Re: jet boats good or bad?
Yamaha has a dealer sponsered test ride program that started this month. Also, All the Yamaha jet-boats use 155mm pump assemblies that were originally designed for use in PWCs. Unfortunately, the jet-boats are about 3-4 times the weight of a PWC, and have a whole lot more “wetted” hull surface area that offers heavy resistant to the pumps. It’s much like using a street-bike engine and tire to push around the mass of a large automobile. The contact patch of a motorcycle tire could be “adequate” for most uses, but it’s traction abilities would be constantly stretched to the limit.
In the case of the Yamaha jet-boats, the use of two pumps on the twin engine machines offers a huge advantage in delivering the boat’s power, but even the twin pumps of these jet-boats are often stretched to their limits by the loads of sheer weight and wetted-hull surface. 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) motor of most twin-engine Yamaha jet-boats tend to run higher rpms than the right (starboard) engine. Because of this , 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. You can install different pitch impellers to compensate for this. Yamaha built all their Jetboats with a “debris cleanout” access plug over the jet-pump intake. This cleanout is a great convenience for owners who drive in areas where there is lots of sea-grass or other floating debris that can make it’s way into the jet pump inlet.
Unfortunately, some owners have experienced high-speed episodes where the cleanout “plug” is suddenly ejected from it’s position over the pump inlet. When this happens, the plug often strikes the access door with such force that it gets pushed open. When the access door is pushed open, the pump plug is often blasted into the air, and vanishes into the open water. As the access door is blown open, it actuates an engine cutoff switch. This creates a situation where the boat decelerates rapidly, while a large column of water is being driven upward through the opening in the top of the water intake. In such a situation, most of that water column will fall into the passenger compartment of the boat…. And pretty much soak everyone. To add to the insult of all occupants taking an instant dowsing … the pump plug is gone overboard, thus rendering the boat close to inoperable .
Just some more thoughts to consider while you are "narrowing"