GA pilot reported while departing a CTAF airport they took evasive action for traffic that departed on a crossing runway resulting in a NMAC. The pilot didn't hear a transmission that the aircraft was departing.

Narrative

Day 0 - At SSI I taxied out from a hangar via B; B6; crossed 16/34; then A for a 04 departure to utilize the longer runway. While taxing I was listening to the CTAF and announced runway crossing. There was no traffic while crossing 16/34. By the time I taxied to the end of A I noted two aircraft using the shorter Runway 34. During my run up an aircraft called departing Runway 34. After my run up; check list review of run up; pre take off; and take off. I announced departing Runway 4. Taxied out; checking for traffic on final; turned straight down the runway; and departed. While on climb out a helicopter appeared abruptly to my right slightly above; near the intersecting runways. I nosed down and maintained the runway heading. The helicopter diverted to the left and continued to climb. This was to the south side of the intersecting runways. The time that passed from the other aircraft announcing departure and the appearance of the helicopter was significant. I am unsure if a 3rd aircraft was departing and the helicopter was overflying the field or if the helicopter called departure prior to executing takeoff. SSI has significant jet and general aviation traffic. It is common occurrence for light local aircraft to use 22/34 and transient aircraft to use 04/22. Nearly every time I fly into SSI there is a bit of chaos around arrival/departure due to the amount of aircraft utilizing both runways and the inability to see traffic on the crossing runway due to hangars and trees. I believe that SSI would benefit from a Tower to improve safety in the operations during the busy seasonal months.