Difference between direct drive and reduction gear?

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RedfireSVT
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Difference between direct drive and reduction gear?

Postby RedfireSVT » Wed Aug 14, 2013 12:56 pm

I am looking to buy a 14x8 airboat with a small block 350 and a direct drive... I don't know much about the difference between DD and reduction other than you get more power out of the reduction drive. Could some one explain the benefits or lack of on the two and will a DD work well with this motor on a 14x8 boat? thanks
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Re: Difference between direct drive and reduction gear?

Postby :) » Wed Aug 14, 2013 1:12 pm

Dd simply means u have no reduction. Has its ups and downs. Dd u can only turn 3000 maybe 3200 on ur prop.. with carmotor u are not making much power at those rpms. But fuel economy and operating cost r lower.. ur boat may or may not run any dry ground. With reduction or gearbox u can pretty much run the rpm u want to. Worse fuel economy but more power and maneuverability.. turn more prop which gives more push..
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Re: Difference between direct drive and reduction gear?

Postby Deano » Wed Aug 14, 2013 1:42 pm

The most defining difference is that a DD is limited to turning about 3000 RPMs.
A gear reduction alows the engine to spin up to enough RPMs to provide usable power.

A 350 at DD RPMs (3000) is not producing very much usable power.
A 350 w/ a gear reduction (5500) will produce usable power.

This is why you see so many big blocks as DD engines.
Cubic Inches make low RPM torque, a factory small black usually does not.

In my opinion, a 14x8 foot hull with a 350 DD is going to be under powered.
That hull would be a good candidate for the 350 and a gear reduction, or
as a DD, that hull would work OK with a 500 ci Cadillac.
For a 350 DD, a smaller hull would be much more servicable, imo.

Suggest you read a lot, get some experience and look a lot before you spend any money.
No disrespect intended, but if a unscrupulous seller heard such a question,
you could very likely end up going for a very expensive first ride. :shock:

Welcome to the forum, you've come to the right place to ask questions !!
There is a [Search] function up towards the upper right side of screen, answers to
most common questions are there. If not, somebody here knows or will find out.
There's more airboat info to be had here than everywhere else put together. :thumbleft:
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Re: Difference between direct drive and reduction gear?

Postby RedfireSVT » Wed Aug 14, 2013 2:13 pm

thanks guys. i guess my next question then would be what would it take to convert it from DD to reduction gear and is that even an option?
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Re: Difference between direct drive and reduction gear?

Postby southern safaris » Wed Aug 14, 2013 2:23 pm

RedfireSVT wrote:thanks guys. i guess my next question then would be what would it take to convert it from DD to reduction gear and is that even an option?


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Re: Difference between direct drive and reduction gear?

Postby Deano » Wed Aug 14, 2013 2:52 pm

The gear box literally bolts to the back of the engine block much like a transmission.

The drive portion is just about as straight forward. Flexplate bolts to engine crank.
The gearbox has a drive plate similar to the flexplate.
They (the flexplate and the drive plate) simply bolt together.

Crank shaft turns --> Flexplate / Driveplate turns --> Prop turns. :)

It really is as simple and straightforward as that.

Given that bit of information, I'm sure that if you scroll through the galleries,
you can find and see enough pitctures of them to make sense of how they operate.

And as Southern has eluded to, the engine stand would require adjustments if its already built as a DD.
If you are shopping, it's likely just as well to look for such an arrangement out of the gate. The upgrade is
not that difficult to perform, but is probably not as cost effective as buying one if you can find one you like.
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Re: Difference between direct drive and reduction gear?

Postby southern safaris » Wed Aug 14, 2013 5:17 pm

Deano.. I have to say you have the most well thought un-biased , clearest and detailed answers I've seen. You are a great asset to this site amist all the jokes and sarcasm and fun that's had I have to give credit where credit is due , no matter how many times the same question is asked or how redundant things may get still the same time and helpful insight every time :thumbleft:
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Re: Difference between direct drive and reduction gear?

Postby :) » Wed Aug 14, 2013 5:53 pm

I agree with buy what u want to start with.. gear box isnt hard to add though. Just extend engine stand out to it.. if u go gear u will still spin prop to the left as looking from back of boat, which is what direct drives spin. The prop on the direct drive would not be enough either way. Gear box changes rotation to right.. so if u bought that boat u would spend another 4-6 grand plus rigging adjustments to get what u really want.
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Re: Difference between direct drive and reduction gear?

