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. 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!!

I have the same boat you do but an 89...
 

It is a wild ride at the porpoise point and unfortunately not easily fixed. The boat likes a good wind blown chop, likes to ride on 4 points. I added tunnel extensions (20") tried 7 props (now running a 5 blade 34" at 6500rpm) raised and lowered the drive (currently a XR shorty with the middle of the bulb level with the lowest point on the outside sponsons). The boat became more sensitive to trim as I raised the drive. IT currently wants to be driven through the mph not just hammered. I also moved the CG around, battery placement, low weight seats. I also have a 540 - with AFR 357CNC heads, the cam is ground for the heads (Lift .725+ and duration 290+ @ .050) a 14:71 pushing 14lbs of boost (water/alky injection, crank trigger...). At my present horsepower level the boat will pass through the porpoise with just a couple of bounces. Before I got to this level the porposie mph just climbed with the speed. As long as the boat had a strong acceleration curve no porpoise slow acceleration curve porpoise. It is now quite the wild ride:drink

Could you tell me what the speed (approx) is that you were able to get over the porpoise??..... around what speed were you noticing the worst porpoise?.... we were thinking the boat may tend to settle at 90mph but not sure.. we are real close to it but haven't gotten past it yet.... Do you have any recommendations??? lab prop? tunnel tabs? shorty? thanks!

25 sxt cat
 

I"ve got a 1999 25sxt, running a 509ci with a whipplecharger
making about 6lbs boost , with a shorty xr and a lab finished
bravo 32 pitch 4 blade. I haven't really noticed much of a
porpoise problem, but I think I know what your talking about.
although in my case, as mentioned earlier, has a little bounce,
but you drive through it, and in my case, it goes away.
(seems to start in the high 70's, goes away in the mid 80's,)
my boat runs 97mph on the gps, at around 5400rpm- and
thats with the stock wimpy 485 lift hyd. roller camshaft! looking to upgrade now, motors out, dropping compression,
installing manley platinum series dished blower pistons, w/
.250 long rods, etc...

i have a friend with a 25. i think all these boats have the same problem, after trying everything possible he decided to remove the rear step in the bottom of the boat. that cured the problem no porpoise at any speed. makes the boat alot more enjoyable and safe.

what kind of material did he us to fill it in.its a big area to fill in. i have a 25 sxt with the same problem. 800hp with a -1 lower unit. depends on the conditions of the water and the wind. but its sucks when it porpises. thanks for the help.
russ

wouldn't a cav plate/trim tab in that location have the same
result? long as it's level with the bottom of the sponson?
just can't see going to all the work of "filling in the step"
if you could do it with a plate

we put bondo in it just to test the theory and believe it or not it stayed in for a couple of trips, after we proved the theory worked i dropped it off to jim at warhawk performace boats in alabama and he glassed in the area and re gelcoated the area. dont remember what he charged but it was well worth it, it mkes the boat alot more enjoyable in any condition. we also tried trim tabs tunnle tabs and any thing else u could think of but removing the rear step was the only cure we could find.