With the introduction of the 2.4 Ghz Radios the 27Mhz and the 40Mhz systems will soon become obsolete, they are however the only ones suitable for underwater submarine use as the 2.4Ghz will not transmit under water.
With the 2.4Ghz system you can forget about wrong crystals; or is somebody on your frequency; dual conversion; incompatibility of some crystals on some radios; interference etc etc. need I go on. The first 2.4Ghz systems were expensive but now a transmitter and radio can be bought for just under £30 there seems to be no point in hanging on to your old 27 and 40 Mhz Radios.... START to update for a hassle free life. Additional receivers are only £10.
It seems that the Planet and RadioLink are no longer readily available. FlySky are a suitable in-expensive alternative but are not compatible with Planet or RadioLink.
A simple,one off binding process for your first time use and your set is tuned to just your transmitter. You can use just one transmitter and have all your boats fitted with receivers which will only receive signals from your one transmitter. Should you for any reason want to add or change your transmitter for the same type you can simple re- bind your receivers again to match your new transmitter SIMPLE !!!!
If you do have a rather expensive 40Mhz Transmitter it is possible to purchase an adaption kit to 2.4Ghz and still retain your transmitter, 2.4Ghz 4 to 6 channel receivers can be purchased for as little as £10 to match the modified transmitter.