Setting a default selected value in DropDownList in MVC3

In MVC3 I have this code at my controller. It retrieves a List of IDs\Names from Installation table and creates a ViewBag

var vs = dba.Installation.OrderBy(q => q.InstName).ToList();
ViewBag.Vessels = new SelectList(vs, "InstId", "InstName");

Now, at my view. I want to render the list in a dropdown list. I used the Html helper that works fine...

@Html.DropDownList("InstId",(SelectList)ViewBag.Vessels, "- Select one -")

I need to set first item in the ViewBag List as a default selected value, instead of "- Select one -" text.

How can I do it?

Thanks in advance!