Postby Deano » Wed Aug 14, 2013 6:01 pm

:) wrote:I agree with buy what u want to start with.. gear box isnt hard to add though. Just extend engine stand out to it.. if u go belt drive u will still spin prop to the left as looking from back of boat, which is what direct drives spin. The prop on the direct drive would not be enough either way. Gear box changes rotation to right.. so if u bought that boat u would spend another 4-6 grand plus rigging adjustments to get what u really want.

This is true, the DD prop not enough either way. Just easier to buy what your after, IF you can find it.

I agree that if you know what you want and can't find it, the upgrade can be done without a lot of grief although there will be an addtional expense, given the box and prop. That's the only reason I would look for one first.
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Re: Difference between direct drive and reduction gear?

Postby :) » Wed Aug 14, 2013 6:11 pm

What r trying to do with an airboat? What r u gonna use it fir mainly?
14 ft Combee. 4 seater, BPE 383 sb chevy, 2.09 stinger, 3 blade ww stump puller.
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Re: Difference between direct drive and reduction gear?

Postby muffled500hp » Wed Aug 14, 2013 6:38 pm

Night and Day difference! Apples to Oranges comparison.
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Re: Difference between direct drive and reduction gear?

Postby Cowboy Rock » Wed Aug 14, 2013 11:39 pm

DEANO for PRESIDENT 2016


You always have good answers,,, Ya' got my vote,,,


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Re: Difference between direct drive and reduction gear?

Postby RedfireSVT » Thu Aug 15, 2013 12:30 pm

:) wrote:What r trying to do with an airboat? What r u gonna use it fir mainly?

i'm going to be running shallow rivers with a lot of gravel bars so depending on water levels i may have to go over areas of just inches of water. i want to be able to get past these spots, it's almost like going on dry ground in the sense that the boat will be on the bottom although technically there will still be water if that helps. i have a jet drive boat that runs very shallow but i can't get past some of these spots especially if there are weeds. so the next option is an airboat but i don't want to get one and find out it can't go any further than my jet drive. the boat is basically new and the price is right ($9500 is what he is asking) so i guess i could upgrade it later if necessary. i'm just not sure if i should go that route or pay more now for the set up i really want.
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Re: Difference between direct drive and reduction gear?

Postby Arnaudd » Fri Mar 31, 2017 1:15 am

Hello! Whith 250 Chevy 6 inline: dd or réduction?
Thank

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Re: Difference between direct drive and reduction gear?

Postby air-roberts » Fri Apr 14, 2017 9:06 am

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Re: Difference between direct drive and reduction gear?

Postby SWAMPHUNTER45 » Sun Apr 16, 2017 9:16 am

Arnaudd it really depends on what you are looking for and how big a rig your gonna push. Up around Cedar Key there are several fellows who run inline 6 Ford 300 in DD set ups and for their purpose it serves them very well. There boats are in the 13ft range and they are rigged on the lighter side most with EMT. They are not hill burners but they are careful with where they run and happy with their boats.

Dollar for dollar the cheapest path to good power is a small block Chevy on a gear drive. Parts are cheap and you can find them everywhere. A very good small block can be home built for a very fair price and every machine shop is familiar with them. If you reach out to OneEyeGator he is proficient with the salvage yard LS platform which is a way to get a reliable performance engine on the cheap. A 5.3 LS on a used gear drive would be a budget powerhouse if your mechanically inclined and able to spend a little.

Having never ran a 250 6 cyl I would expect it to need a very light hull and rigging to perform DD. It is not an optium engine but can meet a need but I don't know how long it would live on a gear drive airboat. An airboat finds an engines weak spots very fast.

Hope this helps

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Re: Difference between direct drive and reduction gear?

Postby Prototype » Thu Apr 20, 2017 9:34 pm

Deano wrote:
:) wrote:I agree with buy what u want to start with.. gear box isnt hard to add though. Just extend engine stand out to it.. if u go belt drive u will still spin prop to the left as looking from back of boat, which is what direct drives spin. The prop on the direct drive would not be enough either way. Gear box changes rotation to right.. so if u bought that boat u would spend another 4-6 grand plus rigging adjustments to get what u really want.

This is true, the DD prop not enough either way. Just easier to buy what your after, IF you can find it.

I agree that if you know what you want and can't find it, the upgrade can be done without a lot of grief although there will be an addtional expense, given the box and prop. That's the only reason I would look for one first.



Deano,
You da man for finding that from :)

Guess he has his gears confused with belts and maybe had too many belts thinking a gear spins left?


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