effec of '-' dash on search engines?

i dont think so

You can ansawer your own question by doing a google search.

Since we all want to be number 1, find the keywords that you want to use. Google them and look for a site with a - in the URL. If you find one in the top 10 then you know its ok. If you dont find one anywhere in the top 200 or so, then maybe you should skip the -

A dash is supposed to be better than an underscore.

Google see hyphend domains as a space.

as far as keyword dense urls go the dash is slightly better than an underscore for replacing spaces and special chars when using modrewrite etc.

i do not know specifically of any instances of a dash in a domain not placing properly. but i would say that if you are competing with domain.com with do-main.com you are most likely late to the game as domain.com would have been the logical choice and the first to be registered. domain history has a lot to do with page rank and the reputation of the domain for assigning page rank to the sites it links to.

most of the time i suggest finding a better domain name if you have to use a dash to get the one you want. out ranking established sites is hard enough without making the job even more complicated. and most often people who are typing in your url will forget to include the dash and you will just send traffic to the other site anyway.

its the same way with taking a .org or .biz name jsut to have the domain you want. the majority of people will type in .com regardless just as they will assume your domain is the one without a dash.

I tried setting up some dashed domains and Google gave me NO LOVE at all... IMHO.. dashes suck and your wasting your time and money.

never seen an underscore in a domain name before

I have a hyphenated domain and get pretty good search results. I havent seen a negative effect whatsoever.