I used to run a 13 3/4 x 15P prop on my 20' Seafari when I had the 115 on it, and it would turn ~5500 pretty much regardless of load. Your 18 is quite a bit lighter, so I would think it could spin a 17" prop up to 5500. I think 5000 rpm (if it's an accurate number) is probably too low for your motor. Check your throttle linkage and cable rigging to make sure the carb butterflies are full open at WOT. You're lugging the motor if it won't reach max rated rpm, and that's harder on the motor than letting it over-rev! The crank on those V-4's is only about 6" long and very stiff, so it's good for about 9000 rpm! Rev's won't hurt it but lugging the motor increases combustion pressures and temperatures and loads on the pistons and bearings and the chances of detonation/pre-ignition, so better to let 'em rev! Don't know what the rated rpm on your motor is, but I'd think you'd like to see at least 5500 with whatever load you typically carry. But before you go screwing around with props, I'd do a couple things:
1. Analog tachs are notoriously inaccurate, so it may be reading low. I'd try to have a dealer check it. Some dealers have a dyno they can hook up to the motor and run it up to it's max rated rpm under load and check the ignition system and tach reading while they're doing it. I had that done to my old motor after it had sat for about 5 years, just to make sure it was in good shape before taking it off shore.
2. Most dealers mount the motors too low! Your AV (Anti-Ventilation) plate wants to be above solid water when you're on plane and fully trimmed out. If you can't see the top of your AV plate, the motor needs to be raised, which will cut drag and maybe buy you some more rpm. On most 20' SeaCrafts, this will put the AV plate about 1-1.5" above bottom of boat. A good SS prop with some cup in it can also run higher on the transom without blowing out in turns.
3. Your min planing speed is pretty high. Most 20' SeaCrafts would plane at about 12 mph if powered with the 300 lb motors they were designed for! That's a big deal because the 18/20 is pretty light and will start to go airborne in 3' seas at about 20 kts, so if you run offshore very much, it's nice to be able to hang on plane at lower speeds. Some props also develop more stern lift than others depending on blade design, and that will help reduce your min planing speeds. Bass boat props are designed to lift the bow for high speeds, while props for offshore fishing/cruising boats are designed to generate stern lift. 4 blade props are particularly good at providing stern lift.
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