trolling w/downrigger weights

[weez]

I'm new to offshore trolling. We tried a 5lb ball at 7-8 knots. That did not work. What should we use or how much weight do we need to get our riggs down 15-30 ft?

We fish from a 23ft sea pro at Jersey's near shore lumps.
Picked up our first tuna. 5 short BFT, 4 skipjack, and 1 small dolphin.

Thanks so much.