Cruising RPM's

HappyKamper

HappyKamper

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Oct 23, 2013
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I have 37' formula with twin Merc 496 motors attached to Bravo 3 drives. Today while speaking to my mechanic. He asked what RPM I cruise at?
I told him 3600 with a cruise speed of 24.5 knots.
He felt that was high. The operating range is 4400-4800. I feel any slower the boat starts to squat and slow.
Anybody have an idea what my cruising RPM should be?
 
If the boat can achieve that 4800 RPM full throttle and typical load, then 3600. If it can't achieve the minimum 4400, start investigating why and adjust to get there. I'd rather have a boat a little under-propped (less pitch than it can actually handle at max) than maxed-out on pitch. The acceleration and throttle response are worth more to me than that last 0.5 KT.

For example, our current engine is rated at 4800 max, can achieve 5000 if I tweak everything. I'm not going to try to squeeze a little more KT and bring max down to 4800. But I don't run it 5000 either... I try not to cruise over 3600
 
75% of WOT is correct but you must be able to hit the top end of WOT. This is where people go wrong. My previous boat which I bought new in 1999 was delivered with too much pitch and could only reach the lower end of specified WOT. I spent three months and lots of phone between Mercruiser and the boat dealer before finally getting the correct set of props. I was told by the Mercruiser rep at their customer service center that he would rather the engine be able to hit the rev limiter as opposed to running at the bottom end of WOT. The reasoning was as you load the boat and add gear, fuel and people it will still perform as designed.
 
See, the mechanic is right. 75% of 4700 is 3525. Clearly you're over-revving at 3600...

It's got that max RPM range and you're achieving nearly the very top. It's all good! If you want to see 4800, check Bottom, Trim, Loading. It'll do it. Then you can have all 3600...
 
Sure , gas marine engines should be able to be happy with the loading at cruise at 75% of attainable WOT RPM .

But that 75% is by no means necessarily the RPM which will provide the best NMPG and max cruising-speed range if that is something you care to factor in . That usually occurs at a lower planing RPM and every boat is different in that respect.
 
Have you ever had the props scanned? Your not killing it by running 3600 rpms but if you feel like the boat wants to squat at any lower rpms that might warrant a prop tune up. If you bring them in for a scan be sure to tell them what your WOT is and where your trying to get it to. A good shop can give you 100 rpms. I know that doesn't sound like much but not only will you get a higher WOT but also better performance through out the entire rpm range. You could also run with a little tab to push the bow down.
 
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