To comply with EC regulations, noise tests can not be ‘self certified’. Our noise certificates have been produced by the Danish governments testing institute, and subsequently backed up by work done by the Austrian authorities.
Anything that moves air makes noise, whether it be a helicopter, airplane, microlite or hovercraft. The action of the air being moved creates the noise and the faster you try and move the air (use more horsepower) the noise is generated.
Vortex Services has taken the problem of noise very seriously over the years and at the start of 2009 we had an order for 4 hovercraft to a company in Denmark, the Danish government insisted that all hovercraft must comply with the EC Recreational Craft Directive (RCD) ; even though they are specifically exempt (at the current time!!)
At that time the quietest single engine craft available from any manufacturer was in the 82 – 84 dba measured at 25m. This is about the same noise level of a large truck going passed you on when you are standing on the pavement. The RCD states that for a single engined craft the noise level must not exceed 75dba @25m and for a twin engined craft the limit is set at 78dba @25m. In sound terms getting a noise level down from 82dba to 75dba requires reducing the noise level by around 50%, not easy to do.
During our R&D testing we were able to reduce the noise by redesigning the engine silencer system, this cut 2dba off the reading. We then did a considerable amount of work on a new fan blade design in conjunction with a Danish fan company Multiwing, although initial results looked promising whilst the noise level was reduced the performance also suffered. We then turned back to our existing fan setup which uses Hascon blades and started to look at a much wider range of fan sizes and blade combinations. It became very clear that an 8 blades in a 12 bladed hub combination running very slowly kept the performance we wanted as well as reducing the sound level down to 74dba.
We felt that there was more we could do and by using a very smart piece of equipment called a ‘sound camera’ (designed and built by a Mr K Oakley), along with using Keith Oakley’s considerable experience we were able to analyses the whole craft and look at where the noise was being generated. Sample picture below showing the craft cross section and the duct, red areas are the point of most noise.
It soon became clear that whilst the movement of air was causing the noise, it was not just caused by the blades, indeed some noises were found away from the craft, being noise reflected off the ground, others were tracked down to areas inside the duct being generated by the splitter plate and then being carried around the duct by the air flow. Other noises we found away from the duct / engine area and were generated in the hull at the front of the craft.
We learnt a tremendous amount during our 7 months of R&D, the results led us to make some fundamental design changes some are listed below along with those already mentioned above:-
The above changes weren’t done all at once but over a period of 12months with each element tested to check that it was an improvement. The result was a craft that no only handled better, went faster but also regularly recorded noise levels below 68dba.
These reading were later backed up by noise testing done when the craft were inspected by the Danish Technical Institute.
We have carried much of this noise development work into our whole range of craft from the single seater up to the new 9 seat craft. Clearly were are not going to get reading down below 75dba for the larger craft but we can and do reduce noise where ever we can.
Our current 2 seat twin engine Storm LE records noise levels at around 74dba well below the RCD 78dba. It is our policy to kepp developing hovercraft and make every effort we can to produce craft that are as environmentally friendly as we can.