Thought I'd revive this thread for a bit.
I recently made contact with Jo Poulter - the engine/prop supremo from Barrus - and he made the long trip down Cornwall to join Brian and me to experiment with Prop sizes on Magellan Alpha.
He connected various sensors and guages to my Yanmar 300 and we set off to check out whether the engine was delivering the right performance through the 1.65:1 ratio Bravo 2X leg and 17 3/4 x 21" aluminium prop. It was also to be a test run for the Floscan fuel meter I had recently fitted.
We went through various speeds from 5 kts, 10 kts, 15kts etc up to WOT which delivered just below 35 kts. Jo monitored his guages and at the end of the test pronounced that the engine was delivering exactly the right power across the range with that prop.
Brian was keeping an eye on the fuel consumption and at WOT we were burning 68 litres per hour - again, apparently exactly the right level.
So - all in all - it seems that that the ali prop/leg/engine combo is right for Magellan Alpha.
Jo did lend me a steel 17 3/4 x 23 inch prop to try as an alternative. I tried this a few days later. The boat was slightly slower onto the plane as you would expect but nothing significant. Top speed at WOT was about a knot faster.
However, the boat felt "lumpy" at low revs doing 5 to 10 knots and there was noticeably strong torque steer at WOT.
Think I'll stick to the 17 3/4 x 21 inch ali prop and get an ali spare on the basis that...
1 - For me, it seems all round performance is about the same as steel.
2 - Ali props are more likely to break blades rather than gear boxes if they hit something at high speed.
3 - Ali props are cheaper than steel ones. A couple of hundred quid spare will pay for a lot of French booze.
We are now scratching our heads and trying to resolve Brian's leg/prop problems. But that's another story and he can tell the tale. If he has the mind so to do!.