m25xp max cruising rpm

[horsemel]

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.

[stevewitt1]

On 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

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.

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

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[Les Luzar]

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! 

[Ron Hill]

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

[Les Luzar]

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. 

[Stu Jackson]

...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

[horsemel]

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.

[Ron Hill]

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

[horsemel]

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?

[Les Luzar]

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!

[Ron Hill]

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