On Thu, 15 Jan 1998 23:00:35 +0100 "Wim E. van Bemmel" writes: >Hello, > >thanks .. I asked EXPERIENCE, no ideas untested ideas. >White lines do NOT stay white, and paint peels off in a seaborne >environment. Electric or magnetic is definitely the way to go. Optical of any sort won't like dirt and oil. I suppose this could serve as a reminder that it's time to clean the engine room. Get the engine manual or talk to a dealer and see if there are any 'hooks' for a tachometer built in. I'm sure it is a frequently requested option. Reluctance sensors based on Hall cells or coils biased with permanent magnets can be used to sense virtually any moving ferrous part. Examples are gear teeth (is there a gear around the outside of the flywheel?), bolt heads or corners on hex bolts or nuts, fan blades on the alternator fan, rocker arms, etc. They have to be mounted securely so the only possible relative motion is from the part moving as intended. It may be easier to attach magnets, etc. to the propeller shaft than to the engine. You may want to have tachometers for both the engine and the propeller, if the transmission is of a type that can 'slip'. When sensing the alternator frequency, use one of the stator phases before the diode bridge. It will swing from one diode drop below "ground" to one diode drop above the positive battery voltage. If the diodes are external, it is very easy to make a connection to the proper point. Alternators for diesel engines may have it brought out already. In fact, this signal may be already brought up to the control panel and just left there for the optional tachometer. Take a look for little gray wires that don't go anywhere and/or RTFM. Sensing the fuel injection could be done less intrusively by connecting a low-pressure sensor to the fuel feed line between the filter and the pump. A periodic negative pressure should occur there as the pump takes up fuel just before each injection cycle. It may be very slight at idle speed when very little fuel is being injected.