New lowers..back to the drawing board
 

I have a 2001 Cig Tiger twin step. I am running IMCO SCX uppers with 1" spacers.I was running a Bravo XR lower with a BBlades labbed 4 blade Bravo 1. The boat came on plane pretty good, had excellent mid range pull, but top end the props appear to be slipping somewhere around 19%. So this winter I installed new IMCO SC lowers thinking the bullet on the XR was causing the problem and thinking that the sharper case from the SC would do the trick. Well my ability to get on plane has gone down quite a bit. I tried a buddies set of custom labbed 5 blade props last year and the boat had better top end but did not seem to accelerate as good. I am looking for some ideas. I still have the 1" spacer in as well.

My question is what's wrong w 18 pct slip

An ideal number on a step boat from what I have been told is 14%. This is about 5 mph.

I agree but sometimes its just not going to benefit you. I am not saying its not worth trying but I am saying you may spend a lot of time trying different things and get the same results.

What I mean is this.

Lets say your spinning a 34 bravo to your max rpm and 18% slip at 90 mph. Now you try a hearing 6 blade and get your slip number down to 14% but cant turn it to your max rpm and still have a top speed of 90 mph. My point is that if you lower your slip from 18 to 15 you may not get that extra 5 mph.

Another way to say it is lets say your running a 30p bravo at 18% and just touch your max rpm. Why not just run a 32 to be faster? Well if the 32 still has the same slip number your likely to be about the same speed but turning 400 less rpms.

Excessive slip beats the hell out of the gears. If you reduce slip with a larger diameter prop or more blades you may reduce slip but fall below your target max rpm. At that point drop pitch to get back up in rpm but keep slip down. All in you may not see the 5 mph but reducing slip by 5% will be very beneficial to drive life.

This is counter intuitive to what I have always thought. Here is my logic, I have read a lot that changing to maximus props will kill a bravo. My thoughts were this would be from the blades hooking up better(less slip) and therefore more stress on the gears. What your saying is that slip atually causes more stress on the gears. For that, I need a better explanation.

More slip causes pitting on the gears. Less slip causes more stress on the shafts and gears.
Kurt told me on the phone a few years ago he was seeing more pitting on gears from stepped hull boats than straight bottoms and it goes along with the higher slip numbers. Every gearset I've had in my hands since, that was out of an xr on a stepped boat has been pitted bad. The couple I've had from straight bottoms with low slip were in much better condition and just worn, very minimal pitting. It's like the slip hammers and chatters the gears. So reducing the slip will help with pitting but will increase the load or stress on the shafts.

Hmmm

You need to take each applications slip vs. rpm goal vs. speed at its own merit and determine the best geometry to get to the desired goal of efficiency and or speed. Propeller geometry doesn't only change rpm, speed and slip but also boat attitude which affects al the previously mentioned numbers.

As far as 15% slip being a go to number on step bottom boats, I've never heard that before. Who came up with that? Any sport style boat we tend to try to get below 15% slip. However, a good point is that on some applications desiring top speed as the main goal, we have created the best top speed with a higher slip % than when trying to harness slip.

Most common cases of this result are true V bottom boats as we are trying to carry the bow high to decrease wetted surface area and are over trimming the boat. Although the slip runs high, the decrease in wetted surface area (drag) is a greater improvement result than the increase in slip and speeds can improve. Slip is the lesser of two evils in this case..

Slip is easier on gears than heavy loads. Pitting from slip? New one to me again. 5 blade props are easier on gears than 4 or 3 blades once the boat catches up to the throttle position. In other words, hammering the throttles to get on plane or flying the boat and coming back in while bouncing off the limiter breaks stuff easier with 5 blades than 3's or 4's but, once the boat catches up, the load / unload pulses on the gears are smoother with more blades.

Brett

Brett
Thanks for jumping in here. You labbed my current 4 blades and boats has awesome acceleration but always slips up top. Since I went from an XR lower to the Imco SC lower the boat does not want to get on plane very easy. Maybe I should try a 5 blade, but what one. I need to go with your loaner program. I spin 5600 with 32 4 blades now. Where do I go from here?

You said it, you made a change in gear case and the plane ability went south.
Maybe the original set up was the best choice to be dialed in?
A 5 blade propeller is a great potential band aid.
You should test any other propellers you can to add to the base line knowledge you have. This will help in the end.
We can discuss this via phone any time.

Brett