I just run a 25 pitch Mazco 4 blade on my 175 Merc and it is on a 18 1/2 ft Champion and it runs about 5600 rpm's at wide open throttle. I also think the 21 to 23 would be better suited for you. Would get you out of the hole better and get you more rpm's at wide open throttle which may even get you more top end speed. You can run closer to 5600 rpm's with that motor. I would call or stop in at Pro Choice or Angler's Port in Warsaw. They will be able to help you in selecting the right prop and may even have a used one and may even take yours in on trade. Mercury has a website that you can enter the info for your boat and what prop you have now and what the rpm's are at wide open throttle and it will give you the proper prop in the differant brand names they have. You may google Mercury props and see if you can find it. I would rather have a great hole shot with a loaded boat and give up a few MPH on top speed. But I do not ever run much over 10-12 miles in most tournaments but move around 15-20 times a day from spot to spot. So I like the good hole shot.