[Chris Burkard]

Yes, the gunnels are cored on your 35 Open. Total thickness should be about 1". Those gunnels are cored with Balsa, so do not screw into them for something as stressing as a downrigger. The minimum would be to thru-bolt with fender washers underneath. Make sure you seal the holes as trit21 mentioned (Use a good silicone based marine caulking - applied liberally!). A backing plate would also be a good idea, but I do not think it is 100% necessary. Fender washers should be fine to handle the 40 - 50 ft.lbs. of force those bolts are going to feel. Measure 5 times and drill once!

Good Luck!