My Volvo D1-30s both limited to 2200 RPMs after alternator install

[nomad7]

I have a catamaran with two 2017 Volvo Penta D1-30 motors. They worked fine a few weeks ago, but now each motor seems be limited to 2200 RPMs at WOT. No smoke, no clogged filters.

My top suspect is the fact that I changed out the alternators on each motor last week. From the factory they ran 12v alternators, I've just changed them to 24v because of a different house battery that I need to charge when under way. Other than simply swapping out the alternators and changing the sense wire to the 24v battery - I've not made any other changes to the motors.

Any ideas of if it is an alternator induced limiter or some other gremlin I've not considered?

Thanks!

NOMAD7

[tkeithlu]

Welcome to the forum, Nomad7. That's quite a starting point question.

[tkeithlu]

Somebody here is going to know more about this than I do, but let me ask a few questions. Does no clogs include your cooling water inlets? What happens when you rev in neutral? Are the 24v alternators huge? Is that 24v battery a lot bigger and dead? I'm having a hard time seeing new alternators and battery chewing up that much power.

[Kiwi Dave]

Hi - same engines on my cat - peak revs fall when props &/or hull is fouled - could this have happened??

[nomad7]

Originally Posted by tkeithlu

Somebody here is going to know more about this than I do, but let me ask a few questions. Does no clogs include your cooling water inlets? What happens when you rev in neutral? Are the 24v alternators huge? Is that 24v battery a lot bigger and dead? I'm having a hard time seeing new alternators and battery chewing up that much power.

Thanks for your response. Yes, the water inlets are clear and I've got good flow when the motors are running. What happens when I rev in neutral is a good question, unfortunately I just let the boat for about a week so that test will have to wait. The 24v battery was charged anywhere from 40% to 100% and no matter the state of charge, the revs were limited...

[nomad7]

Originally Posted by Kiwi Dave

Hi - same engines on my cat - peak revs fall when props &/or hull is fouled - could this have happened??

Hi Dave,

This is quite possibly the issue, the boat hasn't moved at all in the last 3 months as I've been ripping everything apart, this was my first trip out. I'm sure there's some significant fouling by now. But I'm struggling to understand how fowling would alter RPM potential that much - but if you've experienced it, I'm definitely going to keep that in mind.

Many thanks

[Wotname]

Some questions that will help you (and us) resolve the issue.

What was your previous rpm at WOT (with clean hull and prop)?
What is max rpm in neutral now?
Does the drop in rpm come back if you disconnect the new alternator (remove belt or disconnect output wire - before running it etc)?

I suspect the cause will be a fouled prop or hull as it is unlikely the new alternator will be taking a huge amount of hp from the system.

A fouled prop drastically increases the load on the engine.

[Parker9645]

Are the alternator pulleys the same diameter as before? Your tachs may rely on the AC output of the alternator to operate. Check the actual crankshaft RPM with a laser tach. If your readings are off you have to open up the tachs and adjust the pot in there to read correct RPM.

[Parker9645]

The alternator pulleys might not be the same diameter. Your tachs read the RPM by the AC output of the alternator. If the pulleys are the same size then check the actual engine RPM with a laser tach. If the tachs require adjustment you must open the tach and adjust the pot inside until you read correct RPM.

[Bycrick]

Or the new alternators might have a different number of poles internally, so they’d output a different frequency signal for the same RPM. That would make the tach read incorrectly.

[DougR]

The tachometers on the D1-30 engine are driven by an inductive pickup mounted in the flywheel housing counting slots machined into the flywheel.....so it's not pulley size or alternator poles.


As the original alternators were 12 volt, what are you driving the engine electrics with now? 24volts? If so you might be setting yourself up for some expensive component failures. It's possible that 24v into the MDI system could cause a faulty/inaccurate tach reading, but more likely is that it will cause some failures.

DougR

[RaymondR]

Originally Posted by Parker9645

The alternator pulleys might not be the same diameter. Your tachs read the RPM by the AC output of the alternator. If the pulleys are the same size then check the actual engine RPM with a laser tach. If the tachs require adjustment you must open the tach and adjust the pot inside until you read correct RPM.

Most likely cause and best solution.

[Glasshopper]

I have this same motor in my Bavaria.
You mentioned above that you have just 'let' your boat.
Could it be that another renter has run your motors too cold/short and blocked your exhaust elbows? Ive seen this with another D1-30 using OEM (i.e. cast iron) elbow, resulting in same symptoms -his elbow ID was reduced fom ~38mm to ~10mm.

[donradcliffe]

I'd, in the following order

1. verify the rpms with an independent tach
2. Check the props for fouling
3. Change the fuel filters.

[Martkimwat]

Just my small tuppenny worth, are your new alternators rated to give the same amps output? if so you've doubled the load requirement (Wattage) by going from 12v to 24v assuming everything else is equal. Do you still have trouble achieving higher rpm in neutral? 1 hp = 746w, 746/24 = 31Amps and that is assuming 100% efficiency. ie nominal 30Amp alternator @12v is pulling at least 1/2 Hp and at 24V is pulling at least 1 Hp, and more likely double those figures allowing for inefficiencies.