27-30' Forums

  • Boating General Talk Forum
    Do I need to consider new props on larger HP engine? - 21 May, 2019

    We recently repowered out boat with some newer engines and increased the Hp per engine from 275 to 300.  Same Verado engine, same gear ratio (1.85), same weight, etc.  Only difference other than the age of the engines was the slight increase in HP.  We are currently running Merc mirage plus 14" props with a 23" pitch.  The props were original to the boat when I bought it, and seemed to fit the boat nicely under the old engines.  My question is whether there is any need to reconsider re-propping the boat to make use of the extra HP.  

    I'm anything but an engineer, but my simple mind says that if I am turning 4,000 RPM's on a 275 with the same gear ratio, or I'm turning 4,000 RMP's on a 300hp engine with the same exact gear ratio and props, that the boat "should" perform the same as the same props are turning the same speed when traveling at a consistent cruise speed.  Yes the extra HP could be useful in the hole shot to get the RPM's up, and the 300's are more fuel efficient due to the internals of the engine/ upgrades software, but from a prop perspective, has anything changed?  Could I increase the prop pitch 2" to get better cruise economy with the slight increase in HP offsetting the corresponding decrease in hole shot performance?  Maybe its to enough HP upgrade to matter, but I have been mulling it over all day and just couldn't figure out the theoretical solution.

    By the way, in the real world, the boat speed seems to have increased 2mph at 4,000 RPM and the boat will stay on plane at a lower speed by a couple MPH's.  I credit the larger calvitation plate on the new lower unit at helping lift the stern at higher speed and the planing speed I credit to better torque and keeping the RPM's running and not bogging down until a little slower.  So yes there are differences even with the slight increase in HP, but the idea of how to prop the boat has me baffled.....

    Might call the guys at prop gods to see what they say, but thought I'd try to educate myself first before I make the call.  Anyone have thoughts on when to reprop?