HELP! Need a quote as good as this!

I need to find a quote as witty and funny as "A woman needs a man like a fish needs a bicycle".

In the context of "its as much use to us as......... is to a ...........

Can anyone help?

Thanks

I'm told that a common farm expression is "About as useful as teats on a boar hog".

Useless as a screen door on a submarine.