Loss of Power until Battery Switched Off

[Colin Philip]

After hardly using my yacht last season, when I motored her to the yard for winter storage, there was hardly any power.

The yard mechanics have now discovered that when the yanmar engine runs, it turns the props very slowly, even at full power. However, if they then switch off the battery, the power/speed returns. They've replaced both batteries and checked the alternator, but no change. They're now scratching their heads and don't know what to now try.

Has anyone experienced a similar problem, or have any suggestions as to why switching off the battery returns power - before this costs me a fortune in failed attempts to resolve the problem.

Any help/ideas welcome.

 

[jiris]

Very strange indeed. Before trying to find the cause, please, clarify: is the engine running at low revs or its revs are normal and the power is not being transferred through the transmission?

 

[William_H]

The next test to do is to remove the alternator drive belt and check for engine power. Short test as you probably don't have cooling water. If this shows normal engine power then you have an alternator problem loading the engine. It is normal witha small diesel that revs can drop noticeably with a heavily loaded alternator. But then not as much as you describe. If engine is still slow with alternator disconnected but comes good when battery is turned off you must have some sort of engine electrics effect. (no idea here ) olewill

 

[Colin Philip]

From memory jiris, the revs were normal. There was just hardly any power. I'll double check with the yard and see if that's actually the case. Thanks for getting back so quickly.

 

[jiris]

From memory jiris, the revs were normal. There was just hardly any power. I'll double check with the yard and see if that's actually the case. Thanks for getting back so quickly.

If it this case, it would link the problem to the transmission. The question is "HOW?" and the answer depends what kind of trans you have. Some kind of hydrodynamic? Weird...

 

[RMC]

I think I would check the basic stuff first, could be a partially blocked fuel filter or exhaust elbow and the load of the alternator is the final straw.

 

[Colin Philip]

Thanks William. I'll get the yard to try that and see what what happens. Thanks for your suggestion.

 

[Spyro]

If it's revving as normal it sounds like a transmission problem, maybe a slipping clutch but I wouldn't have thought the extra load from the alternator would make enough of a difference to cause the clutch to slip.

 

[Yellow Ballad]

IIRC if the battery has an internal short it can "lock" the alternator so to speak causing a high load on the engine. This can cause a lot of heat and can burn out alternators or the diodes however I doubt you would start the engine if the battery was shorted.

This is a good example of what I mean

https://youtu.be/dv4T49mxtRY?t=19m9s

 

[rotrax]

Just an aside-in normal circumstances turning off the engine battery with the engine running will see the alternator off.

All that electrickery being produced and nowhere for it to go.....................................

The diodes and thyristors dont like it up 'em-they dont like it up 'em...........................

Disconnecting the field coil feed or drive belt would have been a better method.

From extensive dyno testing years ago-mid '70's-an alternator can easily take 4 or 5 HP to get it to rotate at full field excitement.

If you have a low power engine this can make a significant reduction in apparent power and RPM.

 

[pvb]

Werlcome to the forums!

Firstly, turning the batteries off whilst the engine is running isn't a good idea, as it can damage the alternator.

Secondly, a typical small alternator will only use about 1-2hp when running flat out, not normally enough to make a significant difference to performance.

It would, of course, help if were to say which Yanmar engine you have, which transmission, which alternator and which batteries.

 

[pvb]

IIRC if the battery has an internal short it can "lock" the alternator so to speak causing a high load on the engine. This can cause a lot of heat and can burn out alternators or the diodes however I doubt you would start the engine if the battery was shorted.

This is a good example of what I mean

https://youtu.be/dv4T49mxtRY?t=19m9s

That's a wind generator - they use permanent magnets and work differently from automotive alternators.

 

[Heckler]

Werlcome to the forums!

Firstly, turning the batteries off whilst the engine is running isn't a good idea, as it can damage the alternator.

Secondly, a typical small alternator will only use about 1-2hp when running flat out, not normally enough to make a significant difference to performance.

It would, of course, help if were to say which Yanmar engine you have, which transmission, which alternator and which batteries.

I vaguely remember seeing somewhere that on one of those little single cylinder engines there was a device, the oil pressure switch switching the excitation circuit? to allow the engine to start and spin up before the alternator kicked in.
As for the hp of the alternator hp, depends on the state of the batteries, simple math will calculate how much hp needed. After all 1 HP is equivalent to 750 watts ish. So flattish batteries, 40 amp alternator giving about 20 amps times 13.8 equals 270 ish watts? Perhaps 1/3 hp?

 

[Yellow Ballad]

That's a wind generator - they use permanent magnets and work differently from automotive alternators.

Good to know, I assumed they would work in a similar way.

 

[RichardS]

If it's revving as normal it sounds like a transmission problem, maybe a slipping clutch but I wouldn't have thought the extra load from the alternator would make enough of a difference to cause the clutch to slip.

I agree that it does sound more like a transmission problem so the sort of gearbox/clutch is definitely important. However, unless it's a very fancy set-up, I can't see why disconnecting the battery would affect the transmission. The alternator is driven from the crankshaft before the transmission so whatever the alternator is doing surely cannot cause clutch slip or affect the transmission.

I would also have thought that an alternator absorbing this much power (although we don't know how much power) for anything other than a few seconds would cause a lot of heat and probably quickly result in a slipping/squealing/smoking/burning drive belt which suggests that the alternator is a red herring.

Just an afterthought .... I was wondering why the engineer would turn off the electrics anyway as that is usually a bad idea when the engine is running .... but perhaps he/she saw/smelled/heard that the drive belt was, indeed, in some distress?

Richard

 

[Tranona]

I vaguely remember seeing somewhere that on one of those little single cylinder engines there was a device, the oil pressure switch switching the excitation circuit? to allow the engine to start and spin up before the alternator kicked in.

That is on a 1GM with a saildrive and the symptoms are exactly how the OP describes. the engine produces very little power under about 2500 rpm and will not start or run properly when cold with the alternator engaged.

Suggest the OP runs it with the drive belt removed (it only drives the alternator) and of course investigates other reasons for low power such as blocked exhaust elbow - although this is not usually a problem with 1GMs, if that is what he has.

 

[William_H]

IIRC if the battery has an internal short it can "lock" the alternator so to speak causing a high load on the engine. This can cause a lot of heat and can burn out alternators or the diodes however I doubt you would start the engine if the battery was shorted.

This is a good example of what I mean

https://youtu.be/dv4T49mxtRY?t=19m9s

I agree. I saw a long time ago an alternator on an aircraft engine where the centre aux terminal (the one you use for tacho) was inadvertently grounded. The engine was run up then the excitation was applied to the gear driven alternator. The force of electrical load was such that the entire centre section of the alternator (iron core and stator coils) were rotated until the coil connections were broken. So in this case the alternator was definitely locked up, the application of exciter power making it the same as a permanent magnet wind gen. olewill