What Prop on my 1979 18'3" with a DF175
 

Ok, I'm befuddled. I wanted a 15 x 17 stainless 3 blade. My prop guy talked me into a 15 x 19. It will only run 5100 rpms at WOT. Is anybody running my boat/motor combo? What pitch works? Do I really need a 15" or 16" pitch?

Heavy load? If so, I'm thinking you were right on the 17 pitch.

I just purchased a new prop for my 18 last week. The motor is a 99 Johnson 150 so the combo isn't the same but I'll give you the info just the same. My old prop was a SST 15 x 15 and worked well until it spun the hub last time out. It would give me 40 - 42 @ 5400 rpm and a good cruise of 30 or so.
After hearing all the good about 4 blades and forum member EggSuckingDog praising his Stiletto 3 blade, I decided on a Stiletto Bay Pro 3, 14 1/4 x 15. Have to say I love it. In anticipation of that prop's ability to run close to the surface, I raised the motor all the way up. The prop holds the water to almost full trim but I need to lower the moter one hole to get the water pickups down a little. It starts to get hot at full throttle full trim. On the short wide open run, I saw 44 @ 5500 rpm. Thats the fastest the boat has gone and that was even with a full load.
I'll get some better numbers the next time out. I would also say it looks like you need a 17 pitch.

SportPort, no offense but I don't think you'll find anyone running a 480lb motor on a SeaCraft 18. Not sure of the year of your Suzuki but thats the approximate weight of recent DF 175s

A couple of questions:
Is your boat a SeaCraft 18?

What prop (specifically) are you running?

I would recommend going down in pitch at your prop guys expense and make sure the specific model of prop is designed for your set-up.

Clearly you need to drop in pitch, but my guess is that you also need a stern lifting prop to manage the weight of that motor. That means big blades with a low to moderate rake, little cupping and maybe even consider a 4 blade prop.

Good luck

You've got some other problems. The df175 has a 2.5:1 lower unit and should easily swing at least a 20" prop. My 18' with a df90 which has a 2.59:1 lower unit ratio runs a cupped 18" prop to 6300 rpm.

Bits, I suspect your right - motor may be mounted too low or the prop is simply the wrong shape.
SportPort, Identify your prop, and check your anti-cavitation plate height The 175 motor is 130 lbs heavier the 90 but yeah, something else is amiss. Tach setting?