fireboatpilot

OK here's the problem I'm having. 32' Hydra Cat, twin 468's, supposed to be 580HP on dyno w/ Bravo 1 STD drives w/ XR gears. The first time we put the boat in the water w Bravo1,30's full tanks of fuel and 2 people it did 94 MPH on GPS but could only get 4900-5000rpm out of it. Changed props to 28's gained 3-400 rpm's but lost 3 MPH. Same topped off fuel tanks, same people, same water conditions. WTF is going on here? Another problem that I am having is that WOT at just about top speed the boat starts to porpous and continues no matter what trim I attempt to put on it. It's not real bad but it's not good either and shouldn't be. Can anyone help me with this problem(s)? Seems like it's all prop issues am I right?

Ran-Dom 32

I had Brett at Bblades cup a set of 34 Bravo's to help carry the bow of my 31 Cat, seems to help get out of the porpoise range, I'm not familiar with your hull but you may be experiencing the porpoise range for your hull & you don't have enough power to drive through it. Mine starts bobbing around 85 & I either let it fly past 90 & she lays down or I slow down

fireboatpilot

The porpousing starts at about 80 and I think your right about not being able to power through it since I'm topping out at 90 now and only 94 before that. I'm putting the 30's back on and see if I can get the 94 back. I hope to get some input on the loss of speed with the gain of rpm's. It's not making sense to me. I thought I would go faster with more rpm's? Slower is not an option.

Philm

With the 30"s on there pulling 94mph at 5000RPM with 1.36 ratio drives, you have a slip of 10%.

With a 28" pulling 5300 RPM at 91mph, you have a slip of 12%, so it really isnt that much out of the ballpark to lose a bit of speed even though you gained RPM.

Even at 5300 with the same 10% slip, you will still only run a max of 93mph, so still a loss.

Have you tried labbed 30's? That should pick you up a couple hundred RPM and a few mph. they could add some cup to help with the porpoise as well. Say they added 200rpm and maintained the same slip at 10%, that would put you at 98mph. worth a shot if you can find a test set.

Ran-Dom 32

I disagree, my boat porpoised the same after adding -2 lowers, just did it at a higher speed, (you loose leverage to carry the bow when raising the X) this can be corrected by getting the props cupped

fireboatpilot

OK, I'm running 1.5's for the drives. What does that do to my slip %? I need to get the props cupped for lift? and Labbed for? What exactly is Labbed? Stick with the 30's? And for my next novice questions, I Just picked up a pair of Imco Extreme SC's -2 shorties uppers and lowers. 1.5's as well. and I also have a pair of extension boxes that I can put on IF it's going to give me any ride, power or handling advantage? I value all your opinions so please let me know what your thoughts are on using any or all of this stuff. PS- I have a pair of B&M 250's that will go on after I get some more seat time, like next season. I figure they shoould give me enough HP gain to power thru the porpousing?

Philm

Something in your original post is not right. If you have 1.5 drives running 30's at 5000RPM with a max of 94mph, that puts you at 1% slip. This is highly unlikely. the 28" at 5300rpm has a 4% slip, again this is highly unlikely. Use this link.
http://www.go-fast.com/Prop_Slip_Calculator.htm

Labbing a prop means sending it to Bblades or Mercury Racing and having them thin it out, add or take out some cup, change diameter/pitch, and balance the blades. This will give you on average 200rpm over a stock prop the same pitch, thereby increasing your speed. Brett (Bblades) can give you much more info on this than I can. They can tailor the prop to your boat, and pretty much give you a very accurate estimate of your new top speed and increase in rpm.

I would use the Imco's. Are the standoffs you have neutral or 3" offset? Anything to get that X up some should help considering you have std Bravo's right now.

fireboatpilot

Hmmm! OK now I suppose that my new question is ... Is there a way to determine the ratio without breaking the drive down? . If someone could make sense out of all this. PS the boxes are Stellings adjustables. So to summarize. Lost speed gained rpm. Going back to the 30's and have them labbed and cupped for rpm gain and bow lift. Right? Imco -2 shorties should help by raising the "X". Right? Use the ext. boxes or not? And at what setting? And last but not least, HOW DO I DETERMINE WHAT RATIO DRIVES I HAVE? Thanks guys, sorry for being such an amature!

fireboatpilot

OK, well the drives are still on the boat so I'll assume that they are 1:36's like Philm says. That formula made no sense to me. I shoulda stayed in school like my dad said. I get the point about making too many changes at one time. I think I will send these 30's out and get them labbed and see if I have any improvements with these drives. I guess then the next thing is I will put on the Imco's and give them a try before the seasons end. 90's are OK for me I guess this year since I'm still getting used to this Cat hull. Hey Philm, I'll be heading down to the Goose for Aug 1st weekend. If your out there try and look us up. The details are on the Philadelphia thread, Thanks!

fireboatpilot

Scott, thank you too, always good advice! Bill