Tunnel drive performance issue

A thread about this project can be found on this forum, surface drives>diy tunnel drives.

The drives have been in use for several months now and some of the shortcomings like lack of steering efficiency have been resolved by larger steel rudders. Some construction details that were severely critized have so far proven to function very well: the oil filled stern tubes protruding from the transom do not leak oil and no water entered through the shaft seals. The chambers filled with silicone oil provide a perfect barrier.
Absolutely stunning is the fuel efficiency. This boat, originally equipped with 2 140 hp Mercuiser engines and stern drives needed 1 litre of gasoline for every kilometer or 1 gallon for approx 2,5 miles. With VW diesel engines and the experimental tunnel drives it does not 1 but 10 km on 1 ltr of cheap heating oil !! Exact figures I cannot yet give because the fuel gauge still shows 60% but these things cannot be trusted because of their non-linearity.
A minor issue is the rapid disappearance of all 4 sacrificial zinc anodes, but that was to be expected with such a large area of stainless steel. In the next winter I intend to paint all non-moving parts with poly-urethane coating.

A much more difficult issue is the behavior at speeds over 10 mph.
While slowly cruising, water flows around the flanks and circles back inwards near the transom, then mixes with the wake and causes some turbulence. The prop wash is near the surface so there is some splashing which I think is normal for this construction.
Somewhere around 12 mph, while the bow lifts, vibration starts and the expelled water from the tunnels is turning white. With more throttle, speed increases slowly to approx 15 mph, but by then the wake looks like a large white hump with holes on each side and the vibration tells me that I need to throttle back quickly.

Conclusion: the tunnels draw in air from the sides because they cannot be supplied with enough water from under the boat. Not totally unforeseen, because the transom blocks part of the tunnel entrance. Unfortunately I cannot cannot change much there because the 10x12" flanges that mount the drives to the transom are really necessary.
The alternatives I can think of are to fill up the tunnel entrances to provide a more smooth path, add a scoop or submerged spoiler to force more water in or make guides under the hull to prevent water escaping sidewards. But will it be effective and what will be the relation to the extra drag it causes?

I include a drawing of the transom and a photo that I crudely "photo-shopped" to remove objects under the hull.

 

I am no expert, but I think there are a couple of possibilities:
From the transom sketch and actual photo it appears that the V-shaped bottom is forcing the water away from the tunnels as well as posssibily sucking some water out as well; when you displace water away from the props and there is no water to replace it air usually does.
A couple of suggestions:
Weld or bolt on triangular side plates from the front of the rudders along the lower outside edge of the tunnel forward to the area where the protruding stern box meets the transom, you could also attach it to that lip on the box. I would take care to ensure a minimal clearance between the leading edge of the rudder and the rear of the vertical side plate so as not generate additional turbulence. I don't know if doing the same thing on the inside edge of the tunnel would do much, but it might. I would try it on the outside first.
Smoothing the sides, leading and trailing edges of the rudders to reduce turbulence, the same for the rear edges of the tunnels.
It seems you are on the right track so far; good luck.

 

Hi,
interesting project.
Does the water level at the stern lower to the line of the bottom of the boat as you reach planing speed?? If so then your problem is that the propellers are only half submerged at this speed, almost acting as surface props and drawing air in from the stern above the props. This would also account for the vibration you experience as only the lower half of the prop is acting on the water and they are not designed as surface props. The only solution I have seen for this is to continue the tunnels and drop the top down astern of the props so that you form a downward jet of water and the top of the tunnel remains underwater as you reach planing speed. I saw this on a Canadian Harbour Launch that I was part owner of when in College. In her case the prop was well forward of the transom and allowed for the change in tunnel shape within the hull. This solution would require a totally different approach to steering than the one you have at the moment, in fact I dont know how you would solve that one. Possibly small rudders in the prop wash at the aft end of the tunnels??
All the best with the project.
David.

 

I have not been able to reach planing speed yet, so I cannot tell anything about the stern water level.
Of course if the boat would be planing, the water level would be what you described, but the idea was that the tunnels would ensure that there was water around the props.
Somehow I tend to forget that water cannot be compressed. I studied mechanical engineering very long ago; they only paid a lot of attention to aerodynamics where the laws are completely different.
Lt. Holden's idea makes a lot of sense, but his first sentence worries me a bit.

