RPM's

[BradC.]

Does anyone know what the "normal" engine rpm's is for a M25 when the throttle is pegged ?
At full throttle.....I'm reading 2300....is that it ??

[rappareems]

Going to launch today, so have not been motoring yet this season.  However, as I recall we cruise around 2400 rpms and that is maybe half or less throttle.  I would guess your gauge is off.

[tsoko]

Bradley,
The manual says that the M25 tops out at 3000 RPM, and the M25XP tops out at 3200.  If you only show 2300, your tach is off.  It can easily be adjusted.  At the back of the tach is an adjustment screw.  Cruising RPM's usually 80% of max RPM's.

[Jim Price]

Fast Cuise is at 2600 RPM for m 25XP.  I have run 3200 at neutral in slip when doing various maintance checks, etc.  BUT BREIFY!  My '91 model only has 690 hours on it.  Some times I run at 2600 at slip for 10-15 minutes just to get everthing hot and clean out carbons, etc.

[Bradley]

the strange thing is that the motor doesn't seem to be working that hard at all...

even with the throttle pegged....my temp stays at about 150-160 degrees.

I wonder if perhaps the throttle cable has stretched and "full" isn't really full but more like 3/4 ???

[Stu Jackson]

Bradley

I recommend cross checking some other information.  You mention it feels like 3/4 power and wonder about the linkage.

1.  What is the condition of your bottom and how fast through the water are you moving at different throttle settings?

- our M25 cruises at 2200 to 2300 rpm at 6.5 knots with a clean bottom, with a max of 2700 full out IN GEAR moving

2.  How fast does the tach go when you are in neutral and you push the throttle to full?

- ours goes to 2950 rpm

We'll need some of this info in order to help diagnose any other issues not mentioned in earlier responses.  Let us know what you find with your linkage.

We have an M25 with a new 3 inch HX, so the engine always runs at 160.  The older 2 inch HX when clean kept us at 160 unless we pushed the throttle and then it went up to 180.

Lots of different moving parts here, so I'm not so sure we can do anything more than share our experiences with you and hope you find your own "sweet spot."

Stu

[Brad Costan]

(1987 #413)

thanks for the suggestions...I'll check into the tach calibration.

I do have a new bottom....and the speed was in the 6.35 range at full throttle (2300).

I recently changed from the stock 15x11 3 blade to a 15x14 2 blade folding prop and I gained another 3/4 of a knot.  Surprising since it's only a 2 blade...but I suppose it's the 3 degrees more pitch that does it...I did verify speeds vs. GPS and they are pretty accurate.

[tsoko]

Brad,
Is your transom getting black with exhaust smoke?  I would think that it is because the prop you have on now is a LOT more than what was designed for the boat/engine.  It might be the reason you cannot get higher than 2300 RPM.  The OEM prop that came with the M25 is either 15x10 two blade or 15x9 three blade.  Often folding props have greater surface area, so that makes the porblem even worse in your case.

[Brad]

Tom.....you're right, I went and checked the props...I had my numbers wrong...the prop that came off was 15x9.....and the new one is 15x12  (+3 degrees)

Do you really think that's too much ?  

I don't have any smoke or black residue on the transom....

I'm still going to try and see what max rpm's are when in neutral...perhaps that will provide some answers.

[tsoko]

Brad,
The 15x12 refers to inches.  15" diameter by 12" pitch.  The pitch is the theoretical distance the prop would travel in one revolution through water with no slippage.  Three inches of pitch is quite a bit, I believe.  I think I read somewhere that each inch of pitch equates to 200 RPM.  Maybe someone else can confirm that?  Again, that could be part of your problem.  Maybe start with the tach adjustment and work from there?

[Ray & Sandy Erps]

I haven't seen anyone suggest running it at full throttle while in neutral (no engine load) for a moment to see if you can get around 3000 rpm.  That would help troubleshoot whether it's your tach settings, your prop pitch or whether it's dirty fuel filters.

[Brad]

Ron -  the 2 blade I put on (15x12) is a Martec folding prop.....and it seems to have pretty beefy blades (as you surmised)

I just recently bought the boat...so I didn't have much history with the 3 blade prop other than the 10 hour delivery cruise, but we kept rpm's around 2100 and cruised at 6.25 kts.

I immediately went to a yard for a bottom, and while it was out, I put on the 2 blade folding from Martec that I found in a storage compartment down below....it appeared in excellent shape.

Now, at the same 2100 rpm's, I cruise at 7+ but I don't really know what "max rpm's" were before with the 3 blade as I was pretty cautious with the new boat and a 90 mile ocean run right off the bat.

I'll check the neutral/max rpm's and start there and I'll report back in a day or so.

Also, I don't know if this is related (it probably is) but I have noticed an increase in harmonic vibration with the new prop.....I figured it was a result of more bite on the blades...exposing whatever play in the bearings might be existing or perhaps the shaft isn't 100% true....so the increased load is magnifying it.   Sound about right ??

Thanks for all the help....as a new owner, it can be a bit overwhelming trying to learn all the ins and outs of a new boat.

[Brad Costan]

Tom / Ron

I ran the motor in neutral....2650 was tops.

[Stu Jackson]

There is also a lot of previous work on this subject.

A FIND on "engine rpm" gives us:

http://c34.infopop.cc/eve/ubb.x?a=search&s=329609511&reqWords=engine+rpm

Happy reading.

Please take Ron's advice and check the dips.

BUT:  Only if you really care.

F'rinstance, I kinda think of the tach like my fuel gage (omigosh, what blasphemy!!!).  As many may remember, my fueld gage is OUT and I use engine hours to measure fuel consumption.

It's all relative for the tach.  Does the engine sound good to you at certain RPM?  Whatever those feelings are is where the "sweet spots" in your engine's performance reside.  It really doesn't matter what the gage says, it's how it all feels.  On Aquavite, it's 1100, 1500 and anywhere over 2000 where the engine is "purring."  Places in between are somehwat rougher.  It's a three cycle diesel, after all.

It's an analog gage.  Can we measure 2,653.5?  Do we care?

To avoid having to take the darn cockpit panel off:

1.  run engine in neutral to full revs.

2.  read tach

3.  if tach is less than 3,000, take reading

4.  all other readings are pretty proportional to your max reading to 3,000 (i.e., if you read 2,500 at full throttle, then [2,500 divided by 3,000] times whatever you are reading is your real rpm.

So...have I converted anyone or am I still just a wild haired fuel gage weirdo?  {The hour meter better keep workin', though!}

Please don't answer that!  :)

All the best,

Stu

[Brad Costan]

Stu

I'm not overly concerned about the tach being 100% accurate either....I also go more by "feel" and how the motor sounds.

It seems 1200 and 2000 are the happy spots on this one....although the happy spots aren't as happy anymore with the higher pitch prop....the harmonic vibrations are exagerated, that's what caused my curiosity on the whole rpm thing to begin with.....the happy spots are getting hard to find and I was curious what the real "max" was on the rpm's since full power (2300 when in gear) I get the least vibration....but I didn't want to run it wide open if that was indeed wide open.

I'll fool around with it this weekend on the open ocean and see where it likes to be.

thanks...