My Addiction

Im confused, I have a '98 25' Active Thunder, with a Stellings box and a 1.5" spacer which puts the propshaft 3" below the hull(no hook).

When I run a stock 26 Bravo I turn 4900-5000 rpm going 69-70 mph on gps(15% slip).

When I run a stock 28 Bravo(solid hub), I turn 5000-5100 rpm doing 73-74 mph on gps(18% slip)

With these props, any speed above low 60's the boat porpoises, so with both set ups Im porpoising at WOT on calm water. the only thing that stops it is a lot of tab, i have tried tucking the drive and neutral.

My thought is that if I could get rid of the porpoise I would go faster and have better slip numbers. So do I need different props or do one of mine need to be worked?

Too Stroked

So, is the condition caused by the prop or the hull? I'd try a different kind of prop (or two) to see. Have you tried a 4 blade on that hull? Many older deep V hulls really like them.

Older 25' Checkmates (early 1990's) used to be famous for the same kind of behavior. The standard cure back then was to fill and sharpen the trailing edge of the running surface at the transom. From the factory, it was beveled to make it easier to pull the hull from the mold. Believe it or not, this little modification did wonders.

My Addiction

the Bravo's are 4 blades.
the hull has sharp edges, that was the first thing I noticed before I bought it. It does not seem to have any rocker and deffinately no hook.

509 SC

I think he may have meant a 3 blade prop. Steve

Too Stroked

Actually, I meant 4 blades. We've had some pretty decent luck on older boats where the drive is buried and the extra lift of the 4 blade props really wakes them up. Since you've already got one, here's another thought. Not all boats like the extra blades and lift. I tried a 4 blade Spinelli on my Progression and it was downright scary. Too much stern lift. So, back to a 3 blade. Have you tried a 3 blade?

Of course your best bet is to contact Brett at Bblades. I'm sure he can help you.

My Addiction

Is propshaft 3" below hull considered deep or buried? Should I try taking out the drive spacer? Previous owner installed it so the boat would plane better.

I tried a 25" mirage + early on because that was what I used on my Baja 226 when I had this motor in it, It blew out way too easy on the Active Thunder. I was told that was probably due to the extension box?

Could it be porpoising becaouse it is trying to lift the bow too much, then falling off?

Yeah Im hoping Brett will chime in. Im sure I will end up needing his help.

Griff

3" is not deep at all. Actually that is pretty high. If it was deeper, it may even be able to carry bow better, without dropping it. Also, you could try a 5 blade Maximus prop. It will give more stern lift and should the lift the boat more evenly. Maybe some added cup and/or rake to one of you current props would help also.
I know Gordo's 25 used to porpoise on smooth water also. With a decent chop it hardly did it at all. He tried an ext box and said the boat got pretty loose with it.
Slip is not always bad. Higher slip numbers reduce the stress on the drive.

I'm a little confused by your numbers though. Are they both WOT numbers???? The 26 should spin about 400rpms faster than the 28.

michigan troll

What are you runnig for power?
I had the same problem with my 28. I removed the spacer to bring the prop to the surface. I am 1 1/2 to 2 inches below the bottom with the shaft and run a 5blade Max. Problem totaly went away. But you have to have some air in the prop to make it work. Maxximus props do not have very low for pitch sizes.

My Addiction

Griff: The porpoise does seem less with a little chop. That rpm situation is what is most confusing to me also, but I guess the more slip % is letting the r's go higher?

Michigan troll: I have a BBC 489ci, carb, w/ a retro roller setup making around 500hp. Did that setup make a rooster tail?

michigan troll

Sounds like your over trimming the 28 or it has been cut down quite a bit.
Ours throws a big tail from 8 foot full out to about 12 foot at 3/4 throttle. I trim +3 of neutral maybe a little more for full out. But our is a drivers boat. You play with trim and tabs at all speeds to keep it level. They need 1 foot of chop or more for full speed and less tab.
When I purchased the boat it had a 28 Bravo. I pulled the spacer and I had a hard time getting on plane and could not run over 65 without blowing out the prop. The 28 Max fixed everything and now will have it tuned and labbed.