Tunnel drive propeller selection

Hello, new member here. Looking for some input on propellers for a boat I recently acquired. It is a 17’ inboard tunnel drive hull. The tunnel pretty much follows design work I have been reading about on here and elsewhere, transitions from a flat roof to a cylindrical tunnel at probably 40% circumference. The prop shaft is faired into the center of the tunnel. Currently running an off the shelf 3 blade I/O prop, has decent performance but does suffer from cavitation in some cases.

Per the builder of the boat, when they initially tested props they had some difficulty getting something to work. Found that running the blade tip very close to the tunnel wall made for best performance out of the props they tried. This corresponds to test data I have read where 2% of prop diameter or less gives optimal performance. I still need to do some further testing, at this time I am trying to educate myself on this style of propulsion so I can make a semi educated decision on which direction to go.

So in a nutshell, not looking for a specific answer regarding a prop, just some educated opinions/advice or links to data etc. I think there is probably a better performing option than what is currently on there. Everything I have read seems to indicate a long swept blade with a lot of cup, stainless steel, and tight clearance to the tunnel wall. Also seems to indicate 4 blades or even possibly 5 blades. I did read in one linked article that it was not good to have blades entering and exiting the tunnel at the same time, and given the tunnel is 40+% circumference I’d be concerned about using a 4 blade. (Need to measure the arc and get an accurate number for this). Any input or help is greatly appreciated.

 

I'm not familiar with what you are doing, but it sounds a bit like simplified water jet propulsion in the Hamilton style, and I'm thinking your ideal solution lies somewhere between high speed propeller and water jet impeller designs. Do you have photos, diagrams or references please?

 

We deal with the double edged sword of tunnels, helps us get shallow but makes a balancing equation of "least worst ".

I've seen guys go 5 blade high skew recently, I had a 4 blade high skew prop with a decent amount of cup. Although my tunnel isn't that severe, I'll have to measure but I'm closer to 20-25%.

I've a stainless Michigan workhorse and a nibral veem we got second hand. Have way less time with the veem, but it's definitely smoother. Tough call to quantify as to why. The stainless gets put on when chasing fish in the shallows of sand and gravel. Could be the fluke shape, the surface area of the blades, could be build quality... could be how much harder a life the stainless prop has lived....

I do knowy grandfather had a more extreme tunnel hull and his prop test collection was extensive.

 

The shape of the tunnel has a big influence on the prop performance; the devil hides in the details. So before we can say anything, you have to provide info on its lines, and particularly what the edges between hull and tunnel look like.

 

Here is a pic of the tunnel, will get some more and dimensions next time. Tunnel diameter is 16”.

The tunnel is similar in shape to the flat roof shown in this paper

Found this document in another thread, lots of good info here.

The main difference in this case is the boat is a single engine with the tunnel centered in the hull, this study seems to be talking about a twin engine setup.

 

These tunnels have a shape that is thought to be ok for the flow along the hull, but when you introduce the propeller flow, the picture is changed to the worse, just as Comfisherman noted. Most people seem to believe that there are "invisible pipes" in the water, leading the flow straight into the prop, but that is totally wrong!

The screw is creating a low-pressure zone on its inflow side (we call it a hydraulic sink), and this zone is "attracting" fluid from all directions. This results in a three-dimensional flow, which traverses across the sharp edges between hull and tunnel sides, causing a choking of the inflow to the prop; it is simply "starving" from lack of water. In addition, the inflow velocities are varying along the propeller disc area, due to the stagnant flow. Propellers with high scew blades and large blade area plus four or more blades are slightly more tolerant for such bad inflow conditions, but their efficiency is lower than the best three-bladed screw.

To illustrate what it looks like, I prepared a "rough and dirty" simulation, that shows the critical cross-flow, and why the tunnel edges must have a good radius in order to avoid turbulence and pockets of stagnant flow. Note that the source for propeller inflow fluid is a semi-oval transverse area slightly ahead of the tunnel entrance, in this case about twice the width of the propeller and ~half the height. These proportions vary with the propeller thrust loading; at hump speed the cross-flow is more severe. The example corresponds roughly to a 16" propeller powered by 145 hp at slightly over 20 knots.

The upper image shows the flow paths into the prop (the outflow is not shown correctly; it's there just to make the algorithm happy). The lower shows the local inflow velocities into the prop disc. Note the variation!!

 

Very interesting, thanks for running the sim. When you talk about a good radius at the tunnel edges, what does that look like? Are we talking 1” radius or something much bigger? Putting a small radius on the current design would be pretty simple but going big not so much. Also, looking at the velocities at the rear of the prop, the lowest pressure is obviously at the top of the tunnel. Is this why venting a tunnel seems to help in some cases? If so that might be something to look into. Any thoughts on the data in the paper I referenced about not having prop blades entering/exiting the tunnel at the same time? Or no worries about trying a 4 blade? Honestly curious from other folks with something similar what is to be gained/lost by running 4 or 5 blades. I’m not looking for maximum speed, a better overall experience quick planing etc is more what I’m looking for. Maybe a better suited 3 blade would be best bet. Hoping to minimize buying multiple props just to test.

 

A general rule of thumb for radius around a "hole in a wall" outlet is roughly 25 % of the diameter. In the case with the fluid crossing the edge at an angle, you can reduce it to about 20 %. For your tunnel I'd say 20 % of prop dia at the tunnel entrance, reducing to ~10 mm at the exit end.

The outflow region of the tunnel should follow the contracting flow in the exit jet. This prevents air to enter the propeller disc from behind, when the transom is running non-wetted. A correct tunnel shape has a function analog to a Kort nozzle, and it should be created with care (which seldom is the case, unfortunately.......).