Rpms up no speed gain?

[Eddie Ray]

Aq125a 270od 21ft trophy 1985. Once I hit about mid way thru the rpm range. The speed of the boat no longer gets faster as the Rpms go up? In fact it seems as I go up in rpms it might even decrease speed?
This boat is slow anyway. Topping off at about twenty miles an hour. But now she will only do about twelve?
Any ideas. Something in the od maybe?
Thanks in advance for any help .
Eddie.

 

[jimmbo]

Re: Rpms up no speed gain?

prop is sucking air from the surface, prop is damaged, prop is cavitating, rubber slip hub is slipping.

 

[Eddie Ray]

Re: Rpms up no speed gain?

The prop looks to be in okay shape? Not perfect. What would make it suck water? Where and what is the prop rubber? Is that like the rubber inside the prop where it mounts to the shaft? We went thru some sand, and maybe that has damaged the prop in a way that is not noticeable?
Thank you for your help. If you have any more ideas I would be very grateful.
Thank you again
Eddie

 

[Don S]

Re: Rpms up no speed gain?

You probably can't look at the prop and tell if it's a spun hub or not. So make a mark across the hub and the prop and run it and see if the marks are still aligned.

 

[Eddie Ray]

Re: Rpms up no speed gain?

Thank you ,I will mark it and take it out this weekend. I assume ,the marks will not stay alined if the prop is spun?
Thank you again.
This web site is by far the most helpful thing on the Internet.

 

[dennis461]

Re: Rpms up no speed gain?

How long have you owned the boat?
How long has it been a problem?
Have you changed prop size.?
What size prop do you have?
What type oil is in the outdrive?
Are you sure the linkage is putting it fully in gear?

I'm thinking a spun hub would get worse pretty quick.
A bad hub wouldn't get up to 12 MPH.
Cone clutch with the wrong oil could be your problem.

Try this with gloves on and boat out of water, NO key in ignition.
Put the shifter in full forward, try turning the engine over by rotating the prop by hand.
Try it again with shifter in full reverse.
I can do this with my AQ131C-270.
It's how I check the rubber prop hub after running aground :-(

 

[Eddie Ray]

Re: Rpms up no speed gain?

Dennis,thanks for your help. The prop is the one that the book says should be on there. The oil in the OD was changed recently and is what the book calls for,also checked it for moisture and metals . The speed of the boat is probably closer to 8 to 9 . We went into a marked channel that was a lot shallower than it should have been . We went thru a little sand,nothing hard thank god. Have had the problem since. It dose seem to get a little worse. Haven't had it out much since. Couple times. I have noticed that the prop seems to me to have a little to much wiggle in it. Which maybe is the prop being spun?
If I put her in gear and try to turn her. Should I be able to rotate it? I think I tried that and couldn't? Will try again and take mental notes. If the prop was spun and she is in gear would the prop spin and not shaft? I think the last owner reinstalled the prop with no lube. Have tried to get it off,and could not.
Thanks again for your help.
Eddie

 

[Eddie Ray]

Re: Rpms up no speed gain?

Okay I had to cut off the old prop. Took forever,had to split the prop and then cut the center piece into multiple pieces to get it off!
Put on the back up prop. It is just a little smaller?
Took the boat out ,and this prop is not spun . Marked it, and it is all still lined up.
At about 2200rpms my speed is about 7.8 to 8.2 mph. Going up to 2500rpms takes it up a little maybe 8.5. To 8.8. Going up more has no result. Might even slow a little?. It seems to climb to about twelve mph but only momentarily . And then drops ?some times dosn't speed up at all.
I will get a new prop. The one Volvo says it should have is 15/17 have the part number. I will check the book to see how to check the transmission linkage. If anyone has any ideas ,I'm open to try just about anything!
Thanks as always for all the help.
Eddie
I am lost? I will get another prop,right away.

 

[Don S]

Re: Rpms up no speed gain?

Could be the shift cone in the drive is slipping. Have you checked the gear lube? Any water? Does the cable shift the mechanism all the way into gear? Worn cables won't push or pull as far with all the extra slop.

 

[Eddie Ray]

Re: Rpms up no speed gain?

After reading your reply. I decided to check the shift linkage. It did not seem to be fully engaging? In either direction. I adjusted the rod and now it seems to be fully engaging. Before ,when all the way in gear . There was still a lot of travel on the forward reverse adjustment. So I turned the rod two full turns,and now it seems to move up and down all the way. If adjusting this was wrong,please let me know .
Thank you . Very much for all your help.
Eddie

 

[dennis461]

Re: Rpms up no speed gain?

Your linkage should look like this in FORWARD with LH prop setup.
Just about 45 degrees from horizontal on the black piece.

Adjusting the "rod" may not be the right thing to do just yet.
I only adjusted the rod when installing a new cable to make sure the neutral position of the deck throttle control matched neutral of the linkgage.

 

[Eddie Ray]

Re: Rpms up no speed gain?

Could be the shift cone in the drive is slipping. Have you checked the gear lube? Any water? Does the cable shift the mechanism all the way into gear? Worn cables won't push or pull as far with all the extra slop.

Where is the shift cone?what dose it do? Do I replace or fix it? This is starteing to drive me crazy?
Boat will get to about thirteen mph just about to plane ,then nothing? Give it more gas and nothing.
Thanks for the help!
Eddie