Direct Drive vs. Other Drives

Airboats with smaller powerplants.
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TCL Customs
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Direct Drive vs. Other Drives

Post by TCL Customs » Wed Jul 18, 2007 9:57 am

Howdy guys/ladys whoever is browsing im new to the forum and ive been thinking about building myself a mini airboat i fish alot in shallow water and we have trouble getting to our fishing spot with the trusty ol' mercury she's hittin bottem to much and hanging the boat up... i already have an alluminum jon boat hull i think its 14' x 4' or close and im gona get a small motor like between 15-20hp and i see some people are useing direct drive and some are talking about reductions and such i wanted to know wat the purpose is in the diffrent dirve and wat type would be best for me to use?.....
Anybody know where i can get a good goat saddle cheap?

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goldhunter_2
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Post by goldhunter_2 » Wed Jul 18, 2007 10:10 am

well the direct drive is just that and you can only swing so big of prop but a reduction unit allows you to swing a larger prop with that same engine

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Des Moines Boater
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Post by Des Moines Boater » Wed Jul 18, 2007 10:57 am

Most small engines make their full horse power at 3600 rpms. Wood propellers are most efficient at 2700 to 3000 rpms. If you use a small prop to get the engine to turn up to 3600 rpms the prop will probably come apart. I had a 60 horse VW direct drive and probably made 45 horse with the prop I had and it worked fine. If I had a reduction unit I could have used a larger prop and turned the engine up to get the full 60 horse power.

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Post by NorthWestFloridaBoy » Wed Jul 18, 2007 12:08 pm

Des Moines Boater wrote:Most small engines make their full horse power at 3600 rpms. Wood propellers are most efficient at 2700 to 3000 rpms. If you use a small prop to get the engine to turn up to 3600 rpms the prop will probably come apart. I had a 60 horse VW direct drive and probably made 45 horse with the prop I had and it worked fine. If I had a reduction unit I could have used a larger prop and turned the engine up to get the full 60 horse power.


Not all of that is entirely true. One factor used to detrmine a propellers max speed is its length. a shorter prop can be spun faster than a long prop. On my mini direct drive my 42" prop spins up to 4500 rpm with out a problem. All props are more effecient the slower they are spun, when you figure the amount of energy required to turn the propeller versus the thrust put out the slower the the more effecient. that doesn't mean you'll get the amount of thrust you need to move your boat though. so the faster you spin a propeller the less effecient it becomes at transforming the rotational force into forward thrust. I built a belt driven rc airboat with a chainsaw engine and that 18 inch propeller spins in the neighbor hood of 10,000 rpm. TCL customs- I say talk to a propeller manufacturer about your proposed setup when it comes time to pick a prop, and build the boat however you want.

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airduds
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Post by airduds » Wed Jul 18, 2007 2:44 pm

How long is your prop? In general, you want the tip speed to be in the .75 - .8 mach range (600 or so mph) for maximum push, so for a 72" prop that's about 2,900 rpm, give or take.

Now, figure out the max rpm you want to turn your engine (usually based on it's peak hp) and divide that speed by the prop speed.

For example, say you want to run a 78" prop behind an engine that makes its peak hp at 5,600 rpm. Doing the math says you want to turn the prop ~2,700 rpm. Divide 5600 by 2,700 and you come up with an ideal ratio of a little over 2:1.

Also, you have to pick a prop with the right amount of pitch to let your engine turn it at its peak rpm.

So, length determines rpm you want to turn the prop, engine rpm divided by prop rpm determines your ratio, and the amount of pitch you can run (based on length) is determined by the amount of horsepower you have.
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Post by mitch » Wed Jul 18, 2007 6:59 pm

in simple terms as long as you stay in water a direct dr is ok,if you want have a hot rod that will run dry ground you want a reduction unit.there is a big difference with a reduction unit the diff is like a 4x4 truck compared to a 2 wheel dr,a big diff.i have a 383 stroker direct drive with about 450 pds torque,my old boat a standard 350 with a reduction would blow my new boat away!!!yes i will be putting on a reduction unit

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Post by mudkat » Wed Jul 18, 2007 7:46 pm

I have a 20 hp briggs and a 38" 3 blade composite prop ,that dia is perfect for the 3600 rpm they are rated for. I had tried a reduction and a 60" 2 wide blade on that motor and it didn't push any better than the 38" did ,I really think it pushed less.
I'll tell you right now with a 20 horse on that boat you won't be very happy with it.it might plane with one person in it but no more than that.and you will most likely have to get into very shallow water to get it to do that.

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Post by jimmyinpitts » Wed Jul 18, 2007 10:36 pm

4' wide jon boat? Air Power? Gonna wet you'r butt for sure!!! Do us all a favor and get someone to video that for us!! , By the way, the crabs (head lice) will fall off in salt water!! Good luck with your new girlfriend!!!
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Post by TCL Customs » Thu Jul 19, 2007 9:05 am

Posted: Wed Jul 18, 2007 10:36 pm Post subject:

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4' wide jon boat? Air Power? Gonna wet you'r butt for sure!!! Do us all a favor and get someone to video that for us!! , By the way, the crabs (head lice) will fall off in salt water!! Good luck with your new girlfriend!!!



what you talkin about u think it would flip or something? tell me this kinda stuff.. i just want to build myself a small airboat to get into nich fishing spots with shallow water..where a conventional outboard wont get me because of the water depth.. im not trying to race anybody or drive over sand bars...
Anybody know where i can get a good goat saddle cheap?

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Post by NorthWestFloridaBoy » Thu Jul 19, 2007 9:14 am

It ought to do just fine for what you want.

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goldhunter_2
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Post by goldhunter_2 » Thu Jul 19, 2007 11:34 am

TCL Customs wrote:Posted: Wed Jul 18, 2007 10:36 pm Post subject:

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4' wide jon boat? Air Power? Gonna wet you'r butt for sure!!! Do us all a favor and get someone to video that for us!! , By the way, the crabs (head lice) will fall off in salt water!! Good luck with your new girlfriend!!!



what you talkin about u think it would flip or something? tell me this kinda stuff.. i just want to build myself a small airboat to get into nich fishing spots with shallow water..where a conventional outboard wont get me because of the water depth.. im not trying to race anybody or drive over sand bars...


a wider boat hull would float in less water
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papee
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Post by papee » Thu Jul 19, 2007 6:07 pm

goldhunter_2 wrote:
a wider boat hull would float in less water


Yep, I have a low hp setup, mine is 5x16. Mine is not a Jon Boat, it is a Home built aluminum hull. I have had low hp john boats and they will work. As everyone has said, they are not as stable as a flat bottom or wider boat. I can move in 2 inches of water off plane although I do rub on the bottom. If using a Jon boat just make sure to keep all your seating weight low and if you get any speed out of it don't make any quick turns.
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JAMES
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Post by JAMES » Wed Aug 22, 2007 10:43 pm

MY DIRECT DRIVE WELL GO ALL OVER THE LAKE OR HILL 350 WITH 72/36 PADDEL PROP

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goldhunter_2
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Post by goldhunter_2 » Wed Aug 22, 2007 11:12 pm

james ,
First off , welcome to the forum.

there talking about minis build out of wood/glass or aluminum john boat conversations and engines ranging for 8hp to 25hp
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Post by dantheairboatman » Thu Aug 23, 2007 7:49 am

North west i can't take you serious with your new pic. :lol:
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