RPM Sweet Spot

[BAron Sportsman]

V6 Mercruiser Cruiser RPM.

While I know I'm probably about to get shot down in flames with my question but I've just fitted a V6 Mercruiser to my boat and wondering what an optimum cruise RPM for it would be. The con I have for my own question in my head is that every boat is different, length, weight, material etc so I guess my question becomes a bit blind with possible answers.

Sensibly boat engines do a lot more work in a boat to the same engine in a car, therefore I've always been one to set RPM and accept speed or even economy for that matter. I think it's a safer and kinder way to treat it for long term wear and reliability.

Is there a sweet spot recommended for cruise RPM. Merc don't seem to want to answer my question so I thought I'd throw it around with you fine folk.

Boat is 5.5 meters and I believe will weigh around 1400kgs give or take 50. (Yes I do really need to run it over a bridge and get it weighed now I've changed the engine).

Engine is a 2003, 4.3 190hp merc V6 coupled to an Alpha 1, gen two+. Prop is Solas E3x14.8x19R.

I can find WOT answers everywhere but is there an optimum cruise RPM for this setup. I've only done one water trial so far with the new engine and while it appears to go boofing hard up top (No answers recorded on this part yet as I've not opened it right up for the first trial), it felt to me that the engine sounded happiest at around a lazy 2800/2900 rpm. I wouldn't consider up to around 3400 from time to time would see any major increase in engine wear other than chewing a bit more fuel.

I am rather kind on boat engines but at the same time a long haul fishing spot would be nicer if I got there a bit quicker without taking half a day while also not killing an engine. It certainly goes a lot harder and different to the little 2lt Volvo it replaced and a whole heap new learning.

So I guess my question is: Is 2800-3400rpm considered a sweet range or is the range broader than this forgetting fuel economy? Or better yet is there a recommended cruise RPM "Range"?

Thank you once again fine people.
​Cheers

 

[bruceb58]

Only way you will be able to figure this out is using a fuel flow meter and ideally one that is tied in with a GPS to give you a MPG number. For years I thought my sweet spot was at a lot lower RPM than I thought it was until I got my flow meter.

The same exact engine in a different boat could be way different which is why Mercruiser can't help you.

 

[Bondo]

So I guess my question is: Is 2800-3400rpm considered a sweet range or is the range broader than this forgetting fuel economy? Or better yet is there a recommended cruise RPM "Range"?

Ayuh,.... Welcome Aboard,..... I'd say, dependin' on the hull, yer in the right ballpark,.....

Much over 3500 or so, the fuel bill usually goes Up rather quickly,....

An SBC will hum along at 3500 for years,.... non-stop,....

 

[Scott Danforth]

if your looking for an RPM that will make the motor last, generally anything below 4500 RPM and it will be happy spinning along for years.

if your looking for an RPM that will make your fuel last, generally between 3000 and 3500 RPM is the best cruise window, however you will need a fuel flow meter to verify.

for what its worth, i generally boat 60+ miles at 3300 RPM, however I have made the run many time at WOT.

 

[FreeBeeTony]

1st I would think you need to know what your WOT is, S/B ~4400-4800.
I have a SBC, WOT is 4500 and I cruisee anywhere between 3400-3800.
Just for info purposes...........

 

[harleyman1975]

I tend to think that the sweet spot will be determined once you start using it. Don't feel locked into that propeller. If boat struggles to get on plane try a lower pitch (17) if it turns too high (4800+) RPM'S at wide open throttle pick out a higher pitch (21 or even 23). Once you have the right prop then you can look for the most economical cruising range. You want to be on full plane trimmed out with the engine neither lugging or racing. Most find it to be between 3000 and 3600. Just curious...what engine was in it before?

 

[BAron Sportsman]

Harleyman1975: Previous engine was a 2.0lt Volvo AQ130D to a 280 O/D.

Thanks all for input. I'm still believing 2800 to 3400 should prove the right playing field for engine life and economy. It's just so tempting to lean on that stick cos it certainly gets up and boogies but I don't want to kill it....

 

[NHGuy]

Nope, 2800 to 3500 rpm won't shorten it's life.
The stuff inside that motor is capable of over 5000 rpm, but the way it's top end is set up makes it run out of "breath" before that engine speed. Running at 3400 will seem fast and kind of fun because you have a lot more motor now. It's efficient because that's right in the window where there's the most torque.
And it's easy on fuel per mile because you have much of your hull out of the water which reduces drag.