A puplic key algorithm alone (like RSA) doesnīt help you very much whith encryption. You also need another encryption. With that second encryption algorithm (itīs usually a symmetric one like Blowfish) you encrypt your message and the encrypted messageīs key you encrypt with the public-key algorithm.
If you "encrypt" a message with the RSA-algorithm itīs the same than using a One-Time-Pad-key twice: itīs nearly worthless.