Yamaha 60HP OBM FT60DET

[Jaslynfong]

Hi,

Just a query. I have a Yahama 60HP OBM FT60DET which I measured to have a RPM of 6000 (RPM of crankshaft). However, after i installed a propeller guard at the propeller area, the RPM drops to 5800. What might have cause this drop?

I understand that the propeller guard acts as an additional loading to the outboard motor. However, what should have change should only be the boat speed and NOT the RPM? Anyone knows why there is a drop in the RPM?

Thanks.