A White Paper on “Direct Drive Automotive Airboats”

A general, non-powerplant specific, discussion on airboat technology, ie., hulls, rigging, polymer, etc..
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A White Paper on “Direct Drive Automotive Airboats”

Postby WaterWalker » Sun May 13, 2012 1:32 pm

May 13, 2012

A White Paper on “Direct Drive Automotive Airboats”

There has lately been several threads on the Southern Airboat Internet Site about GM LS engine powered “Direct Drive” airboat applications with a stated purpose of competing with the Aircraft style Flat Opposed large displacement engines (Say the O-540 Lycoming angle valve 300 hp AT 2800 rpm as an example) for airboat use.

Modern (water cooled) automotive style engines have demonstrative advantages in airboat applications over even “modern aircraft engines”. Aircraft gasoline fueled flat opposed direct drive engines were and still are designed specifically for smaller general aviation airplanes. Automotive gasoline fueled V style multiple piston engines were and still are designed for automotive use (Cars and pickup trucks).

Neither were designed for airboat use and as such each type has been adapted (read shoe horned) into and for airboats. Which is the “best fit” giving the “best performance “at the “lowest cost”? In my experience and knowledge, it is my opinion that the modern automotive style engine is the most practical, fuel efficient, performance delivering choice for airboats of all sizes.

As an inventor of airboat propeller manufacturing processes and designer of performance winning propeller styles, I feel that I am qualified to state that an airboat propeller, that delivers the thrust for an airboat, has no way of knowing from where it receives the utilized HP at the RPMs it needs. The airboat propeller cannot differentiate as to whether its’ received HP is from an aircraft or automotive engine. It only reacts to proper propeller RPM and Torque which together is the HP being used.

Here-to-for automotive engines develop their peak HP at higher RPMs than airboat propellers require and are optimized for. They have had to use belt or gear style reduction units to reduce the delivered RPMs at higher torque values for an optimized HP at the propeller. Likewise, automotive engines always require transmissions for use in automobiles and pickups.

Aircraft style engines develop their peak HP ranges directly at the RPM values that airplane and airboat propellers are optimized at. The aircraft style engines do this by having much larger displacement values with lower HP per cubic inch of displacement than automotive engines. For example, the Lycoming O-540 has 540 cubic inches of displacement and delivers 300 HP @ 2800 RPM for a value of 0.56 HP per cubic inch at these lower RPMS. A GM ZZ 383 stroker engine with 383 cubic inches of displacement develops 425 HP @ 5400 RPM for a value of 1.11 HP per cubic inch at their higher RPMs.

The Lycoming O-540 aircraft style engine weighs about 435 pounds and delivers approximately 300 HP being about 1.45 pounds of engine per HP. The ZZ 383 stroker delivers 425 HP and weighs about 400 Lbs. This computes to about 0.94 pounds of engine per HP delivered. This is quite a weight savings that improves resulting performance with an automotive engine.

In fuel cost considerations, directly the cost of aviation gasoline is about a third more than automotive pump gasoline. Also because of differences in design of water cooled compared to air cooled aircraft engines the specific fuel consumption of automotive engines is better than aircraft engines. (There has been much previously written to explain these differences in fuel consumption elsewhere). So cost of fuel consumption is generally much better for automotive engines compared to aircraft engines for airboats.

In initial cost, the automotive style new engine is roughly only 10 to 15% of the cost of a new aircraft engine. Within the last decade, I purchased a new Continental twin turbo-charged IO 550 350 HP engine for a kit built experimental Lancair IV P airplane. Even 10 years ago, I had to pay about $145.00 per horsepower for that engine. In the same time frame I have purchased several ZZ 383 Stroker crate engines and have had to pay about $13.00 per HP each for them. $145.00/$13.00 equals the new aircraft engine costing eleven times more per HP than a new automotive engine for an airboat.

To avoid the high cost of a new engine, aircraft style airboats typically use professionally rebuilt but otherwise run out or junked aircraft engines that now can be purchased for about $40.00 per HP (a rebuilt Lycoming angle valve engine of 300 HP for about $12,000.00). For a fair consideration, one should compare this rebuilt aircraft engine to a professionally rebuilt junkyard automotive engine at say $500.00 for the engine and say $3,000.00 for the rebuild to receive a say 400 HP Chevy small block engine. So $3,500/400 HP equals about $9.00 per HP for an automotive style engine that would be expected to give equal or better service life on an airboat.

