This is from the Warlord Sales page on their web site:
“The F-lighter was originally a converted tank landing craft that was being developed for the Kriegsmarine. Up armoured, shallow drafted and equipped with 88mm guns and flak guns, the F-lighter were small silhouette targets that proved very tricky to sink, invulnerable to torpedo attack and capable of dishing out heavy punishment.”
According to Wikipedia, the Draft of the F-Lighter when loaded was 1.4 meters. The draft of an E-Boat is 1.47 meters. Would imply, based on the Torpedo Rules on page 12 of the rule book, that the F-Lighter Large Barge would be Immune to torpedoes.