Fitting larger alternator to Yanmar 3gm engine

[fredrussell]

My boat has its original Yanmar 3gm30 engine with a 30amp alternator. It’s a 3 tonne, 31 foot sporty sort of boat and 30hp is about twice what it needs really so I’ve been thinking about upgrading alternator to make use of some of this excess hp. There’s a 55 amp alternator available- any pitfalls to fitting this do you think?

 

[Fr J Hackett]

My boat has its original Yanmar 3gm30 engine with a 30amp alternator. It’s a 3 tonne, 31 foot sporty sort of boat and 30hp is about twice what it needs really so I’ve been thinking about upgrading alternator to make use of some of this excess hp. There’s a 55 amp alternator available- any pitfalls to fitting this do you think?

It won't be a problem providing you can align the pulleys correctly and they are designed for the same belt.

 

[Alex_Blackwood]

My boat has its original Yanmar 3gm30 engine with a 30amp alternator. It’s a 3 tonne, 31 foot sporty sort of boat and 30hp is about twice what it needs really so I’ve been thinking about upgrading alternator to make use of some of this excess hp. There’s a 55 amp alternator available- any pitfalls to fitting this do you think?

Absolutely no point unless your batteries and system can take the extra output. Is your 30 amp Alt. being overloaded with your present set up, do you intend to increase your battery capacity??

 

[Caladh]

I fitted a 70 amp Balmar and Smart regulator to same, many years ago to some effect. No issues fitting.

 

[B27]

Some of the old 30Amp and similar alternators are also quite low regulated voltage.
To see what benefit you might get, you need to fit a decent voltmeter and ammeter and see how often your alternator is limiting the system.
Depending on how you use your boat, how much solar you have etc etc, you might find it makes a lot of difference or very little.

A bigger alternator may give the belt a hard time.

Personally, the house battery (single) is 105Ah and the recommend charge rate is 20% of that. I rarely see more than 30A from the alternator, but I do get 14.3V at the battery at the same time as 20A, at lazy cruising RPM, if the battery has been used a fair bit.

It's different if you've got big house banks, windlass and all that.

 

[fredrussell]

Absolutely no point unless your batteries and system can take the extra output. Is your 30 amp Alt. being overloaded with your present set up, do you intend to increase your battery capacity??

My house bank is now more than twice the capacity it would have been when boat left the factory, so I do feel the extra output would be put to good use.

 

[Sandy]

It might drink a little more fuel.

 

[Alex_Blackwood]

My house bank is now more than twice the capacity it would have been when boat left the factory, so I do feel the extra output would be put to good use.

If your existing 30 amp alternator is charging the batteries without being overloaded, getting exceptionally hot etc. then just upping the capacity of the alternator will not make any difference. As Sandy says it may drink a bit more juice as it may be a bit heavier to turn The batteries will only take what they require. Have you measured or otherwise observed what charging current your present system takes if the batteries are a bit low on charge?
If you do as Caladh has done and add a smart regulator as well as a larger alt. you may charge your batteries a bit quicker.

 

[B27]

It might drink a little more fuel.

You don't get anything for nothing, but I doubt anyone would notice the change.
There's perhaps the same number of Ah to be generated?

If the existing alternator is nearly always giving 14.4V at the battery, you'll likely see no benefit.
If your solar is adequate to top up your battery on 95% of days, there's not much benefit.
If you run the engine a lot less amps is still enough amp-hours.

If you use a lot of power and want to recharge by alternator only, with minimum engine hours, then a better alternator will help.

But I would be looking at solar first.

 

[Fr J Hackett]

As has been mentioned if you do fit a larger alternator of which there is no physical problem with the caveats also pointed out it would be prudent to fit a smart regulator to maximise it's benefits and ensure the batteries are charged correctly this however is a different question to that which you originally asked.

 

[Caladh]

If your existing 30 amp alternator is charging the batteries without being overloaded, getting exceptionally hot etc. then just upping the capacity of the alternator will not make any difference. As Sandy says it may drink a bit more juice as it may be a bit heavier to turn The batteries will only take what they require. Have you measured or otherwise observed what charging current your present system takes if the batteries are a bit low on charge?
If you do as Caladh has done and add a smart regulator as well as a larger alt. you may charge your batteries a bit quicker.

Yup that’s the reason I did it, having fitted an electric windlass and did much more anchoring.

 

[fredrussell]

Op here. Many thanks for advice. I’ll stick with the 30a original for now I think, as I’ve got 160w of solar on boat. I sail uk year round, extra output was for charging 220 a/h bank in winter, but in reality I’m never that long from shore power in the winter months.