A new ZZ 383 stroker engine at say $6,000.00 and 425 HP would be $14.00 per HP.

Until now, to realize the benefits of an automotive engine for an airboat one had to use a reduction unit. This of course adds some cost but still in my opinion, the numbers still heavily favor automotive for the most practical cost/performance ratio for airboat use.

None of this discussion allows for ethereal considerations like sex appeal, mystics, bragging rights, boat style, “my father had one”, “aircraft boats are better to frog from”, “I like the sound better” and so forth. Some people buy Hummers, some Corvettes and some buy a Suburban. Others buy practical pickup trucks and so it goes. There will likely always be A/C airboats and that’s a good thing for those that will always like them and are willing to “pay the price”.

Now let’s look at the current Direct Drive experiments. I have just reviewed the performance charts of the GM small block V-8s and several of the new LS engines. In at least a dozen of the examples that I looked at the HP rating for each engine clustered at about 220 HP at the maximum desired 2800 RPM that propellers like to spin at with diameters that apply to typical airboat use. Try it yourself. Go to www.crateengines.com and look at the charts for each performance engine that you like and see the HP line at 2800 RPM.

To directly compete with an aircraft engine airboat with say a Lycoming O-540 angle valve engine delivering 300 HP @ 2800 RPM, a motor build mechanic would have to do considerable “magic” with “engine building tricks” to just get even with aircraft in a direct drive application. Remember the limit for comparison is 2800 RPM max. If the LS engine is spun higher, then spin the aircraft higher and it will generate even more HP as well, but the “wear” goes way up also. Generally higher specific performance with an engine equates to lesser service life for that engine at peak performance use.

To abandon the benefits of a reduction unit transmission for automotive engines for an airboat is to pay for performance that you forever “haul around”, but don’t use.

Yet the benefits of building say a 200 to 250 HP performing airboat using direct drive with an automotive engine could be a “good thing”. The engine weight to usable HP would still be about equal to a good aircraft engine. The specific fuel burn would be better than an aircraft style and fuel cost would be less. The engine service life at 2800 RPM and lower would be terrific. Parts and repair would be more reasonably available. And of course the initial cost to build the boat would be much less.

Could a “built up” direct drive LS engine airboat outrun a good Lycoming O-540 engine driven airboat. My bet is likely yes if its’ engine “build” had achieved more than say 300 HP at 2800 RPM. Would it be cost/practical? Will the builder share the total real cost for a customer to replicate such a boat?

What do you think about it?

David Wine
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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby plumcrazy » Sun May 13, 2012 2:32 pm

the (main) problem .... the automotive style engine cannot withstand the forces the prop puts directly against the rotating mass , and also the the enormous difference in size of each combustion stroke or quench area between the two engines, so at 2800 the a/c is the king of tq
in dd. could they be equal. maybe if you had two cylinders fire at once on the small block, and mount a large cone bearing to hold the thrust :idea:
Last edited by plumcrazy on Sun May 13, 2012 3:23 pm, edited 1 time in total.

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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby Bruce » Sun May 13, 2012 2:37 pm

cant go wrong with 18-2000 horse with a 2 speed supercharger
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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby glades cat » Sun May 13, 2012 2:38 pm

A fair assessment Mr. Wine
Many of us started with direct drive SBC's or Caddy's and tried to squeeze every last bit from them.
They provided many years of affordable reliable riding. Mine never left me stranded...only stuck. lol

AV gas is expensive and it seems to go awfully quickly. A casual ride can cost $50 easily.
Aviation piston engines are also remnants of a time when America built stuff that lasted...my Continental is over 50 yrs old. That's difficult to dismiss and neat to show to non-airboaters...an old airplane engine on a boat.
Even though I haven't had to hand-prop my engine in the woods yet, due to a dead battery, it's reassuring to know I still can. It must be worth a couple of ounces in the balance.

