Heavy flatwounds
 

I use Ernie ball power slinky 5 string set 50-135 round wounds on my fretted basses. Are there any flat wound strings that are the same sizes for my fretless?

http://www.juststrings.com/

They should have something to suit your needs.

La Bella and Roto make 52-110 flats which are, I think, as close as you are going to get to 50-135.

The experience with heavy flats will be very different from your heavy rounds, I assure you. :)

50-110 for a 5 string set?

Also how are they going I be different?

Sadowsky Black Label SS flats....believe the larger of the two gauges is a .135 B.

Here is a link for D'addario XL chrome flats....their B is .132:
http://www.daddario.com/DADProductDe...32__Long_Scale

get these and add the low B

http://www.juststrings.com/fnd-9050m.html

I use 9050's and, even though the low B is a 128, it sounds much better than the D'Ad Chrome low B, and it still has tone after a year on the bass.

They tend to be higher tension for the same gauge, ie you'll want something lighter than the rounds you normally use. Jasper is probably right when he suggests the Jamersons, they are pretty damn heavy. At least they'll be a good starting point, if you don't like them you can always go heavier.

The Jamerson 5 string set is .052 through .128, and will be much stiffer than you are used to with your super slinkys. To get approximately the same tension, I'd consider Sadowsky Black Label Flats in teh Light set: .040 through .125. Flats are stiffer than rounds, by an order of magnitude, especially when compared to super slinkys, so you really want to go at least one set lighter than you are using now, maybe even more.

For the best prices and service, I'd go to Bass Strings Online. Jason (the owner) is a supporting member here, and is really good to other members.