Changing Gear Ratio...Prop Change Required

[bcovecrew]

I have a 1989 Regal Sebring with the 470 (3.7L). Last year was a 2020 type year for my boat. 1st time in the water...charging system bad (second time with this). Replaced with an alternator kit this time. 2nd time in the water. Not a new condition I’m sure, but now noticeable. Had the fix the head gasket. 3rd time in the water. Outdrive toast. Haven’t repaired that yet and it is why I am here. Decided to repair in Spring and in talking with my mechanic, he suggested a new outdrive (upper and lower) from a company in FL with a 3 year warranty. Recently I reached out to him to see when we ought to purchase and he said he had a used outdrive he was rebuilding that he felt good about, but it was a different gear ratio. The current gear ratio is 1:81 and I use a 21 prop (I have 2 spare). The gear ratio on the rebuilt drive is 1.98.
I can save $500-700 with this rebuilt outdrive (depends on how many spare props I want...would probably only do 1).

What experience can others share with a similar change?
Is there a chance the difference is small enough I can continue with the same prop (21)?
Assuming I have the right prop (whether it is the 21 or another), are there any other downsides to this type of change?
Any suggestions as to what prop pitch I might want to try if the 21 isn’t it?

Thanks in advance!

 

[WesNewell]

Going from 1.81 to 1.98 GR will require going to a 23P prop to maintain the same rpm as you have with the 1.81 GR. However, if your rpm was on the low side with the 1.81gr, then it might still be within rpm range with the 1.98gr.

 

[bcovecrew]

Thank you. So if I go this route, keep the 21 on check the WOT RPM vs specs (4200-4600) and if I am not in that range, move to the 23 and try again?

Where might I see other side effects? Will it be on the low end (pulling up skiers, getting to plane) or high end (maximum speed)? I’d give up max speed to not lose on the original get up and go.

Anything else to be concerned about with this kind if change?
Thanks in advance.

 

[Sea Rider]

What's the current passenger load. light, medium light, heavy, state number of passengers that plan going boating with ? Wot test the 21 pitch prop and report the wot rpm achieved independently of the 1.81 or 1.98 gear ratio you choose to stay wth.​

Happy Boating

 

[bcovecrew]

Boat certified for 8. Usually 4-5 people on board. Think maybe 700 lbs of people. 20 gal of gas. Normal other equipment. I would say medium load. We rarely fully load and if we do we usually are not going to ski or wakeboard.

Middle of winter here. Trying to decide now on the replacement unit by understanding the pros and cons of the change. Won’t get it on the Lake likely till early to mid May.

 

[Scott06]

Thank you. So if I go this route, keep the 21 on check the WOT RPM vs specs (4200-4600) and if I am not in that range, move to the 23 and try again?

Where might I see other side effects? Will it be on the low end (pulling up skiers, getting to plane) or high end (maximum speed)? I’d give up max speed to not lose on the original get up and go.

Anything else to be concerned about with this kind if change?
Thanks in advance.

Yes try the 21” check wot rpms. You may over rev so back off the throttle when you get to 4600.
going to steeper gearing will improve ( all other things being the same) holeshot and will limit top end, hence going up in pitch to compensate.
Five years ago i swapped a 5.0 for the original 4.3 in my boat, I kept the 1.81 drive , just had to step up from 19” to 21” pitch. Saved me from sourcing a ‘correct’ 1.62 ratio drive that would have come on a 5.0 from the factory

 

[Sea Rider]

So the first wot test was done without any reported wot rpm data due to not having a tach on the dash board, not working properly or thought that was uselees knowing about ?

Yep, try the 21 pitch prop with current 1:81 gear ratio at wot on flat calm water cond loaded with 4-5 souls on board at full hammer down, need to know what's the max wot rpm achieved. This test is done for a very short time run, not an issue if over revving a bit past 4600 wot rpm. Does the motor counts with an over rev limiter ?

Happy Boating​

 

[bcovecrew]

So the first wot test was done without any reported wot rpm data due to not having a tach on the dash board, not working properly or thought that was uselees knowing about ?

Yep, try the 21 pitch prop with current 1:81 gear ratio at wot on flat calm water cond loaded with 4-5 souls on board at full hammer down, need to know what's the max wot rpm achieved. This test is done for a very short time run, not an issue if over revving a bit past 4600 wot rpm. Does the motor counts with an over rev limiter ?

Happy Boating​

No first wot test due to the outboard completely failing. However i think the previous wot from memory was in range of 4200-4600. I just never thought to record it before. So now no way to test it now.

 

[Sea Rider]

No first wot test due to the outboard completely failing. However i think the previous wot from memory was in range of 4200-4600. I just never thought to record it before. So now no way to test it now.

If so, the current 21 prop pitch and the 1.91 gear ratio are doing their homework right, there's nothing to improve. Assume the 400 wot rpm difference was due to having one more/one less soul on board ?

Happy Boating​

 

[bcovecrew]

I am sorry for the confusion on this. I have a boat that had an outdrive with a 1.81 gear ratio that uses a 21 prop and I think it got in the spec range of 4200-4600 rpms at wot. I say ‘I think’ because I never recorded it. That outdrive has failed and I am faced with deciding how to replace it.
It is an alpha 1 gen 1


Option 1 is brand new with the same gear ratio. $1,700 installed

Option 2 is a different rebuilt unit with a 1.98 gear ratio (rebuilt by my mechanic). $1,000 installed

Option 3 is to buy a used or rebuilt with same gear ratio, but unknown condition. $?

So considering 1 and 2 right now. Do I save $500-$700 (depending on how the prop ends up) or do I pay the extra and be done with it.

I put a good bit of money into it last summer and wouldn’t mind saving the money If there are no bad side effects once the right prop is in place.

 

[Sea Rider]

With option one at $ 1,700 brand new you count already with the current 21 pitch prop.

With option 2 will need to add a higher pitch prop if the 1.98 GR revs the motor higher than with the 21 pitch. The extra cost $$$ will depend if going for an alum or an SS prop.

With option 3 buying a used rebuilt lower unit could end in buying a Pandora Lower Unit. If I had to choose for my peace of mind Option 1...

Happy Boating​

 

[Faztbullet]

You can get a complete upper new with 3 year warranty for $695..complete drive for $1395

 

[bcovecrew]

You can get a complete upper new with 3 year warranty for $695..complete drive for $1395

The way it failed, we need the complete unit. Add tax and shipping and install and that is the $1,700. See pic.