Identify inboard boat with prop in tunnel which cannot strike bottom ?

I remember way back running across a boat design that somehow was an inboard
but did not have a problem in shallow water because the prop did
not stick down below the bottom of the boat. I am wondering how
the prop stayed in the water instead of spinning in air ?
I know there was a tunnel under the boat hull but I am certain the water line
on the hull was much lower than the top of the tunnel. Does this design
have a specific name which will allow easier searching ?

 

"Tunnel drive" (for example).

http://www.boatdesign.net/forums/surface-drives/diy-tunnel-drive-20433.html

 

Penn Yan, never saw one that ran right.

 

The Coast Guard had quite a few 22 foot Penn Yans with tunnel drive. They were ok in forward, but would not back up in a straight line, if at all. However they were pretty rugged and could take a beating. We used them to patrol offshore power boat races.

 

From experience: the key word is "nightmare"!

 

Mersey class lifeboat has tunnels, now replaced with Shannon class with jets.

 

The Bluewater coastal cruisers have tunnels,I think the props stick out a few inches.
The bow is lower in the water than the props,so if the bow clears you know the props will.
A regular poster here has one.

 

Bayliner for many years had prop pockets in boats around 35 to 45 feet.
But they found that the tunnels impacted lift and dropped that idea.

 

To make a tunnel(s) big enough to keep a prop hidden, you have chopped out a lot of volume. The hull would really need to be designed starting with the tunnel in mind, to be successful.

 

Correct
Additionally there is a reduction in planing surface as well as a reduction in pressure as the velocity of the water speeds up into the tunnel.
While there are advantages to a tunnel protected propeller, optimization of lift is not one of them

 

I think the boat you are looking for is the Atkin Rescue Minor.

 

http://atkinboatplans.com/Utilities/RescueMinor.html

Robb White's variant:
http://www.robbwhite.com/rescue.minor.html

 

The Atkins tunnel hulls are very limited in top speed. The best ones can do 25 MPH, but most are slower. Tunnel design isn't an easy thing to do right, but prop protection is pretty easy. Maybe it would be simpler if you describe you concerns and see if more conventional methods can address them.

 

Thanks for the reply...
I see that I do not have a need for tunnel hulls but was simply seeking education
and had no idea what their purpose was. .... much in the same way it took me
a while to figure out what a needed log was as relates to inboards.

 

There's always this approach:
http://www.mshs.com/images/Propulsion_Power_Briefs/Big Dipper.jpg