POLICE have issued a photograph of Hobart yachtsman Bob Chappell in the hope the public can help in their investigation of what they say is a `very suspicious' disappearance. The 65-year-old radiation physicist hasn't been seen since Monday when he was aboard his 16m ketch Four Winds moored off Marieville Esplanade at Sandy Bay.Detective-Inspector Peter Powell from Hobart CIB said this afternoon that the circumstances of Mr Chappell's disappear ance were ``very suspicious''.``We don't believe there's any chance this man has disappeared by accident or some kind of misadventure because of the obvious sabotage that's been caused to the vessel,'' he said. ``There's been efforts made to sink the vessel . . . if Mr Chappell had suffered some accident, that damage wouldn't have been done to the boat. ``So we believe we're certainly dealing with an act of sabotage.''Divers today again searched the Derwent River near where Mr Chappell's 16m yacht Four Winds was moored when he disappeared on Monday.Officers have also re-examined the foreshore and the police vessels Freycinet and Observer have been patrolling the area.Police were alerted to the situation when Mr Chappell's yacht was reported to be sinking off Marieville Esplanade at Sandy Bay early yesterday morning.When police arrived at the yacht, moored in the Derwent River near the Sandy Bay Rowing Club, there was no sign of the owner.Marine police pumped the yacht out and discovered a cut in the toilet inlet pipe.The vessel has been examined by forensic experts but the results are not expected before the end of the week.Four Winds was towed to the entrance of Constitution Dock yesterday afternoon. It is understood that Mr Chappell, who was the chief radiation physicist at the Holman Clinic at Royal Hobart Hospital, bought the yacht in Queensland last year. Anyone with any information relating to the matter can contact Tasmania Police or Crime Stoppers.