I've always heard 75% of max RPMs. For example, @ 5200 RPM max, that's a cruise @ 3900 RPM.

The formula is different for carb engines the 12-12.5 number isn't for carbed engines it is just for mpi.

I have twin procharged engines and I just consider cruising anything without having boost. For me that is anything under 4000 to 4200 rpm. Mine will very on speed depending on water conditions and load. It also depends on where I run the tabs and drive for a good ride. Most of the time 3500 to 3800 from 66 to 68 mph.

As you can see from the variety of responses here, cruise speed is an arbitrary and meaningless number. You can just ignore it.

Michael

i like post 6. boobies flailing. means the smaller busted your woman, the faster your cruise speed is. also need less left tab. it is defined though as best miles per gallon speed. so you gotta compare gph and mph...note; idling on 1 engine doesn't count...

I agree with the "lots of factors" idea. My cruise speed and RPM will vary with the fuel and passenger load. My 382 likes to cruise at about 3800 and with a little tab with a full load of people and fuel. I do listen for the secondaries and back slightly off from them opening. When I am lighter its lower. Water conditions play in also.
Just my .02

Bravos break most often between 3-4K rpms so I keep it above that

anything under 3500rpms with cubic inches and twin blower motors or you will need a fuel bardge and all major gas cards at your conveniance

Not sure I understand when people say hurt the motor. For the most part stock engines (Merc for example) have an operating range. It says nothing about "hurting" the motor when using a motor at any point in the operating range. Sure at WOT you are burning a ton of fuel and maybe accelerating wear a bit. But as long as the engine is operating within its normal parameters for any given RPM, you are not "hurting" the engine.

I do know some boaters that absolutely cringe when they push their boat motors toward WOT. I guess it is the noise that freaks them out?

Marine engines are tough! Don't be afraid to use them from idle to WFO. Just keep an eye on your gauges.

Cruise? It's the point where speed/fuel efficiency is at its peak for any given boat. There can be no "standard" as there are too many variables.

My 382 with 500 EFI's cruises at 3800 rpm's and is running at 43-45 mph depending on water conditions. I always wanted to change prop pitch to see if I could obtain a better cruise speed at the same rpm's. My max wot rpm's are 5400.