Iran threatens Mediterranean closure over Gaza, without saying how | Reuters
Does not seem very feasible to me, since there are no countries near the Mediterranean Sea that are likely to be very friendly to Iran, with the possible exception of Syria, and Syria's seaports are a short flight away from Israel.
So I checked on
Islamic Republic of Iran Navy and that led me to
List of current ships of the Islamic Republic of Iran Navy I was surprised to discover some submarines there, because submarines are the only feasible kind of warship for such threats. Surface ships are too visible to spy satellites.
Iran has four long-range subs: three Russian-made
Kilo-class submarine with a range of about 12,500 km and one locally-made
Fateh-class submarine with a range of about 6,700 km. All four subs are diesel-electric, meaning that they must be refueled.
Iran has several naval bases -
What a New Naval Vessel Says About Iran’s Ambitions at Sea - New Lines Institute and
Iranian Naval Forces East-of-Hormuz strategy - Nato Defense College Foundation - with its main one being at
Bandar Abbas at the Strait of Hormuz.
The shortest route to the Mediterranean Sea goes through the Bab el Mandeb strait, the Red Sea, and the Suez Canal. But Egypt is not likely to let Iran's subs go through that canal.
But can a sub sneak through? It could do so by staying submerged the whole time, but I wouldn't bet on that. I checked on
Suezmax and that canal has no locks, unlike the Panama Canal. A canal lock is a segment of a canal whose water level can be varied to get a ship between neighboring parts that have different water levels. So getting into and out of a lock will not need to be done, and one does not have to worry about sharing one's lock space with a ship that is legitimately using it.
The Suez Canal's limiting draft (depth) is 20 meters, its limiting air draft (height) is 68 m, and its limiting beam (width) is 50 m. A Kilo-class sub has a length of 72 m, a beam of 9.9 m, and a draft of 6.2 m. Using the schematic picture in the Wikipedia article, the while line in it is 6.9 m above the lowest extent of the vertical tail fin, the top of the hull is 9.8 m, the top of the sail is 14.6 m, and the topmost projecting part 19.4 m. So this sub will barely be submerged in the Suez Canal.
That means going the long way around, like the ships that are being threatened by the Houthis. Going in a straight line as much as possible, the distance between Bandar Abbas and the Strait of Gibraltar is nearly 19,000 kilometers, too much for a Kilo-class sub. "Straight line" being a geodesic, for a sphere a great circle.
It will be necessary to refuel the sub somewhere along the way, so Iran's leaders will have to twist arms in Africa, like offering them discounted oil in exchange for letting Iran's subs refuel.
The farthest an Iranian sub's forward base can be is about 6,000 km, because the sub will have to do a round trip to and from it. That is the distance to Nigeria, but Nigeria is already an oil producer, so buying a forward base with oil won't work. That might work with closer countries, however.
The Strait of Gibraltar is a natural choke point, like the Bab el Mandeb, but none of its neighboring countries are likely to be friendly to Iran. They are all NATO members, or in the case of Morocco, closely cooperating with NATO. The same is mostly true of the Mediterranean Sea more generally, with Syria, the only likely ally, too close to Israel for their comfort.