Any general message that is sent to competitors is also sent to the list of contacts.  If you add coaches to the contacts list, they receive all communications except communications directed solely at the competitor, such as notice of a protest being filed or notice that the PRO has responded to a scoring inquiry.

So if you go to the Competitors list from the main menu on the Event Panel, you'll see the Messages link at the top.  Click on that and click Custom Message and you can send all competitors and contacts a message.

Thanks for the immediate response. I think that if I write a custome message I cannot choose only one coach. You may consider this option as the young sailors (9 to 13 yrs) do not all have mails, so is better to contact with the coach. 

Vasilios,
A solution, especially if the competitors don't have email adresses or mobile phones, might be to use the coach email adress and phone number in the infos of his competitor. That way, they will also receive infos directed at the competitors (probably they will receive x times messages directed at everyone).