Hello

[Ian Brooks]

Hello all

I happened across this forum and found some interesting stuff here. My interest is hovercraft, but of course we share some similar technologies in our craft. I was particularly interested in airboat props, and wondering if they would have any advantages for us on hovercraft?

We use a lot of 35hp briggs engines - my own craft has one for thrust (separate 13hp briggs for lift), coupled to a multiwing 7w fan of 60" diameter. Ducts of this size are pretty troublesome though, so I am wondering what prop might be suitable.

Noise is a key issue for us in the hovercraft world too - so if anyone has data on noise & thrust for any engine / prop combinations I would be interested to hear it. My own Briggs 35 / 60" 3 blade 7w fan / 2.8:1 reduction drive gives about 195lb thrust and about 80dB noise at 25m (~80ft)

Ian

[Duane Scarborough]

Ian,

Welcome to the forum. A hover craft is essentially a mini airboat with a much more complicated hull, right?

I've thought about using a ducted fan like many hovercrafts use on a mini airboat. My understanding is that a ducted fan is a little more efficient thrust wise than an open prop, and a good bit quieter as well.

I've never seen a side by side comparison though. Maybe you have something to share on that?

Duane

[Ian Brooks]

Ian,
Welcome to the forum. A hover craft is essentially a mini airboat with a much more complicated hull, right?

Pretty much.... bit more complicated and bit more squishy!

I've thought about using a ducted fan like many hovercrafts use on a mini airboat. My understanding is that a ducted fan is a little more efficient thrust wise than an open prop, and a good bit quieter as well.

I've never seen a side by side comparison though. Maybe you have something to share

UK hovercraft traditionally use small ducted fans, but this is mainly driven by small racing craft where they like to keep a low thrust line and don't care too much about noise.

There seem to be a number of things to think about in the duct V prop issue:

Noise: Reducing noise dictates that we need larger diameter fan/prop as this reduces the tip speed which has a big effect on noise. It turns out that small multi-blade fans also create high frequency harmonics that are pretty objectionable, whereas large slow turning 2 or 3 blade fan/props create low frequency noise that people find more acceptable.

Thrust: If you do the maths, it turns out that bigger is better whether its fan or prop size. The bigger it is, the more static thrust we get per HP

Duct length: In theory the duct can provide a useful additional thrust "for free" BUT this only works if the duct is long enough and well designed. For a duct to work, it needs to have a length at least equal to half its diameter. The duct on my craft is about 12" long and 5 ft diameter; in short it does nothing other than look pretty! The effect of fan/prop size tends to outweigh the practical duct effect in all "real" installations.

So it seems that the best thing, for noise and thrust, is the largest possible fan/prop, BUT once the duct gets over about 45", it is no longer possible to make a reasonable duct that can be fitted to say a 14' craft. For example, a 5' duct should be at least 3' long, and this 5'x3' "sail" is going to cause huge problems in any kind of a wind. When I first built my craft, the duct was 16" long... in any tailwind over about 20mph, the craft would just "shuttlecock" (turn round to face the wind) and was uncontrollable! The duct had to be reduced to 12" long, and I had to fit much more effective rudders to make the craft workable. And now that the duct is 12" long, it simple does not work, so I might as well have used a prop and saved all that hard work...

So in short, if one is limited to a thrust device under about 45" then a ducted fan could be useful and there may be some benefit from the duct effect if it is well designed. However, if the device is to be over 50" or so, then the duct size becomes a limiting factor and the better solution may well be an open prop, especially as by the time the duct itself is taken anto account, a given craft will accomodate a larger open prop than ducted fan.

My feelings are that the airboat world probably has it about right in using the largest, wide blade prop that fits, and I'm considering fitting one to my craft, but would like to get a feel for the issues/performance before I jump in.


Ian

[Duane Scarborough]

Ian,

Wow ! I didn't know about the length to diameter ratio. But it sure makes sense. We learn something every day. A 67 "prop would need a substantial duct, huh?

In the prop world, large diameter, high pitch, and slow speed does seem to be the way to go.

Thanks !

Duane

[Ian Brooks]

Ian,

In the prop world, large diameter, high pitch, and slow speed does seem to be the way to go.

Duane

I think so.

What props do you guys use in the 30-40 hp range? I have an ambition to replace my ineffective ducted fan with a prop, I can accomodate up to about 62". Its diven by a Briggs 35, and "quiet" is important. What thrust might such a set up generate?

Ian

[Duane Scarborough]

Ian,

I've got a 67" Whirlwind Mini Prop. That's their standard 65" prop, with an extended hub, which makes it 67". They also make it in a 63" version. That's got the standard hub, but they cut 1" off of the end of each blade. They "MAY" be able to make a 62" version. You'd need to contact them to find out. They also make a 3 blade version, for higher HP applications.

http://www.whirlwindpropellers.com/a...duct_mini.html

Of course there are many more options out there. The point is that all prop manufacturers have something to offer that you won't see on their website. Spend some time looking at what they offer, then call them to see if they can help you. All that costs is a phone call, or e-mail inquiry.

Search IVO prop, Warp drive, Ultra prop, Arrow Prop, etc. There are several more, but those are the ones that come to mind.

Duane