Tunnel Tab Size

Johnny Venomous · 03-15-2005, 08:04 PM

I have a 30 ft. Cobra cat with outboards 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 the tab too long or short. Any ideas or theories would help.
Thanks

mmwalters · 03-15-2005, 11:25 PM

In making the tab to long as I did, I overloaded the trim rams. I am changing to trick marine rams that have 3/4'' larger bore. Stay with the 18'' Also ckeck your prop clearance at tab down and drive trimed up.

Sean H · 03-15-2005, 11:28 PM

what does your boat do that you need a tunnel tab? and i am surprised that the tabs at the sponsons had no effect on the boat...

Johnny Venomous · 03-16-2005, 01:11 PM

The boat gets the infamous hop up around 80 and I have been told that a tunnel tab will help a lot (both, to get rid of the hop and help the speed big time, I don't know how true it is, but I have been told that my hull should gain 12-13 mph by adding the tab. This hull has a hard time packing air in the tunnel and keeping it there). I can't get the boat over 85 without having a major problem with the hop and have to back off the thottles. The boat reacts well to trimming the motors, so I really never use the tabs(they seem to help the hop a tiny bit but it takes mph off the top end, so I'm going to try the tunnel tab).

mmwalters · 03-16-2005, 02:31 PM

Johnny V The tunnel tab eliminated the porpoise on my motion 30' and added maybe 1-3mph on the top end. The tunnel tab was the best addition I could have installed on my hull. I think 12-13mph is unrealistic

Johnny Venomous · 03-16-2005, 09:15 PM

Cool that's what I wanted to hear, I think if I can get rid of the hop I will be able to top out the boat safely without having to back off the thottles and get a true top speed with the motors reved all the way out.
How big was your tab on the Motion, did it span the entire tunnel, and what was the length ?
Did you have sponson tabs at all, if so did you really use them?
Thanks

mmwalters · 03-16-2005, 11:12 PM

JohnnyV yes the tab was the full width of the tunnel in my case 44'' and I cut it 26'' long . My trim cylinder were 1 1/4'' which was to small the new cylinders are 2''. Hopefully this will help stop the tab from creeping back up

mmwalters · 03-16-2005, 11:21 PM

Johnny read the thread started by Shawn titled Trim pump pressure relief valve

Johnny Venomous · 03-17-2005, 10:12 AM

MM,
This is great, I love all the feedback you are giving me. How thick was the aluminum plate you used. This is almost exactlly what I am trying to achieve.
Thanks

mmwalters · 03-17-2005, 01:15 PM

Johnny The aluminum plate I used is 1/4'' and has worked well. I saw some race cats in key west that used 1/2'' lexan for the tunnel tab. I don't know the size or your merc rams, try them and see how they work. I think trick marine cylinders were in the range or $265 ea. powder coated more if polished. If you can keep you tab short 18''-20'' your merc rams may work fine.