What gear ratio should I go with?

[g-tech]

I am repowering my Donzi Classic 18 with a 340hp 400ftlbs torque chevy V8. It's cammed pretty good with max power around 5,500. What drive ratio should I select for my new mercruiser drive. Any prop suggestions would be appretiated as well. Also the motor I have is a long block only, anybody out there want to get rid of some manifolds, mounts, etc.

g-tech
G-TECH motorsports, LLC
g-tech@verizon.net

[Formula Jr]

I would go with a Bravo even if its more expensive. Of those, two would work.
The 1.5 and the 1.36: People have different theories on this. Here are mine:
With the 1.5, your drive is spinning slow and to compensate you'll need to run a big pitch prop like a 27 to max out your top end. These don't "Hook Up" as well at moderate speeds or getting on plane. You will really feel this when the prop "bites," it will feel like a sling shot from mid-range up. A slower spinning lower unit also means less wear in the long run . But you could be boxed in by the drive ratio trying to make a high pitch prop work well. I've seen alot of Volvo drive boats that have tons of horse power, yet run out of prop.
With the 1.36: The drive is spinning faster so you would use smaller pitch props, like 22s and 24s. But you have the option of working up in pitch to match the engine and there are far more props to try out in this pitch range.
I'd go with the 1.36. You can always prop up to what the engine can pull. A Bravo will easily take the torque. Not so for an Alpha at 340 hp.
With the 1.5 you could out run the commonly available props if you are getting good power up to 5500 RPM.
It depends also what conditions you boat in. I like big waves lots of air time. This means I want to hook up quickly on landing. And not max out the last mph. So I like the 1.36 and smaller pitch props. This hooks up faster with the big changes in running speed.
On mostly flat water, the 1.5 will work just as well with a big pitch prop.

[Donzigo]

.............an alternative view.

I've had two Bravos and one Alpha with the 1:30 ratio and they have all had to be changed to 1:50. I have had a Z-25 (4500 lbs) and the Z-33 (10000 ilbs). I have run 23 Mirage on all. The smaller ratio gears just won't hold the torque.

But, then again, there are those on the board more savy than myself, when it gets to the fine tuning issues.

Good luck with whichever your choice is.

Richard

[Formula Jr]

Donzigo.
He's running an 18. If he was running anything bigger, my answer would be like yours.

[LKSD]

My guesstimate is a 1.36 with a 23p mirage +

Jamie / Lakeside

[BigGrizzly]

The only reason to run a 1:36 is the boat is too fast to spin the largest prop. The fact is the 1:50is faster with a bigger prop than a1:36 is with a smaller. Check the BAM web site. The fact is the 1:36 is weaker. JR, the drive is faster so the engine speed is slower you save engine not drive! As for props the Volvo guys have this wired. PROP for you G-tec with a 1:50 drive and a 25 pitch puts you at about 70->72 mph a 26/27 may be too much prop. the engine you have is comparable to 377 scorpion package or a good 350 mag. You will get a number of suggestions today on who's prop to use mine is a Turbo1 or a Lightning/TXP by Precision props Rootsy mite suggest a Quad 4 by Hydromotive and various people will recomend a Lab finished or blu printed something. Start with a 25 Turbo1 and go from there. The handling that YOU prefer is what is important not what the rest of us think or do, we can only guide you from trash. Some even still like the Marage series, me I will never use one again. I borrowed my old Marage back and realised most of my old handling problems were it. The choice is yours, I am not giving a sales pitch but I do sell Turbo ans Solas props unaer my company big grizzly performance propellers, yes all small letters. Remember opinions vary and every boat is always a little different. These are my opinions but I can say are backed up with data of some sort and I have been dooing this a long time and have tested many different boats in my time. Rember Donzi puts together a similar setup with a 1:50 ratio as well do several other companies. What JR said about acceleration won't affect your cobination in the least. I am speaking from experience not text book. Big Plus youwill have the same drive and ratio as everone else. Remember for speed use a GPS not the spedo because they can vary as much as 3 mph the same day due to BP and Temp. In my opinion if you don't use a GPS your speed doesn't mean much to me.

[Formula Jr]

Randy, are you coming to our little event on the 29th? In the pacific northwest, and, Will you bring a boat? These are waters you have never been in.

[LKSD]

One reason the 1.36 is weaker than the 1.50 is it is not as storng of a torque multiplier as the 1.5 or 1.65 ... The internals should be just as strong on the 1.36 as its 1.5 counter part. I have a client running a pair of them on his 35' executioner with 496 mag ho's.. The drives are still together since 03' & he beats on it.. Unless I am from mars the fountain is heavier & more powerful then the sbc in an 18c.. I do agree with you Big Griz on the prop thing. Its a matter of personal comfort & tuning likes/dislikes (to each their own).. :)

