how do tunnel (drive) hulls work?

Not catamaran tunnel hulls, but the tunnels on jon boats for running in shallow water. What pulls the water up above the surface? The outboard prop sits well back from the hull, so I don't see it working like a pump. Seems to me that if you just towed the boat or set the outboard off to the side of the tunnel the tunnel would still fill with water. Is this correct?

At this point I'm guessing it has to do with surface tension and the water wanting to run along the surface of the hull even as it slopes up from the surface of the water.

 

A realted comment of my experience. Back in the 50's I worked on a Penn-yan tullel hull boat.
It was difficult to steer. It just didnt want to turn sharply and small changed required a lot of time and Rudder.
It did go straight very well though.

I'd like to see your drawings or photos of a Jon boat with the Tunnel.

 

try:

http://www.iboats.com/Alumacraft_Boats_mv_1546_aw_dd_tunnel/nb/moi109053

http://www.iboats.com/G3_Boats_1860cct_dlx/nb/moi102884

 

thanks, a really uninformative and bugging site!

 

he asked for a pic, and that's what I found. What's wrong with that?

 

Seems like they didnt want to give away any construction secrets by showing photos of the tunnel eh?

This thing I worked on was huge. About 28-30 feet, made of wood. The bottom timbers were 3 or 4 inches thick. It had a Six Cylindere Diesel. It was all work deck with just a cuddy with a windshield and another helm at the stern.

I'd bet the Jon boats would have enough weight to get down below the top of the Tunnel. It looks like the theory is to run on the outsides of the tunnel like a pair of sponsons. It probably goes pretty fast, but I just dont see the usefulness of that tunnel.

 

I had a hand in helping develop this vessel:

http://sail2live.com/images/boat.htm

It has no problems turning quickly in shallow water.

This is the brand:

http://www.explorebeavertail.com/212jet.html

 

Each link had a pic of the tunnel. May not be enough to design by, but it gives the idea.

As I understand it these boats are slower with a tunnel than without. The reason for the tunnel is to run in shallow water. I haven't seen one run but I thought the tunnel would be above water level when up on plane.

 

I was amazed looking at your photos. I'd never seen an Aluminum boat under construction before that one. I was immediatly curious about bending the boat parts into shape for welding.

Dreamer c'mon back with the Answer. Was that Tunnel designed to let the boat run in shallow water..specifically?
Or was the idea to funnel the water to the thruster?
Or was it designed for a Jet drive on the foot of an outboard to sit at the output of the Tunnel?

The photos I saw had a Squared portal to the Tunnel.
So you'd have Vorteces and Hysteresis to deal with right outside that tunnel not to mention the arguments going on inside the tunnel.
By the time that flume of water hit the prop of the outboard, the water would be less dense (full of bubbles and foam) and of less use than a prop down below the boat.

I'm curious of what the Sales value of that tunnel was based on.

 

yes.

There is a video out there showing these boats shooting up streams (more like rapids) and skimming across water that's only inches deep. In fact, at speed these things are designed to actually 'skip' across the ground. I don't think they're too concerned about the inefficiencies of the design.

I'll see if I can dig up the video. It's from one of their competitors.

 

Thanks, did they use a Jet drive on your boat?
Was the Jet down in that tunnel or just behind the tunnel?

I did fish in a three lake chain where each lake was a few feet different in elevation. That tunnel would be just the thing for going back up that elevation.

 

Ahhh, tunnels...

I am running a boat with a tunnel, or, more correctly, pocket tunnel. The boat itself is a mod-v flat bottom aluminum center-console jon boat, about 24.5 feet (26 feet if you include float pods, or sponsons, on back of transom) long with 60-inch bottom. Tunnel is about four feet long, fourteen inches wide, six inches deep. Motor is Merc 115hp four-stroke. Boat floats in eight inches of water with motor tipped up. With motor down and jackplate in full up position, it will jump on plane in 12 inches of water and run in six inches of water for short periods, even while carrying 1000lbs of weight.

The trick to tunnel hulls is in the propeller. The propeller blades need a lot of area to deal with the aerated water. This is why a typical four-blade propeller make a better tunnel prop than a typical three-blade prop. Better yet is a prop with long, swept blades, often called long-eared props. Even better is a prop with a LOT of cupping. The prop I run is a 3-blade Baumann prop with heavy cupping and performs much better than the Merc Trophy four-blade that I originally had on the boat.

I highly recommend the floatation pods (sponsons) on a tunnel boat. They make up for the lost buoyancy created by the tunnel and help to float the boat level (important in shallow areas) and get the boat on plane quicker (again, important in shallow areas).

Adding a tunnel to a hull will generally cost you about 3-5mph in top speed. Adding a vent (small hole at the leading edge of the tunnel) will get most of that speed back. This works because a tunnel basically creates a vacuum underneath the boat. Adding the vent breaks the vacuum and "releases" the boat from the water's suction underneath.

Back to the prop: With the heavy cupping, the prop does act as a suction pump to pull water through the tunnel. If I hit WOT from a standstill, the prop spins up quickly without the boat moving much, then water is pulled through the tunnel and the boat takes off quickly and jumps up on plane, usually within a boat length.

Another issue with these types of boats is their relatively poor handling. Turning can be slower, and reverse thrust is minimal compared to a non-tunnel hull. The lack of reverse thrust may be a product of the tunnel, but is also likely due to the heavy cupping used on a good tunnel prop.

So, yes, for the most part, these "pocket" tunnels you see in jon boats are designed to allow the boat to run in shallow water. These boats will usually run shallower than they will float in, so you can get yourself into trouble if you're not careful.

As for tunnel design, many say the best tunnel design is a half-pipe, but you rarely see those in production boats, except maybe some of the Texas flats boats that are built in fiberglass. Some tunnel designers say the tunnel should be around one-third of the boat length. If the tunnel is too short, it will not prime properly and you get too much air in the water. If it is too long, you start to get air coming in from under the front of the boat while it is on plane. The idea is to get the least amount of disturbance going into the tunnel so that the water is less aerated. Propeller tunnels are typically five to six inches in depth, and jet tunnels are typically three inches in depth. Jets will not work on prop tunnels, and props will work but not as efficiently on jet tunnels.

That's about all I can think of off the top of my head. Hydraulic jackplates are a must for best performance. Tunnel boats are a pain to set up properly, but once properly set up, they can be truly amazing. If you regularly run in water less than a foot deep, or you run in turbid rivers where you can see no more than a few inches below the water's surface (like I run in), then a tunnel boat is a good investment. Otherwise, stick to a regular hull.

 

Hey thanks for the input Mr. Idiot , that's good information.

When up on plane the top of the tunnel is above water level, right?

 

Good stuff Mr Idiot.
By your description the Penn-Yann I worked on must have been a Half-pipe. The tunnel was formed just about the length of the boat, shaft drive and the prop was all but completly up into the tunnel, right at the end and the Rudder was just at the end of the tunnel.

It wasnt a hi-speed boat like yours is. The Weight to HP ration on this was probably negative, but it moved with power at 10 Kts or so.

Yours sounds like it would work good in most Alaskan rivers during salmon season, except for the Kenai which has a 35 Hp restriction on it.

 

Good point about the sponsons. If I recall, we ended up adding sponsons to the design above.