Possible alternator problem?

[colhel]

A quick visit to the boat last week with smart clothes on so couldn't investigate properly.
When I initially started the engine, tamd41a, the voltage meter on the dash showed 13-14 volts ish. But then after a few seconds the rev counter stopped working properly, the ignition light and water temp light stayed on but I could get them to fade by increasing the revs to about 2500 rpm.
Any ideas please?

 

[Portland Billy]

Did the. Voltage meter drop at idle revs but pick up when you increased ?

 

[colhel]

Yes I forgot to say that in original post.
No matter how many rpm the engine did, the voltage meter on the dash stayed at 12v.
I never checked anything else with a multi meter, but that's my next step.

 

[volvopaul]

Hi col if you have a split charger relay or diode pack which being a single engine boat I’d guess you have the feed to the yellow exciter wire could be no voltage , normally there is a fuse inline to protect the feed from the domestic batteries , test for voltage engine off At that point on the alternator.

 

[colhel]

Thanks Paul I suspected it would be good to look the smaller stuff first rather than charge straight into getting the alternator rebuilt.
Would the diode pack/relay effect the rev counter and temp warning light?

 

[Daydream believer]

Slipping belt?

 

[colhel]

Slipping belt?

I couldn't turn the alternator pulley, so I'd say no, but cheers for the idea ?