I'm keeping an eye on what Chuck at Airboat Pro's is doing with the Ecotec 4 cyl.
That should be a great lightweight power system.
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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby cntry141iq » Sun May 13, 2012 2:52 pm

I have spent a lot of money testing things back in the days not so far gone ... I was sitting in a engine builders shop one evening after watchin him play with a dyno .. and I posed the question to him .. How much hp can be had out of a small block at 3000 rpms ??? he replied .. plenty .. if you have the money .. I can build them to run high or low .. naturally over 400 hp will be pretty hard and very expensive to attain ... I sure wish I had of given that a little more consideration ...
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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby Muddobr » Sun May 13, 2012 3:45 pm

Two words polluted waters

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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby Daddy Dave » Sun May 13, 2012 4:03 pm

All here know you are a strong proponent of SBC's for airboats. When matched with some sort of reduction unit, they are the answer for our newest segment of larger "ride & party airboats". And, as Radtech and others have observed, DD large displacement / low RPM design based Cadi engines on longer airboats out-perform the GPU family of opposed cyl. power sources. That particular family of airboats boats will never operate where most AC boats frequent on any regular basis.

In our practical world, AC engines still provide the most reliable power source for the most popular sized airboats - those up to 14 feet. The ignition systems are redundant and the fuel delivery systems simple. In more than 45 years of continuous ownership, I have only had to rebuild an AC engine once after a sinking. Most folks who run with me will say I'm not easy on equipment.

EVERY SBC that I know of has to be "refershed" at least every couple of years. No matter how efficient modern auto engines are, they all must have a radiator and that is their weakest link. One Kissimmee bean will render the most expensive LS all aluminum wunderbar impotent. Lots of airboaters run to remote and essentially inaccessible corners of forgotten Florida - it's where our manna source resides. Reliable and simple AC mechanical systems have been PROVEN to perform for hundreds upon hundreds of extreme use hours without failure across the spectrum of thousands of different airboats..

IF the Cadi head system were more bullet proof and IF a reliable auto engine based cooling system could be developed, then an automotive based DD airboat powerplant would compete apples for apples at 3K design peak RPM in the real world most of us airboat in. Or, IF a modern aluminum LS auto engine series could be developed that reached peak performance at 3K RPM, then other options would be practical, IF a cooling system that weren't so fragile were developed at the same time.

IMO, "one off" non-stock powerplants for average airboaters are not practical -- regardless of the theoretical engineering advantages. The conversations about this are good reading -- in a theoretical universe.
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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby :) » Sun May 13, 2012 5:13 pm

I don't believe a cm will ever do as good as a ac on a smaller boat, say under 13 to13 1/2ft. After that u will need a cm but u gonna need the reduction. If ur gonna run a smaller boat and have the initial cost, u can't go wrong with a good AC motor. Now Mr. Wine if u design a prop just for a dd ls or sb application where u could achieve even 4000 rpm, well now we got ourselves a ballgame! I know the Ac guys can make more at more Rpms too but not for very long. Some them now r say 3000 Rpms for an allotted take of time. A car motor can run those Rpms all day everyday. I like ur topics, always interesting and educational. That 3000 rpm thing is killing the DDs. They need just a little more and still be noise tolerable. These r jmo,
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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby Whitebear » Sun May 13, 2012 7:57 pm

I look at what engine will get the job done with the least mods when I'm looking at an automotive engine and I just haven't seen a great one yet. The Caddy does better than most. Before it became made of unobtanium, the old Buick 455 CID engine was king of the DD hill.

Even the Caddy wears hard on the thrust bearing as a DD engine but the Buick did not. In DD configuration the Buick was easy to modify for DD Airboats if you knew what to do. It was 4 simple mods, that anyone with a wrench could make. Now this does not extend to using the Buick as a re-drive engine because of its weak oiling system for high RPM, but under 3K RPM its would last with all the aircraft engines. Sadly the engine is to a greater degree, simply not available any more. It was slightly lighter than the Caddy, definitely a smaller physical package.

However it still suffered from a weight disadvantage in smaller hulls. While I have put them on a 10x7 glass hull I personally felt a 12x7 or 13x7 was the best hull for the engine but was also not ideal for the weight. Done right the Buick had a propensity to cool off when idling at frogging speeds so periodically ya needed to pump it up and get some heat back in it ro it would stall on acceleration like nearly any COLD engine will.

I still think the 4 cylinder 100-200 HP AC engines are the all around best for the smaller hulls. 10-13 feet, i.e.. 12' x 7'. I also am watching the current development of the Echo Tec engines for smaller hulls, with an eye on final price tag installed ready to run. I further have not followed closely Daves new LS DD engine as close as I would like. Maybe some info is still not out yet on it but it seems to be coming along fine with the mods for the thrust plate.

