Numbers don't lie, but they can be confusing. Interpreting gauges requires knowing what "Normal" looks like for that specific engine.
1. RPM Analysis
The Golden Rule: An engine must reach its rated RPM (e.g., Yanmar 4JH = 3600 RPM).
Low RPM? It's usually the propeller. Over-propping (pitch too high) kills engines by overloading them like driving a car up a hill in 5th gear.
2. Temperature Curves
Diesel engines love 180F-195F.
Too Cold (140F): Thermostat stuck open. Bad for efficiency and carbon buildup.
Too Hot (210F+): Cooling system blockage. Check the raw water intake, impeller, and heat exchanger.
The IR Gun: Trust, but verify. Shoot the thermostat housing with your infrared thermometer. helm gauges are notoriously inaccurate.
3. Oil Pressure
Low oil pressure at idle is common on old engines. But it should jump instantly when throttle is applied.
- Cold: High pressure (oil is thick).
- Hot: Lower pressure.
- The Danger Zone: If pressure drops while RPMs increase, stop the engine. You are grinding bearings.
4. Vibration Analysis
You don't need a vibration meter; you have feet.
- Shaft Vibration: Frequency increases with speed. usually felt in the floorboards. Indicates bent shaft or damaged prop.
- Engine Miss: A rhythmic "stumble" at idle. Indicates a bad injector or compression loss in one cylinder.
Conclusion
Record the numbers every 5 minutes. Build a table in your report. "Idle: 600rpm/160F/40psi -> Cruise: 2200rpm/180F/60psi -> WOT: 3000rpm/190F/65psi." Data protects you from "It ran fine before you broke it" claims.