The problem I think may have been it was very cold.
Just below freezing, but on race day it was about 10 or 11 degrees.
I almost did not run in the race, someone had strung ropes across the river.

RBT had little floats out side his house, with rope strung between them just below the surface.
They must be part of the Anti Grockle defence system, installed by the local WI.
Or might be left over from the war, to stop invasion.
The day before the race whilst checking out the opposition, I fell into the trap and went speeding through the minefield

Andy could not believe my response was just "bother"

I had to limp back to Fowey at 12mph, then break out the tool box to trim the blades.
Make do and mend.
Slightly less blade area did not make any difference, it might even have raised the engine speed slightly.
I will need to try a slightly smaller prop, as I think I am still slightly over proped.
H-A