airboat engine

would a subaru engine from a 95 legacy be a good choice for a small airboat?

 

Most air boats I have seen in my neck of the woods in FL use bigger (chevvy small block) or out of time aircraft engines.

Small diameter or only modestly geared air props are not very efficient at pushing , so bigger motors are the norm.

Maybe if the airboat is small enough?

There are specific air boat sites that might have simple rulles of thumb.

Whatever you decide PLEASE USE A MUFFLER!!

FF

FF

 

I've heard of the Subaru flat-four motors being used successfully for hovercraft, so there's no reason to suspect it wouldn't work in an airboat. Are you talking about the 2.0, 2.2 or 2.5 L motor?

Keep in mind that even after you marinize it, the thing is still a car engine at heart- thus, it will not appreciate having the throttle left wide-open for hours on end. Don't count on being able to use the full peak horsepower rating that the engine produced in the car.

I concur with Fred that you need a good set of mufflers... now and then, someone tries to build one of these things with a straight-out exhaust. Doing so may gain 1 mph or something, but will also have the neighbours fighting over who gets to take the first potshot at your new toy with a hunting rifle.

 

I have heard that the four-cylinder engines (e.g. - Subaru) run nearly neck-and-neck with the V8 Chevy engines when it comes to airboats. They are a different animal and have different power requirements than a land-based vehicle. Properly propped, a turbo'd Subaru may easily outrun a poorly-propped V8. Weight is a real killer in airboats, and a smaller lighter engine makes sense if it does the job adequately.

Mufflers help the noise of an airboat, but much of the noise also comes from the propeller tips themselves. At full speed, many of them break the sound barrier at the prop tips. Solution would be slower-turning props, which would require greater pitch and thus more power to drive them. The newer four- and five-blade composite props seem to be much quieter. If one had the ability to alter the pitch of the prop, like on a helicopter, power needs would likely be significantly reduced.

Also, traditional airboats are inherently dangerous because they typically have that huge engine mounted high and the driver sits up high, so you have a very high CoG, and the boat can easily capsize. I'd suggest mounting the engine down low in the boat and drive the prop with a belt (becoming more common on modern airboats). Make sure the radiator/cooling is mounted up high where the airflow to the prop can keep it cool.

As an aside, airplane engines make great airboat engines because they produce lotsa torque at low rpms, like a diesel engine, but are still lightweight. But, airplane engines are expensive and expensive to repair, and use expensive fuel. Diesel engines are too heavy. This is why automotive engines are popular in airboats, and most use reduction drives to drive the prop, to get higher torque at lower rpm. You would just need a greater reduction drive for the higher-revving subaru...

 

One thing you might do is to bolt an outboard on the hull and go for a ride.

Each 10 HP will roughly eat a gallon per hour.

See what speed the hull is comfortable with, and find the fuel burn in GPH.

Every HP will push roughly 20 of thrust , so if its 2gph with the outboard that's 20 hp and roughly 400 lbs of thrust.

An air propeller of small diameter will be great IF it produces 5 lbs of thrust per hp.
5 into 400 is 80 , so you would burn 8GPH with the air prop and need an engine with 80 Continuous HP.

ROUGH , but should get you in the balpark.

Speed and weight are expensive!

FF

 

In order to work effectively you need to have a gear reduction or belt drive to reduce the speed of the prop. The prop swept area needs to be about 1 square foot for each 7 hp, and the tip speed needs to be held down below mach 1. What ends up happening is that for about 250hp you need a prop that is around 78 inches in diameter, and the prop speed needs to be about 2900 rpm.

If you use a big V8 you can make that much power at that speed without a gear reduction. Also since the speed is down the engine has a good chance of living for a while. If you are using an engine like a Suburu you need a gearbox or belt drive and the engine is spinning at high rpm's and it isn't going to last very long. The big 460 Ford is popular for airboats for this reason.

Aircraft engines are expensive and light, the big Ford engines are heavier but cheaper, and the Suburu's are light, but it is doubtful that they will last in this kind of use.

