Porpoising
 

I'm running a Active Thunder Tantrum with a 500 HP. Bravo 1 drive with a labbed 28 pitch Bravo 1 prop. Mercury racing did the work. At 5250 rpm I'm running 75 mph on gps. The boat wants to porpoise starting around 65 mph mark. I also see some chime walking as well but this was reduced a little after i placed the 28 pitch that Mercury racing worked on for me.
When I purchased the boat it was running a Bravo 1 26 pitch and running at the 5300 rpm range and hitting 74 mph. This prop porpoised bad. I have no idea what was done originally to the prop other then the probably added some cup to get the rpm down.
What can be done when it comes to props to eliminate porpoising and gain top end?

Thanks in advance,
Mike

Re: Porpoising
 

Mike,

I have calculated your slip with the 28P Lab Bravo I and you have 16% slip. This is a little Higher than I would like to see for your boat. You have 14% with the stock 26P Bravo I propeller, which falls into the acceptable range of 10-14%. So with this I think that you should consider working the 28P Bravo I Lab propeller and increase the cup to see if you can take the edge off of the porpoising. It is also possible that a 5B propeller may assist in decreasing the behaviors as well. They will create more lift and require a different amount of trim, possibly affecting the behavoirs....not sure how though. Testing is key in your situation, hopefully finding a propeller to take the edge off.

Julie

Re: Porpoising
 

throttle up-
does the cupping of a prop inprove a porpoise problem? assuming that the hull is flat on the bottom......what is actually going on when a boat does porpoise?
kevin

Re: Porpoising
 

If the 28 Bravo 1 decreased the porpoising after the Lab Finishing, I wouldn't add more cup to it. You stated the 26 porpoised worse. The 26 would have more cup than the 28 since it wasn't Labbed. Also the slip % on a Labbed prop can be higher than a box stock prop. A 5 blade is going to lift the stern more and if the prop can't hold the bow now, 5 blades may be worse. Be sure to try a 5 blade before you purchase one.

Have you inspected the bottom of your boat to make sure the running surface is true? Also how deep is the prop?

Re: Porpoising
 

EYEBURN

I started to write the last post and had to walk away so it got posted without seeing your post.

Porpoising can be caused by numerous reasons. Boat bottom hooks and rockers are key factors. Prop depth and prop choice can be a large factor too.

The propeller attempts to raise the bow by leveraging the stern. As the weight increases the prop looses its ability to support the bow and down it comes only to start the process over again and again.

Re: Porpoising
 

I was told by Mercury Racing that shorting the prop barrel that it would create more stern lift and possibly decrease the porpoising.
Your thoughts?

Re: Porpoising
 

bbladesprops,
I missed your last question when I read your reply.
My boat was built by active thunder for Smith Brothers Race Team. It was raced in the F1 series in it's first year.
Not sure what was done with the hull. I know that it has no hook, might have a touch of rocker.
Are you referring to my X dimension when asking about my prop depth? Not defendant on how to measure that.

Re: Porpoising
 

Who at Merc Racing told you that about the barrel? Scott or Ron?

If you have the chance, give me a call. I can't give away all the trade secrets on this board. It's obvious by some peoples comments on other threads, I'm already giving away too many secrets. Hehehe.



(just kidding, I know this forum is here to help and we all do the best we can.)

Re: Porpoising
 

Will a rocker cause a boat to chine walk?? My Powerquest has what I would call two rockers, one on each side at the transom right where my tabs are located.

Re: Porpoising
 

Okay if we are talking porpoise, here's my question. I have a Formula 311 running 502 MPIs through Bravos. It spins stock 26 Bravo 4 blades to 4600 rpm light at 70 mph. BUT I have to run tabs to about 6 in any but the smoothest water or it constantly porpoises. So, do I need more or less bow lift and should I think about maybe a 24 or labbing the 26's? Props are turning out right now.