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Snowman

2 Different Subaru boat ideas....

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After following the threads on subie powered boats, I got to thinking about some other ways to do it. In SE Alaska where I live, air boats are extremely popular for getting around in the shallow rivers. I've also heard these craft called "fan boats" because that's basically what it is. You take an airplane engine and prop, and stick it on the back of a small aluminum boat. So I was thinking maybe I could take the EA81 sitting behind my shed, somehow mount a prop to it, stick it on a boat, and go like mad. Anybody tried this or seen others do it?

 

The other idea that came to mind was a high speed catamaran. Picture this.... Two skinny little hulls that are just big enough to keep the thing afloat, joined by a couple of cross braces. Stick an EA-series motor in the middle connected to a high-speed prop, either direct drive or through a transmission so that you could gear down for starting off and run a really big prop. Is this crazy talk?

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I have seen FAN Boats before. There was one in "WATER BOY"

 

The pontoon boat sounds a little scary in rough water. I guess it depends on how high you plan to set the platform.

 

How about a hovercraft?

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Yeah, high speed runs in the cat boat would definitely be restricted to calm days.:rolleyes:

 

 

Anybody here see that episode of Junkyard Wars where they built a powerboat out of fuel drums and dropped in a big old Land Rover diesel? So many ideas, so little time....

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the main thing that needs to be kept in mind is that a marine engine can be tached out all day and be fine. An auto engine is connected to a tranny so that it will reach high speed without being tached out, because it can't handle that kind of a pounding.

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the main thing that needs to be kept in mind is that a marine engine can be tached out all day and be fine. An auto engine is connected to a tranny so that it will reach high speed without being tached out, because it can't handle that kind of a pounding.

That's only sort of true. An automotive engine can generally be safely run at its peak horsepower for any length of time so long as fuel remains available, the oil doesn't break down and the engine stays cool. The problem is that transferring energy to water in order to achieve forward motion is far less efficient than applying the same energy to pavement. So, speed in the water is all about turning the prop faster (for a given blade pitch). Easiest way to do that is to start with a high-revving engine. That's why most outboards are 2-stroke engines and many of them are based on motorcycle engines. There are lots of boats powered by Chevy 350's. Most of them make use of some sort of stern drive or inboard/outboard drive system. Lets them have a gear box to step up prop speed in relation to engine speed. Much like the re-drives used by the aircraft crowd to get a propeller spinning really fast.

 

If you figure a stock EA-81 makes peak power around 5,000 rpm then you can select a prop based on available power, displacement of the craft in question and planing efficiency of the hull. For a given engine speed, the only way to move the boat faster is to use a more aggresive blade pitch. Problem with that is it uses more power and, beyond a certain point, is less efficient due to cavitation. In an air boat, you can get around some of that because you don't have a cavitation problem to deal with.

 

In an air boat, you can simply set a governor to run the engine at a certain speed regardless of load. Then you use a variable pitch propeller to change speed. Wanna stop? Feather ir. Wanna haul rump roast? Max out the pitch. USe an aircraft redrive to get more speed out of the fan.

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dosn't the redrive lower the speed of the prop? cause you don't want the prop tips going faster than Mach, and at 5000 rpm i think they would be going way past that.

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Kelly, I can take you to a place north of Wasilla where they build airboats from ea 81 and 82's. They like the older subbie motors because of their durability but more for their weight.They smoke other boats with a 6 blade prop.

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Kelly, I can take you to a place north of Wasilla where they build airboats from ea 81 and 82's. They like the older subbie motors because of their durability but more for their weight.They smoke other boats with a 6 blade prop.

Heck yeah! I wanna check that out!

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