I am about to order a Comprop as a backup, and to use in the rare occasion that I have someone else in the boat. I much prefer to fish alone, so I can go my pace and not worry about positioning the boat to allow someone else spots to fish, but for tournaments and to help with gas, I will sometimes bring a friend along.

Currently I turn a 3 blade SS 14" x 19p prop and get approx 5100 rpm out of my 85 hp johnson.

The comprop is a 4 blade. Should I drop down to the 17 pitch like I should for a 3 blade to get my rpms up? I have read that a 4 blade will cause you to gain rpms, but the Comprop description says to use a similar size and pitch for same results.

I've never used or had a desire to own a composite prop. Stickly as a cheap, light weight emergency spare, they may serve for that purpose, as a usable prop, I'd have some serious reservations about that one. If I was going to buy a spare prop with the intentions of using it when the boat is loaded heavy, it would at least have to be an aluminum prop but personally I would still get a good used stainless prop with a high rake. The cost would only be a few dollars more and a hellava lot better prop.

As for pitch. I think your motor is all ready turning 400 rpm below factory recommended max rpm, if so you really need to go down to a 17 just to get where it should be now. In the same props, dropping from a 19" pitch to a 17" pitch will normally increase the rpm approx 300 rpm with everything else the same. If your motor is already turning 400 rpm below the recommended max rpm, then you might want to look at going down two sizes for a load carrieng prop. All makes of two stroke motors perform much better and last much longer if turned near the max recommended rpm. If the factory says the max is 5,500, you want it turning 5,300 to 5,500 rpm at WOT with your normal load. I kills a a two stroke motor to load it 300 or more rpm below the max, they are not made to load down like you can your car motors. Other than most DFI motors, it's better to let one turn 100 rpm over max than 300 under. Actually, I think I would try to borrow a 17" prop and try it, you will be amazed at what getting the rpm up where it should be will do for the overall performance of that motor, then condsider getting a good prop in that pitch and using you're current prop as a spare. Most dealer will let you try their used props and some will have new props you can test run.

Now as for going from a three blade to a four blade prop. Most four blade props have a smaller diameter so you generally get the same performance/rpm out of a 21" pitch four blade as you do a 20" three blade. Where things are going to get messed up is trying to compare different materials and different pitch's. Running a stainless prop, and going to a composite or aluminum, the stainless blades don't flex and lay back like the aluminum and I don't know what the heck the composite blades will do, but because of this, It's not uncommon for a 19" aluminum to turn a couple of hundred more rpm than the same diameter and pitch stainless.

What I've tried to do is educate you a little on props because condition (bent, knicked blades can change rpm) of your prop and has an affect the WOT rpm and not knowing diddly about composite props and the diameter of it, other than I wouldn't waste my money on one, I don't have a clue as to what size composite prop you need.

I'll include a pic of my prop, I think its in pretty good condition.

With only an 85 hp motor, will i notice a big difference between aluminum and SS?

Yes, even on my 25 merc, I notice a difference in performance between the aluminum and SS. Where you would notice the biggest difference is if they make an 18" raker for that motor. or a 19" or 17" renegade. The SST props are strickly utility props and offer very little in the way of performance gains. The rakers and gades give more bow and stern lift than the SST, plus the motor can be raised more. The reason I said an 18" raker (they only come in even numbers) over your 19" SST, with the increased lift, and if you raise the motor a couple of holes, most likely you would see as much as five mph more, which would mean an increase in rpm's. I haven't messed with those old four bangers in so many year, I don't know what's available in props for them.

I've never seen much use for those compprop's myself.  I guess if you wanted a light weight spare they might make sense but you can pick up a used aluminum for the same price and actually use that thing if you had to, I'm not so sure you could do that with that composite prop.  Ya know, you could scrounge around on Ebay, bass fishing homepage or bass boat forums and come up with a decent, used, back up stainless for what you'll spend on that composite.