engines, conning station and battery.
Capable of firing a broadside of at least four heavy weapons on either broadside, with the aggregate weight of the broadside cannon to not be less than 105,000 lbs (i.e. two 11" Dahlgren and two 15" Dahlgren, or larger numbers of smaller rifles)
Minimum contract speed 12 knots.



Internal discussions have hashed out many of the details of the expected roles for these ships. The reason for the specification in terms of resistance is the recognition that the 4.5" solid, backed armour of the Warrior at New York better resisted fire than the 11" laminated unbacked armour of the Monitor at the same battle; otherwise the intent is to allow designers to do their best within the restrictions.
The sea service ironclad is intended to be a battleship, the coastal service ironclad is intended to fight enemy forts, the harbour service ironclad to protect friendly ports, and the two riverine ironclads are intended to protect the Ohio river and tributaries (of which the Ohio in particular is now the US-CS boundary) while also having the power to adequately fight the river.
Notably the gunboat design is essentially supposed to resist field guns only - the 32-lber smoothbore would cause progressive damage.