I was going to say idle speed is set too high, OR, the prop is pitched incorrectly. Not that I know props and how they work, but too much pitch? I believe would potentially be the issue.
For trolling, 3-5 hp motor would work. You will not need 10HP to go 2-4 knots in reasonably smooth water. Throw some seas into the mess, along with some current, not we need more like 5-10 minimum, and you might as well use the diesel for these conditions.
She drives me boat,
I drives me dinghy!