 

CDK,

From the drawing & picture it appears that you do not have a "tunnel" really just a shroud around the props off the back of the boat. Do you have any better pictures showing have far the props are from the transom? From what I am seeing you will need surface props. It appears to me that you would only have about 1.5 blades in the water at a time with the rest out once you start getting some speed. With that you need to start thinking 4-5 blades or cut a tunnel into the bottom of the boat. If i am wrong about the tunnel please advise.

 

Hi Nomad,

Better pictures I cannot provide, just more of the same. The heavy shadow makes it hard to see the innards; I should have used a flash.
The tunnels are almost 2 ft long. The props are in the center of the half-circle, but the flange that mounts the tube/shaft to the transom is offset vertically to gain height in the engine bay. So the prop shafts are angled down approx 6 degrees.

When I started with this project a couple of years ago, I took into account that fast planing might be wishful thinking. Now that we regularly make day trips at 6-8 mph at 25% throttle I realize how slow that is compared to what the boat did with stern drives.
If need be, I will change to surface props, but not before I have explored alternative improvements.

 

Without being there looking at the boat & tunnels and seeing it run it's hard to tell what all needs to be done.

The tunnel at 2' seems short for the prop height, what does the front of the tunnel look like?

 

A simple drawing of one tunnel. From the transom edge upwards the tunnel is closed, so water has to come from below. It does, in fact, but not in adequate quantities when speed is increased above 12 mph.

 

I believe the tunnels are definitely too short for proper water supply at higher speeds, thus leading to prop ventilation. Easiest fix is likely a change to the props. Look for either surface-piercing props, or props with a large swept blade area (e.g. - long-eared blades) and lots of cupping. The larger blade area and extra cupping help the prop to get a better "grab" and hold onto the water longer; therefore, they tend to work better when air gets into the mix.

I think it will be difficult designing appendages to direct the proper amount of water to the tunnels, as this may change with speed and will likely alter hull efficiency. But I've been wrong before...

 

Hi,
the more I look at the arrangement you have the more I think that the tunnels are wrong. Attached is a sketch of how I think they should look in order to prevent air from getting in to the tunnels and causing the problems that you have. The tunnels need an extension forwards to allow a smooth entry of water to the props. They also need an aft extension to close the tunnel down to the planing waterlevel so that no air can flow back to the prop to cause cavitation when planing. In order to do this you will need struts to support the shafts close to the props, and rudders just aft of the tunnel extensions in the prop wash. A lot of work I know, but that is what I think you need to get this arrangement working properly.
All the best with the project.
David

 

Diwebb, the layout you propose is impossible. The engine bay simply isn't long enough to cut off a piece of the transom and bottom, I need every inch there is. But I do could construct something IN the tunnels to guide the water to the props.

When I started this project, I looked at various commercial designs, like Simplicity Marine ans Saro. This is what Saro says about their tunnel drive:

"TUNNEL-DRIVE
SARO invented the tunnel drive. This is really a half a tunnel, or hood, over the top of the propeller. The tunnel reaches into the water on both sides and prevent air from being pulled or sucked in. Now only water is filling the tunnel, and a solid stream of water is accelerated to push the boat forward."

This I do not understand.
Their tunnel in fact also is just a shroud around the prop and it isn't longer than the ones I've made. Yet they claim there is a solid stream of water, where I only see heavy cavitation as soon as speed increases to the point where the props push the water out faster than it comes in.
Furthermore, my tunnels are cylindrical, theirs are narrower at the entrance so they take in an even smaller volume.

How can changing the props improve the performance? If insufficient water is available, a different blade shape cannot change that.
My props are Michican Dynajet style, designed for "a non-cavitating to a partially cavitating environment". They were made in Italy on special order and it took 4 months because they rotate in different directions. I hate to spend more money and waiting time unless I'm convinced the problem is solved with different props.

 

Some props will run better in aerated water than others. Otherwise, surface-piercing props would make no sense.

Tunnel (or, more accurately, pocket tunnel) boats are common in areas such as the TX gulf coast, although these are typically small, fast planing hulls. They use props that are specially designed to run in the aerated water that the tunnels produce. They typically have extra or double-cupping on the blades to get a better bite on the water. Also, a long-eared (skewed) blade will be in contact with the water for a longer time, thus delivering more thrust when you have some air in the mix (I don't believe you will ever get rid of all of the air in any pocket tunnel design).

