Tunnel hull cavitation

I bought a used 18 foot allweld wide flat bottom boat to restore and use in skinny water. I put a short shaft Mercury 70 hs motor on it. It had the tunnel welded closed and i took a grinder and opened it back up. The first time i tried it at the lake it cavitated so i adjusted the engine a bit more forward ( or down at the lower end ) and it helped some. I had my partner sitting in front of the console and when I had him move just back of me the boat planed out and ran about 10 miles faster. It did dig deeper into the water in the back though. When I cut out the tunnel I left a bit of a lip at the entrance to it because the fuel tank is right above it and i did not want to take the chance of accidentally cutting all the way through and into the tank. The lip is about 1/2 inch out into the cavity and may be diverting the water back instead of sucking it up into the tunnel. I just don't know what else it might be. Can any of you enlighten me on what might be the problem? Also when i had the engine tilted up a bit the water was not getting to the water pump inlets. With it tilted more down it does get water to it but not as much as it should, probably because the tunnel is not full. I don't want to change the center of gravity by moving the console back until I rule out the tunnel or something related to it out. please help. All comments will be appreciated.
Thanks
Strawberry

Re: Tunnel hull cavitation

I recomend using a grinder to take the 1/2 inch lip out. Its only a half inch but could be making a lot of differance in tunnel water flow. Also, I think that the cavitation would be pretty severe if the water pump is not pumping properly. Performance issues would be your main sign to make a change. If the boat responds even moderatly, planes out and maintains speed without roaring out, your pump is getting water. The cavitation plate should be even with or 3/4 inch above the top of your tunnel.

Re: Tunnel hull cavitation

I will try the grinding part as soon as it quits raining, which will be in a week or so. I am thinking 4 blade prop but will have to find out what size pitch to get. I am also thinking of a Jack plate for it with a 5 inch set back. That way i can adjust it up or down to suit the sea conditions better. I do not have tilt and trim but don't really think i need it. Also, right now the cavitation plate is about 3 inches below the top of the tunnel and is about 4 inches above the bottom of the boat. The tunnel is about 2 feet long. As the situation is right now i cannot raise the motor anymore than about 1 1/2 inches and cannot lower it at all. I will keep in touch. Thanks for the suggestions.

Re: Tunnel hull cavitation

I am just about to complete a purchase on a G3 1860 CCT, the dealer told me that a manual Jack plate is all that is needed and that once I "fine tune" the location of the motor height in the tunnel, I shouldn't need to adjust again. Is that correct? or should I get a hydrolic plate so I can move the motor up and down more frequently? Thanks.