xp25 rpm's

Terry Forshier · May 03, 2006, 01:44:14 PM

The 25xp diesel in the 1988 C34 I just purchased seems to run well but under full throttle will do only 2500 rpm. The motor was recently checked out by a mechanic and pronounced goog to go. The prop is the factory installed 3 blade, bottom and prop just cleaned and tuned. My old Yanmar seemed to have more reserve. Is this the norm? Motoring at 5kts standard? It does not seem to strain, it just goes to 2500 rpm and stays there. Terry

dave davis · May 03, 2006, 02:46:06 PM

I also have a 1988xp but I have a two bladed folder. With a clean bottum and light winds I cruise at 2000 and the GPS and the meter show 6knots correcting out for current. If I push it to max, I get 2500 rpms and show about 6.5 knots.
Hope this gives you another point of reference.

Good luck, Dave

Jim Price · May 03, 2006, 03:01:24 PM

Full throttle should be much more than 2500 RPM, assuming your tach is correct. You may want to so a search on this subject as I have seen several threads on tachs reading wrong because switches set wrong plus a lot of other variables.

I have 25XP and just today I had a complete review by my mechanic concerning a vibration between 2000 and 2400. Good new is we determined vibration is exactly same in gear and out of gear - bad news is live with it or change out the old style engines mounts (will do this fall).

All this leads to while testing, we ran the engine out of gear and in reverse up to 4000 RPM and all the ranges in between to isolate the vibration. She can do that much in RPM.

Now the next point, I have a 3-blade fixed prop and the best I get is about 6.1 / 6.2 (in lake) at 3000 RPM. Above that point I am just wasting RPM with no speed increase. Mechanic explained I could get more if prop was repitched but I would loose low speed torque and control. Apparently, there is enough range of pitch that you can be pitched for torque (heavy current environment) or speed. I am apparently pitched for above average speed and average low RPM handling. Your prop could be pitched for open water / heavy current use thus lower speeds, assuming your engine is really running higher than 2500.

Just a couple of thoughts from an absolute "non-expert".

Stu Jackson · May 03, 2006, 04:38:35 PM

A search on Teleflex will give you some ideas. Also try searching on calibration.

The check:

Engine in neutral, what's that max rpm? Engine in forward, what's the max rpm holding the throttle wide open (it will "slouch" back to a lower rpm when you let go of the throttle unless you've really tightened up the friction bolt).

It's also tied to the alternator.

It's really all relative.

Ron Hill · May 03, 2006, 07:19:38 PM

Sails : The first thing you need to check is to see what RPM your engine is really putting out!! Then check that against what your tac says(as it's getting it's signal off of the alternator - which may not be set right!!)
Read all of the old Mainsheet articles and you'll find that most of us with a 3 bladed prop have added extra pitch.
All of the questions that you've asked have been answered starting back in 1989!! Good stuff in those Mainsheet tech notes!

Mike Denest · May 04, 2006, 09:11:10 AM

I get 6 knots at 3000 rpm. Of course my tach at static shows 500 rpm! It's on my list to correct.

Earl Miller · May 04, 2006, 09:28:22 AM

My 1989 C34 cruises at 6 knots at 2500 rpm with a three bladed prop. The one time I tested the max rpm it was 3200.

Terry Forshier · May 04, 2006, 01:15:56 PM

I have an adjustable alternator on the boat. Turning it on and up, it whines and increases the charge from just a few amps to 18. I think now that when the charge is turned up it must increase the load and take away some of the power of the engine. I am going to vary this next time I get out to see how much drag it actually adds. Also I was reading the kts from the display from the little paddle wheel indicator on the signet ystem. This I think may also be off some and the former owner now says that that system always reads less than his GPS.

Ron Hill · May 04, 2006, 06:35:09 PM

Sail: The side load from the alternator could be a large as 1 - 2 HP. The tac will slow down as the engine slows. I'm not too sure about Signet Knot/Log, but it MAY be adjustable to the GPS speed like my DataMarine?
You still need to know what your real engine RPM is at a couple of 3 points (1500/2000/2500) and see just how far off you tac really is!!

Terry Forshier · May 06, 2006, 12:53:39 PM

OK, yesterday I went out and motored around. When the alternator was set to give a high charge the motor slowed appreciably, down to 2400 on the tack. As I lowered the charge or output the motor gained rpm's. At the point I turned off the alternator I was doing 2800 to 2900 on the tack. This variation was the problem I was concerned about and now I know what is causing it. Incidenmtly this is a new tack last year and was supposed to be set up with the new alternator system by P.O.

Ron Hill · May 06, 2006, 07:24:12 PM

Sail's : I haven't made my self clear. You need to put a special TAC on the engine to find out what the engine rpm REALLY is against what your tac instrument says!!
That special tac attachés to the center bottom drive pulley of the engine and only gives engine rpm!!

sojourney · May 07, 2006, 08:44:27 PM

As others have mentioned, your tach is probably wrong. Most mechanics have a special tach that looks sort of like a strobe light. They put a white mark on the front of the lower engine pulley and can measure RPM'sit in less than a minute. The XP25 should be able put out about 3200 rpm under way. If it isn't, it could be your prop is pitched too high. I think 15x10 (3 blade) is what most folks find is the most efficient.
Your alternator will take a some power but it shouldn't be as much as you're describing unless it's very large. I have a 100 amp alternator with a Balmer MC-612 and it's never done what you describe.