m25xp max cruising rpm

horsemel · November 16, 2012, 06:18:13 PM

I've been through the tech wiki and done a search under m25xp max cruising rpm I am curious about what rpm you cruise at to get the best speed under power. The engine manual says to run up the engine to 3000 rpm when getting the air out of the anti-freeze and I do this with success. However the previous owner said not to cruise at more than 2600 rpm. In most seas we don't get over 5.7 to 6 kts with our three bladed prop at 2600 rpm. With seas of 2-3 feet on the bow we are lucky to maintain 4 kts. The engine manual is moot on the point. In other posts I see that many of you do over 6 kts consistently. What kind of rpm do you cruise and what is the max rpm I should consider for being under power?

Joe Holmes · November 16, 2012, 07:01:26 PM

I typically cruise at 2600 rpm,and get 6.5 knots or so depending on conditions. My boat is equipped with a 15 x 10 three-blade (fixed) prop. I expect others will advise you to check the actual rpms with a strobe, as your tach may be wrong.

Stu Jackson · November 16, 2012, 07:09:09 PM

I agree with Joe. Also find that "sweet spot" so your teeth don't get vibrated loose.

We do some time at WOT 3000 rpm each trip out. If my knot meter says what I remember with a clean bottom, then I always know what the condition of the bottom is. I check that against when the diver is due the next time.

We motor out 45 to 60 minutes each cruise, so we've gotten to understand it. Sometimes I have been known to doddle a bit at LOW rpms, usually when coming home, saying, "I don't wanna go home, I wanna stay on the boat!"

Ron Hill · November 17, 2012, 11:26:56 AM

Mark : The normal "rule of thumb" for a diesel engines' continuous operation - is 80% power.

In an emergency you can push it up to 95% power. I'm sure that at 100% power you'll be blowing black smoke out the exhaust and have little to no gain in speed.

Don't think I'd use the 95% power for more than 30 minutes and I'd keep an eye on the temperature gauge
My thoughts.

horsemel · November 17, 2012, 06:48:21 PM

Ok so it sounds like 2600 rpm is the magic number from what you say. I have read that a 15 x 10 fixed prop is what most have. Is tha the diamter x pitch or pitch x diameter? How do I measure the pitch? I think I will check with a stobe as well just in case the tach is off. I can do the prop this winter and then check the rpms once we get back in the water in the spring.

horsemel · November 17, 2012, 06:59:32 PM

Just answered my own question by googling how to measure a prop. Doesn't sound too dificult.

Stu Jackson · November 17, 2012, 09:15:24 PM

Mark, the prop should have a stamp on it, clear to read. Mine says 15x10 (fixed three blade).

Ron Hill · November 18, 2012, 12:17:21 PM

Mark : As Stu said the prop should be stamped on the aft hub with the diameter X pitch.

You may have to remove the prop or at least clean the hub to read the markings, especially if you or someone painted the prop. A thought

Les Luzar · November 26, 2012, 05:54:22 PM

Just to confirm what everyone else is saying, I have a 15x10, 3 blade prop on my 87 and I have found the cruising sweetspot to be between 2600 and 2650 RPM. My boat runs smooth and cruises between 5.8 to 6.5. On average I would say the I get between 6.0 and 6.2, but keep in mind that speed depends on which way I am going relative to current, wind, and sea conditions. But I have noticed that above 2650, the engine runs faster but speed generally does not increase. It is my understanding that the original prop on our boats was a 15X9, which by the way was marked on my prop hub. With this pitch, I usually cruised at 5.8 to 6.0. After repitching I got an additional half knot or so at 2650. And after repitching, the prop shop remarked my prop hub with 15x10.

stevewitt1 · November 26, 2012, 06:20:16 PM

Just wondering. I certainly agree with the premise of most all comments in this thread. I might run mine slightly faster, 2650-2700 and speeds seem as others posted influenced by wind and wave conditions. I too have 15 x 10 3-blade, but here's my question/comment.

I have a good friend that was previously the Service Manager at a Kubota distributor/dealer mostly agriculture. They use a number of 3-cyl engines in small tractor, mower applications. The consensus with these, virtually all powering hydrostatic systems, is to run them just ever so slightly back from "wide open throttle" The innuendo is that the governor system maintains rpms that are where the engine should be.

I'm not trying to over simplify the issue, nor am I anything close to a mechanical engineer. This is just what I've been told the equipment owners are being told to do.

Steve

visit us Atwww.ocontoyachtclub.com

Stephen Butler · November 27, 2012, 06:26:41 AM

Have heard the same from equipment suppliers, and in fact, used to recommend the same when in the business. When powering hydraulic systems that have some type of pressure/flow regulation system, many manufacturers recommend setting a high rpm and letting the system adjust itself to meet pressure and flow requirements. In a mechanical system such as ours, where the prop can cavitate and the displacement rule rules, this is a much different application. As most have said, we cruise nicely at 2500 rpm and sometimes go as high as 2650 rpm. Hope this helps.

Les Luzar · November 27, 2012, 09:33:46 AM

Here is the way I see it. By observing your engine, you can learn a lot. At 2600 to 2650 RPM, my engine runs at 175 degrees which is it's normal operational temperature on my boat under cruising loads. At this RPM, my boat optimizes its speed, engine temperature is stable, and the boat runs smoothly. I am consequently content. If I increase the RPM from here, my boat seems to work harder, speed does not increase, the engine temperature rises over 180, and sometimes I can observe some white smoke coming from my exhaust. So for me, I like the 2650 RPM for optimal performance on my boat. Observe your engine, and I'm sure you will fine your sweet spot too!

Ron Hill · November 27, 2012, 05:21:01 PM

Guys : When you start talking about RPM, please clarify that you are talking about ACTUAL engine rpm - measured with a laser tack.

The tachometer gauge on the engine instrument panel takes it's signal from the alternator and it notoriously BAD on giving accurate rpm.

Mark Sutherland · November 27, 2012, 06:04:24 PM

HorseMel, I replaced my engine panel, and I KNOW that my tach is not properly calibrated. The "book" cruising RPM is 2600, which is 86% of the 3000 max RPM. On Stu's advice, I determined my "as displayed" cruising RPM by 1)Observing my "as displayed" RPM at full/max throttle, in neutral water/weather conditions(mine reads 3700 max), and 2)I multiplied that figure X 86% to yield an "As displayed" cruising RPM of around 3200 RPM.

Les Luzar · November 28, 2012, 09:31:40 AM

On that same thought pattern, my open throttle is 3,400 RPM on my Tachometer, which is 78% of my indicated tach reading. That is good enough for me. Also, my engine vital signs seem in line with that speed. The main point is not to baby your engine and make sure you get it up to operational temperature to burn your fuel efficiently as well as maximize your speed. But "spare time"! (other than sail). Don't forget to go sailing first!