What problems?
Can you state some actual issues rather than vague generalizations?
I certainly can, most attempts to use single centrally mounted engines, encounter the problem of aerated water entering the prop. Asymmetric tunnel hulls, less so. This necessitates getting the prop down lower, requires a fairing ahead of the leg, or a modified leg, both of which create drag. The aeration problem can be worse in turns. As I mentioned, some smaller cats have been successfully fitted with singles, but when you get into larger boats, the geometry of your typical outboard. makes it difficult to get it low enough to get into clean water, you would have to have a wide central pod , and your cat is turning into a tri-hull, with the "congestion" in the tunnel becoming a problem. It is possible that the 30" leg outboard might overcome these problems, but 25" won't cut it, except in the smaller boats where it scales better with the geometry of the boats. You could get away with a slimmer pod, with a 30" motor. One little 5 metre cat appeared to work with a 25" leg motor, but they needed 5-bladed stainless props to defeat the aeration issues. But that boat was also available with twin engines, and more buyers went that way, possibly because they could be pushed harder than the single engine version, as when the boat got a bit airborne, as these cats easily could, the centrally mounted engine's prop, being a little higher in the water column, would "let go" sooner.
