View Full Version : Different Outdrive Ratios

06-18-2012, 11:33 AM
Looking for some guidance here if anyone can help. I have searched through many threads looking for information on the difference between outdrive ratios but have not been able to find one discussing repercussions of slapping on an outdrive with a totally different ratio than previously ran. I am looking to purchase an SS outdrive for my 18 TR. I hear it is a bolt on 5-8 mph which excites me. that being said, the one I am looking at has a different gear drive from what I am running now. My first generation Alpha has a 1:5. the particular drive I am looking at is a 1:32.

Is it as simple as re-propping to compensate for this different ratio?

Am I going to lose any top/low end versus going with a 1:5 ratio ss?

thanks in advance!

06-18-2012, 11:52 AM
I don't have any personal experience w/a SS, but I don't know anyone who has seen those increases w/no other changes.
In fact, some have seen none at all.

IMHO, a stock Alpha w/an additional 100 HP will generate a pretty big increase.
SEI sells their after market Alpha replacement drives w/an incredible warranty for $1,295.00.
I think it may be cheaper to go the huskier engine route than trying to find and maintain an unobtanium SS.

06-18-2012, 12:56 PM
I really don't want to burst your bubble, but on a 20' MINX. I saw NO increase in speed.

I did however suck up every possible weed getting into my boathouse.


06-18-2012, 03:47 PM
So long as you are reasonably within the range of available prop pitches, I'd think you could just re-prop and find something that works. (Whereas, if your prospective 1.32 gears would require dropping down to a 15 inch pitch or something, that'd likely be more of an issue.)

The more I hear people report about their prop observations, the more convinced I am that you just have to try a few. So many variables.

I would not expect anything near a bolt-on 5-8 MPH. I suppose it's possible, but I would try not to get my hopes up on something like that. Back to the props/variables thing, there seem to be a lot of very different results for different people. Some alpha SS setups sound like they work wonders, and as some have noted already, some don't seem to do much at all.

There may be different schools of thought about turning steep props more slowly versus shallow props more quickly, though the former would seem to offer at least one clear advantage. Dropping from 1.5 to 1.32 and changing nothing else would be going the opposite way.

Best of luck with whatever you decide. Curious to hear how it turns out.

joseph m. hahnl
06-18-2012, 06:48 PM
????? not sure what just transpired. So an SS is a shorter Alpha drive than a the standard Alpha drive. It is designed to be more of a surface piercing drive. It is using the heavy duty gear set with the ratio 1:1.32. This ratio is designed for high torque motors such as a big block. In theory the prop is going to slip more because it's so close the surface of the water. Therefore the even though the the ratio is greater the prop slip should null the Rpm loss. If any thing you'll need to increase pitch to get the slip out. Because you need to add more pitch to prevent slip your speed would increase. Hence the gain in mph.Note what George said parts are scarce and expensive. But I think if your running it on a small block the 1:1.32 should hold up pretty well

06-18-2012, 08:00 PM
My 87 alpha1/454 330 hp setup has the 1.32:1 ratio. I run a quicksilver 21p (I cant remember the size, maybe 14.5??) cleaver @ ~63-65 mph @ 4300-4500 rpm. I can feel the prop spinning 90 to nothin when idling around the docks. Original parts since 1987 and the internals still look new.

I hear the reason for the 1.32:1 is because the two gears are closer in size, making them more similar in strength than a 1.5 or 1.62 gear ratio. It sounds feasible, so I'll accept it.

06-18-2012, 08:27 PM
Hadn't ever heard that about the slip going up that much, but it's interesting. (I have to say I'm skeptical that the slip would go up so much with the 1.32 alpha SS that the prop pitch should be the same as a 1.5 alpha, but I have absolutely no experience with them to actually know. Just seems like jumping from say, 10% slip at WOT, all the way to 20% slip would be a lot.) Hmmmm...

06-19-2012, 06:03 AM
Let me say it like this, these hulls really won't go faster than about 65 until at least 400 or more HP (for an 18) is utilized to lift the hull further out of the water and reduce wetted surface.
I don't know of anyone who had any real, consistant, measurable gain just by adding a shorty and maintaining stock power.
The reduced surface area of the shorty only adds to the reduced area of the planing surface benefits when enough power is available to get the hull "over-the-hump" in the 75-85 MPH range (for the typical Donzi hull).
There're several here that tried this route and ended up w/quite a bit more power to make it work.

Besides, I think you'll find there's quite a few more props available for 1:47-1:50 ratio drives.

06-19-2012, 09:49 AM
Wow, this is all very good information here. I have been told by several donzi owners that this would be the answer to get me into the 60's. For some reason, I can run at only about 57 no passengers, half a tank of gas. Maybe I should try to play around with some different props first as I ahve an awesome hole shot presently.

