Her is the answer I found. It talks about a twin engine Yamaha jet boat but the theory is the same for a SeaDoo twin.
Twin Engine Pump Loading
The phenomenon described above affects the twin motor jet boats in an entirely different way. The water intake surfaces on the bottom of twin motor hulls are on angled surfaces on each side of the hull. This “angled water entry” gives an effective entry angle that mimics the pump loading of turning the boat. This means that (while driving in a straight line) the drivers side pump (starboard) is receiving water at an angle that mimics a left hand turn (thus loading the engine rpms down). At the same time, the passenger side (port) impeller is receiving water at an angle that mimics a right hand turn (resulting in higher rpms and a closer cavitation threshold). This is why the left (port) motors of most twin-engine Yamaha jet-boats tend to run higher rpms than the right (starboard) engines.
Getting past the whole theoretical aspect, the functional result is what’s important.. Because of the phenomenon described above, the right side pump is generating more actual thrust than the left pump at any given rpm. This causes the steering wheel to constantly be “tugging” toward a left hand turn, and at the same time causes the left side engine to rev higher than the right engine in order to generate the same thrust.
After weeks of testing, we eventually resolved all these problems (on twin motor Yamahas) by fitting better design impellers on the pumps, and applying a noticeably steeper pitch to the left hand side prop. The end result is engines that turn virtually identical rpms all the way through the throttle movement range, and an end to the steering wheel constantly “tugging” to the left. In most cases, the “off-the-shelf” pitch of the props we chose were not right on the money, so we custom pitch each pair that we sell to the specs that worked best during our on-water tests. Installing these “staggered pitch” aftermarket impellers is one of the most effective improvements that you can make to your twin motor Yamaha jet-boat.
