I think there are too many nuances to live with many of the dumbed down rules of thumb. On my Volvo’s I was able to observe that increasing the cruise rpm on a heavy boat loaded with dive gear actually saw an almost immediate drop in exhaust gas temps. We would tend to say that the engine was run “harder” because more throttle was applied, but empirically we see that the engine was actually run easier. Rpm by itself is not a bad thing, it’s all about load.
Think about pulling a weight up a hill with a rope directly or with a block and tackle. You will do the same work. But with the block and tackle, you will pull the rope that much further. If you use an engine and increase the rpm with the block and tackle, you will do the same work over the same time, but with much less load per revolution. Clearly rpm is not the whole story.
In my case, my engine would lug more at lower rpm, than higher, just as when you put your car in 2nd gear it lugs more than just before you shift into third.
So when we prop a boat, we often like to claim that getting more rpm out of the higher rpms is better. Because this is setting a very low load at high rpm, ensuring the load at the lower rpm we actually run at is done without lugging. At least, as far as it goes.
Surely these nuances presented themselves are incomplete. Even so, trying to make a blanket statement with just these few is pretty challenging. What if we throw turbocharging into the mix and achieve Intake pressures capable of a much more efficient fuel burn and overall engine efficiency?
Lots of the advice, even the dumbed down variety, carries some valuable insights. But it only goes so far and when people try to turn it into religious style dogma, much of the value vanishes. There is a lot more going on than simply running at 30% or 80% of throttle, sometimes, maybe, probably.
Think about pulling a weight up a hill with a rope directly or with a block and tackle. You will do the same work. But with the block and tackle, you will pull the rope that much further. If you use an engine and increase the rpm with the block and tackle, you will do the same work over the same time, but with much less load per revolution. Clearly rpm is not the whole story.
In my case, my engine would lug more at lower rpm, than higher, just as when you put your car in 2nd gear it lugs more than just before you shift into third.
So when we prop a boat, we often like to claim that getting more rpm out of the higher rpms is better. Because this is setting a very low load at high rpm, ensuring the load at the lower rpm we actually run at is done without lugging. At least, as far as it goes.
Surely these nuances presented themselves are incomplete. Even so, trying to make a blanket statement with just these few is pretty challenging. What if we throw turbocharging into the mix and achieve Intake pressures capable of a much more efficient fuel burn and overall engine efficiency?
Lots of the advice, even the dumbed down variety, carries some valuable insights. But it only goes so far and when people try to turn it into religious style dogma, much of the value vanishes. There is a lot more going on than simply running at 30% or 80% of throttle, sometimes, maybe, probably.