PLT OF AN SMA SEL MADE AN EMER GEAR UP LNDG OFF ARPT AFTER THE ENG QUIT.

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DEPARTED HILTON HEAD (49J) IN RTE TO VNC; FL. DEPARTED PATTERN TO THE SW. CONTACTED SAV APCH FOR FLT FOLLOWING - - AT 4200 FT ENG DEVELOPED NOISE AND LOSS OF PWR -- CHKED MIXTURE; MAGNETOS; CHANGED TANKS; TURNED ON BOOST PUMP -- NO CHANGE. CALLED ATC AND REQUESTED VECTORS TO NEAREST ARPT. AT THAT POINT NOISE GOT LOUDER AND ENG LOCKED UP BUT PROP CONTINUED WINDMILLING. APPARENT BROKEN CRANKSHAFT. DECLARED EMER. TRIED TO FEATHER PROP. NO RESPONSE! BY THIS TIME WE WERE AT 3200 FT AND SINKING FAST. TOLD PAX AND ATC WE WERE GOING DOWN. THE MARSH LOOKED LIKE THE BEST OPTION. HAD PAX PREPARE WITH PILLOWS AND HAD THEM UNLATCH DOOR. TOLD PAX AND ATC WE WERE GOING TO DO A GEAR UP LNDG IN THE MARSH. AFTER LNDG EVERYONE EXITED THE AIRPLANE AND WAITED ON THE WING FOR HELP TO ARRIVE. MY WIFE HAD A SMALL CUT ON HER HEAD. MY SON AND I WERE UNINJURED. MINOR DAMAGE TO ACFT FROM LNDG BUT DELAYS IN GETTING PERMISSION FROM THE FAA AND THE INSURANCE COMPANY TO REMOVE THE AIRPLANE ALLOWED ENOUGH TIME FOR THE TIDE TO COME IN AND DESTROY THE PLANE.