Re: Why does my diesel not rev when in gear?
One item nobody has mentioned is the exhaust elbow. If a diesel will rev without load but not with load a clogged exhaust elbow is one of the most common causes. It is also the least expensive fix. Diesels need clean fuel, compression, and air - both in and out. A clogged elbow will restrict the revs as much as a clogged air intake and over a few years it is a common place for build up crud to cause a problem.
A barnacle covered prop will do the same.
As far as starting the batteries probably contribute to the problem, and may along with too small wiring and a bit of corrosion on connections be the bulk of the problem. I re-wired my Yanmar YSE-8 with new larger cables - 1/0 vs the original 4 awg - and my starting time went from about 15 seconds or more to about 5 seconds.
Living aboard in Victoria Harbour