Tunnel Tab Size

[Johnny Venomous]

I have a 30 ft. Cobra cat that I am removing my trim tabs from the back of the sponsons, and moving them inward to make a tunnel tab out of them (the boat really didn't perform any better using the tabs). My question is: How long should I make the tunnel tab from front to rear. My tunnel is 4 foot wide and 1 foot deep at the transom and my tabs are 18 inches long. Also what are the pros and cons to making it too long or short. Any ideas or theories would help.
Thanks :confused: