A proper prop

When Starcross had a new engine fitted in 2008 I was advised by Norbury Wharf Ltd that the new engine didn't match my propeller, but that as it was "only just" outside the recommended range to try it and see how I got on. The only problem I found was that the minimum speed of the boat with the new combination was a little too fast for comfort, even with the engine revs at the minimum setting, which made me feel guilty when passing moored boats sometimes and made manoeuvring a bit tricky. It did, however, lead to a considerable improvement in my manoeuvring skills, particularly as I regularly have to moor a 51-foot boat in a 52ft space at Norbury Junction.
However, after a few adverse comments from visiting steerers I decided to do something about it and last week I had a new prop fitted. I now know, from reading on the internet, that propellers are measured by diameter (blade tip to centre x 2) and pitch (blade angle). In theory, the pitch of the propeller represents the distance that one revolution would move the boat before allowance is made for the resistance of the water. Hence, my old propeller was a 17 x 15 (all measurements in inches), whilst the replacement is a 17 x 12 which means that each revolution should propel the boat only 80% as fas as the previous prop, reducing the minimum speed by 20% if I've understood the maths correctly. It certainly seemed like that when I took Starcross for a test run on Saturday and she definitely seems to handle better as well, although I'll only know if its "money well spent" when I get the bill! 
The two propellers have a third measurement, although in both cases its 1.5 (as in 17 x 12 x 1.5) but I must confess that I'm ignorant as to what this means (anyone know?).


Oh! and if anyone wants to buy a used 17 x 15 x 1.5 right-handed prop then I'm your man.