Proping around an incorrect gear ratio

[adogdrian]

1989 23' searay weekender, 7.4 liter, 454 mercruiser. I got the boat a few years ago and I have spent allot of money and time trying to diagnose the issue with it. At WOT which is 4600, I'm only going about 10 mph. I thought it might be a spun hub, but its not, upon further investigation it looks like someone at some point put an outdrive for a 165 hp, mercruiser on it, with 1.98 gear ratio, I'm pretty sure its supposed to have a 1.5 on it. Is there any way to correct this by adjusting the prop pitch? It has a 14" x 23"P on it now.

 

[thumpar]

That drive you have is actually for a 3.0l. You would have originally had a 1.32. I don't think you will find a prop that is going to work well. You would at least want a 1.5 ratio drive to try to get close.

 

[Bt Doctur]

or find 1.32 gears

 

[adogdrian]

Damm

 

[Bt Doctur]

A SEI upper drive? No warrenty with that motor and you must have a soft throttle hand

 

[Rick Stephens]

You may also be able to get a shop to swap gears to a better ratio. Whatever you, with a 454 you gotta be gentle not to break it.

 

[nola mike]

At WOT your RPMs are correct but your boat isn't moving. You should be spinning at a million RPMs with that setup. Getting the correct ratio drive will just make the boat lug...
My guess is something's wrong with the engine.

 

[stonyloam]

OK, just a minute here. At 4600 rpm with a 23” prop, even with a 1.98 gear ratio you should be going about 45 mph. Something is SERIOUSLY wrong with the drivetrain. Not the engine. Sounds like your coupler may have stripped splines. If your prop hub is ok, that is most likely the problem.

 

[jimmbo]

An Alpha drive isn’t robust enough to handle a 7.4 liter. Look for a Bravo or a Bravo 3 drive. One of the problems buying a used rig, you don’t know what some unscrupulous owners will do to unload something.

 

[thumpar]

An Alpha drive isn’t robust enough to handle a 7.4 liter. Look for a Bravo or a Bravo 3 drive. One of the problems buying a used rig, you don’t know what some unscrupulous owners will do to unload something.

Mercruiser built the 7.4l with an Alpha one for a few years.

 

[adogdrian]

OK, just a minute here. At 4600 rpm with a 23” prop, even with a 1.98 gear ratio you should be going about 45 mph. Something is SERIOUSLY wrong with the drivetrain. Not the engine. Sounds like your coupler may have stripped splines. If your prop hub is ok, that is most likely the problem.

coupler splines look fine.

 

[nola mike]

OK, just a minute here. At 4600 rpm with a 23” prop, even with a 1.98 gear ratio you should be going about 45 mph. Something is SERIOUSLY wrong with the drivetrain. Not the engine. Sounds like your coupler may have stripped splines. If your prop hub is ok, that is most likely the problem.

If the coupler stripped or it was a drivetrain problem, he should rev waaaay over 4600 rpm. He should be hitting 4600 well before full throttle. If the engine isn't making full power OTOH...

 

[Scott Danforth]

Mercruiser built the 7.4l with an Alpha one for a few years.

and most of them kachunk...kachunk....boom....as the gear set blew up.

the big blocks need a bravo. the alpha drive could be made to survive, however the stronger gear sets were really pricey.

spun hub is easy to look for, only needs a permanent marker to draw a line across the hub and prop nut

coupler need to have motor pulled.

 

[stonyloam]

Well maybe it’s the coupler itself, but you would think it would be burning. At 4600 rpm your prop should be turning about 24-2500 rpm, that would put you at 40+. If you are only getting 10-15 mph that says your prop is only turning about 1200 rpm. Something is slipping big time. A broken driveshaft or stripped gears you would not go at all. The only places it can slip are the coupler and prop hub, seems like the problem has to be one or the other. I guess you could remove the cover at the top of the outdrive and look at the gears, but it seems like that would be a go-no go situation, but strange things happen.

 

[adogdrian]

Well maybe it’s the coupler itself, but you would think it would be burning. At 4600 rpm your prop should be turning about 24-2500 rpm, that would put you at 40+. If you are only getting 10-15 mph that says your prop is only turning about 1200 rpm. Something is slipping big time. A broken driveshaft or stripped gears you would not go at all. The only places it can slip are the coupler and prop hub, seems like the problem has to be one or the other. I guess you could remove the cover at the top of the outdrive and look at the gears, but it seems like that would be a go-no go situation, but strange things happen.

The coupler might be slipping I guess I could mark the coupler and the prop then run it to check. I wasn't able to spin the prop by hand with the throttle down, but obviously I'm not putting as much force on it as the motor.

 

[adogdrian]

A SEI upper drive? No warrenty with that motor and you must have a soft throttle hand

I'm not sure if the upper drive is SEI or what SEI stands for on the note. I'm still learning boats. I got the serial numbers and called Mercury to get the gear ratio. Also realized someone had taken a sharpie to black out the 165.

 

[porscheguy]

If it says 165 then it’s a 1.84 ratio. Should move along well into the 40s. You’ve got either a spin prop hub or the engine coupler is done.

 

[harringtondav]

Not contributing much here. But the whole set up seems off. Like Scott Danforth said, big blocks couple to Bravos. I'd buy or borrow a prop close to your 23P, and see if the boat planes up to rule out a slipping prop hub. If not, I'd suspect the engine coupler splined sleeve is slipping in the rubber bushing, or the bushing is slipping in the steel engine coupler hub. The Alpha has everything spinning by spline and dog clutch engagement. I can't imagine any partial torque without a heck of a lot of noise and racket.

If it is a rubber coupling slipping, it will eventually fail completely and deliver near zero torque.

 

[harringtondav]

Borrow a prop close to your pitch, and see if you can plane up. If not, I suspect the engine coupler is slipping, either the splined sleeve in the rubber bushing, or the rubber bushing in the coupler hsg. Alpha drives are 100% spline and dog clutch engaged. I can't see anywhere to slip, just a lot of ugly noise.

 

[jones01m]

adogdrian, this is what I would do after listing to all the input.

1) pull the drive and lay it on some support blocks (easy)

2) flip the yoke 90deg and have someone stand on it. Torque on the prop nut. If it moves relative to the yoke (not just a little by taking up backlash, etc.) your problem is in the drive. If you toque hard and something sounds like it breaks, it was already broken. recommendations have been provided here to do something about that drive anyway. The drive does not seam likely for slip but at least you would know. If you cannot get movement in drive, move to 3

3) remove yoke at u-joint and place in engine coupler. figure out how to get some torque on the yoke. Have a friend hold engine pully with wrench on nut. torque the yoke. If you get relative movement, or slip, it is your engine coupler.

there is something that is just providing friction through the drive train. you need to see if you can overcome the friction and determine where the slip is.

This is just what I would do...