Continuing my trawl thru the old club mags in the loft I find confirmation that a sub 80dba cruiser is possible in this report I wrote for the sept 73 issue: (Eureka was a viable 2 seat cruiser with three 9bhp stihl engines)


The thrust engines on Eureka have now been equipped with large absorption type silencers on the exhaust system and an expansion box common to both engines has been provided for the air intake.

No significant reduction in the cruising performance has been noticed. Noise Tests were made on Eureka following the Danson Park Race meeting (14th July 1973) using a borrowed Bruel and Kjaer Sound Level Meter No. 2205. The observations were made at a fixed station 25 meters (paced out) from the craft with the craft rotated into each of four positions and with the engines at approximately half power, which is the normal cruising level, and at full power.

The noise level at tick—over was measured at approximately 60 dba at 25 meters and the noise level at the drivers head from the port thrust engine only with the microphone facing the engine was 85 dba at tick—over and 105 dba with full power.

Results :—

Front Port side Stbd Side Rear

Lift only 78 72 68

Port engine only slow 68 66 68 69

fast 77 75 73 72

Both thrust engines slow 70 70 72 70

fast 80 78 77 72

All three engines slow 72 75 71 70

fast 81 80 79 75

Note : lift engine exhaust faces forward, Thrust engine exhausts face the rear.

In general the conclusion is that the noise level is approximately 80dba at 25 metres at full power and 70-75 dba at cruise power. This compares favourably with the HCGB racing limit of 95 dba and the cruising limit of 86 dba (at full power) and shows that this limit can be achieved with quite simple measures. "