To hide the nav bar use this code
[[self navigationController] setNavigationBarHidden:YES animated:YES];
To show the nav bar you can use this code
[[self navigationController] setNavigationBarHidden:NO animated:YES];
And here is the doc's that might be helpful my friend
Hope that helps you man.
EDIT
Here is a github project for hiding the tab bar. Hope this helps you.
https://github.com/idevsoftware/Cocoa-Touch-Additions/tree/master/UITabBarController_setHidden
Let me know if this is what ou are looking for and if you need more help man.