Thrust tunnel

[Jasen]

I've searched and searched but with little luck. The little bit of info I have found is really scientific and about useless to me. I'm trying to find some information on thrust tunnels.
This may sound goofy, maybe it is, maybe it's a waste of time, but I'm planning on building a peddel power boat and wanted to fit a thrust tunnel to the drive unit. The drive unit is actualy like a drive unit off an outboard motor and will also be the rudder.
I'm just not sure about placement in relation to the prop. and length of tunnel, prop. clearance, etc.

Thanx,
Jasen

[tom kane]

Perhaps a Kort nozzle is what you are after,they are fitted to outboards as well as large craft.They are used for saftey on small boats.

[marshmat]

A few of the lake freighters around here have Kort nozzles; they boost the efficiency of a large prop considerably but these boats have props maybe 240" dia. On outboards they're more to keep prop and solid stuff (logs, people) apart.

Generally, such gadgets are fitted as close to the prop as possible without letting anything touch. They usually completely enclose the prop when viewed form the side. Other than that, you might find some books or drawings but it's mostly trial and adjustment. Given the patience to work the math I would think you could get a decent approximation... but for your pedal boat, I think you'd be easier going with a plain large-diameter, low-pitch prop.

[Jasen]

I helped install one on a tug many years ago and just thought it might work on a small prop. I can see your point Marshmat.
Just trying to squeeze every oz. of thrust out of each crank of the peddel.
Sounds like it might be more work/math/experimentation then it's worth.

Thanx,
Jasen

[tom kane]

Fitting a kort nozzle can give the advantage of a larger prop with out the extra weight of a larger prop and shaft etc.,

[marshmat]

That's one of the cleanest outboard-engine nozzles I've seen, Tom (a lot of the ones I've seen look rather crudely built).... just curious, how did it perform compared to the plain outboard?

For a displacement hull like a pedalboat it could be beneficial, perhaps someone here has experience with these things (?). I'd think that on plane, though, you'd want an unobstructed flow to the prop, so perhaps it's a low-speed thing?

[Jasen]

Tom, Thats exactly what I was looking for. Guess I used the wrong terminology.

With the design of my drive unit it really wouldn't be all that hard to experiment with it. Might just give it a try.

[tom kane]

The Kort nozzle is a local make here in New Zealand and fitted to a classic (about 18 HP) 1960 Anzani Electric start dual rotation outboard motor,to protect the propeller as replacement would be difficult.It is usually fitted to a light aluminium 12 foot boat and performance change is not really noticeable.No speedo.Any added appendages must effect top speed.Kort nozzles are normally associated to lower speed application.

[Jasen]

(Kort nozzles are normally associated to lower speed application.)
Thats me....LOL