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?
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.
I agree with Joe. Also find that "sweet spot" so your teeth don't get vibrated loose. :D
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. :D
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!" :cry4`
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.
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.
Just answered my own question by googling how to measure a prop. Doesn't sound too dificult.
Mark, the prop should have a stamp on it, clear to read. Mine says 15x10 (fixed three blade).
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
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.
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 (http://www.ocontoyachtclub.com)
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.
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! :thumb:
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.
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.
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! :thumb:
Thanks for all the great input. I see that I need to check the prop size and then check the rpms with a strobe and check that against the tach reading. Once I figure what actual 2600 rpms reads as my tach and have the right prop, I should be good to go. I won't get to my boat to check the prop until after Christmas. I will do the tach test in the water so that waits until launch in the spring. I will keep you posted.
QuoteOn that same thought pattern, my open throttle is 3,400 RPM on my Tachometer, which is 78% of my indicated tach reading.
I'm sorry Les, but I don't understand that statement. Is 3,400 your WOT tach reading? So, do you mean you cruise at 2,650 which is 78% or the 3,400?
A earlier post stated 3,000 as their WOT. To be honest my brain, which doesn't function that well, seems to remember somewhere around 3,100 - 3,200 as my WOT but I wouldn't bet the dink on it.
So, given the presumed inaccuracy of the alternator driven tachs we seem to have, what are our choices for accurate tachometers? I'm not familiar with a laser tach. Are they just a diagnostic tool? Can they be installed? Is taking an accurate reading by one just a snapshot in time or can we make sound conclusions going forward after reading one.
The other comment was
QuoteObserving 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.
Is is safe to assume an potentially over reading tach will be consistently proportionately fast at every rpm?
All you experts are throwing so much data out here my simple brain is having a hard time absorbing it all.
Steve
visit us at www.ocontoyachtclub.com (http://www.ocontoyachtclub.com)
Steve,
I was only trying to share the fact that WOT read on a boats tachometer will vary between each boat based on a variety of factors, and as Ron Hill has indicated, is not the actual measured RPM as done with a laser tack. Also, I believe that the Universal WOT spec is 3,000 RPM but as you have seen from my post and Mark Sutherlands post, there are differences in WOT tachometer readings, his being 3,700 and mine about 3,400. But I never believed that this is my true RPM. It is only my reference point RPM. Also, I did notice that after I had my prop re-pitched, my maximum tachometer RPM reading decreased, probably due to the increase load with the repitch to 15x10. The main point of all of this is that diesel engines like to run, and should not be babied. This is where the running your engine at 80% throttle rule of thumb comes in. Our engines are more efficient and "happy" working hard than at low idle speeds. That is why I suggested that you know your engine and operate it accordingly. I can tell that at 2,650 tachometer reading, my engine runs smoothly, gets to the proper temperature and seems happy. I can also tell that if I give it more throttle, it will rung at a higher RPM so I know that I am not running at full throttle. That also does not mean that 2,650 is a magic number, 2600, 2650, 2,700 RPM are all probably fine for my boat. After six years of operation, she is still running fine (knock on wwd). Sure the best thing would be as Ron Hill suggested and measure your RPM with a laser tack, and adjust your tachometer. In lieu of that, run it up to full throttle, and then back is off some to what you feel is about 80% and observe the vital signs of your engine, sound, vibration, speed, and temperature. If it all sounds and looks good, then you can be happy that you are out on on the water... My motto is "Go Sailing" I hope this makes sense to you! :D
Guys : On checking your actual engine rpm against what the engine tach insturment says, you do not have to be in the water (just connect a hose to the raw water intake so it sucks up water from a container - to run the engine!) Also the engine does NOT have to be ingear (fwd or rev)!
I found that there is no kind of symeterical curve on the error - between the tach insturment and actual rpm. it can vary as much as 100+ rpm at one setting and 50+ at another!?!
I've tryed to change the dip switch settings (see my Mainsheet articles) and found it easier to just make a chart that shows gage vrs actual at about 6 /7 rpm points.
A few thoughts
Ron,
Very interesting. Thanks for your continued input..... Almost everything I learned about my boat is from this site, and I am continually amazed at how much knowledge is shared between our members. :thumb:
Quote from: Ron Hill on November 28, 2012, 05:30:15 PM
...you do not have to be in the water (just connect a hose to the raw water intake - to run the engine!)
If you run your engine out of the water connect the rw pump inlet to a bucket and put the hose in the bucket.
Like this: http://forums.catalina.sailboatowners.com/showthread.php?p=873073&highlight=winterizing
just back from the marina that is repairing our hail damage from last summer. Sure enough, we have a 15 x 9 prop so I need to look into repitching the prop. Also found out today that the marina we sail out of (not the one doing the repairs) is in foreclosure so now we have to wonder what is in store for us in that regard. Why can't we just put the thing in the water and go sail. Also, we spent about 15 minutes watching a coyote make its way along the frozen shore of Lake Erie.
Mark : Posted this before, but with an M25XP I 'd suggest that you repitch to 10 or 10.5 inches. You have an older engine so I wouldn't go all the way to 11".
Again, find a prop shop that uses "Prop Scan" for your repitching. A thought
Ron, I put in a call to Black Dog Props today. Hopefully they will return my call. I definitely want prop scan after talking to the local shop yesterday. They talked about heating the prop and then being able to "get it close". I did not read about going to 15 x 10.5 or 15 x 11. If I read correctly, just about everybody is having good results with 15 x 10. Is there some advantage to the extra .5?
Mark,
For what it is worth, I read all of the posts, and after I thought about it all I re-pitched to 10 inches as opposed to 10.5 because I was afraid of adding just a little too much extra load on my engine. Since most people had positive results with 10 inches, I took that route. I like to run my engine at 80% of open throttle, which in my case is about 2,800 RPM. With my new pitch of 10 inches, I noticed that 2,750 to 2,800 is fine with my temperature gauge remaining at 175 degrees. This was the normal operating temperature with my instrument panel. However with the new pitch, above 2,800 my temperature rises to 180 to 185 degrees, which I attribute to the additional load on the 10 inch prop. And at this increased temperature I noticed a little white smoke as well. So I run at 2,700 to 2,800 RPM all day long maintaining engine temperature and I am happy with the additional half knot. With my 9 inch pitch, my engine did not increase in temperature as much. So there-in lies the delimma. It is important to run your engine and work it at its specified cruising RPM for it's internal operational health. However, I also have a 90 amp alternator which also adds additional load to my M25XP. You don't want to be under pitched, and you don't want to be over pitched! More food for your thought..... 10 inch pitch works for me. Let us know what you do and your results. It may help someone else in the future. It is always good to pay it forward!
Mark : When I took my prop to Black Dog 10?/12? years ago to have it checked (prop shop did a 10" job years earlier) they found that two of the blades were at or over 11 inches. I wrote this up in a Mainsheet tech note article.
Black Dog suggested that if the engine ran well, not overheating and I didn't have black smoke out of the exhaust with that screwed up pitch I had, that I might as well go for 11" rather than the 10.5" that I was asking for. So I went for the 11" pitch and all worked out well. I probably ran that engine at 11" prop pitch for about 2000 hrs till the seals started to leak.
As a caution I only recommend 10.5" as your engine has more than a "few years under it's belt" and you might have a large alternator tapping off an added side load, so I stick to my recommendation that 10.5" be the maximum.
Think about what Les says and make your choice. A few thoughts