To give you an idea of how effective these props are, many of these boat owners (and more manufacturers) are putting vents in their tunnels to allow even more air INTO the tunnel. This typically enhances performance on a planing hull by eliminating the vacuum created by the tunnel and allowing the boat to quickly jump on plane as well as gain 3-5mph on the top end. Too much venting, however, can lead to water pressure (lack of) problems on an outboard engine.

You might check out www.baumannprops.com - they do custom prop work and should be able to put some cup on your blades. There may be a local prop shop in your area that can do the work also - ask around to see who folks at the marina recommend. You want a prop designed to operate in a "ventilated" environment. A good prop shop should be able to modify your existing props to be better able to do that. You may be amazed at what a difference a change in props can do for boat performance.

Take a look at this page http://sites.mercurymarine.com/port...26_49291:126_49299&_dad=portal&_schema=PORTAL
If the link doesn't work, try this: http://tinyurl.com/6cpche
This is a good page to read through the fundamentals of how props work and how you can use that knowledge to improve your boat's performance.

 

I think the problem has been well defined as air leaking into the props when you increase speed. This is what I think is happening.

At low speed, the transom is submerged and edges of the shrouds are are also submerged and the low pressure in front of the prop is insufficient to draw in air. As speed increases, the transom starts to clear and the tunnel edges become at or very near the water surface. Now it takes less "suction" to draw in air under the tunnel edges at a time when the suction is greater. No wonder the wake is frothy white since it is a foamy mix of water and air. The inboard edges of the tunnels actually appear to be above the hull bottom which can only make the problem of air entrainment worse. This is probably where the ventilation starts to occur first.

Lengthening the tunnel would help but not cure this problem. You also need to run the tunnel shrouds down into the water further to make sure that they stay under the surface. The rudders help in sealing these edges but are not enough. I suggest welding some plate to the tunnel edges and extending it forward under the hull a bit. Make sure the rudders are as close to the tunnel edges as possible. I think this will help a lot but that short tunnel will eventually limit what can be done. If the old transom forms the front of the tunnel, it would probably be better to fair this up to the top of the tunnel and reduce turbulence in front of the props.

In the usual tunnel set up, the hull itself seals the tunnel edges but here, there is nothing to do that since they extend beyond the hull.

I think this is a very good experiment with some good ideas.

 

1st Tunnels are just extensions of jets/nozzles, etc. The more you can wrap the circumference the more efficient your system becomes. Any energy from outward flow is prevented and thus transferred into propulsion. With tunnels, jets or nozzles the premises is that they are low enough to prevent ventilation.

2nd. As soon as you try to get up on a plain you are shot. Tunnels are for semi displacement, as soon as you get up on a plain you are in essence looking at a surface drive system. You have a system that is designed/built to do one thing yet it is encountering design requirements associated with a completely different school.

3rd. I can’t quite tell but it looks like you are sending a lot of disturbed water to the props to start with. By the looks of your sketches the forward side of the tunnel is squared off…that has to send a lot of disturbance to the props.

You will never be able to make tunnel extensions long enough… if you did the owner would not want to have to pay for the slip. Go out on any boat on a plane and then picture how far you would have to extend something beyond the transom to prevent air (ventilation).

Recommendation
1) Major fairing of the inflow, tunnel forward of props will help
2) Don’t know if you will accomplish what you intended, tunnels and planning.

 

Village_Idiot, thank you for the links.
It would be very nice to have something like Baumannprops.com nearby, but
there isn't. This is another part of the globe, where the local
commercial activities are frying fish, opening beer bottles and selling
phony brand name articles to tourists. For anything serious you have to
travel to Italy, Germany, Holland etc. I found one guy here who claims he
can repair props but doesn't have a decent workshop to begin with.
I do suppose that better props could make a marginal difference but I'm
certain there is much more to be done.

tom28571, as I answered to diwebb previously, fairing the transom to the
top of the tunnels is no option because that's where the drive flanges
are bolted to the transom. But what I could do and seriously contemplate
is to put an insert in the tunnels that should have the same effect. A
warped oval plate that guides the water to the props, made from glass
fiber and the space behind it filled with polyurethane foam.
There is still some leakage between the stainless steel flanges and the
transom; I hope that will also be resolved then.
Plates extending forward on the sides of the tunnels I will also make
from grp. Should they have the length of the tunnels or extend under the
boat's bottom?
I attach a sketch of what I plan to do (the red part).