Excessive prop slippage help, please

[Robbie Racer]

The good news is that the new (rebuilt) Larry Peto 580 blower motor (1160 hp at 6600 rpm) out of Chris Kellog's previous boat is now installed in my 25' Carrera Cat (with light weight lay-up) and it runs fantastic. It makes much better power than my previous (now hurt) 598" motor. But, I still have what I think is excessive prop slippage that I could sure use some help with. I took it out with Kenny to Canyon Lake last Wednesday and tested it. With 4800 ft. corrected air density, we ran 116 mph at 6800 rpm with my lab 30 prop with the 1.30-geared B-Max. I think that is about 22% slippage. As I have had to since I have owned the boat, I had to trim the out drive to 8 on the blue water indicator scale to get the boat to take a nice porpoise free set). That afternoon I towed it to my rigger (G.S. Marine) in Corona, CA. On Thursday morning, Greg and I took it to near by Lake Elsinore, which had much better corrected air of 2500 ft. I challenged Greg to see if he could improve the performance because I believe that I'm losing mph due to the excessive prop slippage caused by the excessive trim in order to get the boat to quit "bouncing" or maybe porposing. We put on a borrowed custom made lab 33 and spun it 6200 RPM at 116 mph. I think that is still 22% slippage. Greg didn't trim it all the way to 8 and didn't go past 7 but the ride was still "bouncy" or what I call porpoising, which is a little scary at that speed. Greg says it isn't a porpoise but it is a sideways rocking caused by rollers from a couple of other boats that were out there. He says all cats of this size ride like this. I'm dubious about this statement but have not ridden in other cats at high speed and would like some feed back from other cat owners on this issue. Bottom line, he was not able to improve performance and with the better air, the performance fell off a little. We then switched to a lab 32 and spun it to 6500 at close to 117 mph, which is still about 22% slippage. I think if I can reduce the slippage to 10%, top speed would be in the mid 120's or maybe low 130's. I also have a Hydromotive 31 semi cleaver, which I want to try to see if that helps, but we were not able to test it as yet. Greg suggested a nose cone on the B-Max might help. Do any of you guys have any suggestions on how to either reduce the prop slippage or how to get the boat to take a set without having to put in so much up trim? Any help would sure be appreciated.
Thanks, Keith
By the way. The guys at Larry's Engine and Marine were great to work with. Larry and Kenny went above and beyond the call of duty helping me with this project. I highly recommend them for this kind of project.

[SVO 540]

Robbie Racer,
You have some serious HP. 1160HP and a light lay-up. That is some power to weight ratio.
I am not that familiar with the Carrera Cat. First, I am not buying the "sideways rocking". My best guess is the problem is somewhere in your outdrive. I don't think it is the prop and I don't think it is your boat. Here are your options as I see it.
Try tunnel tabs. The tunnel tabs will not fix the problem but it may offset it so the boat doesn't bounce.
Try a nose cone on your Bmax.
Try an IMCO SC drive with nose cone. Select the correct length lower unit so your x-dim is located in a "proven" position.
I recal a recent post in here about changing a Bmax to the IMCO SC drive and the IMCO drive picked up about 8 MPH and the slip factor went way down.
Good luck. Feel free to contact me if you want to discuss in more detail.
PS When your boat is dialed in it should run in the low 130's.

[HeavyHitter]

Keith, I don't buy the rollers explination. I thought I had a porpoising problem 'till I looked at where I was trimming at speed. I was only up 1.7 degrees. Got braver on the up switch and it's gone away (way different than the jet boat). My prop centerline is 2" below the keel. Prop slip is huge until I get it around 70 mph. As I get over 100 it gets down around 8% but I've had to back off the throttle to get there. I'm turning a 14.5" x 30 Mazco RE4. The thing accelerated like a pig. I think I was slipping the clutches in the tranny when I was putting the 1100 ft-lbs to it. Ended up smoking the clutches. Just got the tranny back from BAM. I'll see if it makes a difference soon. Some other slip numbers I know of are, 22 Daytona 5% with a 3 blade Mirage 125+. Prop CL is about 4" below keel. 40' Skater was running 14% with labbed 5 blade cleavers. That boat still ran 167. His CL is 2" above keel. right now most of the boats are comming with CL 2" above the keel. Not sure if they ever really hook up unless going blindingly fast.
Wally

[Robbie Racer]

SVO-540 & Wally, thank you for the feed back and suggestions. I may indeed have to go with some tunnel tabs. I want to try the nose cone for the B-max first to see if that is causing me the 22% prop slippage. I did measure the distance from the prop centerline to the bottom of the keel. When it is in the #8 trim position (where I have to run it to keep it from porpoising) it is 2" below the keel which is the same as your boat Wally. My 22% slip is at 115 or so mph. If it only did it below 70, I wouldn't care as much. I asked Greg to order a nose cone and if that doesn't fix the slip problem, then I will try a different drive. SVO, do you know what distance below the keel that your prop centerline is and what your slip is around 115 mph? Your boat has way better performance with similar power than mine has. Thanks again, Keith

