PLT OF AN SMT TWIN STRUCK THE PROPS DURING LNDG FLARE WITH GEAR UP AND SUBSEQUENT GAR.

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BAY APCH DIRECT TO ENTER L DOWNWIND RWY 29 AT ORABU 2K BAY APCH; HDOF TO SJC TWR ABEAM TWR DOWNWIND ASK SJC TWR FOR RWY 30R. SJC TWR CLRS CHANGE TO RWY 30R; 'FOLLOW CESSNA ON FINAL.' REDUCED PWR TO 15 INCHES; 15 DEG FLAPS; TURN BASE; NEED TO SLOW FOR TFC; PUT GEAR SWITCH DOWN AND FLAPS TO 35 DEGS; OVERTAKING TFC ON FINAL AND CLRED TO LAND BY TWR. ASK FOR CHANGE TO RWY 30L TO AVOID TFC ON FINAL. TWR APPROVES CHANGE AND CLRS TO LAND RWY 30L. HAVE CONVERSATION WITH PAX ABOUT RWY CHANGE. REDUCE PWR TO 12 INCHES AT FLARE; PLANE BEGINS TO SINK; GEAR HORN GOES OFF. IMMEDIATE APPLICATION OF FULL PWR; NOSE PROPS AND PROPS STRIKE RWY SURFACE. INFORM TWR THAT I WILL GAR; AS GEAR WAS NOT EXTENDED. TWR APPROVES AND SAID THEY HADN'T NOTICED LACK OF GEAR. I NOTICED GEAR SWITCH HAD BEEN BUMPED TO CTR (OFF) POS. CLB TO TPA; EXTEND GEAR SUCCESSFULLY; CLRED TO LAND RWY 30R; COMPLETE LNDG; TWR SAYS 'GOOD CHKLIST?' TAXI TO PARK. FACTORS: 3 POS GEAR SWITCH (UP; OFF; DOWN); NO GEAR HORN ABOVE 12 INCHES MANIFOLD PRESSURE OR FULL FLAPS NOT LINKED TO HORN; LACK OF CHK ON MY PART (3 IN GREEN); MULTIPLE RWY CHANGES ON FINAL; PAX DISTRS ON FINAL. THIS PLANE HAS BEEN PREVIOUSLY LANDED GEAR-UP BY OTHER PERSON; SAME SWITCH POS (OFF). PROP STRIKE WAS MINOR; NO ENG STOPPAGE. NO FUSELAGE OR WING DAMAGE.