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View Full Version : Bow drops & prop looses bite lower 90's



KeithRichards
05-24-2001, 07:41 PM
96' Allison xb2002 2.5 260 HP. with a 15" drag mid. Stock gear case w/bob's nose and tab. 10" rapidjack and transom wedges. The prop shaft is about even with the pad with the motor trimed to the running position. This is with the jackplate all the way down and the motor in the lowest holes. Boat runs great till about 92mph then you feel the prop loose bite and the bow drops. Have tried diffrent props. Motor too high ? Not enough set back ? Help... That 100mph mark is close and I'm ready to jump over it.

johnnyracer
05-24-2001, 08:11 PM
What kind of prop and rpm? What kind of shape is the prop in? It should be wanting hook with ya at those speeds if the prop is turning loose that bad.

KeithRichards
05-24-2001, 08:45 PM
Right now I have a 14x26 Merc clever bolted on turning about 8100 RPM @ 90 MPH. Also have a 141/2x32 clever that turns in the low 7's at that speed but does the same thing. Both props are in exelent cond.
I get some chine walking when coming off speed but nothing scary.
My first thought was blowout. That shouldn't happen with a nosecone on . Right ?

JTS Racing
05-24-2001, 09:19 PM
How much steering torque do you feel in the wheel just before the problem occurs? Who installed the cone and tab? Sounds like the cone is put on at a slight angle to the right or upward. Props get their highest precentage of bite in the first 1/4 of their stroke. If something is disrupting the water flow at this point, it will not reach peek performance. Since you have tried different props and the problem still occurs, take a close look at your lower unit.

KeithRichards
05-24-2001, 10:20 PM
Can't really remember the torque at that speed was to busy holding on. Seems like it was driving easy with less pull. At lower speeds it does pull. Bought lowerunit from a big name on Scream&Fly. Don't want to hurt anybodys buisness by giving name. They have been good to me. How would I check nose for straightness?

TODD2002
05-25-2001, 01:14 AM
DO YOU HAVE A BLOWOUT RING?

Bobalouie
05-25-2001, 08:45 AM
I think you may need to look closely at your setup. The numbers you posted equated to 15.61% inefficiency. This is 2-3 times higher than it should be. Are your speeds measured w/ gps or radar, or are they from the speedo? You may want to go to http://www.allisonowners.com and ask your question on their message board. There is at least one quy over there that has a very similar setup as you.

Bob

johnnyracer
05-25-2001, 09:14 AM
Clever question for you guys that know. Do you need to run a clever as high on the transom as you would normally with other props such as chopper style props? Seems that the way a clever lifts in the stern that it would need to be a little lower to keep some blade efficiency. I don't have any experience with them , just asking.

RichS
05-25-2001, 10:55 AM
I think you would be better off with some kind of a chopper prop to try and lift the bow. Sounds like the cleavers are just setting the bow down and lifting the stern.

Bobalouie
05-25-2001, 11:16 AM
Cleaver props were originally designed to operate in a surface piercing condition--Meaning 1/3 to 1/2 the total diameter of the prop should be out of the water. The following is straight out of Merc's Prop book:

"The cleaver style of blade is specifically designed to operate in venilating, surface-piercing applications. To install one on a boat not capable of speeds in excess of 70mph, or to bury the propeller under water, as in a standard installation, could noticeably reduce performance, rpm, and boat speed"

I agree with RichS--the cleavers will not provide bow lift (which you dont need in an Alli anyway), and will instead provide lift at the stearn. Allis typically get very squirrly (according to J. Tiger Jr in a past B&WB mag) when run with a cleaver. Allisons do not have a completely flat pad. It has a 4 deg V of its own, which is one of the reasons they are so hard to balance when running. That 4 deg V gives you all the stearn lift that you need--you wont need a cleaver for it.

Try a 28 or 30p Lightning ET or Chopper 2--thats what most Alli guys are running for speed. I wouldnt Jack your motor to the moon either to make that cleaver work correctly--your propshaft wont take the abuse for very long.

Bob

[Edited by Bobalouie on 05-25-2001 at 11:24 AM]

jimmyb
05-25-2001, 01:09 PM
one more point to add... your stock gearcase wont live long with a 260 hp motor pushing it to 8000 rpms. Do you have a torque tab on your gearcase? If not you might be crabbing very badly at speed, which is why your efficiency sucks.

also, i would try lowering your motor a bit... this will help minimize your chance of blowout. High speed blowout in an alli aint very fun.

Dave@DunnWright
05-26-2001, 06:32 PM
Keith, what are you referring to as a stock gearcase? What you are describing is definitely the opening act for the prop blowing out and the dreaded hard right turn to rear it';s ugly head. I had the same set-up you are running and had no luck at all with a cleaver prop. I tried some really nice ones too. get your motor down to about 3/8" prop centerline below the pad. Bolt on a good 28-29 chopper, preferably Allison style cut. If the prop continues to lose bite i would suspect the lower unit combination. That boat is very sensitive to the length of the gearcase. I had a CLE modified by Darris with a 260 EFI on my xb2002 and it would drive and handle like a dream at 106-107. that lowr unit had aton of work done on it. I bolted an early 200 style EZ shift with Bob's and blow-out ring and the boat dropped 10 mph and would drop the nose at 96 mph everytime. A blow-out ring is critical and a good Allison torque tab with the right congiguration at the last 1" of length helps drivability and bite also. By the way, even with the good set-up a 30" worked cleaver stone walled at 100mph. Good luck.

TODD2002
05-27-2001, 12:23 AM
I am running a 30 p lightning et on a allison xb 2002 with a 225 promax and 1991 xri 200 gear case with bobs nose cone with 9 inches of setback and i am turning 6800 rpm with the bullet even with the pad without any trouble. you might want to try a 30p lightning, 29 srx or a chopper2 give that a try and see what you come up with

[Edited by TODD2002 on 05-27-2001 at 12:29 AM]

KeithRichards
05-27-2001, 06:52 PM
Mounted the GPS. Tried my last prop a 24p Yamaha. This Yamaha is a blast out of the hole and real easy to get some air time. Top end was 90mph gps @ 8400 rpm ,91 on spedo. This speed was under my problem area , could have wound out some more but did not want to break anything. Gathering info at high speed is not fun. Watch water,check spedo & tac, where's that other boat going,oh no a little chine walk,what's the pyro say, is that a wave ahead, then try to pick out the speed reading on the GPS.

Looks like I'm in the market for another prop. My wife doesn't understand all these props. You have a prop why do you need another? Will post again when I Try another one.

Listed a 14X26 clever on Ebay if anybody is interested

Keith

D rocket
05-29-2001, 11:42 PM
Get a round ear prop srx,ET, choper,ect DUNN is right 3/8 above center line always works . Torque tab and straight gearcase a must . Is your skeg too thick at the front? 8" setback is best over all and 10" is fastest with out loosing handeling. Thanks DeWitt