I am not trying to be a smart a$$ or start a battle of words, its just my opinon as far as what I am going to say next so please dont anyone take offense....As far as Bam & some others nothing against them & I am not pushing any parts in this post either, BUT I think the Donzi crew in general would be better off trying to plug a site supporter first.. For example, I dont care if it's not my shop all the time,I know price & policies dont always fit everyone's budget or needs. Although I do try to make my prices competiive within reason. If I cant meet someones needs I will be the first to tell them and try to reccomend them to a place that may. I dont know everything or do & sell everything ( I do do a lot though)..
It's just my opinon that site supporters / advertisers on the site should be plugged first.. For example: Plug Livorsi, Teague, Victory, etc.. etc.. If a legit marine related business is being plugged here I think they too should be a paying advertiser.. It's only fair in my opinion.. Unless no one on the site handles what a member is looking for..
I don't think it would be right when I plug my shop if I didn't financially & verbally support the sites I participate & plug my shop on. It's just part of having a busines & paying the dues that go along with it.. Again this is just my view/opinion....... Jamie

[undertaker]

Jaime would you stop working and go run that new boat of yours:yes: :shades: :shades: :shades: :shades:



Undertaker:bonk:


PS Sorry did not mean to hijack thread:bonk: :bonk: :toiletpap

[LKSD]

Jaime would you stop working and go run that new boat of yours:yes: :shades: :shades: :shades: :shades:
Undertaker:bonk:
PS Sorry did not mean to hijack thread:bonk: :bonk: :toiletpap

Hey Ed. Donzi Kristin here. There will be no boating for us today. Hopefully tomorrow. Jay stop working? Right! Does a bear sh@# in the woods??? Happy Dad Day to you. K:)

PS: He is really hell bent on patronizing your business.:bonk:
Sorry I sidetracked from the post as well.

[Formula Jr]

Are you going to run on Lake Champlain exclusively?

[LKSD]

He simply said to check the BAM site and use their slip calculator.... :boggled:


:D True, But I was also trying to civily reference his plug.. : "The choice is yours, I am not giving a sales pitch but I do sell Turbo ans Solas props unaer my company big grizzly performance propellers, yes all small letters."

No problem... Im over it... Im' not mad, it just didn't sit well with me as a paying advertiser & contributor.. It was just an observation & opinion of mine.. I spoke my mind on it & it's over with..

I am trying to remember lifes too short to worry about trivial stuff in the grand scheme of things.. I sure there are things people dont like about my posts at times. Life would be boring if we all thought the same. As long as people dont personally try to attack one another I encourage others to voice their point of view. :) ... J

[MOP]

I had gotten into this same subject awhile back, I recounted my experience with an old SK boat I had. I did not have anyway to get speed results, but here is what I did. I went from near direct dive to a fair jump up IE: over driving a lower smaller pitched prop. The boat seemed to run about the same speed, I lost a little out of the hole but the handling improved which I attributed to reducing prop walk. Before the swap when ever I got air borne upon landing the boat would snap to one side at times scaring the crap out of me, the snap to one side was greatly reduced after the swap
.
When I first put the BravoX on the beast the only prop I had handy was a 21 Mirage which handled very nice but let the engine spin to high, rolled high in turns but did give a dynamite hole shot I tried a couple of 23" wheels they got the RPM down and the speed up but hurt handling. I asked Randy's advice and went to a Turbo 4 blade still 23" again RPM dropped and speed went up a tad, I would like to try a 1:36 with a lesser pitched prop to do away with the prop walk and maybe a little of the roll up in left turns.

According the Merc site the 1:36 is stronger makes sense as there is more gear face contact, their quote below:

1:5 and 1:65 is higher reduction!

The downside to a higher reduction in the upper is that there is more torque on the lower gears, vertical shaft and prop shaft. In the case of a Bravo there is also more load on the gear floor.

A 1:5 or 1:65 will load the lower unit more , by moving the torque up into the upper and taking some load off the lower which Merc states as being stronger are we not better off. What are the real trade off's in speed and handling etc?

[Mr X]

horsepower in = horsepower out... therefore... in order to reduce torque at the propeller you must increase shaft speed... this alleviates the load on the internal components of the drive... at the negative of increased frequency of dynamic loading.

the 1.36 drive is seen as stronger overall... not because of larger gears or stronger materials but because the torque transmitted through the gear train is decreased due to less multiplication... the 1.5 ratio may very well BE the strongest of all in the alpha ranks as far as gear ratios because of the HD gearset available for the 1.5 ratio setup....

340 crankshaft hp is gonna put you somewhere between the 350 mag mpi and the 6.2 mx (320 @ prop). soooo 23P 3 blade iwth a 1.5 bravo... 25P 3 blade with a 1.65....



Ding Ding Ding! We have a winner! :yes: EXACTLY!

[Formula Jr]

Lets keep G-tech's application in mind here. An 18 is a realtively small boat to push with a Bravo. He could even get away with using an Alpha and gain 2 mph over the Bravo. The Alphas all got broken when they were mated to larger heavier boats, like Z21s, and that bad era in the late 80's still haunts our collective knowledge base. But who the hell wants to screw around with the Aplha's brain dead method of shifting anymore? Plus if G-Tech does do some more performance upgrades to the engine, then the Alpha is getting into the caution zone.