Likely one thing will continue in airboats and that is that there never will be a best engine for everyone for any boat. Demands and expectations are so vastly different between operators I just can't see this happening at all. Which indeed leaves room for lots of products with a wide diversity of appeal and applications.
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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby dusty burch » Mon May 14, 2012 10:39 am

sound like a bunch mumbo jumbo I am tired about hearing about a damn car motor if u want to run one get one and be happy but real airboats still have aircraft motors

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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby sidejobs » Mon May 14, 2012 12:19 pm

DD caddy 4 Life ! 8)
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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby WaterWalker » Mon May 14, 2012 5:08 pm

BigDaddy:

You have “walked me back” a bit about aircraft engines. Of course, as I said, “there will likely always be aircraft engines on airboats”. My first airboat was a commissioned aluminum 8’ X 16’ Combee built boat on an Alumitec hull with a Lycoming O-540 angle valve engine and a wood prop. Also, I have owned several general aviation airplanes with “flat engines”. They are good engines. Its’ just that they burn a lot of expensive Av gas and expensive Av oil.

The rest of the airboats that I have commissioned or personally built over the years (about a dozen) have had automotive engines because of the benefits that I have described in this post. The “Concept Boat” that “Plumcrazy” is currently putting time on has its’ twin small radiators placed inside the decked over hull in a bulkhead. The incoming air is screened to protect from such as your Kissimmee River Bean and other trash. There is a lip behind the prop that produces a Ventura effect to draw air through the hull and thus through the radiator.

I ask you to read the recent post about (AV gas $$$$$ ) that has comments that bear out the reasons that new airboats are increasingly using modern automotive engines with reduction units.

The body of my post was to state that these modern automotive engines as factory stock engines do not produce the needed HP (RPM X Torque / 5252) at about 2800 RPMs, and as such, will not make good direct drive power plants for airboats without each one being highly modified. This brings me again into agreement with you. “One off” highly modified engines for airboats are not practical or affordable things.

Anyhow, “Long live the aircraft engines”. They will have to live long because fewer of them are being built originally for airplanes so that they can later find their way as re-built run-out engines for airboats.

Lucky are the ones that own a good aircraft engine and a good prop to go with it. Two of our very successful propeller models, The Cypress and the Signature blades were designed for the power/RPM ranges of aircraft engines. There are many thousands of them out there doing well.

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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby BHB » Mon May 14, 2012 5:41 pm

I believe the secret to the DD cm is a prop that is able to spin 4500-5500. If something like that is ever developed or even possible the aircraft motor will be a thing of the past but until then I'll stick with my tried and true lycoming.
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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby poobought » Mon May 14, 2012 5:42 pm

dusty burch wrote:sound like a bunch mumbo jumbo I am tired about hearing about a damn car motor if u want to run one get one and be happy but real airboats still have aircraft motors


Then keep your ass in the AC only section you XXXXXXX XXXXX.

Mod Edit: NO NAME CALLING.

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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby todd glover » Mon May 14, 2012 5:48 pm

Even if you can make an ls motor turn the same prop as a 540 av you first have to make it cost less and it has to be complete prop adapter radiator ready to run same way you buy a 540 after thats done now you have to find a market most people want something you can resale later a 520 won't sell as fast as a 540 will so it better run alot better or be alot cheaper or nobody is buying it car motor gearbox great car motor no gearbox same prop 540 please

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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby Daddy Dave » Mon May 14, 2012 6:40 pm

For your typical 15' x 8' ride & party boats, nothing beats a CM / reduction, Most of those airboats are never used the way traditional AC powered hunt boat rigs are. FSU Rich and RUAGATR are two probable exceptions. Your "Concept" airboat has lots of unique features as I have seen first hand and Plumcrazy is one fine test pilot. But Plum damn sure isn't driving "Concept" following AC boats through Gardners and St. Johns or Citrus / Sumpter as they hunt off the beaten paths. A couple of years of that will let folks know how the power systems compare in similar applications over a reasonable real world hard use extended time span.

If a CM / Reduction powered rig were available that did not sound like two metal skeletons in an orgy at idle, I would have that system on my gator hunting airboat. I'd love to buy cheap fuel for my business airboat. But, mature alligators are critters who have mastered survival and they quickly drop from sight as reduction rattling approaches and I can't seem to find the stomach to suggest rotten road kill to my clients as gator bait.

