For trolling, which we do a lot of, there is another way .
On a RIB, fitting a downrigger is pretty much a non starter, however, over the years we perfected another technique which seems to work just as well
Use a standard Tube / wrecking booms (the correct name is a Guy's boom ) and quite a long trace of 6 to 12 ft ....... the important bit is to spend a little time adjusting the lead size to the depth you want to fish. We find leads of 2 to 4oz are generally good enough. The way to check it is correct is to set the boat going at just above idle (I use mercury Troll Control), start with 4oz and stream out 50 - 100 yds, if it is bouncing bottom on a tight line, then reduce the amount of lead until you can just feel the lure working.
On rough ground you can do the same with a Portland rig, but you need to keep the rotten bottom to less than 3 inches .............
All we do is set the clutch to just about non slip on the ratchet and listen for the reel to scream ..............
I have two rod holders just for trolling on top of the A frame, I set one rod at 50 yards and the other at 100 yards
The takes are normally savage !