My Beneteau Oceanis 370's ST-50 depth sounder stopped working correctly. It flahes at 38 feet or flashes at 0 feet. Raytheon thought it might be a dirty transducer so we scraped the bottom a few times yesterday by diving below. It still blinks steadily at 0 feet. So, we're thinking about installing a shoot thru fishfinder or depthsounder. The marine store asked me if our hull was fiberglass or cored (with fiberglass.) He said if it was core then a shootthru would not work. So, is my hull fiberglass or is it cored fiberglass??? Thanks in advance! Ron