battery

85Amp/h would probably start it, but why not chuck a 120/140 in...

The engine is a standard Volvo car engine (2.5 litre 5 cyl, used in many cars!) but your boat may be standing far more than a daily used car...
 
For a dedicated engine starter battery the CCA (Cold Cranking Amps) value is more important than the AmpHour capacity.
It's not difficult to find a full size battery with a CCA of something approaching 900 Amps - which is more than ample but allows plenty of scope for deterioration over time. I expect such a battery would have about 100 Amp Hours capacity.
No point in paying extra for much more than 100Amp hours - they would probably be leisure batteries which are not necessarily good for engine starting and the extra Ah are not really any benefit .
Check your existing battery as it probably has a sticker on it with the Ah and CCA capacity. Also make a note of the existing battery terminal arrangements in order to get a replacement with the same arrangement and make fitting simple.
 
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