Re: Twin engines vs one single engine
If you are speaking of low speed "holeshot," ability to pull skiers up, etc. I doubt you would be able to feel much difference between a dual engine installation and a single engine installation of equal, total horsepower. You might get a bit more "umph" out of the twin rig just because the setup might be somewhat under-propped to insure planing ability on one engine, should the other one fail. That said, you could obviously go with a lower pitch prop on the single, if your operating parameters make a "lighter" pitch more applicable to your needs.
One thing that I would say about a twin installation for water skiing on a smaller boat, is that the skier might be less likely to "wag" the stern of the boat. Having a pair of thrust lines off of center, will have a bit of a stabilizing effect on the boat laterally. As to how much of a factor that might be, I would say that it would be hard to predict. The advantage would also be negligible on a larger boat.
As to drag, I agree that a twin installation of equal horsepower creates more drag, and reduces maximum speed. In my experience, however, they don't affect low speed situations in the same way, simply because the amount of parasite drag relative to the speed of the boat is low. The issue becomes more pronounced at higher speeds because drag doesn't increase in a linear fashion. Instead, what happens is that the rate of increase in drag speeds up as the boat goes faster, resulting in a proportionally higher amount of drag relative to boat speed.
Beyond those issues. the real disadvantage of twin installations is cost. Everything about powering the boat becomes more expensive, from acquisition to mainteneance to fuel burn. I really like twin installations a lot, but converted my present boat from a pair of 35hp ERudes to a single 65hp Johnson. Its hard for me to tell how much of a speed difference is due to the change in motor arrangement, because the ERudes were horsepower rated at the powerhead and the Johnson is rated at the prop, but my boat does go faster now. It also burns a lot less fuel, and I only have one motor to maintain.
