snip
Earlier in the season with three on board, at one point the largest
member of the crew (I'd guess ~20 stone) decided to sit in the roof of the
fore cabin for the return journey. This has a VERY significant impact - I
lined the boat up with
the middle of the river and, with a very slight infrequent adjustment to
the
helm, it stayed on course - very desirable behaviour but never before
witnessed with this boat.
snip

Without being able to offer "old sea dog" help, all I would advise would be
to remove the ballast in the bow if it is easy to get at and start again. I
guess the original thoughts were that the diesel is heavier than the Volvo
resulting in the stern sitting lower. Do you know the difference in weights?

I'm honestly a novice and may get slated for this idea but... although trim
tabs are designed for planing boats, would they also help to give a bit of
lift to the stern rather than filling the bow with ballast and getting the
boat to sit deeper and deeper?

If the worst comes to the worst you could try taking your big friend out
more often!

Good luck,

David