need help deciding what pitch prop to get??

[matt88]

Motor is a 1973 85hp merc. Prop on it now is a 21pitch but is not in good shape at all. Merc is hung on a 15ft ski boat with a dry weight of 750lbs and gross weight of 1400lbs. Top speed with this prop is 35mph at 5600 rpms. The motor height needs to be readjusted again as well. I can trim it out to the hole before the last one but there is a lot of spray coming from the cav plate so I guess I need to raise the engine a little more. When I got this data the boat was fully loaded with fuel,gear,etc... The motor may actually turn a few more rpms because after my helper bailed on me I discovered I had the engine running a little lean and it seemed to pick up a little power after I tuned the carbs on the water instead of the driveway. Need to test that again I know but I just wanted a second opinion before I buy a prop. I know I need to straighten out a few more issues like the motor height,etc... but I have to get this damaged prop off the boat to get some accurate numbers so is a 23 pitch enough or should I go for the 25?? My goal here is to achieve as much top end as possible with a full load of fuel,fishing gear+2 people,me and 1 passenger.

[bifflefan]

1400 lbs is not much with an 85 HP.
Something is not right with your numbers.
I think the 21 should do fine if it was any good. My old 16' with an 85 HP used to run 40+ with a 17p at about the same to less RPM if memory serves me. Mine had plenty of power and speed. I never tried a 21p but had a 19p that I used a couple times.
Is there a prop shop in your area that will let you rent, or try before you buy?
They can usually fix your old prop for way less than a new one.
Got the motor height right before you start with the prop.

[matt88]

The numbers on the data plate read
85
750
1240 (instead of 1400)
Im assuming that this means 85 max hp
750 net hull weight
1240 gross.

I have talked with two people one that had a boat like mine (with a 75hp)and one that has a 17ft hydrostream with an 85hp merc. Both have told me that my boat should do 55-60mph. Im attempting to be conservative with the 23p because with my 2.3 lu gear ratio that would only put it in the low 50mph range. I would like to see higher speeds but I need a prop that I know I can use now and try to fine tune later. Your 85hp probably had a 1.78 gear.