I had really hoped the "new" belt drive was going to offer the answer, but it too seems to require the attention of an expert technitian to function without premature unbeltulation. Not really a practical solution in my airboating world.

So, I'll either have to find a means to afford AV gas or, I'll have to find a way to utilize a 4 stroke outboard to power my gator guiding boat. The price of an efficient auto powerplant, propeller, reduction and the sounds and vibrations they create at idle and under load as a power source currently prevent that option as being the best for my personal application.

Different strokes for different folks.
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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby dusty burch » Mon May 14, 2012 9:37 pm

poobought wrote:
dusty burch wrote:sound like a bunch mumbo jumbo I am tired about hearing about a damn car motor if u want to run one get one and be happy but real airboats still have aircraft motors


Then keep your ass in the AC only section you XXXXXXX XXXXX

Then u keep ur ass in the automotive engine section (DELETED TEXT)

Mod Edit: Edited and deleted parts to conform to SA Rules.

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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby Whitebear » Tue May 15, 2012 12:27 am

Any more rules violations and I'm deleting posts with no notice.
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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby poobought » Tue May 15, 2012 7:08 am

dusty burch wrote:
poobought wrote:
dusty burch wrote:sound like a bunch mumbo jumbo I am tired about hearing about a damn car motor if u want to run one get one and be happy but real airboats still have aircraft motors


Then keep your ass in the AC only section you XXXXXXX XXXXX

Then u keep ur ass in the automotive engine section (DELETED TEXT)

Mod Edit: Edited and deleted parts to conform to SA Rules.


I'm not the one complaining about the thread.

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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby Southern Style » Tue May 15, 2012 3:08 pm

Very good posts, But a run out A/C is less than a junk yard 350 ( now a gear box, radiator, plumming, bigger prop, and all the extras) with no additional rebuild and work.. I agree, once you pass 13' 6" or 14' hulls, aumotomotive is definately the way to go !!!! They get heavy and need alot more HP. But again, a light weight aluminum hull with a 6 banger aircraft is really hard to beat !!

A/C vs LS DD: I think was settled at Maccassin Island last Thursday...
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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby sidejobs » Tue May 15, 2012 3:17 pm

Southern Style wrote:Very good posts, But a run out A/C is less than a junk yard 350 ( now a gear box, radiator, plumming, bigger prop, and all the extras) with no additional rebuild and work.. I agree, once you pass 13' 6" or 14' hulls, aumotomotive is definately the way to go !!!! They get heavy and need alot more HP. But again, a light weight aluminum hull with a 6 banger aircraft is really hard to beat !!

A/C vs LS DD: I think was settled at Maccassin Island last Thursday...

There is a lot of "trickery" going on out there. How about a real dd challenge ? Real ride boats with some weight in them ? Say your boat vs. Your buddies dd caddy ?
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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby Southern Style » Tue May 15, 2012 3:22 pm

OUCH... Sidejobs............ Who let you in the room !!!!!!!!!!!!! :lol: :lol: :lol: :lol: :lol: ... Scotty is sitting here laughing !!!!!! I am just a A/C guy, and that Caddi is still a beast !!!!!!!! Caddi 500 is a torquing S.O.B. !!!!!!!!! Why has nobody run a boat like your old one (Scotty's) out there? And that is an honest question.. I always hear of a LS, or a 383, or whatever.. I would love to see a 500 Caddi stretch it out...

And when I said "hard to beat".. I meant price wise and weight wise.. I enjoy riding at a low RPM and enjoying the scenery.. No racing for this guy !!!
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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby sidejobs » Tue May 15, 2012 3:46 pm

Lol. Just messing with ya. That boat has outrun gearboxes and 540's. Of equal size. It's def a ride boat.
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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby Southern Style » Tue May 15, 2012 4:06 pm

YEA BABY !!!!!!!!!!!!!!!!!!!! Its STILL a beast !!! Carb jets were gettin' clogged, Spittin and Sputtering but Dusty cleaned them out. Running back up to speed....

I would definately bet that boat has !!!!
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Re: A White Paper on “Direct Drive Automotive Airboats”

Postby Uhairball » Tue May 15, 2012 6:12 pm

So please explain to me why you can't buy a prop that is capable of
turning 4000+ rpm's??


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