[Robbie Racer]

After testing it more this weekend and getting a video taken of it at full speed, I don't think I'm triming it too far up. At 8 on the blue water indicator is only 6 degrees of up trim. I now don't think this is excessive as my rooster tail was not nearly as high as I thought it was. It was only about 6 or 8' high. When I hit 8 (or 6 degrees of up trim) the porpoise went away and the boat took a nice set. The problem is that I'm still getting 22%+ prop slip. I have a friend that has the identical boat with the same power and he is going 134 mph with 8% slip. The two major differences are: He is running a XZ drive vs. my B-Max and he has a "cupped" prop. I don't know if the cupped prop is the ticket or not, but he is going to send it to me to try. If that doesn't work, it sounds like I'm going to have to try a different drive. Anybody else have any experience with a "cupped" prop? Thanks, Keith

[HavasuDreamin']

I have had some experience with cupped props, albeit not on 115 mph I/O tunnels, but on 90 mph O/B tunnels. I believe that all props have some cup to them, but you can have more cup added to a prop which in theory is suppose to cut down on slippage and make the prop hook up better. I did not have a gps at the time, so I can't give you any accurate figures, but with the same motor my rpm came down slightly which leads me to believe it was hooking up harder. Unfortunately that prop is now at the bottom of Havasu (sheered prop shaft). I think you are heading in the right direction. I think it is going to be a series of trial and error before you find out the culprit to your slippage problem. One more thing to consider when comparing to your buddies boat witht he XZ drive.....it takes more power to turn the B-Max than the XZ or the IMCO SC drive, so you may not get the same speeds with the identical boat and motor. Good Luck.

[SVO 540]

Robbie Racer, I will need to get the measurement you are asking me for, however, I don't think it will be of value to you. This is because I have a standoff box. If you don't have a standoff, the comparison will be of no value.
I think your high slip is a combo of trim setting at WOT and the bmax. I wish I could remember the topic but I know there was someone who posted comparison data between a bmax and an IMCO style drive. Slip was improved by switching to the IMCO.
The more you trim out your boat, the more slip you are going to have. You have enough HP and MPH to drive through the speeds where more trim is needed (60 - 90 mph range). Let me try to explain.
A cat gets its speed by packing air in the tunnels. This causes the boat to lift giving the effect of a lighter boat, less wetted surface and less friction in the water. A cat will porpoise when the boat packs enough air to cause it to lift but there isn't enough speed and air packing to get the boat to stay up on top of the porpoise cycle so it falls back down only to repeat over and over and over etc. When this occurs you can help the boat stay up on top of the porpose cycle by using positive trim. This in effect helps to lighten the nose of the boat so it can carry itself more easily. Also at this speed positive trim will increase the gap between the deck of your boat and the water. Said another way, if you lift the nose of the boat, there is a larger opening to pack air. Your cat can take a bigger bit of air, thus packing more air.
A high HP cat can achieve enough speed so that you don't need to use positive trim to help pack more air. At 100+ mph there is enough air getting packed so you don't need to cheat it with trim.
I suggest you go try this. Slowly take your boat up to 80 or 90 mph and hold it there without any porpose. It is ok to be trimmed up to +8. When the boat has a nice smooth set, squeeze the throttle to the stops and at the same time back out of the trim. Try to get the trim back to about +2. If all goes well you will get a new top speed. If you can do it on the same pass, try to back the throttle off so you are running about 95% of WOT. Hold it there until your speed stays constant, then get the stats to calculate your slip. I know you will be suprised.
I will be in Havasu June 8 and 9 and would be happy to meet up with you to discuss further. Also, a friend of mine has asked me to go to Elsinore with him next week to do some testing. If that happens, we could meet out there and see if your boat reacts to trim settings like mine does.
I have also found my boat to be quite a bit faster at Havasu than it is at Elsinore.
Good Luck. SVO 540 - OUT.
[This message has been edited by SVO 540 (edited May 31, 2002).]

[Robbie Racer]

Thanks for the feedback guys. SVO, I wish that I lived near Havasu so that I could hook up with you there or at Elsinore but I live in the San Jose area. I will try to do some of the tests that you suggested on my next outing (June 9th) with the borrowed cupped prop and see how it works. Greg has the nose cone ready but needs my lower unit to install it but I'm not ready to down the boat for a week or two just now to get the nose cone installed. Thanks again, Keith