Re: bayliner capri 1700 90hp prop slipping
13 X19 is just about right for your 90 Force on a 17 foot boat. If I remember correctly, the 85 is rated at 5250 rpm and the 90 is rated at 5500 rpm. Your RPM should be somewhere in the 4500-5500 range. If you want to tow a skier, drop down to a 13 X17 pitch prop to pop the skier out quicker.
While increasing the pitch would seem to increase speed, if it bogs the engine down past its rated rpm, it will actually deliver less speed.
I think the 15 pitch prop is way too small for your application--your engine speed is up and mph is lower--correct? You would only need that prop if you wanted to tow the Queen Mary. Your mechanic has unintentionally mislead you. Lets give him the benefit of the doubt. Maybe he thought you were carrying very heavy loads.
You might be able to get away with the 15 on an 85 because the lower unit on the 85 is 1.78 to 1, but the lower unit on the 90 should have 2-1 gears (nothing wrong if it doesn't. The 2-1 gears allow the engine to rev up a little more and produce the extra 5 hp.)--bigger reduction so it takes a little more pitch. To double check this, turn the flywheel by hand. It should turn twice for each single turn of the prop.
If the usual load is just you and your wife without a bunch of other weight, you might be able to swing a 21 pitch prop. Bayliner is not a particularly heavy boat. You might find your speed to be the same or even a little less than with the 19 at wide open throttle. Hard to say without trying though.
