4600 plane speed and RPM with 450C engines

[Gargiulotime]

So the expected fuel burn 2200 rpm @ 19-21 knots is .9 per engine ? Or combined.

[SteveP]

For my boat the combined is a little less than 1 NM / gal at cruise. I get a little better at hull speeds.

[kenyu73]

I'm getting 23.5knots (27 mph) at about 2450 RPM WOT. 2001 450s, 80% fuel tanks.

What's the benefit of having higher RPMs for a few more knots/speeds? I'd suspect these should be the factory props. I'd think I wouldn't want to get more RPMs which would be more things moving faster in the motor, right? Or are you thinking less RPMs means more resistance somewhere?

[SteveP]

Go to boatdiesel.com or seaboard marine sbmar.com and search on 450 diamond. If you are not getting 2600 (preferably 2650+) you may be over propped, which puts more demand on the motors. This results in higher exhaust temps and possible valve damage. There are several tests you can run such as unloaded WOT to determine if it’s something external to the engine (e.g., props/dirty bottom) causing the problem. You should also validate your gauges with readings directly from the flywheel.

[Glenn111]

First step...verify the accuracy of your gauges.

[kenyu73]

Originally Posted by Glenn111

First step...verify the accuracy of your gauges.

I totally agree. Though I feel they are somewhat accurate as both motors are reporting similar RPMs.

I'm more of a weekend warrior with my boat so I only go about 11mph (pulling a dinghy) so I rarely get on plane. I mainly posted this question as I was getting a survey done.

Over winter I plan on setting up a MFD and installing an engine analog to NEMA kit so I'll find out what the true RPMs are next year.

[Glenn111]

Your motors will LOVE you for years to come with 2650+. Home Depot is where I bought my digital/laser tach.