possessives - How to make the genitive of a person's name with "OF"?

We built an engine for the boat of Mr. Sander

or

We built an engine for the boat of Mr Sander's

?

Answer

If you had to choose one of them, you should say "the boat of Mr. Sander's". But I don't think you should use either in contemporary English. I would say

a boat of Mr. Sander's,
this boat of Mr. Sander's,

but not

*the boat of Mr. Sander's.

As was noted in the comments, it is much better to use "Mr. Sander's boat."

Why is this? I suspect because "Mr. Sander's boat" already implies that Mr. Sanders owns only one boat, so this phrasing is shorter and simpler and means the same thing. If you want to replace "a boat of Mr. Sander's" using a possessive before the noun, you need to say "one of Mr. Sander's boats", which is longer than "a boat of Mr. Sander's"; both are fine in contemporary English.

If you use the indefinite article, it's clear (to me) that you should say "a boat of Mr. Sander's" and not *"a boat of Mr. Sander".

And if you use a different noun, you see that "a painting of Mr. Sander" and "a painting of Mr. Sander's" are both grammatical, but mean quite different things.