So a Bravo is the recommended path for the long view.

Now, we have to know what G-Tech wants to do with the boat before figuring anything else out.
How does he want to run it? What are the normal water conditions of his local area? How many people does he normally take along?
We never ask these kinds of questions. We go immediately to "This is how you get the last bit of MPH out of your set up." Thats not how I addressed his question. I addressed it in extremely general terms, almost to the point of exaggeration, just to frame the basic concepts. Then the experts want to point out just how wrong I am?

So we are a little pre-mature here talking about diameter thus pitch.
And I at least am waiting for more info on what its like to run Lake Champlain,
if thats his main lake and if he wants to take the whole family along and go skiing.

[Pismo]

1.5 tried and true.

[g-tech]

I appretiate everyone's comments. To answer some questions I did think about and alpha for a milliseconds becuase of the reduced wieght and drag coefficient however I can't deal with old school dog shift and I may increase horsepower in the future to a fast burn 383 motor with 425 hp and 460 ft/lbs in which case an alpha just couldn't handle it. Has anybody dropped a big block into an 18? I have a 427 with alluminum heads that I could use but I wonder about the added weight making it sit to low at idle. I can use that fast burn 383 and get more power that a 496, however as for as torque there is no replacement for displacement and boat's love torque.

I thought about how I would use the boat and I am really interested in midrange accel. Top speed is nice but want that push into the seat from 20 to 60. Lake Champlain can get pretty rough so it's unlikely that I will be traveling anywhere near max speed for long distances.

Since I am repowering from a King Cobra I have the oppurtunity to relocate the drive hieght. Any suggestions on that. I was thinking cavitation plate 1 inch above keel line. I would not be apposed to going to a on the boat water pickup and seapump with a closed hub prop and very high drive mount if it will make a difference keeping in mind that top speed in really not that important to me. If it would help in accell I would do it.

[boldts]

No expert here, but my 22 Classic which is bigger, heavier than the 18 Classic came with a 1.36 Bravo1. Before my ownership, boat had a ProCharger Big Block in it. When I purchased, boat came with a carbed 454 MAG. Boat came with a 23" Mirage (not the +) Boat's handling caractorists were atrosous. No fun to drive and basicly was a full throttle or no throttle boat. Pulled the Mirage off and replaced with a 22" Turbo1. Smaller diameter prop and 1 less inch of pitch. Boat gained 3 mph on gps and handling was like a sunday afternoon drive in the country. No bad habits. A real joy to drive. Later added a nose cone from Hydromotive to the Bravo1 and gained another 1.5 mph due to the ability to run more trim with no cavitation. Boat felt like riding on a flying carpet. It just floated the bow with no adverse handling traits. It was fun and a real thrill to drive throughout the rpm ranges.

What does this mean for G-Tech? I don't know. It's just my experience with a 1.36 Bravo1. Good luck setting up you 18 Classic. I personally like the Turbo1 prop and no, I don't sell them. What you get out of the box will be ready to run. No tweaking needed. No labbing needed unless your into spending more money.

[MOP]

Scott makes me like the idea of trying a 1:36 on mine even more, I may get away with a 21" or better yet a 20" 4 blade. Hmmm the wheels are turning a little more.

Phil

[Mr X]

Don't waste you time. I did it ,and the handling was HORRIBLE!
Almost undrivable at top end.

[Formula Jr]

I took one look at Lake Champlain and thought to myself, "this lake has potential for some monster wind effect waves from the south." Then I looked up some of the wave height buoy data, 5 footers! Oh my god, that sounds like a lot of fun!!!

So we can brachet the application now.

Excelleration from 20 to 60. HP gains expected up to 425.
I don't think raising your X is going to do you any good.
If anything, I'd be going the other way and put in a drive extention.
This will leave your drive in the water at least a little bit longer.
Airtime = less power to the water and less control.
And we need to think about hole shot in case you come off plan in this kind of close train, wind effect water. You want your hole shot to be almost instant.
Putting in the 427 might be, as you have surmised, too stern heavy for this lake. And if you install a drive extention you'll be putting some weight further back anyway.

Good luck!

[Mr X]

Yea, thats the way to go!! Get that drive DEEP in the water!!
That baby will scream!! :wink:

[Formula Jr]

"I thought about how I would use the boat and I am really interested in midrange accel. Top speed is nice but want that push into the seat from 20 to 60. Lake Champlain can get pretty rough so it's unlikely that I will be traveling anywhere near max speed for long distances." GT.

The client has just told us what his likely application and envirornment is going to be......

Given that, why still advise raising his X? Or going to a cleaver or a surface prop, or any of the other methods that ring out the last mph on relatively flat water. By flat, I mean anything up to one and 1/2s and/or rollers or the occassional boat wake.

I'll make it easy for the experts to be sarcastic. :propeller

I'd recommend a Bravo III, and a Stern Jack combined with the 383.

Then I'd say add K-Planes, 150s K-Planes.

In car analogy, some people here are suggesting a Nascar. I'm suggesting a Baha 500 racer.

:wink: