Gearbox Gear Ratio Question

Automotive powered airboat discussion.
Post Reply
WarDuck
Southern Airboat Member
Posts: 5
Joined: Mon Nov 08, 2021 7:21 pm
Location: Palm Beach, Florida

Gearbox Gear Ratio Question

Post by WarDuck »

I have a 12' Diamondback hull with a 350 DD and I'm looking to add a gearbox. My question is what ratio for that motor? I know a 350 crate motor can only do so much, but I have people telling me a gearbox will still make a world of difference.
User avatar
kwanjangnihm
Site Supporter - IV
Site Supporter - IV
Posts: 2287
Joined: Sat Mar 30, 2013 8:23 pm
Location: Bartow FL

Re: Gearbox Gear Ratio Question

Post by kwanjangnihm »

I would think a 2:1 would be a good choice for a stock 350.
flcracker9 wrote:
Wed Jun 05, 2019 8:04 am
Birddog247 on this site runs an old 350 that he pulled from a van, 2:1 gearbox, 2 blade R, on an aluminum hull, and I've yet to see him sruggle on the hill with 2-3 people. The belt box in question weighs more than his gear box, but the combo operates the same. Set-up would dictate how it performs, but the generic question was "anyone running a 350 with a 2:1 box", and I'm sure there are plenty of boats running that combination. Set up right, it can be a point and go combo.
He'll cut your throat, baby, stick you in the back, drive off in your Cadillac.
He's more trouble than you think, he'll kill your sugar, leave you in the drink.
WarDuck
Southern Airboat Member
Posts: 5
Joined: Mon Nov 08, 2021 7:21 pm
Location: Palm Beach, Florida

Re: Gearbox Gear Ratio Question

Post by WarDuck »

Thanks kwanjangnihm. I was trying to do a little research on here last night, but didn't come across that post.
User avatar
Slidin Gator
Site Supporter - IV
Site Supporter - IV
Posts: 1301
Joined: Sat Feb 03, 2018 11:33 pm
Location: Jupiter Farms, Florida

Re: Gearbox Gear Ratio Question

Post by Slidin Gator »

WarDuck,

The thing that makes the 2:1 so intriguing for many DD upgrades is the potential to use the same prop with a belt drive. Most 350 motors make max power at 5-5.5K RPM which matches max power on 70-74" port rotation props.

Depending on the hull, present rigging and prop plans, a larger prop and more gear may be in order. More information will improve your feedback here.
I still think I-10 is the Mason Dixon line.
2013 Mike Stossel Boats, 13' x 7' deck over, IO-540, 74" NGQ less winglets.
WarDuck
Southern Airboat Member
Posts: 5
Joined: Mon Nov 08, 2021 7:21 pm
Location: Palm Beach, Florida

Re: Gearbox Gear Ratio Question

Post by WarDuck »

IMG_0072.jpg
Slidin Gator, thanks for the reply. I actually prefer not to do a belt drive, though I know that's the cheaper route. I was thinking of either an Ox or a Stinger. Right now there's a 72" 2 blade Water Walker on it, but I know I'll need to change that out for a right hand prop that better suites the setup, whether it be a 2 or 3 blade. I'm new to the ownership side and knowing the ins and outs of the different setups.
User avatar
kwanjangnihm
Site Supporter - IV
Site Supporter - IV
Posts: 2287
Joined: Sat Mar 30, 2013 8:23 pm
Location: Bartow FL

Re: Gearbox Gear Ratio Question

Post by kwanjangnihm »

WarDuck is your 350 stock or built high performance? Remember you will need to move your rigging forward ~12" to accommodate a gearbox. What is the maximum diameter prop you can fit inside your cage? If you are planning to rebuild or replace the 350 (ie: stroked 383) in the near future, a higher gearbox ratio would be in order like Slidin mentioned above.
He'll cut your throat, baby, stick you in the back, drive off in your Cadillac.
He's more trouble than you think, he'll kill your sugar, leave you in the drink.
One Eyed Gator
Southern Airboat Member
Posts: 1386
Joined: Wed May 30, 2007 11:33 am
Location: Ocala,Fl

Re: Gearbox Gear Ratio Question

Post by One Eyed Gator »

Remember that if you're keeping your motor at the same height. The prop height will move up for a gearbox and even higher with a beltbox.


2.09 is a decent ratio depending on how the motor was built and the cam.
User avatar
Deano
Site Supporter - V
Site Supporter - V
Posts: 4670
Joined: Thu Jan 06, 2005 11:16 am
Location: Inverness, FL

Re: Gearbox Gear Ratio Question

Post by Deano »

I am very surprised that the consensus of opinion thus far would appear to be so close to 2:1.
To my way of thinking the smaller (HP wise) an engine is, the MORE it will benefit in the real world by using more gear.
Using gears like a 1.77 or 2:1 are better suited to bigger (HP wise) engines that are challenged to run reliably at elevated RPMS for any extended period of time. Because your 350 small block doesn't suffer from that type of affliction AND will produce more power at an elevated RPM, it would only make sense to me to use more gear than has been heretofore proposed.

I started to respond to your previous post in another thread, but it was to late for me to finish that night and I didn't get back to the board till now. I'm inserting/copying that post here in it's entirety, because it's still good info and will likely provide you with food for thought, if not 'new to you' information.
In an earlier thread, WarDuck wrote: . . . I'm new to the ownership side of airboats as a whole. I have a 12' Diamondback aluminum hull with a 350 DD and a 2 blade water walker prop. I'm looking to add a gearbox and change out the prop to maybe a 3 blade right hand prop. I will most likely go with an Ox, but my main concern is what ratio? I know a 350 crate motor can only do so much, but I have people telling me a gearbox will still make a world of difference.
The info you're after could be broken down into two different categories; those that are a given, and those that maybe could be debated.

