Battle of Horten Harbour

The Battle of Horten Harbour was an engagement early on in Operation Weserübung when the Germans launched an amphibious assault on Karljohansvern, the Norwegian naval base at Horten. After being initially repulsed by Norwegian naval units, German troops headed overland to outflank the base, forcing it to capitulate.

Background
Karljohansvern, 25 miles up the Oslofjord, had been Norway's main naval base since 1819. On the morning of 9 April two operational ships were in the harbour: the minelayer Olav Tryggvason and the minesweeper Rauma. Approximately 40% of their crews were on shore leave at the time. Also present were the dilapidated coastal defence ships Tordenskjold and Harald Haarfagre, as well as the incomplete torpedo boat Balder, which took no part in the fighting. The commanding officer at the base was Admiral Smith Johansen.

Shortly after midnight Admiral Johansen informed the captain of the Tryggvason, T. Briseid, that there had been a battle at the mouth of the Oslofjord, and it was likely that the enemy ships had breached the outer fortifications. Briseid was given orders to open fire on any hostile ships. Johansen shared his intentions to send the Rauma and the Otra out to sweep the fjord. Briseid dispatched one of his ship's motor boats to Langgrunn where it told the merchant ships at anchor to extinguish their lights.

The German forces allotted to attack Horten were from Kampfgruppe 5. This consisted of the torpedo boats Albatros and Kondor under the command of Siegfried Strelow and H. Wilcke respectively. They were accompanied by the motor minesweepers R17 and R21, heavily laden with troops from the light cruiser Emden. Admiral Oskar Kummetz ordered the ships at 03:30 to detach from the main force and secure Horten.

Battle
The detached force arrived at the mouth of the harbour at 05:35. The R boats proceeded to make their landings while the torpedo boats held back. Olav Tryggvason, which had been guarding the entrance since 03:15, initially let the vessels pass. The ship soon realized its error and opened fire. The minesweeper R17 was struck just as it pulled alongside the quay, sinking in the shallow water. The ship's depth charges detonated at 06:15, causing further casualties. Rauma returned and began dueling with R21. Rauma was damaged and its gun crew was wounded, so the ship retreated to the naval base for medical assistance. Two sailors were killed. R21, also damaged, disengaged.

At 07:30 Albatros tried to directly intervene by heading into the harbour. The boat's course was constricted by the narrow channel it was using, and soon began taking fire from Olav Tryggvason. The Albatros responded with its forward battery, but it jammed after eight rounds and the damaged ship was forced to retreat.

At 08:00 Emden moved over to support the Germans at Horten, sending around 50 soldiers ashore to attack over the land. Outflanked, Admiral Smith Johansen surrendered the base 35 minutes later.

Aftermath
With Karljohansvern now subdued, the Germans could focus on taking the forts at the mouth of the Oslofjord. Albatros transferred its soldiers to R21 so it could provide gunfire support for an assault on Bolærne. That afternoon the ship attacked, but was driven off by 10 rounds from the fort's 5.9 inch guns. While maneuvering in the fog to avoid being hit the ship ran aground, becoming a total loss.

All the ships captured at Horten were brought into service by the Kriegsmarine. Olav Tryggvason became Brummer, after a training ship that was lost in the campaign; Rauma became Kamerun; Tordenskjold and Harald Haarfagre were transformed into AA batteries; Balden was completed by the Germans. All ships were returned to Norway after the war.