how to calculate the optimum RPM for a Mercury 150 ph 4 stroke?

Hi

We just bought a Mercury 150ph, 4 stroke engine with an 14.5x19 pitch prop that we put on a 10m fiberglass boat. I would like to know what will be the optimum RPM with a goal of a midrange cruise/fuel economy?
thanks.

I think you will need a prop with less pitch.------------------Motor may not be big enough for a 30' boat.

http://www.boattest.com/engine-review/Mercury/9500038_150-FourStroke_2013-ref=YT buried quite a way down in article are the numbers you are looking for...but note what racer said in his entry..if you got too much boat for the engine you may never reach optimum..

if you run a 4 cycle at max rpm then expect to use the same gas as a 2 stroke...in fact i burn more than a 2 stroke...my boat is propped at the optimum..6k top...merc says the best economy considering fuel burn and miles traveled is at 3k..i cant run that slow without going nuts so normally run 4-4.5k.....

all that bullchit put out by salesmen about 4 cycle economy is just that..bullchit...

It says what the engine's WOT RPM should be right on the transom bracket where the serial number is. Their's really no good way to "calculate" what pitch prop you will need when you're re powering an existing boat with a different HP outboard. You're going to want to just test it out as is, and prop it out accordingly from there. Each inch in pitch will change your RPM by about 200-300.

So as an example, let's say you test out the boat with the new 150 and 19 pitch prop. You get it on plane and at full throttle, fully trimmed out, you're getting a maximum RPM of 4800. Let's say the engine calls for around 5,500 RPM. That means you're going to want to step down to something like a 16 pitch prop (of the same diameter) to get within range.

General rule of thumb for "cruise rpm" is somewhere around 2/3's to 3/4 throttle.

On older two strokes it was somewhat easy to find - you used a buddy and ran with the top cowl off. When you got to max timing advance (by watching the linkage) and still had some throttle left you were at/near your max cruise rpm.

With electronic ignitions you don't have any "linkage" to watch anymore.

As has been mentioned (but the salesman never tells you), once you get above about 3000 rpms a 4 stroke sucks every bit as much gas as a 2 stroke of similar horsepower.

So once you are "propped" correctly I might run maybe 1000-1200 rpms "below" your max rpms and see how she does on gas.

If you are really looking for that sweet spot the only sure way is to install a fuel flow meter which not only helps find your best rpm but also allows you to tweak the trim for best "gas mileage".

I have one on my 26 footer (which is mahogany over fir construction - nowhere near as heavy as fiberglass - powered by a 140 Ev turning a 15 pitch adjustable prop that rarely gets changed to anything above 16.5 pitch) and even saving .3 or .4 gallons per hour adds up pretty quick over the course of a season.