How much top end would you folks say I should expect form my boat? stock 350 mag 18', only 300 hrs on the engine runs very strong, almost perfect compression, normal conditions, no passengers half a tank of gas.....I thought 57 was kinda tame. With the correct prop, can I achieve the 60's in your opinions? Should I try that first?

CHACHI, I am sorry to hear that! what a waste of time and $. I would obviously want to avaoid that scenario lol. Is there anyone on here who has had success with slappin on an ss drive? Anyone? I feel like I have come across this topic so many times on thi ssite saying how great they are.....

Thanks all!

06-19-2012, 10:24 AM
Patrick, you really need over 300 HP to get into the 60's.

A new engine w/at least 400 HP will/should get you into the 70's w/some prop shopping.

joseph m. hahnl
06-19-2012, 05:35 PM
You need to evaluate what you got for numbers. Pitch, RPM, MPH then you determine how much slip you are getting from the theoretical MPH to the actual MPH. A prop calculator will figure it for you.

You can never go wrong with a top end kit to increase HP :wink:

06-20-2012, 07:16 AM
To give you a comparison, my 18C with 350 mag mpi, 300 hp, with 1.5 drive ratio, and a 23 pitch mirage, would do about 68 mph.


06-20-2012, 09:28 AM
"To give you a comparison, my 18C with 350 mag mpi, 300 hp, with 1.5 drive ratio, and a 23 pitch mirage, would do about 68 mph.


Seems to disagree with many above posters..

06-20-2012, 10:16 AM
I'm scratching my head--is it possible there's some 18C versus 22C confusion going on in this discussion?

"You really need over 300 HP to get into the 60's" sounds more like a 22 than an 18 to me.

"These hulls really won't go faster than about 65 until at least 400 or more HP (for an 18) is utilized to lift the hull further out of the water and reduce wetted surface." I thought 415/425 should make for maybe 68-72 in a 22, not an 18. I'd think an 18 would reach 65 with far less power, more like Bob's numbers (18C, 300 HP 350 mag, 68 mph).

I wonder if the "True Lies" thread has some 18 numbers to work from...

06-20-2012, 02:46 PM
Actually, I was using my Minx experience as a basis. It was 2' longer and 200 or so #'s heavier than an 18.
A minx really needs 300 hp to do 60.
A stock 260 hp 350 would not do it unless everything was optimistic to the extreme.
Mine had a new 350 Vortec which everyone agrees produces 315-330 hp or so and would do 63-65 MPH on most days.

But I still believe if the 18 in question goes 57 and it's propped correctly, the answer isn't an SS.

06-20-2012, 03:55 PM
Thank you for all the varying inputs folks. Yes, once again an 18 foot classic.

I would have hoped that being so close to 60 George, I would be able to be pushed over the edge with a supposed bolt on device that would do so. In your opinion it would do nothing.

We have CHACHI who also had the 20' minx George was referring to in hsi ecperience who saw no gain at all.

Tidbart has a very close comparison with the same boat/engine combo just pumped up to 300 hp. Bob, what kind of drive do you have?

It seems like there isnt anyone that can jump on here and say, "yes I slapped that baby on and wow I got 4mph out of it." If so, can you share your experience with us? The science behind it makes sense to me. Nose cone and a shorter drive with less drag....pretty basic physics you would think, but the proof is in the experience of those on this gerat forum in my opinion. And so far its not very convincing towards the SS being a viable option........

06-20-2012, 04:41 PM
Just a couple of notes about 24* Donzi hulls.................
Deep Vee hulls tend to have more drag (surface area) than other types of hulls.
Everything else being equal, and using an outdrive, it'll take more power to get to a good cruising speed,
i.e.; 35-50 MPH, than a hull w/15* deadrise, or a flat bottom.
The large round keel on a Donzi, or other hulls that decended from the mind of Ray Hunt, acts as a modified pad.
The faster you go, the more lift the keel generates.
If the transom corners start rising clear of the water, the surface area starts to reduce at a pretty high rate of change.
My whole point is, reducing surface area is key to increasing speed. So, your hypothesis is correct as far as it goes.
But the shortcoming of the idea is, you can reduce the area of the bottom planing area (and thereby increase speed) at a higher rate by adding power, than by putting on a shorty. The reduction in area by installing a shorty is fixed, the reduction in bottom planing area is almost infinite.
This is why the addition of a shorty at 80 MPH is more rewarding than at 60 MPH. At 80 MPH, the bottom planing area is already reduced to a point that the shorty and its smaller parasite drag is more meaningful.

06-20-2012, 07:15 PM
patrickie, what prop are you running now?

06-20-2012, 09:15 PM
Some of this stuff just don't jive! My 22 with 300 HP did a little over 60 check photo, bringing to near 400 with the 383 it now does 65.5. There have been a few 18's with 300 that went past 65, to me something is amiss with his setup. For starters I would go through the engines tuning spec's, check the hull for any possible problems.

