mcollinstn

okay, stick with me here...

You ran 89mph @ 5100 revs with 33x4 labbed mercs BEFORE engine work.

After engine work you turned 5300 revs with the same 33x4 labbed mercs.

Is this correct? That's an increase of 200 revs. That takes about 75 horses per motor.

So if you are running the same props, and the same drive setup, same boat weight and fuel load, same basic trim angles and tab settings, but you are turning 200 more revs, then you are going faster.

Period.

So either your tachs are off, or your speedometer is off. Or you are running against the current or you were running with the current before and are not now. Or something.

The fact that after engine work you got prop blowout suggests that either you now have more power at the sticks (thus blowing out the props easier), or you have a damaged leading edge on a prop or something creating an air pocket off the hull (delaminating gel or something). Maybe one or both of your props got tweaked while it was being worked on. Have you checked em?

There is something fundamentally amiss in the explanations here. The devil is in the details. Let's walk thru this slowly.

mikes280

how deep are the cooler pickups sticking down , maybe they are causing some drag . also the increse in toque may be ausing the prop blow out getting on plain

Steve Zuckerman

I have the same setup on a 28' single, with great results.
I think your low end blowout is due to increased horsepower and torque. Sounds like you are making more horsepower @ WOT too, evidenced by your 200 extra RPM, even though your MPH is the same.
I think if you had just added some cup to your 33" Bravos you would have reduced your planing blowout problems, and locked up your props better on top end, reducing slip and gaining MPH. If your MPH is the same with a 200 RPM increase, your slip #s increased. That is not unusual with high pitch Bravos.
I also think you are under pitched on your 5 blades. I would think 29" or 30" P5-Xs, blueprinted to your boat/engine/drive
combination would have worked better.
I run a 30" Bravo and a 28" Hydro P5-X, both blueprinted by Throttle UP, and the P5 is 1-2 MPH faster at all RPMs.
Good luck,
Steve

doubletake

zuckerman, thanks for the input! Who would i contact for the props you suggested. Not familiar with a P5-Xs. Also, would you recommend me going with a new 5 blade or cuping the 33's i have. Thanks again.

doubletake

Mcollinstn, i appreciate your comments very much! I have checked props and hull- no damage at all. I totally agree with your comments and just can't figure it out.Do you think my props are also slipping at high end rpm's and need to be cupped or should i try a different 5 blade? thanks!

doubletake

I've hear from a few people to contact Tomcat. If anyone can help me to get his info, i'd appreciate it. I don't know how to find him on my own! Would love to get phone, email etc. Thank you very much!

doubletake

mike, i only have one pick up and its a clam shell that only sticks down approx .5inch. I was advised by my mechanic that i only need one pick up and one sea strainer to feed both motors for the intercooler?

PatriYacht

How is water pumped thru the intercooler? Are you sure you have good flow thru the cooler? Some people run water pressure guages on there intercoolers.

tomcat

Adding 50 HP per side would be good for about 3.5 mph and that's exactly what your RPM increase says you got. I don't know why your speed readings don't agree with this.

With a pulley change and the intercooler you are making more torque down low. If your drives were high from the factory and on the verge of blowing out before, the extra torque down low is blowing out the props.

Sure the five blades fixed this and I'll bet the boat accelerates better but it's rare for five blades to be faster than four. Maximus blades are big, P5X is the next smaller blade area, P5 the smallest. More blade in the water, more than you alsolutely need to absorb all shaft HP, is usually slower but you have to balance this with driveability at lower speeds.

Borrow some P5X props and try them. I would also try your original props with drive spacers (to lower the props into the water a bit more) to solve the planing problem. I'm sure someone here knows a good rigger in your area with a selection of spacers and props to help you sort this out.

I'm also wondering if a bigger cam wouldn't help by taking away some of the low end torque and adding it up top? Next time you're in the engines you could consider this.

Finally if you have access to a wide band Air/Fuel ratio (ask your rigger) check it at WOT. Maybe you're running really rich. Wouldn't be the first time that a reprogrammed ECU needed adjustment.

Steve Zuckerman

doubletake,
I would talk to Matt and Julie at Throttle Up. They are very knowledgeable, and have done a great job for many of the offshore racing teams and poker runners. They did a nice job on my props.
Your Bravos should be able to be modified to work better with your new setup. The Maximus can also be modified to perform better. Re the Maximus, I think you have too much diameter and not enough pitch. I know most people are reducing their diameter to get RPM. I don't know if they can be effectively repitched. The Hydromotives (P5 & P5-X) seem to work better out of the box, and modified in most applications I've seen.
www.throttle-up.com
I also hear good things about BBlades, but I have never purchased anything from them.
As mentioned by the other guys, you may also have some tuning (ECU) issues.
Regards,
Steve