25' sxt Warlock
 

We own a 1995 25' SXT Warlock cat. Needing help with a porpoising problem. We bought the boat last summer with a 310HP chevy in it.. Obviously we didn't want the motor just the boat. So, we took out the 310HP (65mph)motor and replaced it with a 540 BB Chevy W/ patriot aluminum heads. Running a 30 pitch bravo 1 prop. (bravo 1 drive).. When we are running the boat it runs,drives great until we get up to around 80mph. At around 80 we get a bad porpoising problem:eek:. We have GPS'd the boat at 88mph... BUT from the 80mph range it is a wild ride.. I'm pretty sure if we could get the porpoising problem to go away we might be able to break the 90mph mark.. Any suggestions would be greatly appreciated.. Thanks in advance!!

mrfixxall
 

try borrowing a 5 blade or see if you can borrow prop that spins the oppisite direction.

You will need to put tunnal extention on the boat . Dana marine has a real nice set for around 700.00 .

Is the extention you refer to for the stern drive to lower the prop further??

Check the bottom of the boat for hook. I have heard of this before with this hull. They had the same "porpoising" problems that you are. He changed props and installed a tunnel tab. "Porpoising" stopped only after removing the hook in the boat bottom.

The boat will pack air in the tunnals on the big end. If you get to much lift you loose that air out the back of the boat wich makes the boat lower back into the water. This also will slow you down. We just made a set for my buddys DCB they worked great now I'm putting them on my Daytona. If you want to make your own make them the same width as your tunnal 12 to 14 long 1/4 inch drop .We use turnbuckles to adjust them if needed.

I owned and ran a 25sxt with 850 hp supercharged. Will look up some pics and notes. I didn't run a tab,but ran a stern power drive with a merc #3 cleaver..The boat best was 128 and ran 125 at Loto.The hulls with bravo drives seem to have more problems,, I'm thinking drive height,prop design and the engine on the trans hurts the CG. The SternPower drive set the prop back as far as a stand of box with a bravo.
The person that purchased the boat also installed a 540 NA and the boat ran in the 90s on a rope.Give us some more information..

540 is naturally aspirated with 850 demon. Bravo 30" is stock. Bravo 1 is stock. I have no trim tabs and steering is cable but I do plan to add full hydraulic. I did use 4' level in 4 different places at back of sponsons and they appear straight for that distance. I also wondered if spacing prop downward would help leverage but I know this adds drag. I'll look into extentions as the description offered by PLUMB CRAZY sounds very much like my problem. Thanks for any and all suggestions.

mrfixxall
 

try a differant prop first, my friend a eliminator 25 and the thing porposed so bad at 80 it felt like the boat wanted to leave the water. i put a hydromotive 4 blade on,problem went away,my prop gut said that the boat was to light in the front when it got aired out and the bravo 1 prop had to much rake for that tunnel boat..

We have a friend that has a 31 hydromotive 4 blade and we did try that prop with the same outcome.. a tiny bit faster (less than 1mph) and still had the same porpoise problem. I think we are going to try the tunnel tabs that PLUMB CRAZY mentioned... hate to experiment with $800.00 but that really sounds like it could help the problem. Any other suggestions would be greatly appreciated and when we do the tunnel tabs we'll let you guys know the outcome.. thanks again!

25 warlock
 

I have one of these boats and have fought porpoising forever..
I run a 1150hp 540 (AFR 357CNC, matched cam, 14:71 @ 14psi)
34 5 blade (1.5 gear) spinning 6500rpm+... Did not get there over night. Larbe Tunnel tabs, raised drive (raised!) straight bottom, shifted CG and it is very sensitive to trim and power, The boat must be driven through the mph ranges and its transition points. Likes a good chop- On a glass surface I can make this boat porpoise at any speed if there is no acceleration event or if I do not watch the trim. The chop is more forgiving and the boat is a different animal.

Did the tunnel tabs make an appreciable difference. I understand they help on some cats. I am looking into installing them but have issues with swim platforms and exhausts and was wondering if results are worth the effort and expense. Also am relative newcomer and not familiar with CG-can you define for me. Thanks for the input.

Do you think the tunnel tabs made a difference?? thanks in advance!

What RPM are you running at top end ?

5100 RPM... 30 pitch bravo 1........... any suggestions?? thanks in advance!!

Have you tried a smaller prop and more RPM ?

We did use a 26P that a friend loaned us... As far as we can remember the porpoise was still there.. Can you explain how more RPM's would make less porpoise?? thanks in advance!

Have you tried trimming the boat up? Cats need air and will porpoise if run flat.

Sometimes the torque load can affect how the prop carries the boat. With my DCB I usually run it up to about 50, raise the trim to where I run at top end, back off for a second and then hit it. It will porpoise between 60 and 80 if I run it up slow. The smaller prop would accelerate quicker and maybe run though your problem area.

My buddy has a DCB we put a set of tunnel tabs on it and all the porpoise went away !!!! SIMPLE FIX LOW COST IT'S JUST MONEY.

I tried tunnel tabs on my 22 Daytona, when they were adjusted down far enough to do anything, it would slam on every wave and occasionally stuff the front end under. Back them off an 1/8 and it would porpoise again.

Do you have tunnel tabs on your boat? thanks

No, don't care for them.