COMMUTER ACFT ON OVERWATER FLT HAS ENG PROB. DIVERT LAND.

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WE WERE DSNDING FROM OUR CRUISE ALT OF 5000 FT THROUGH APPROX 3500 FT ENRTE FROM SJU TO STT. WE HAD BEEN CLRED DIRECT TO STT VOR. WE HAD BEEN CLRED PLT'S DISCRETION TO 2100 FT. APPROX 20 DME FROM STT VOR WE EXPERIENCED A SUDDEN DROP OF TORQUE ON OUR L ENG; FOLLOWED BY A LOUD BANG. IT WAS APPARENT THAT WE HAD LOST PWR ON OUR L ENG AND PROCEEDED IMMEDIATELY WITH PHASE 1 ITEMS FROM THE COMPANY'S EMER CHKLIST. AFTER FAILED ENG WAS IDENTED AND CORRECTIVE ACTION WAS TAKEN; WE SECURED THE FAILED ENG. NO PRIOR INDICATION WAS DETECTED BY THE CREW; SUCH AS LOW OIL PRESSURE WARNING OR FIRE WARNING. INITIALLY THE L PROP FEATHERED. SHORTLY AFTERWARDS IT UNFEATHERED. I IMMEDIATELY INSTRUCTED MY COPLT TO TURN R AND FLY DIRECT TO THE ISLANDS CAYO NORTE AND ISLA CULEBRITA. AT THE SAME TIME I REVIEWED THE ENG FAILURE PROCS. THE PROP WAS NOW PERIODICALLY OSCILLATING FROM FEATHERED TO UNFEATHERED POS. AS THE PROP UNFEATHERED; THE ACFT WOULD DROP L WING AT THE SAME TIME IT WOULD INITIATE A STRONG L TURN. I SAW FLAMES COMING OUT OF THE EXHAUST STACK. I INSTRUCTED MY COPLT TO TURN FURTHER R AND AIM FOR THE ARPT AT CULEBRA (CPX). MY NEXT ACTION WAS TO DECLARE AN EMER TO SJU CERAP; BY SAYING 'MAYDAY; MAYDAY; MAYDAY; THIS IS ACR FLT YYX WE HAVE AN ENG ON FIRE AND WILL ATTEMPT A LNDG AT CULEBRA.' ATC'S FIRST RESPONSE WAS TO TELL ME DIRECTION AND DISTANCE FROM STT ARPT. I SAID THAT WE WOULD NOT BE ABLE TO MAKE STT; AND WOULD TRY CPX. I ALSO ASKED IF THEY WOULD PLEASE CONTACT COMPANY. ATC THEN ADVISED US TO SWITCH TO CPX UNICOM FREQ. I ADVISED ALL CPX TFC THAT I WAS INBOUND WITH AN EMER AND WOULD ENTER TFC PATTERN ON A L BASE FOR RWY 13. I MADE 2 SUCH CALLS; THE LAST AS WE CROSSED THE SHORELINE. DURING THE ENTIRE TIME FROM FIRST INDICATION OF ENG TROUBLE THE L PROP CONSTANTLY; BUT NOT AT A SET RATE; FLUCTUATED BTWN FEATHERED AND UNFEATHERED POS. AS THE PROP UNFEATHERED; THE DRAG INCREASE WOULD DISRUPT THE FORWARD MOTION AND THUS THE ACFT WOULD LOSE AIRSPD AND ALT. MAX LOSS OF ALT RECORDED BY THE COPLT WAS 2000 FPM. ONCE FEATHERED; WE WERE ABLE TO MAINTAIN ALT AND AIRSPD. AS WE ENTERED THE CPX TFC PATTERN. I ADVISED THE PAX THAT WE HAD LOST 1 ENG AND WOULD MAKE AN EMER LNDG. I INSTRUCTED THEM TO BRACE AND PREPARE FOR EMER LNDG. I ALSO ADVISED THE FO THAT HE HAD PERMISSION TO LAND ON ANY SUITABLE LNDG SITE; AND NOT TRY TO 'STRETCH IT' TO MAKE IT TO THE RWY. I HAD POINTED OUT SEVERAL SUCH SITES; INCLUDING A BEACH; A ROAD AND A GRASS AREA. OVER THE THRESHOLD WE LOWERED FLAPS 20; SET THE ACFT ON THE RWY; AND BROUGHT THE ACFT TO FULL STOP BY MEANS OF BRAKING. WE COMPLETED ENG SHUTDOWN CHKLIST; SECURED THE ACFT AND EVACED THE PAX. AFTER ASSURING THE SAFETY OF THE PAX; I CONTACTED ACR OPS AND ADVISED THEM OF OUR SIT AND LOCATION. COMPANY DISPATCHED ANOTHER ACFT TO ACCOMMODATE THE PAX AND SENT A MECH TO INSPECT THE ACFT. HE CHKED THE L ENG AND FOUND EXTENSIVE INTERNAL DAMAGE AND THAT THE MECHANICAL LINKAGE BTWN THE COCKPIT PROP CTL LEVER AND THE PROP GOVERNOR HAD SHEARED BEHIND THE GOVERNOR. I HAVE LATER LEARNED THAT ONE OR MORE OF THE MAIN BEARINGS IN THE ENG HAD FAILED.