Hi everyone, I'm a new boater and I've face a few issues on my motor
yacht. My boat weights 18.1 tonnes, I have a twin yanmar engine
400 horse each. The yacht is a stern drive and does not have rudder
. Here is my problem. When I'm at 2500 rpm
the speed is only at 10 plus 11 knots. When I increase the rpm
to 3000, the speed actually drops to 9 plus 10 knots. Before I did a full engine
servicing I could reach up to 16 knots at 3300 rpm. What happened? The boat's manual say I can reach up to 24 knots but not I'm struggling to hit 12. Do give me your valuable suggestions. Thank you.