I know what you mean by “lowering the cruise revs” however this is normally detrimental to Performance, economy and reliability unless you are underpropped and hitting the rev limiter.
After 27 years of setting up pleasure and racing boats here is my thoughts on dropping revs to achieve lower revs at any speeds. Also attached extract from Mercury’s book called “Everything you need to know about propellers. (Good Read) When wanting to lower the cruise revs (like we do by changing into overdrive in a car) the outboard motor will normally be labouring harder and whilst lowering the Revs, you will have to increase the throttle position thus using more fuel. The litmus test for if your boat is propped correctly is ” At full throttle with a normal load, trimmed up correctly it should be as close to the higher spec of the manufactures WOT rev range or as close to the rev limiter as possible.”
If your motor is designed to make say, 60hp at 6000rpm and is over propped and can only pull say 5000rpm it is only producing approx 50 HP so by over propping you are leaving 10hp on the table. The problem with over propping is it makes your boat sluggish to get onto the plane and is even worse when you increase the normal load with more gear, people etc. If the same over propped boat then had dive gear and extra passengers it might only pull 4000 at full speed meaning you are only utilizing maybe 40 of your available 60hp. Personally I like to be able to just touch the rev limiter when trimmed right up (giving highest top speed with normal load) and then I know I have maximin HP and Torque available to get best acceleration (Skiing or bar work) and also load carrying capacity for when lots of fish onboard or dive gear etc. Another added benefit is in rough seas your boat can plane at a slower speed so you can get home more comfortably Your boat will handle better and be safer in adverse conditions as it will have more throttle response when you need it.
If you have a highly powered craft, then you can afford to slightly over propped, but you should try to be at least at the lower recommended WOT range e.g. 5500-6000 which can be found in the owner’s manual or online. FYI each extra inch of pitch of an outboard prop will drop the revs between 200-300 and most styles of outboard props increase in 2-inch increments. However, this difference will be more if the boat is way over propped as you are also losing hp and torque. Hope this helps Just remember there is also gear ratios, diameter and rake to consider when selecting a prop for your boat and your personal needs. This should be done by the manufacture / dealer however I have seen an alarming amount of over propping so always good to do the check yourself on a nice calm day for peace of mind.
Cheer Ross Christensen.