throttleup

John,
Dave is right. A more accurate pitch for your 30 lab Bravo's is 28.5. So your slip is more like 11% at WOT and 6% at cruise. The 6% number seems a little low but the 11% seems about right.

You should expect about a 4% decrease in slip with a 5 blade. That does necessarily mean you will gain 7 mph. The prop will be more efficient but it still takes a certain amount of power to move a boat through the water at a certain speed and if the power is not there then the speed won't be there.

You would have nothing to lose by trying a 5 blade. If it works and it gives you the results you are looking for then great. If it doesn't then you haven't lost anything. Everyone I have talked to who has tried the P5X has been impressed with it's performance. I think you should expect the same.

Matt

rv

Matt,
I have a question regarding your reason for selection of 28.5 instead of 30 for the Bravo prop. I understand that the true measurement of the blade is 28.5. However, this does not account for the cup that is added. Doesn't cup add into the equation since that effectively adds to the pitch, in this case about 1.5". It seems to me that the dynamic movement of the blade through water including cup should be figured on the efficiency calculation.

What am I missing????

Thanks,
Rick

throttleup

Rick,

Thanks for the question.

I have measured many Bravo 1's. I measure them with the Hale MRI Computer measuring system. They are measured on 5 different radii, 50%, 60%, 70%, 80% and 90%. The measurement is from the very edge of the trailing edge to the very edge of the leading edge, which takes the cup into consideration.

A lab finished Bravo 1 typically has the cup reduced which effectively reduces the pitch even more.

For example, I just scanned a set of 36 Bravo 1's lab finished by Mercury, they had cup added and they measured only 34.5" of pitch. Without the additional cup they would be closer to a 34" pitch.

That's why we always have to go down in pitch when going to another manufacturer other than Mercury.

Matt

cobra marty

OK with the 3" shorter lower you are even with the bottom of the sponsons. If you put the blackhawk on, just bolt it to your gimble, where will that put the propshaft relative to the bottom of the sponsons?

Interesting enough on my Cobra the propshaft is 5" above the bottom of the sponson but I have a true tunnel with no center pod.

Airpacker

Marty,I have a BH drive on my 25 daytona. The prop shaft is about 3 inches above the bottom inside edge of the sponsons. It makes for a major lack of trimability below 60 mph until it starts packing but it has run 91.1 gps with a 500efi thru 1.5 gears and 31 BH props at 5100rpm.

rv

Thanks Matt for the reply, I understand now. The cup really is taken into account when you measure the pitch with your scan tool.

Rick