DIY tunnel drive

opps!!!! i mean I DID NOT want to cut out for tunnel for that reason you said about weakening the bottom

 

With a shrouded prop under the boat shallow water operation is limited to the prop diameter plus shroud and clearance, say at least 18" for a 14" prop. The angle of the boat changes a bit because of the diesel and gearbox, so I estimate the shroud will touch the seabed in a little over 3 ft. of water.
If that isn't shallow enough you have to put the prop behind the transom and build something over it to keep the air out. With my tunnel drive the prop protrudes approx. 4" under the bottom, but my experiments show that the tunnel length must be between 3 and 4". If you are an experienced metal worker you could make a stainless extension 2" long, flanged to the transom and a small cutout in the bottom, between 1 and 2" long gently sloping downward. On top of the sloped part you can weld flat strips to connect the two beams that must be cut to make room for the tunnel.

 

Lovely work,haw much was the cost to fabricate per 1 propshaft..

 

Seems you are beyond the stage where you can make any changes to the tunnel but on a boat I built which is micro drafting (runs in 2-3 inches) the tunnel is about 54" long and starts wider forward tapering and rising aft above transom by about 6 inches. It's for outboards, the props run above water surface and have an extra antiventilation plate which extends down around prop ...but some of the principals are probably similar, you need length of tunnel to fill and taper to pressurize and on the jacked outboards we use heavily cupped props that don't slip at all even in turbulent air mixed water.

 

In my experience I have seen only two ventilated tunnel type.

On Hatteras boats it is used behind the propeller to reduce the propeller pressure pulsation on the hull (see the enclosed picture) the ventilation is on the side down on the water except during the hump when the boat need high thrust.

Other use was for surface or cupped propeller see the following patent Power Vent Technologies, Inc a normal propeller can have reduction of thrust or vibrations because of suction side intermittent ventilations.

 

CDK many thanks for posting your design; it looks great. I was wondering if you might be willing to share some part numbers that you used. I've been reading the Parker rotary seal design guide and I suspect that a great many of their parts would be fine but given that you've had success with your ideas I'd love to know more. Mostly interested in the seals and bearings so I can look up the part numbers and get an idea of the specs. Fellow engineer here, but EE not ME so I have to try harder and think more.