The engine runs on 2 cylinders operating while at the lower speeds. I assume the designers designed this function for Great Lakes trolling or such where the majority of the outing is trolling....fuel economy in mind. The lower 2 cylinders get fuel mix but not enough to light off and produce power....just lube.
The spark/fuel linkage is setup to bring all cylinders on line, providing power, at around 1800-2000 RPM. Depending on several things, the transition could be hardly noticed.
As Faztbullet said, you need to under prop...use less pitch than you would use under identical circumstances with a regular 4 cylinder engine, to get your engine RPMs up before you really need full power. Hole shot needs to be firewalling the throttle with the engine "tucked in" go help in getting the bow of the boat down to bring the stern up to aid in acceleration until you get the skier up on the water. Then you can cut the throttle and trim out to a position that brings the bow up slightly for best performance. Speed depends upon the qualifications of your skier and other non related things.
Being under propped, expect to hit your 5250 RPM revolution limiter ( a little module in the engine in the 4 cylinder engines) at a lower speed than would be the case if you had a prop installed for a non-water sport application and lightly loaded boat. It only takes a few minutes to change props so having a "speed" prop handy isn't unusual...I have such....and prop nut torque is 55 ft-lbs.
Other thing on the prop is the design. You want to get through the transition region as soon as possible to get all 4 cylinders on line in addition to getting RPMs up as fast as is reasonable considering other things. One thing that helps that is to get a "ported" prop...holes in the barrel just under the leading edge of each blade. In aluminum, Turning Point Hustler makes such...in SS, lots of folks. Porting the prop gives the advantage of exhaust gasses added to "new water" in the prop slipstream reducing the resistance into which the engine is trying to accelerate allowing it to accelerate faster.
Since HP is {Torque X RPMs)/5252 you want your RPMs up fast to cause both to rise fast and get your skier up out of the hole and on the water fast. Once up and running, the holes will seal off with flow by water pressure and your RPMs will drop down as your speed is increasing, prop locking in, and then increase as speed increases just like the holes weren't there. I think a 3 blade ported prop would be your best option, diameter is determined by your lower unit size....13 to 13+" on that 115 lower unit, and pitch to suit your usage... on a 750# hull and a couple of folks with a 3rd in the water, a 17-19" pitch leaning toward the 19 for your application would be a good starting point if I were doing the buying. The Hustlers sell on this site for about a hundred bucks. You can miss your target and redirect without spending too much money on a closer guess if you miss....performance testing is the only way to really know what prop works with what engine/boat/usage situation. Just remember for the holes to work best for you, when the skier gives you the nod, engine tucked in, and FIREWALL the throttle.
