QuestionI wrote to you on several occasions previously with regard to the 20-hp B&S Vanguard Twin V fitted in my hovercraft.
I live in Malaysia.
Nothing has improved so far but I must admit that I have learnt a lot from our previous communications. In the meantime, I have still to resolve the problem and I thought it as well to write in again with a full account of the problem, what we have communicated and what I have done so far.
In order to achieve maximum lift capability, the fan must spin at a very high speed. On the outset, the engine power rating of 20-hp @ 3,600 rpm and the lift fan size/pitch angle are as per the build plans. I tried two types of fans, the standard 4-bladed fan and the heavy duty 8-bladed fan, both purchased from the reputable hovercraft designer.
With the hovercraft in full trim (fitted with the skirt) resting on a flat ground surface, I managed only 2,700 rpm with the standard 4-bladed fan. At this speed both butterflies in the carburettor are already fully opened.
I accept the fact that the fan pitch and the pressure in the skirt would generate a physical resistance to the engine, which may shave off, say 500 rpm. Still that would in fact, leave me with 3,100 rpm.
(Idling speed is rather high, say at 1,200 rpm).
At this point, I was wondering whether
1. adjusting the speed governor could compensate for the loss of 900 rpm (3,600 rpm - 2,700 rpm), OR
2. whether it would be too much to ask for the additional ‘900 rpm', OR
3. whether I was wrong in thinking it could be done, in the first place.
Anyway, based on your instructions, I went for the governor crank and the governor lever to make the necessary adjustments, but the ‘slackness' was too little. Rotating the governor crank (tried rotating both ‘in the same direction' or ‘anti direction' as the throttle) with the throttle linkage held in high speed position, made no difference at all to the maximum engine speed.
Can you please confirm whether it is in fact possible to increase the maximum engine speed to compensate for the load imposed to the engine? If so, how do I do it.
AnswerYou can increase the speed with the governor but only to an extent.
When we set the governor on the engines we are setting the speed with no load on the engine. So when I set an engine to 3600 rpm I make sure the blades are not turning on a rider mower so as not to place a load on the engine.
The 1200 rpm idle is correct. With the fan dis-connected from the engine you should be able to get 3600 rpm. Most Briggs engines will actually tolerate 4000 rpm without suffering damage but you really do not get enough HP gain with the added 400 rpm.
I'm guessing that the fan is too much for the engine. What is the minimum HP required by the fan? Will the engine run at 3600 rpm with the fan disconnected from the engine?
Let me know.
Eric
