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Old 05-04-2012, 06:13   #1
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Props and Engine RPM

Hi all,
I am in the process of buying a FP Belize and my sea trial and the marine Suveyors trial showed a split in engine RPM at full throttle.
Port engine full throttle 3250 RPM (3078 hours on engine and 16 x 12 2 blade prop we believe)
Stbd engine full throttle 2825 RPM (2009 hours on engine and 16 x 12 2 blade prop we believe)
Port engine has 120A and 55A alternator
Stbd engine has 55A alternator.

Are there any Belize owners out there that can let me know what RPM you get at full throttle and what prop do you have?

Thank you
Graham
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Old 05-04-2012, 07:41   #2
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Re: Props and Engine RPM

Greetings and welcome aboard the CF, Graham.

It seems counter-intuitive to me, that the more heavily loaded Port engine would spin faster than the more lightly loaded Stbd engine.

A 12V alternator requires about 1 hp per 25 - 30 amps of output power + about 1-2 hp for increased engine operating loads.
Hence, I’d expect the extra 120 Alternator (on Port engine) to be using an extra 4-6 HP from that engine’s propulsive capability.
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Old 05-04-2012, 08:14   #3
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Re: Props and Engine RPM

Quote:
Originally Posted by GSwannell View Post
Hi all,
I am in the process of buying a FP Belize and my sea trial and the marine Suveyors trial showed a split in engine RPM at full throttle.
Port engine full throttle 3250 RPM (3078 hours on engine and 16 x 12 2 blade prop we believe)
Stbd engine full throttle 2825 RPM (2009 hours on engine and 16 x 12 2 blade prop we believe)
Port engine has 120A and 55A alternator
Stbd engine has 55A alternator.

Are there any Belize owners out there that can let me know what RPM you get at full throttle and what prop do you have?

Thank you
Graham
Graham,

Our Belize set up is similar to yours with 16 x 12 two-blade props except the large alternator was installed on the stbd engine at the factory. Are you sure you didn't mistype your configuration? I never liked how that alternator put a heavy load on the engine at startup with depleted house batteries so I disconnected it after installing 520 watts of solar panels. It was poorly installed at a slight angle to the engine anyway so threw Vee belt shavings all over the engine room.

Can't speak to full throttle RPM's as I don't usually run above 2400 RPM cruising speed. The owner's manual lists 2600 RPM but that was for 11" pitch props that came on the boat. I downrated by 200 rpm for the higher pitch based on a recommendation elsewhere on this forum.

Does the mismatch in RPM's continue after the batteries are fully charged? If not, the extra load from charging may be the answer, although 425 RPM seems excessive even for that. If so, look at mechanical adjustments on the throttle stops. Our engine RPM's are quite closely matched with the throttle levers in the same positions.
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Old 05-04-2012, 08:18   #4
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Re: Props and Engine RPM

If the batterys are charged to full capacity, I would not expect there to be additional pull from the alternators, most alternators will not put out there full output unless the batterys are drained. I would look for other factors that would explain the differance in load RPM. I would look at the props first, maybe one is fowled or has some damage. If I could, I would swap props then test again. Other factors could be fuel related, maybe one engine has a bad fuel injector, fuel filter, or other fuel related problem. One engine may have low compression in one or more cylinders. There are a lot of reasons why one could have an RPM difference, it could be something as simple as throttal linkage or stops set differnent.
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Old 05-04-2012, 08:23   #5
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Re: Props and Engine RPM

You can try getting the prop re-pitched on the starboard engine to a lower pitch. That should even out the RPM's. But even RPM's aren't necessarily going to solve your problem if there are different pitches to each prop. The lower pitched prop will simply displace less water.

I concur with other posters regarding the load the alternator is taking on the engine.
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Old 05-04-2012, 08:31   #6
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Re: Props and Engine RPM

Maybe there is something wrong with the starboard engine? Bad injector or low compression in one cylinder? That would reduce max power in that engine and make the engine unable to pull max RPM.
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Old 05-04-2012, 08:47   #7
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Re: Props and Engine RPM

I'ld check the calibration of you tachs.
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Old 05-04-2012, 08:57   #8
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Re: Props and Engine RPM

Ill second the calibration of the tachs,
get someone with a photo tach to check the actual engine speed, if the tachs are alternator driven it's possible there is a mismatch between the alternator and tach.
Also, what engines? how fast are they supposed to go?
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Old 05-04-2012, 09:41   #9
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Re: Props and Engine RPM

Thanks very much everyone for the quick replies. I feel part of a great group here and see there is a lot of experience to draw on.

No I have not made a typo, and it does seem strange the larger alternator is on the port engine which has the lower indicated RPM. We do not know if the props are matched at this time as the cat is back in the water. I am having a Yanmar mechanic look at both engines next Tuesday where he will do a 250 hourly service and check injectors etc. (also adjustment of fuel shutoff valve, throttles etc). Compressions have already been checked and are OK.
When we do another sea trial I will try will no electrical load -good suggestion. Also swapping props is a good idea.

My concern is to find out whether there is an engine problem before I take delivery.
Regards
Graham
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Old 05-04-2012, 10:27   #10
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Re: Props and Engine RPM

Quote:
Originally Posted by lancelot9898 View Post
I'ld check the calibration of you tachs.
If the engines sound the same, I'll third it! As stated, unless the batteries were low there shouldnt be mch difference. Did you run the engines a good long time? Maybe an hour? What kind of engines are they? There are a lot of possibilities, could even be a cable adjustment. You should send some more time ferreting this out...
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Old 05-04-2012, 11:28   #11
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Re: Props and Engine RPM

Quote:
Originally Posted by GSwannell View Post
... No I have not made a typo, and it does seem strange the larger alternator is on the port engine which has the lower indicated RPM...
Quote:
Originally Posted by GSwannell View Post
Port engine full throttle 3250 RPM
Stbd engine full throttle 2825 RPM
Port engine has 120A and 55A alternator
Stbd engine has 55A alternator.
Did you intend to say the Port engine, which has the HIGHER indicated RPM?
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Old 05-04-2012, 15:41   #12
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Re: Props and Engine RPM

Oops Yes the the port engine has the higher RPM
Graham
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Old 05-04-2012, 16:30   #13
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Re: Props and Engine RPM

Quick and dirty check of the alternator load hypothesis: remove the drive belt from the big alternator before running the comparison...

Cheers,

Jim
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Old 05-04-2012, 16:50   #14
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Re: Props and Engine RPM

Could also be the fuel injection timing. Is the engine slightly louder than the other in the engine room?

CJ
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Old 06-04-2012, 00:56   #15
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Re: Props and Engine RPM

I would have thought that when one engine was 10% faster a balanced boat would not motor straight. I too would check the taco. Or the boat is not balanced and the difference in rpm make the boat to go straight.
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