"Morgan - don't get me wro
"Morgan - don't get me wrong here, these are extremely fuel efficient motors compared the "older" versions.
I did read a "non-scientific" test that a guy did when he replaced his old 90 horse, with an ETEC 90. Had a flow meter on board and tried to duplicate conditions to compare - so relatively accurate I imagine.
He found at cruise speed he got 50% better fuel economy, getting 4 mpg on the new motor and closer to 3 mpg at top end.
You have to assume that either motor you are considering would use more than that simply because of the increase in horsepower - but it does work on a "curve" - and the 175 wouldn't use double the 90....
Anyhow, an ideal prop for your set-up using the 150 horse, would be either a 19 pitch 3 blade (top speed about 47mph), or a 17 pitch 4 blade.
With the 175 you could move up to a 21 pitch 3 blade (top speed about 50mph) or a 19 pitch 4 blade.
Depending on your preference/application the 4 blade could be more econmical if you are looking at running more often at cruise speed than top end.
But in any case you could be looking at burning upwards of 10 gallons per hour to run either of these motors..."