Yeah, I'm thinking that after 11 years the OP has got this figured out.
Nonetheless, there are some significant issues with grossly over-sizing the auxiliary as in the original question--and, yes, 100hp is grossly over-sized for a sailboat that displaces 11 tons.
Hank mentioned some of the biggies; prop shaft, heat exchanger
all have to be considered. Beyond that, in calm conditions it is only going to take 15-20hp to move the boat at hull speed
. Going into wind
and waves it might need 45-50hp at most. Meaning that you are either going to be constantly running the engine at just above idle speeds, which is bad for a diesel
engine in the long-run, or you are going to be wasting huge amounts of fuel running the engine at higher speeds just to push the bow wave a little harder. Not really a good situation.
Oh yeah, and the engine probably weighs nearly twice as much as an appropriately sized one, so you're carrying a few hundred pounds of extra weight in the stern for no good reason. Extra weight, extra expense, extra fuel; all for no benefit whatsoever.
I really do not understand why so many people out there think that putting a huge engine in a sailboat is a good idea. It is a SAILBOAT, for God's sake! You're not going to go skiing behind it! And, of course, they think these gigantic engines make the boat worth so much more than a smaller one would when, in reality, for those who understand the drawbacks, it makes the boat worth so much less.