The first major thing that IS A GIVEN, is that adding an appropriate gearbox and prop WILL make a world of difference. You have been given solid advice in this regard. Given prop speed restraints in the real world, automotive engines in general only run at a skinny half throttle at DD rpms. Obviously, this not the way to attain the most power available in any application, let alone an airboat where maximum thrust is the prime objective.

The second thing that IS A GIVEN, is that the BEST GEAR RATIO for any given application will be the one that allows the engine to run in it's native RPM range, while at the same time, allowing the propeller to operate in its most productive rpm range. These are the three variables that need be considered (and be independently adjusted) to arrive at the desired end. With an existing boat, where one of these variables is predetermined, the two remaining choices simply need to be made with that preexisting one in mind.

The third thing that IS A GIVEN, (and often overlooked reality), is that blade count should not be strictly approached with the premise being that 'more is better'. When you add an additional blade you are increasing the mass, weight, and blade surface area that all add to, and increase parasitic drag. This 'increase in overhead' must be overcome HP wise before any net gain in thrust will be realized. This kind of translates to IF you can hold the engine back with X number of blades (2), you don't NEED or want X+1 number of blades (3).


Now, in your case, you already have the engine, so the two variables to be adjusted are your gear ratio and propeller selection.
This being the case, you can define either one first, as long it provides for a proper second choice to "fill and connect the gap".

IMO, using a 2.68 gear would be advantageous because it will provide more available torque at the prop shaft were the rubber doesn't meet the road, so to speak. Generally, the only time to not use more gear would be if it would entail spinning more RPMs than the engine needs spin to make the prop happy (read 'most productive'). In the case of your 350 this shouldn't be an issue, so that is what I would use based on ultimately producing maximum thrust. Now bear in mind, you're using the gearbox as a torque multiplier, NOT an RPM enabler.

Take for example, a 15' Rivermaster hull with an 500 Cadillac using a 2.09 gear to turn an 80" two blade Super Wide prop.
This boat was a little on the heavy side for what it was, but ran quite respectably for the amount of money not spent toward that end.
Considering said prop could push that boat dry with a load, that would be plenty of push for your boat, don't ya think?

Now consider the gearing to spin the same prop in these two different scenarios:
Scenario One - Cadillac BB with 2.09 gear > WOT = 4,200 Engine RPMs turned said prop 2,009 Prop RPMs
Scenario Two - Chevrolet SB with 2.68 gear > WOT = 5,384 Engine RPMS turns said prop 2,009 Prop RPMs
It should be pretty clear that the 350 is much more in it's native power range than it would be a Caddy rpms.
Even if it didn't want to turn 5400, you could add a little pitch and get it where it wants to be, while still maintaining the gear advantage.

If you want to see that this isn't my opinion alone (about the gearing, not the prop), [Search] some of Waterthunder's posts.

A couple other questions/comments:
You said "crate engine", do you know how many RPMs your current cam will allow it to turn?
What does your cage width measure? ie. How big of a prop will it accommodate?
How much room is there currently between your forward rigging and the bow compartment

Note the comments made earlier about engine mounting. It's more desirable to lower engine rather than compound top heavy on a 12' hull.
IMO, if it's possible, it's usually easier and far preferable to build a new engine stand and scab your existing rigging to the front of it rather than attempt to alter what you have to make it work. This allows you to not only position the prop where you want it, but will also provide a lower center of gravity (which will help where torque roll is concerned).
"The suppression of uncomfortable ideas may be common in religion and politics,
but it is not the path to knowledge; it has no place in the endeavor of science."
- Carl Sagan
WarDuck
Southern Airboat Member
Posts: 5
Joined: Mon Nov 08, 2021 7:21 pm
Location: Palm Beach, Florida

Re: Gearbox Gear Ratio Question

Post by WarDuck »

Deano I really appreciate that input and knowledge. Very well explained and easy for this newbie to understand. As for the prop distance, I only have 3" from the prop at the skinniest part of the cage on the sides so I can't go up in prop length from 72". But, luckily I think my rigging can be moved forward as is and I'll just have to lower the engine stand to allow for the gearbox height. I'm looking into Stinger boxes.
User avatar
John Fenner
Southern Airboat Member
Posts: 2985
Joined: Tue Feb 14, 2012 12:26 pm
Location: Miami(Cuba) Fl.

Re: Gearbox Gear Ratio Question

Post by John Fenner »

The main question that needs an answer is, what was the 350 blueprinted for, to spin 3000 rpm? Or was it just a 350 stock engine thrown on it adapted to take a prop? That said, there is much more to just dropping in a gearbox and a prop, the engine needs to be blueprinted for its application, proper torque curve that the prop stalls the engine at the optimum RPM, so the next question is, do you really need a gearbox?
Or do you need to blueprint the direct drive engine to turn the prop at it's proper torque curve, search DD builds, CS-711 camshaft in this website, there are many things to build a DD that will out perform an aircraft engine, much less the added weight of a gearbox and reworking the rigging to accept a box and the expense of of a prop.
I never finish anyth,,,.
Post Reply

Return to “Automotive Power Only”