Just read your other posts . . . .
Given the age of the boat (and I assume engines) the issue could be loss of power or over propped, or boat weight gain. Mirage props are for big boats, and 23 pitch is a bit in the high side, but not too much.
I would do the following things:
Calculate prop slip at various speed and rpm levels. If you have the gear ratio of the outdrives and can record a few rpm and speed points along the way up to WOT, which is 3800 rpm and 44 mph, then you can calculate prop slip. I can do that for you if you like, just give me the data. If the prop slip results look OK, then . . .
Check throttle linkage for full travel and opening of secondaries . . If that is OK, then,
Do compression test and cylinder leak tests on both engines.
Cooling system pressure test.
If those thing look OK.
Go with or 19 or 20 pitch props to see how it performs. You could probably pick up a used prop on eBay and then re-sell them if they do not pan out.
If all that fails . . . List the boat on Craiglist (kidding).
