Hi Andy,
I'm not sure what size my prop is but I've deduced through the final law of engineering i.e 'when all else fails,read the manufacturers instructions', that having eliminated everything else, all that's left is that my prop is too large.. The old engine, a 1937, 16hp Lister CE had a maahoosive flywheel and generated a huge amount of torque,so was able to turn a big prop. The Sabb, although twice as powerful doesn't have such a large flywheel so isn't as torquey, and as a consequence is a bit smokey...its a bit like trying to drive off in third gear... all good fun though... It does make a lovely noise though...
also, it's fitted in a 57' narrowboat...