props
 

Yes, it's possible. Technically, as long as you can swing the prop safely (meaning it doesn't hit anything like your anti-ventilation plate) you can use any prop diameter. Now before I get a ton of corrections, remember I said technically.... the fact is there is much more to choosing the right prop for your boat than just diameter. But you can use just about any diameter you want as long as you don't over-rev or under-rev the engine.
The proper way is to find out what your engine's WOT (Wide Open Throtle) specs call for and then find a prop that matches those specs.

For example, lets say your engine manual says that WOT should equal 5600 rpm. So you start out with a 15p prop and you find that the rpms are 6000. You would then go to a 17p prop and see what that resulted in. The idea is to dial in, as closely as possible, to what your engine's specs say are ideal for you engine. Maybe the 17p was still a little over reved, but a 19p was way under-reved.... you could then increase the diameter of the prop from a 14 1/2 to a 15, but keep the same 17 pitch. See? Everything works together. There are other things to consider also, like rake, cupping, aluminum vs SS, etc., but for now just start out simple and find out which of the 2 props you have gets you closer to your proper WOT spec. In your case, since they are both the same pitch, the smaller diameter prop will give you higher revs than the larger diameter one.

Good job!:clap:
Take the time to digest what Destroyer posted.
Each design facet changes the props performance.

Thanks you guys are the best. I have a few things to learn... but have all summer to lean them..

I think to much... the anode does not interfere with the prop...

To run the larger diameter props MERCURY sells a smooth anode without a fin on it to clear them. And props are available in varying diamters. Larger diamter props tend to be for slower boats that need more blade area for bite at slower speeds. What engine and drive do you have in the boat? The 17 pitch may be a tad big for that size of a boat if it has a small block in it.

and put a new anode on if you have not. better safe than sorry

I have small block 350 chev with a merc alpha 1 outdrive.

I am willing to buy a new prop as I suspect the preveious owen might have the wrong prop on it.... I was going to see if I could take the boat and water test it with different props, if possible..

Try the 15.5X17 and see how it runs first. That big of a boat will like a large diamter prop(like a MIRAGE) or a 4 blade to get enough surface area so it will stay on plane at slower speeds. As for an anode, get teh smooth MERC one for it if you replace it. The one with a skeg fin on it will need trimmed to clear the larger diameter props.