The noise requirement at the ponds for Nitro and Gas boats is 87db @ 10 meters. This is a National guide line for all boats that run in NZ and we are very diligent in controlling this as we are in a populated area and we must make sure that all boats meet these requirements.
Most boats bought ready to run have to have an after muffler or a built-in silencer in the exhaust system.
Here are some designs that may help you.
Silencer by Barry Jessop
Silencer by Wayne Lester