My buddy has a 14 ft. deep vee Gruman with a tiller. We tried them off the back but found it was too "crowded" back there so we moved the down riggers forward. Actually mounted them on the oarlocks. Worked out perfect. He can easily use them alone and run the tiller too, or there's plenty of room for a mate to run things too.
fishnmachine would you be able to get pics if possible thanks
Most people are going to tell you to mount them as rearward as possible. I went against the grain and mounted them 4' forward of the stern to allow me to use them (and mount them) easier and I'm pleased with it. The only time I think it would be a problem is if I ever went to the big lakes and fished deeper than 120 feet. Then I would probably have an issue with blowback and making sure I kept the cable out of the kicker prop. First season with this setup and still working the bugs out, I may feel different in the future.
Off the rearOn each corner
I dont think youd have a blowback issue if you use a heavier ball.
see pg 12http://store.cannondownriggers.com/media/document/3397136rf_UniTroll_EasiTroll.pdf
I may not have a blowback issue even with my 10lb'ers, I certainly don't when I've got 150 feet of cable out at 2.5mph. I also don't have electric riggers (yet, but I will) so I have no interest in much heavier balls or fishing deeper water than 120 feet. Mac's link shows blowback calulations. Pretty handy charts actually.