1985 allison xst 26p chopper vibration

[sasquitch]

Hello all, i have a 18.5' 1985 allison xst with a mercury 220 converted to carbs(wh22s). I have a 6" set back jack plate, the prop shaft is 2" below the pad. When I bought this boat it had the 26p chopper prop on it. I could not get the slip # down with out raising the motor. So I bought a 25p trophy and ran it 2 1/2" to 3" below the pad. It ran great and the slip went from 22% to 16%. I installed water scoops and now have great water pressure 25psi at wot. I raised the motor to 2" below the pad installed the 26p chopper and took it out for a test run today. With the chopper, I now have a bad vibration through the hull. I did not do a wot test because the more rpms the worse the vibration. I have talked with other allison xst owners. They have ran choppers even or above the pad. Do I have a prop that's out of balance? not enough setback for motor height? I ran this prop a couple of months ago at 3" below the pad with no vibration. I ran it to 5000 rpm today and the water pressure was 20psi. Should I go up? come down? Take the prop off and reinstall it with different splines? I really appreciate any advice. Thanks. Brad

[Wizard]

Brad if it vibrates with only the Chopper on it then it's the prop thats out of whack. If you put your Trophy back on and it still vibrates then you probably have a bent prop shaft. Chances are the Chopper is bent and out of balance. Try another one!

[sasquitch]

Thanks for you help Wizard, I ran the trophy with no vibration one week ago. Is it typical for a prop (chopper or others) to show there out of balance when they surface. How high can I run the trophy? I know I can go up a good bit more with the water pressure I have. Thanks,Brad

[patchesII]

i'm with wizard. sounds like the chopper is a little out of whack. is there anybody around you that you could borrow another chopper from to try. with your hull and motor combo you really should save your dollars up for a bobs nosecone. then you could raise the motor to where it needs to be and run some faster props. that rig should run some good numbers after you put a nosecone on it. my dad use to have one of those boats when i was a kid. i still think their really cool boats

[sasquitch]

Thanks patchesII, I have been told by other members that I should try to get my slip numbers down before purchasing a nosecone/lwp. I have the scoops on it now with great water pressure. I think I could go up another inch or so and still have enough pressure. I don't know anyone in this area with a chopper I could use. There is a guy that repairs props in my area. I think I'll give him a call and have the chopper checked out. I was thinking of adding the 2" setback spacers from bobs. I think adding the spacers should help with the slip? My chopper is in great shape no dings or dents. I guess it could have been out of whack all along. I just never noticed it until I got the motor up. Thanks again, Brad

[patchesII]

if you do get your slip numbers down, which you should, it will put you right into blowout territory. a stock lower unit is only good to around 83-85 mph before blowing out. your rig should be easily capable of that. the slip numbers do sound really high. the 6 inches of setback is not the problem. it should work fine on that boat. wondering if your tach is right. what are your rpm's and speed. are your speeds from a gps

[sasquitch]

Hey , The last test with the 25p trophy 4 1/2"below the pad was 63mph at 5800 rpm . I am using a handheld gps. The tach looks like it came with the boat. I added the water scoops and raised the motor to 2 1/2" below the pad and the trophy didn't carry the bow very well. I did try to trim, but any trim at all would shoot a huge rooster tail. I didn't record any numbers with the trophy at 2 1/2" below, but I know it wasn't any huge increase, it seemed to be slower. So that's when I thought I would try my 26p chopper and now I have the vibration. If the setback is not a big problem. What else can I try with my current lower unit ? I know where I can buy a 28p trophy for a good price. Should I try more pitch? Have the chopper checked out, and try it again? Come down with the motor height with the trophy until it carries the bow better.Thanks again for any suggestions. Brad

[patchesII]

i'm assuming your measuring height as centerline of propshaft in relation to pad. that trophy should have had more than enough bowlift. what kinda load are you carrying in the nose of that boat. those allisons are none short on bow lift. have you laid a straight edge on the bottom to check it for hooks or any other problems. keep the answers coming. i'm curious about this one.

if your only turning the 25 5800 you don't need more pitch. the 25 trophy should put up some big numbers and create more lift than you probably need

[sasquitch]

The pad looks straight and flat. It does have a strange design . It is like a v all the way back to the in the middle. On the sides about 3' or so from the back it has step flats. The v is only about 1' or so across in between the flat areas. The v comes all the way to the transom, no step. I have turned the chopper 6100. I think I can turn more than 5800 with the trophy. The private lake I was testing on is kind of small. I am measuring from the center of the prop shaft. I have a trolling motor ,battery and some life jackets in the front. With the trophy or the chopper deeper is has great bow lift. I was thinking maybe the bow is waterlogged. I cant see how water can get into the bow area. maybe for the live well filling/draining pipe. I do have some water come through the bow drain hole every time I take the boat out. Its not much but enough to make me wonder where its coming from. Thanks for the help. Brad

[patchesII]

i'm assuming there's no damage to the lower unit that could be messing up water flow to the prop. can't remember if theres a livewell in the front of that boat. if there is, is it plugged off. you shouldn't be carrying any batteries in the front of the boat. they should be in the back. the xst my dad had would run around 80 with a bone stock 150 xr2 and a 26 chopper and had tons of bowlift. he ran his with a stock lower unit and it would blowout if really pushed so maybe it would run a hair over 80. i'd say do a closer inspection of the bow area with the boat in the water to make sure your not carrying water up there. like you said, it could either be waterlogged foam(not sure if theres any up there) or you have a livewell hose leaking and putting water in the boat which i think you would immediately notice when you took the boat out and pulled the plug. sorry if i keep spitting out random thoughts. at least none of them cost money:D

[sasquitch]

Thanks, I appreciate the random thoughts. It doesn't have a live well in the front just in the rear. There is almost no water coming from the boat when I pull the plug. The only inspection I can take of the bow is through a 1' hole under the center console liner. I think I will plug the inlet pipe on the lake side of the transom and see if I get any water coming from the bow drain hole. I have talked to many people about the boat and they all say that's a fast hull. Its driving me crazy that I cant get it to break at least 70mph. It is a learning experience and somewhat fun. I just cant figure out whats slowing me down. I think I will move the battery, put the trophy back on ,plug the live well pipe.,unload the boat of all fishing stuff and give it another run, maybe Monday. Do you think with the hull not having a step , that maybe the 2" of setback might help? When I put the trophy back on should I run it at 2" below the pad,where the center of the prop shaft is now or lower it . I not sure how high a trophy can run? There is another member I talked with last year who has had the same boat. He was running a 2.5l mercury with 9" of setback ,lwp/nosecone about even with pad. He told me I needed more setback due to the lack of a step.

[patchesII]

if you have the 2 inch spacers it would be worth a try at this point. if you move the motor back to 8 inches lower the motor back down to the 3.5 inch area and start moving it up from there. the extra setback will probably help now that i think about the comment you made that if you trim it up too much you just make a big rooster tail.

[sasquitch]

Thanks for your help patchesII. I will let you know what happens.

[sasquitch]

Well the rain has kept me form testing. Does anyone have any other suggestions or opinions. Thanks Brad

[sasquitch]

any opinions ?