Do I have wrong gear ratio?

[liquidlew]

I just discovered that I have a different drive/gear ratio than what is supposed to be in my boat. According to several manuals, I am supposed to have a 1:84 in my 1989 Alpha 1, 4.3 v6, and found out I actually have a 1991 drive with 1:50 ratio. Does that explain why I never get over 4100 rpm (range is 4400-4800)? Running a 3 blade 19p or a 4 blade 17p, would a lower pitch solve the problem, or should I think about changing it back to the right ratio, and how hard/ costly is that? Engine runs great, new tune up, compression good, etc., so I have ruled that out.

 

[83Evinrude]

Re: Do I have wrong gear ratio?

I actually have a 1991 drive with 1:50 ratio. Does that explain why I never get over 4100 rpm (range is 4400-4800)?

I believe the 1.5 ratios are for V8 engines, a lower pitch prop would probably help but your 700rpm from max.

 

[HT32BSX115]

Re: Do I have wrong gear ratio?

Do I have wrong gear ratio?

Um, yes. You do. How fast did you go at 4100?

A 1.5 ratio is more suited to a V-8. (read MORE torque)

You could go to a lower pitch prop with limited success....

The problem is that the WOT RPM (about 4800) for the engine, also coincides with a similar most efficient RPM for the prop.

In the extreme, running a "VERY wrong" ratio which results in using a prop pitch that "works" prevents running the prop at IT'S most efficient RPM.


Where you might use say, a 19P 3 blade to get the correct engine RPM (4800 with the correct drive ratio)) you would have to maybe run a 15p or 14p prop with a 1.5 Ratio because the 4.3l engine doesn't have the torque to produce that RPM .

The prop could be turning a LOT more RPM than it's most efficient RPM.

Turning a prop at a very high RPM (in the water) produces a LOT more drag and in the extreme will cause cavitation unless it's designed to turn high rpms (labbed props)

It isn't very cheap to do a gear-set swap. It's sort of like a complete rebuild since you have to check all the pre-loads (or you should)

You might consider just selling the drive and buying an SEI. you could probably get $600-800 for your drive (assuming it's in good condx), then get an SEI drive of the right ratio and get a 3 year warrant to boot!!


Now,

Having said all this, if you can borrow a 15p 3 blade stainless prop, give it a try and it might get you where you want to go .

If you're happy with the approx 35-40 mph you'll probably get at 4800, then you can stick with it......

If you continue to run your engine with a prop that gives you 4100 at WOT, you will damage the engine eventually.

(if it were my boat, this would be an excuse to find myself a 350 V-8 and swap it out!!!)


Regards,


Rick

 

[liquidlew]

Re: Do I have wrong gear ratio?

Wow, thanks for the detailed info Rick! I see 40-45 on the speedo, but I don't go there too often, mainly just cruise at around 2500 rpm. I've had the boat for 4 years and just recently discovered the mismatch drive. Knew absolutely nothing about boats when I first got it, but I sure do know now! I could never figure out why I couldn't get into the correct rpm range with everything in good working order. Found out about the drive by accident, had a bad upper yoke seal and had it replaced last Dec. Mechanic told me what the 1:50 meant when I asked him and he too said I had the wrong ratio and the 1:50 was for a V8. Good idea on getting an SEI and selling mine, since I just had it resealed and new impeller to boot. So just to be sure, 1:84 is the one I should get?
Oh, and cool idea about the V8, but I think it would sink my 18'!

 

[HT32BSX115]

Re: Do I have wrong gear ratio?

So just to be sure, 1:84 is the one I should get?
Oh, and cool idea about the V8, but I think it would sink my 18'!

Oh, I'll bet your boat was available with a V-8!!!

My previous boat was a 1966 18' Mark Twain that Dad bought new!

It could be ordered with a 120 4cyl up to and including a 409 V-8 310HP Mercruiser and everything between!! Of course my brother and wanted Dad to get the 409!!!!


We settled on the 150 (230cu-in 6) I sold the boat in 2005. The hr meter quit back in the 80's at around 1800 hours or so..........


Anyway, I would give a 15p prop a try and see how it runs. It might just run good enough to satisfy you and not "lug" the engine! It certainly wouldn't "hurt" anything to turn that prop a little faster than normal.


