Changing propeller pitch

[TernVI]

Are you sure or am I reading it wrong? Are you saying that if I run the engine at 2000 rpm and motor with the wind at say 6 knots of speed (stw) and then turn the boat against the wind with a drop of speed at 5 knots the consumption will increase? Why? I would expect that only if you increase rpm in order to maintain the 6 knots will increase the consumption.

It's amazing how little people understand of their engines.
The marine diesel engine, basic non electronic flaour, has a centrifugal governor which adjusts the fuelling to maintain a set speed.
In neutral it will rev to 3000 and use very little. Add some load, it increases the fuelling to attempt to keep the speed correct.
Diesel engine specs are full of numbers about how accurately it will keep the same rpm when the load changes.

The more headwind, the more power it takes to turn the prop at the same speed (rpm).

 

[Buck Turgidson]

For anyone not sure, think about slip.
Fixed pitch propellor, fixed rpm so what is different? The speed of the mass of water flowing past the prop.
Thrust is the difference between the water velocity in front of and behind the prop. It's higher the slower you go at a fixed RPM and therefore the fuel required must also be higher. As you accelerate you offload the prop, less load - less torque required - less fuel flow.

If the engine didn't have a governor then at a fixed throttle position you would hear the rpm hunting as the prop loads and unloads. Like bogging down your car if you ease the clutch too quickly or don't add gas when you approach a hill.