The medical vessels' ensign (which apparently is only used by a single hospital-ship covering the Sahara coastal fishing fleet and moving to the Bay of Biscay during the tuna fishing season) is a national flag with the coat-of-arms replaced by a centred, white Maltese cross occupying the full height of the yellow stripe.
José Carlos Alegría, translated by Santiago Dotor, 06 Dec 2000
Album des Pavillons 2000 shows among others the Spanish sanitary/medical service ensign. Only one ship flies this ensign, the Esperanza del Mar, based at Las Palmas (Canary Islands) most of the year. It belongs to the Instituto Social de de la Marina (social institute of the merchant navy).
Firstly, the depth of the concave ends of the cross' arms. According to the image in the 1977 Decree, the maltese cross occupies the full width of the yellow stripe, but immediately beneath the image it specifies "Altura de la Cruz de Malta: 3/8 de a." i.e. "Height of the Maltese Cross: 3/8ths of a." where "a" is the flag's width (height). I believe what this means is that the "height of the cross" referred to here is the distance between two opposed concave (i.e. inner) ends of the cross' arms. Measuring on the image the full length of the arms gives of course half the height (width of the yellow stripe) i.e. 1/2 of "a." However measuring the distance between two opposed concave ends of the
cross' arms gives exactly 3/8ths of "a." This results in much less deeper ends than in José Carlos Alegría's image, which resembles more closely the maltese cross as used in Malta (for instance on the civil ensign) and by the Order of Malta.
Secondly, the meeting point of the cross' arms. On José Carlos' image, the
four arms of the cross appear not to meet – they look like four spearheads. On the image in the Decree, not only do they meet, but it is even possible to inscribe a square with sides equal to 1/25th of the flag's height within the meeting point.
Thirdly, the Decree says the cross emblem is "blanco y ribeteado de negro" i.e. "white fimbriated black."
Obviously the remaining question is how does the actual ensign look. Since there is apparently only one being flown (that of the Esperanza del Mar), a picture or a witness' report would be interesting.
Santiago Dotor, 21 Dec 2000
The only vessel which is entitled to fly the medical service ensign, the Esperanza del Mar, flies the plain civil ensign (with no coat-of-arms). Source: a picture of the said vessel in my files.
Aingeru Astui, translated by Santiago Dotor, 19 Feb 2002