Whiskey Dick

They say that drinking too much alcohol makes you have a limp dick. I believe that is true but found out that different types of alcohol have different effects. I found that when I drink whiskey (2-3 glasses) is usually enough to make me feel tipsy but I can still keep a normal errection. However whenever I drink beer even half a pint I find that I have a problem keeping a hard on. This seems to be the case with some other alcohol. I know that its alcohol that you are consuming either way but I wonder if there is a reason for this.