ORIGINAL: bradfordkay
The whole Guadalcanal campaign was within torpedo carrying range of Rabaul, and yet I can only come up with information on that one attack on US shipping. Something must have held them back - it was a while before the Cactus Air Force was strong enough to be a real deterrent (remember, they did keep coming to Lunga to bomb the airfield).
-I don´t consider 660 miles an effective range for a torpedo bomber attack. You must locate target before you launch an attack, and Guadalcanal was relatively far from patrol planes. It takes about three hours for Betties to reach target, not counting the time needed to refuel, rearm the bombers, during all the time the position of targets can change drastically, so your recon data is not reliable, unless you´re attacking docked ships. I don´t know what was the pattern of ships operations in Guadalcanal, but I doubt if the USN kept them too much time exposed to enemy air attacks, unless they had adequate air cover. It´s possible the Japanese felt they would have been more useful bombing the airfields (a target that doesn´t need to be located and doesn´t move...). It seems that when US ships started to operate closer to Rabaul the frequency of attacks increased, I don´t know. But it may be also the fact that the Japanese felt their bombers were too vulnerable for torpedo attacks vs even unprotected ships (as the result of the mentioned attack suggests), and it wasn´t worth to use them for this purpose unless a really good target was in range (like a cruiser or carrier).
-Here is the report of that Betty attack
http://www.angelfire.com/fm/compass/G4M.htm
The first attacks on Allied forces to be made following the Guadalcanal landings were carried out by G4Ms flying from Rabaul. An attack on the US transports by 26 Betties was to demonstrate to Allies and Japanese alike the vulnerability of the G4M to anti-aircraft fire. At least 17 Betties were shot down. One aircraft damaged by gunfire made a suicide crash on the transport George F. Elliott. The resulting fire destroyed the ship - this was the only damage inflicted by Japanese air attacks in reaction to the Allied landings.
G4Ms operated throughout the six months of fighting on Guadalcanal, suffering heavy losses. By early 1943 the Japanese Navy had developed new techniques for night torpedo attack. These were put into effect on the night of 29/30 January 1943 in the Battle of Rennell Island, in which Betties torpedoed and sank the heavy cruiser Chicago. G4Ms repeatedly harassed US task groups in night attacks from this time until almost the end of the war, occasionally inflicting heavy damage - for example in February 1944 when a Betty torpedoed the Essex Class carrier Intrepid after Task Force 58's raid on the Japanese base of Truk in the Caroline Islands.