The Ironclad Numancia (
https://es.wikipedia.org/wiki/Numancia_(1864) ):
From 1864 to 1870 the armament consisted on thirty four 68 pounders (200 mm) and a ram. The iron protection was 130 mm thick. She had a top speed of 14 knots.
She participated in the Pacific war against the peruvian and chilean navies. In those years with very fast changes in the world navies the country that incorporated some of the latest battleships improved its position in the ranking of world navies. Spanish navy was fourth but chilean navy was very close. They had a really good navy.
She lead the attack on Valparaiso and then the battle of Callao where seven Spanish ships (the Numancia, 5 more frigates and one corvette) fought a peruvian navy with 2 monitors and 3 gunships and 56 land based guns. The outcome of the battle is not clear. If we take conservative sources of the era, the Peruvian forces suffered around 300 casualties and several batteries were damaged or destroyed and the Spanish navy suffered 43 dead, 83 wounded and 68 lighly wounded (most of them in one of the unarmoured frigates).
Given that the Numancia (and part of the fleet) returned Spain after circumnavegating the globe, I do not think the damage taken was serious.
Apart from that she participated in a weird civil war in Spain.
The Spanish Navy in the 1860s:
We have an american source of 1860
http://www.nytimes.com/1860/11/16/n...e-berenguela-spanish-vessels-gulf-mexico.html
However in a few years (as I mentioned) the composition would change from that to:
* From 1862 to 1868 we have the construction of the ironclads Numancia (40x68pd and 14 knots), Tetuan (40x68pd and 13 knots), Arapiles (2x250mm Armstrong, (4+1)x200mm Armstrong, 10x200mm and 12 knots) Vitoria (30x 68pd and 13 knots), Zaragoza, Sagunto (10x220mm Palliser, 3x180mm Palliser and 12,5 knots) y Mendez Núñez (4x220mm and 2x200mm, speed 11 knots)
* From 1855 to 1868 we have the steam frigates: Berenguela, Petronila, Princesa de Asturias, Blanca, Concepción (20x68pd, 8x32pd and 6x16pd and 12 knots), Lealtad (20x68pd, 8x32pd and 6x16pd and 11 knots), Nuestra Señora Del Triunfo (20x68pd, 14x32pd and 1x220mm and 11 knots), Resolución, Nuestra Sra. Del Carmen, Villa de Madrid (30x200mm, 14x160mm, 2x150mm and 15 knots), Gerona, Almansa (30x200mm, 14x160mm, 2x150 and 2x120mm and 12 knots), and Navas de Tolosa (30x200mm, 14x160mm, 2x150mm)
The US navy in the 1860s (
https://en.wikipedia.org/wiki/Union_Navy#1864–66 ) apparently had much more ships, but most of them unsuitable for a war on open sea.
It was a brown water navy, with lots of slow monitors. Yes, they had big guns, but in few numbers and unable to operate far from the US coastline.
If you pit that against an oceanic navy (of a similar size) of faster ships with more guns (although smaller), you have that the US navy would have to fight where the Spanish navy would dictate, concentrate on key areas of the american coastline to protect from Spanish raids or be destroyed ship by ship...