Cheers,



Rick

 

[tfret]

Re: Do I have wrong gear ratio?

I have a 19' Dixie with 5.7L V8. My WOT rpm is at the bottom of the specified range with a 3 blade 19P prop. Its an old boat I bought used and I've wondered before if the outdrive is original or not. How can I confirm the gear ratio to be sure it's right? Can I manually turn the engine crank pulley and count revs of the prop or something like that?


Ooops, please disregard this post. Already answered in previous thread. Sorry.

 

[liquidlew]

Re: Do I have wrong gear ratio?

Thanks again Rick! For now I'm gonna snag a 15p off ebay and see how that works, get me thru the season anyway and figure out what to do in the off season. I can't figure out why the PO put a different and wrong ratio in that boat, especially since the guy was over 250 lbs. and an avid skier? Don't make a lick of sense to me, he may have trashed the original drive and maybe someone gave this one to him? I don't know...

A V8 would indeed be fun, but with the 4.3, my boat already sits kinda low in the water at the stern, plus, even with the wrong gearing, it still has enough power for what I use it for, mainly just cruising and occasionally pulling the little ones in the tube, but I will definitely address this issue. Thanks again for the info, really cleared up a major mystery for me!

 

[HT32BSX115]

Re: Do I have wrong gear ratio?

A 15 p prop might just work "ok", just understand that it won't be as efficient as the right ratio & right prop.....


Do you just run it in a local lake? (like Castaic or Pyramid or Piru)

 

[liquidlew]

Re: Do I have wrong gear ratio?

99% of my water time is spent out in the Colorado river at Parker or Havasu. I've had it out here in the marina (ocean) twice, but I'm not a big fan of salt water, except for surfing! I'm definitely gonna address the gear issue, only gonna use the 15p to get me thru the season, it won't be permanent. I just wish I would have found this issue when I first got the boat, hopefully running it for 4 years with the wrong ratio hasn't hurt it. It still seems strong, compression good, etc. I'll be taking it extra easy on it now, even more than before, I'm real picky about my stuff and always try to do everything right.

 

[HT32BSX115]

Re: Do I have wrong gear ratio?

It probably hasn't "hurt" it yet.....

It would be a problem if you had a severe detonation event.

Lugging an engine (low RPM with very high (or WOT) throttle settings) with severe detonation, have been known to punch holes in pistons, burn valves, and burn the electrodes right off a spark plug!


This is why most (or all) of the engine manufacturers suggest "propping" to the upper end of the RPM range for WOT.

but I'm not a big fan of salt water

Yeah, me neither.....back in the 60's after my dad bought the 66 Marktwain, we went out to Anacapa island and back.... I think we spent the next 2 days washing the salt out of the boat....That was the only time we ever had it in salt water!!

 

[mcleaves]

Re: Do I have wrong gear ratio?

Could always get a new SEI drive with warranty for a little over a grand and then sell yours. I think V-8's eats those more often than fours so you ratio should be in demand. It's almost summer and a good time to sell a drive.

Just my .02

[EDIT] - Just saw Rick's comment suggesting the same. Sorry for the repeat.

 

[liquidlew]

Re: Do I have wrong gear ratio?

It probably hasn't "hurt" it yet.....

Hmmm... I did have the heads done 2 years ago, mechanic said I had 2 bent valves, could that be a result of wrong gearing? When I asked him what would do that, he said over reving. It had low compression in one cyl when I bought the boat (I know, shame on me for not checking that!:redface When I finally did have the compression checked, tests confirmed it wasn't the rings so I took the heads in, engine has been great since, just checked compression and it is same as 2 years ago when I put it back together.

I've never heard any detonation at any throttle range, engine always sounds great, but now I will keep an eye (or ear) on it!

 

[HT32BSX115]

Re: Do I have wrong gear ratio?

Hmmm... I did have the heads done 2 years ago, mechanic said I had 2 bent valves, could that be a result of wrong gearing? When I asked him what would do that, he said over reving. It had low compression in one cyl when I bought the boat (I know, shame on me for not checking that!

I have never heard of a detonation event bending a valve (seeing as how the valve is closed when the "kaboom" happens!!).....I suppose anything is possible....


You will better protect the engine by allowing it to turn in the correct RPM range. It may not run the prop as efficiently as it could, but it won't hurt anything........