It is all about pump loading.
In the words of immortals of Group K:
http://www.groupk.com/yjetboat.htm
"About Pump Loading
All technicians that build high performance PWCs quickly learn a few fundamental rules about PWC pump function. As you engage a jet-pump driven watercraft in to a high-speed (power-on) left hand turn, the engine/pump gets very heavily loaded, and engine rpms are usually “pulled down”. However, making a right hand turn at exactly the same speed results in no rpm loss at all … and often results in an rpm increase, often accompanied by mild cavitation.
The reason for this phenomenon is that the entry angle of water coming in towards the impeller has a big effect on how well the pump get “loaded”. In left hand turns, the water comes toward the prop at an angle that very effectively “loads” the pump with water, and allows for very minimal “cavitation”. A hard right hand turn has exactly the opposite effect because the water is coming in toward the prop in a way that is very “unfriendly” to pump loading. Anyone who has driven a single engine Exciter 135 will experience this phenomenon in a big way. Using aftermarket impellers with a better blade/hub design can greatly reduce the cavitation experienced under all conditions …including in right hand turns. But even the best designed prop cannot eliminate right-turn cavitation altogether.
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."
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