 

Although my airboat experience is limited to designing and building the surfboard airboat here: http://www.channel4.com/science/microsites/S/scrapheap2007/challenges_7.html my main interest and experience is with designing light aircraft.

To get good low speed thrust, which is what an airboat really needs to perform well, then you need a big prop geared down to turn relatively slowly, just as the post above suggests. There are reduction drives available to fit the Subaru, as it's a fairly popular home built aircraft engine.

If you want a very light boat, then you could do worse than take a look at some of the better microlight/ultralight engines. The engine on that surfboard scrap creation linked to above was just 46hp, driving a 52" two blade prop via a belt reduction. The power was such that I couldn't use more than about 1/4 throttle, for fear of flipping the thing.............. This is what 1/4 throttle looks like:

Jeremy

 

That is what I was planning. Mount the engine as low as possible to keep the center of mass low and then use belts and pulleys for reduction and turn a big prop. As for the reduction I was planning on using double or triple industrial "B" V belts. How do you mount the prop? Do you have any picture of this? I was thinking of a hub on a shaft supported by 2 pillow block bearings with the double or triple pulley in between, but I don't know if the forward push of the prop/shaft would creat a problem with the pillow bearings. How much push are we talking 100lbs , 1000 lbs...Do you know of any prop suppliers?

 

You can get an off-the-shelf reduction drive with prop hub for the Subaru engine that will save you a lot of work. I've designed a couple of reduction drives, one using a gearbox and one using a polyvee belt, plus I've worked on a few with HTD belts. It's a challenge making a drive reliable, due to the high peak torque pulses from the engine and the propeller inertia. You can easily get all sorts of bizarre resonances that cause things to break (guess how I know that......).

My advice would be to buy a well-proven unit, unless you want to spend hours tinkering and changing things to get good reliability.

Here are some manufacturers of Subaru reduction drives and prop hubs that might be useful:

http://www.maxwellpropulsion.com/

http://www.ramengines.com/id5.html

http://stratus2000.homestead.com/files/stratusnew/ea81descript.htm

http://www.foxcon.com/product.html#topic4

A hunt around will probably throw up a few more manufacturers, this was just a quick search based on those I've heard of.

I'd advise against using plain V belts for a few reasons. Firstly, you will need a lot of them to transmit the required torque, which will cause problems with bearing loads and finding matched sets of belts. Secondly, V belts are not that efficient, they tend to waste a fair bit of power in applications like this, which means they get a bit warm. Thirdly, they slip when they get wet, something that's pretty likely with spray flying about.

If it were me, and I were determined to go down the DIY route, then I'd go for a big HTD toothed belt or maybe a wide polyvee belt, if you can get one that will take around the 80 to 100hp that this engine will deliver. A good source for big belt drives is the homebuilt hovercraft people. They frequently use big HTD belts to drive propulsion fans/props from hull mounted engines. Here's a link to a UK company that stocks pulleys, frames bearings, shafts etc, but I would imagine a bit of Googling may turn up someone similar closer to where you are: http://www.avdw32.dsl.pipex.com/transmission.htm

When it comes to props, you need to make sure you get one with very good leading edge protection, as spray and sand can do a lot of damage. The air boat suppliers are probably the best people to talk to, as they understand the problem and tend to stock props that last well under these conditions.

Good luck with your project.

Jeremy

 

You'll need 6 large V-belts to reliably transmit the power on hand. Reduction drives for propellers are not good novice engineering projects; if you design one yourself your first try will almost certainly be a failure. Real engineers attempting this usually fail out of the chute; why should you expect to fare any better?

Buy a reduction drive already in production and save yourself the aggravation (unless you just thrive on that sort of thing )

When you factor in the cost of the reduction, even a 'free' Subaru engine will not be a free airboat propulsion system.

In the end, it may not be any cheaper than buying a good running (but unairworthy) Lycoming O-320 or O-360 on the used market. Then all the engine's systems will be simple electrical and mechanical (magneto w/points, carburetor induction) with NO advanced electronic whatsoever, thus repairable in the field (even on the water, should that become neccessary )

That's why they remain so popular.

Jimbo