Design help, making a tunnel

[cyclops]

Something to keep in mind. A true tunnel hull at FULL efficient speed and thrust is definately a jet drive boat. There is NO actual difference. Same dangers when crossing very shallow water. Do you know the 2 biggest dangers?

[tom kane]

The designer of the Hydrodrive tunnel post no 10 (a local boatie) fitted an outboard through the top of a hydrodrive for experemental purposes,none were successful.The inboard version sold well localy and overseas,and was used in ocean racing boats,(two gallons a minute).I tried tunnels in constant deadrise V hull with an outboard fitted behind the tunnel,the plane angle increased, because of planing area loss and the boat did not want to turn,and when it did turn,as the boat rolled into the turn,the edge of the tunnel interfered with the prop.It is a big ask,expecting the water flow to follow the tunnel line at any high apeed.At low speeds they may be ok.The idea of having full water flow over all of the prop at any speed would add to drag,as the top of a prop contributes nothing to thrust,but is just drag.Modern open jets have a means of diverting the water flow to the prop.

[bananas]

Hi Tom

An inch of hard water over the top of the prop causes much less problem than an inch or two of mixed vapor and air over the last few percent of the prop. A fully submerged, highly loaded propeller cannot tolerate ventilation. The 40-Percent or so thrust lost and the prop tip jet damage to the bottom is much worse than a little tunnel drag. I am talking about 1-Inch of clearance on 40-Inches of propeller.

The transition to surface mode is far more difficult than the design of a good tunnel.

The Hydrodrive dances very close to the patents of Don Smith's Powervent. And I suspect that the Powervent performs much better. I have driven it, it runs from 100-Percent submergence to 50-Percent submergence smoothly with just the throttle. And at speeds over 50-MPH.

Ciao

Max

[tom kane]

Rigging Reccomendations for Shallow Draft "Flats" Boats
www.sundanceboats.com may be of some help. The Hydrodrive was a New Zealand 1960 patent NO 126778 a surface propulsion drive. You can not patent a tunnel it has been done before.There may be some new and novel aspects that may be patentable. Many leading American power boat operators dismiss the need to ventilate a surface propulsion as it happens naturaly.

[Firerob]

Was this project ever completed?
I am now doing the same modification to my 19' Carolina skiff.
I used the dimensions for the pocket tunnel that were listed.
20" wide 36" long and height 6" tapered to 5"
Will be glassing the tunnel this week.
Hope this works out.....

[kengrome]

Firerob,

William Atkin and son designed "Tunnel-stern Seabright skiffs" in the 1930's and later. Most of their designs were for inboard power, but I designed one of my own specifically for an outboard engine:





In the inboard designs the two side tunnels come together beneath the aft end of the hull, but in this one they remain separate -- because this makes more sense for outboard power.

Boats with bottoms like these are generally not capable of safe speeds beyond 20 knots, but they are very safe -- and theoretically very fuel efficient at 15 knots or less. Their props are fully protected too of course, since they are located behind the aft end of the "box keel", which usually has a flat bottom. As long as the outboard's propeller bottom is slightly above the box keel's bottom it should never encounter any damage when you're moving forward.

[Firerob]

Wow that is a serious tunnel, it looks like it can run pretty shallow. I like the looks of the pocket tunnel, maybe I wont lose so much displacement. I figured I'll be losing less than 140lbs of displacement with the pocket, can't imagine what I'd lose with a full tunnel.
I hoping to gain seven inches on step with the pocket and jackplate.

[kengrome]

Wow that is a serious tunnel, it looks like it can run pretty shallow.

5-6 inches at design displacement -- and this is the measurement all the way to the bottom of the prop ...

I hoping to gain seven inches on step with the pocket and jackplate.

You're going to change the prop too, correct? Most outboards running on jackplates use differrent props.

I actually have a design for a genuine Texas Scooter type flat's boat with a flush deck and a specially engineered tunnel that requires a surface drive propeller. I designed it for inboard power -- not only so cheaper engines can be used, but also to elminate the jackleg, tilt and trim, and all the other expensive adjustable items on similar boats:





I also wanted to get the engine off the aft end of the boat so the captain and his fishing partner can walk clear around the boat and play the fish from any position -- without that engine or any of that other crap getting in the way. Just a nice, clean little boat with an inboard engine, a diesel if fuel economy is important.

The idea here, not shown, is to have a center console with the engine installed in the lower part of the console and accessible from the front, thus the reason for the yellow drive shaft running so far forward.

Note that there is no stuffing box required in this installation because the entire drive shaft is located inside a shaft tube that has its inboard end far above the waterline. Actually the other end is also above the waterline too since this boat spins a surface drive propeller. Here's an image of the ZF surface drive unit overlayed onto a profile view of the side of my boat so you can see how it works:




I'm not actually going to use the ZF drive unit, this picture is just to show how it might work if I were. The geometry matches up rather nicely, so I'm hoping that my simplified but similar drive system will work like the ZF might.

Even though my design is supposed to make the boat simple and cheap to build and power, it's a relatively innovative design with some high-tech features that are not exactly obvious. For example, it has pressure relief tubes built into the boat's structural framework that allow air that gets trapped inside the tunnel to be expelled rather than being mixed into the water. This keeps the prop in cleaner water so it can get a better bite with less aeration and therefore better thrust.