06-23-2012, 03:30 PM
I like the 18C that does 68mph with a stock 300hp motor, Hmmm..

06-24-2012, 11:23 AM
[ QUOTE=$originalposter]{$pagetext}[/QUOTE]

what is the difference between my numbers and the ones that you post for your 22?

Not trying to be argumentative as I am going from memory. I sold the 18 in 2005. Speed could have been 67 for all i know, but it flew under the right conditions.

06-24-2012, 04:00 PM
We have estimates of high 50s to high 60s for 18s with 300hp. It all just sounds odd.

06-24-2012, 05:13 PM
the Alpha would/could/should be faster than the Bravo..

06-24-2012, 11:26 PM
[ QUOTE=$originalposter]{$pagetext}[/QUOTE]

If you wanna talk about this with someone that has actually played with the 18/Bravo/Imco shorty with stock power, PM me and I'll give ya my # and we can chat or you can keep getting the advice of some that wanna speculate.

06-25-2012, 05:04 AM
Two things;
As I said previously, my speed estimates were based on my Minx experience, not an actual 18C.
I think Patricke needs to work w/his boat as it is, an engine tune up would help, I'm sure, and to try a dozen props or so. I believe he can get his boat in the 60's pretty easily w/o installing an SS. After all, that was his original question.

06-25-2012, 06:08 AM
My MINX with a 21 cleaver (I think), 350 MAG would run 61-2, lightly loaded of course.


06-25-2012, 09:31 AM
My MINX with a 21 cleaver (I think), 350 MAG would run 61-2, lightly loaded of course.


that'sd exactly what mine ran with the same prop and engine combo

06-25-2012, 10:13 AM
Jim, I thought I read somewhere you had put an Alpha SS on your 18 2+3? If you did, what were the results?

06-25-2012, 10:23 AM
immediate +6 to 7 mph with one person, same day tests, only +2 or +3 mph with 2 people due to lack of sufficient bow lift before cavitation... 18 loved it

+0 results on the Minx, hated it

06-25-2012, 01:11 PM
[ QUOTE=$originalposter]{$pagetext}[/QUOTE]

there have been a couple of bone stock 18 / 350 mag combos that ran high sixties low 70's . ranman i believe was running those numbers and a long time ago Mr.X had an 18 that ran a little over 70 .

06-27-2012, 11:09 AM
Sorry guys been away for a while. So Jim this is after adding an ss drive on your 18 with a stock 350 mag? If so, this is the first positive response to the SS so far on this thread! :thumbsup:

Here is a pic of my prop. I know nothing about what it means if you guys wish to speculate and shoot me some ideas:


"QUICKSILVER 48 89800 A5 19H"

06-27-2012, 11:36 AM
That is a old 70s-80s style cleaver 19", you can do better.

06-27-2012, 11:46 AM
[ QUOTE=$originalposter]{$pagetext}[/QUOTE]

this is good to hear, talk to me brother!!


06-27-2012, 12:26 PM
you need a 23 Cleaver for best top end with an SS or a Turbo 22 for best cruise and efficiency

it will dance a little with the Turbo at WOT, but it is dead rock steady at WOT with the cleaver, however the midrange cruise rpm with the cleaver was 4-5 mph less vs the same cruise rpm with the Turbo

the handling with the SS was night and day better

06-29-2012, 11:52 AM
[ QUOTE=$originalposter]{$pagetext}[/QUOTE]

this is great information thank you! Does it matter what ratio the particular ss drive is with these particular props you mention?

06-29-2012, 12:03 PM
my drive was a 1.50 ratio

for a 1.32 you would subtract 2" of pitch from either selection

06-29-2012, 12:07 PM
Excellent you porbably just saved me a bunch of time and money with this info brother thank you.

06-30-2012, 07:36 PM
John the red Donzi 18 from Mattituck has about 300hp with an SS running my old 4 blade Ballistic re-pitched to 23 it runs close to 75. IMO running an SS on a Minx or 22 is a totally different ball park, both boat require a fair amount of trim. Running a cleaver on either one is self defeating, cleaver produce stern lift the opposite of what those hulls need. I have a high X being an old Tempest 22, a 23 cleaver got me a hair over 63. Replacing it with a 23" 4 blade Turbo Ultima brought me to 65.5, also the cleaver was very squirrely I was glad to pass it on at a loss!!!!

07-02-2012, 03:52 PM
[ QUOTE=$originalposter]{$pagetext}[/QUOTE]

Wow. those are some great increases. Im about 50 HP shy I would imagine of the 300 mark which I could eventually get to with some mild engine mods. Add an SS drive into the mix with the correct prop and it sounds like I'm in the 70's based on what you're saying.