Although the hovercraft flew along the alley at the back of the house, it clearly wasn't finished.
Power valve control
The powervalve controller needed sorting out, otherwise I'd be stuck in low power mode, or have to use a manual control.
The business end of a Yamaha powervalve
The Yamaha Power Valve System (YPVS) varies the timing of the exhaust port. At high rpm, the power valve is adjusted automatically, to further improve top end performance.
An "apple-core" shaped valve (shown left) in the side of the cylinder rotates to alter the timing. This valve is rotated by a pair of bowden cables which are pulled by a servo motor assembly elsewhere on the hovercraft (or bike). The servo motor is controlled by an electronics module which senses the tacho signal from the CDI unit, and drives the motor one way or the other. A potentiometer (variable resistor) inside the servo motor unit, feeds the rotation position back to the power valve control box so the electronics knows what setting the power valve is at.
Yamaha probably located the servo unit remotely from the engine to reduce the vibrations, but why isn't the electronics module combined in the same box as the servo unit? Even better, they could have put the CDI in there too, thereby cutting down on all that wiring and iffy connectors.
The wiring from the controller module to the servo unit was unclear - some of the colours of the wires didn't match, but after a little dismantling and inspection I sorted things out. When the electrics are switched on the motor pulls the cables one way then the other, then it settles down. Now, when the engine reaches 6,000 rpm, the controller senses this and progressively adjusts the exhaust valve, which gives a power boost, and allows the engine to rev to over 10,000 rpm. Some people say it feels like a turbo boost, but of course it works in a completely different way.
With the servo unit working well I built a bracket and fitted it to the side of the cockpit in front of the exhaust pipes, using two of the screws. which hold the right hand deck panel in place. This location, and the fact that the servo is mounted on rubber bushes means it should be isolated from the engine vibration. A dust shield over the front would probably be a good idea.
A lot of the construction regs are about safety. There is a requirement that the transmission shaft is restrained so that if the bearing blocks fail then something else will prevent the shaft from flailing around - at 10,000rpm. This simple bracket is clamped between the bearing support platform and the bearing block (via the holes in the bottom). The transmission shaft passes through the large hole with 5mm clearance. Nigel and his flypress came to the rescue again!
As well as needing to finish it off mechanically, I needed to give it some serious testing and tuning, and that meant I needed a wide open space. Fortunately I found a local famer who let me use one of his many fields.
Taking it off the trailer
You can see the 6 fan blades in this shot. When the engine is running, the fan blades turn so fast that they seem to disappear.
And they're off - just about.
The hovercraft started off OK, a bit sluggish, but it did hover, and it did accelerate.. The rudders seeemed to have an effect - although I couldn't really drive very well due to a total lack of experience. The engine was revving up to an indicated 10,500-11,000 rpm, but I wasn't making a huge amount of progress, then I started to slow down, and eventually came to an unintentional stop in some longish grass. I had lost lift and couldn't get it back. The grass where I had started from was shorter than where I had stopped. Did the long grass mess up the skirt contact with the ground? Probably. Did it produce more drag? Probably. But what was the main problem?
I knew that the clearance between the fan blade tip and the duct was around 10mm, or more, and 5-7mm was the recommended tip clearance. Was this why the lift was poor?
The blade tip clearance was poor at the bottom of the duct - where it really mattered, but also the clearance was uneven , ranging from 4mm to 11mm.
I decided what I really needed to do was reduce the clearance in specific areas, so I applied some glass fibre to the bottom of the duct, reducing the clearance to around 4mm in that area.
Would adding more blades provide better lift and load the engine at the same time?
Back to the trailer
I was not happy about the way the trailer supported the hull - the skirt segments often caught on the lengths of wood, and I wasn't convinced they would take the load, or the dragging.
I replaced them with a sheet of marine plywood (pricey or what). While I was messing with the trailer, I also made up a heavy duty trailer board frame from lengths of steel. This slots into the back of the trailer once the craft has been loaded.