Originally Posted by Longranger5
One captain told me that his yacht is not safe to run below 1800 rpm at 18 knots since the turbo charger stops below that speed.
I suggest that if anything makes that yacht unsafe it might be the captain.
The turbo does not "stop" and the output of a turbocharger is related to engine load, not rpm. If he can maintain 18 knots while reducing rpm he has a planing hull, a magic boat, or is operating well beyond hull speed in which case reducing rpm would be good for the engine which is probably well and truly overloaded.
You should obtain the engine manufacturer's data on engine output vs rpm to determine the performance you should expect. Each engine maker publishes a performance chart based on the use of a "well matched" fixed pitch propeller. This chart will tell you how much horsepower the prop absorbs at a given rpm and tells you the fuel consumption at each point. The chart has data points for operations within the manufacturer's acceptable range.
As far as the actual speed and range of the vessel in which the engine is installed, that is a whole 'nuther animal that only sea trials and operational history can determine.