Atkin tunnel revisited

It's been a long time since I've been on this site. Always an enjoyable and informative time. Recent increases in the price of fuel have reestablished my interest in Atkins box keel design work. A very informative thread boatdesign.net/forums/showthread.php?t=16066 ( I hope I did that right) was very enlightening.

I have an old book at home, a first edition by Atkin that my old man bought as a young man, that I have recently been rereading. Much of the interest buzzing over the internet has to do with some of the seemingly extraordinary claims of fuel economy. I'm not formally trained as a naval architect but I apprenticed as a shipwright as a young man and spent 15 years fishing, I know my way around a boat, LOL. In any event this is not about me.

Reading Atkin's book closely I'm struck by three details.

1. He consistantly placed these boats in a performance envelope from 10 - 20 mph. Being content to achieve moderate speed.

2. He was very particular in specifying particular motors, shaft speed,
and propellers.

3. The design of the keel was refined over the years, he indicated that was in the interest of cleaning up the flow of water to the propeller.

To me this indicates a concentration upon the art of matching propeller requirements to motor torque curves.

Recent recreations of his designs or designs inspired by him have had varying success regarding fuel economy, some impressive others not so much.

What I am wondering about is whether a good measure of the rather extraordinary economy claims may stem from superb choices matching propellers, gears, and motors.

I know you guys have already had some good discussions regarding box keels but if you're not all sick of it by now I'm interested in other's opinions and ideas.

TollyWally

 

The one thing that stands out about all the Atkin engine choices is that they were NEVER OVERSIZED.

This means the engines were run at a very efficent range , even tho most were antique gas engines.

Today the advertising dept seems to select the engine size , not the boats designers.

The use mostly no reduction gears was probably a compromise to keep the prop diameter small , to enable beaching. Today there are many programs to optimize the prop , so efficiency may not suffer much.

FF

 

There's quite a bit of interest in Rescue Minor - Here's the URL to some pics of a company building at least one as well as a neat video showing her at speed and close in. http://www.seaislandboatworks.com/boats/rescueminor.htm

 

Hi TollyWally,

The current theory seems to be that Atkin tunnel-stern boats are designed so their propellers are surrounded in and pushing on already-moving boundary layer water and therefore re-capturing some of the energy in this water, whereas other boats try their best to keep their propellers in still or 'clean' undisturbed water as much as possible so they cannot re-use this energy and therefore it goes to waste. Naturally the more energy you can entrap and re-use the more efficient your boat will be.

Robb White's boat was very fuel efficient but probably that was mostly due to its extreme light weight. My Tolman Seabright Skiff could end up weighing twice as much so it won't get the mileage of Robb's boat, but I'm hoping it does okay.

I don't think so. If anything I think today's propulsion experts could make the Atkin boats even more efficient today than they were in Atkin's time.

I think it's mostly the way the bottom of the boat traps and re-uses the energy-filled boundary layer water that makes these boats more efficient myself. I also believe that a boat that does this does not need a box keel. I've done a recent design that gets rid of the box keel completely and it seems to me that it might be every bit as efficient as the Atkin tunnel-stern boats. But I have aother boats to finish before I build and test a prototype of this new design, and with my other obligations I'm not sure when I might be able to get to it ... but hopefully before retirement!

 

Thanks to all who have replied,

Fred,
I know what you mean about the motors not being oversized. As stated previously I wonder if partof the "magic" comes from skillful optimization of motor torque curves. I was unaware of the reduction gear angle.

Do you suppose that the careful choice of a motor that matches torque to correct shaft speed without the inevitable loss of mechanical effiency by running the power through a reduction gear could be responsible for some of the noted fuel burn claims? If so it makes the idea of correctly choosing motors even more remarkable.

Artemis,
Yeah I've seen that video, pretty cool!

Ken,

Perhaps my grasp of boundry layers is too limited to quite follow your reasoning. It would seem that most inboards with a wheel situated somewhat forward of the transom would also be operating in this boundry layer. I'm not really sure I understand this energy filled boundry layer thing. Atkin's tunnels seem more like built in trim tabs to me.

Remarks by Robb White about the tunnel filling with water once the boat starts moving make me think that perhaps the column of water the propeller is working in is more solid sort of. Kind of like a supercharger cramming more air into a piston cylinder. I remember that Atkin writing in the 30's spoke of refining his box keels to give a cleaner flow of water to the wheel. Is this what you meant by an energy filled boundry layer?

Stick a motor in that Tolman hull of yours and have some trials. If you're operating a boatyard in a semi third world country it can't be that expensive to have some of the boys mount a motor in a skiff. I know you're busy but if you can deliver on the concept you have a viable product.

Again thanks to all who have replied. I know there are some smart people here who could add to my limited understanding of fuel consumption and hull performance. Make something light enough and it becomes easier to do anything. LOL, the devil is always in the details. A sneeky suspicion in the back of my mind is that those Atkin boats represent a lot of small details done right.

 

They may operate in some boundary layer water, but not as much of it as an inverted vee bottom boat, and the more of this water a boat can push on the better in terms of efficiency. This is probably why the inverted vee aft is more efficient.

Some boats designed to be similar to the Hickman Sea Sled take this effect to the extreme by using the entire bottom of the boat to trap the energy filled boundary layer water beneath the boat and funnel it toward the propeller. Most of these boats are also designed to be high-speed planing boats too of course, but I think the concept is the same, just to different degrees and with different top speeds in mind.

Most boats do not have inverted vee aft sections, so they generally allow / encourage the boundary layer water to be pushed laterally away from the boat. Regular vee bottoms and rounded bilge bottoms are like this, and in my opinion they reduce the propeller's ability to push on the forward moving boundary layer water. This results in their pushing on still water which is naturally less efficient than pushing on water that's already moving in the same direction as the boat.

I think the tunnel pocket in the Atkin boats has little to do with efficiency, and much more to do with preventing the squat most power boats experience as they accelerate from a standstill to planing speeds. My view is that the aft tunnel pocket creates an area where the propeller's pumping action suctions the aft hull tightly to the water's surface thus preventing it from rolling to a large degree.

If the boat is 'stuck' to the surface like this it will feel exceptionally stable, probably much more stable than many other boats its same size. Robb White commented about this effect when he said his boat felt like it weighs 10,000 pounds. I attribute this to the suction effect in the aft tunnel pocket which holds the boat tightly to the surface. Others have argued with me about this suction effect so maybe I don't know how to explain it properly, but even though I many not be able to describe it to others I still have a good understanding of it myself.

It can be solid water and still mostly boundary layer water. There's nothing to prevent the accelerated boundary layer water from behaving exactly as you described here.

Yes, and because he improved these designs as he went along Shoals Runner might be considered the 'best' of his tunnel-stern designs since it was the last one he did, coming some 200+ (or was it 500+) designs after Rescue Minor.

The boat is not finished yet and my helpers are busy on other projects, so the issue is not as simple as it might seem.

That's what some people say but I have my doubts. I think the high cost of fuel is not affecting boaters as much as some people (including me) thought it might by this time, and now I wonder if there are enough American boaters interested in such a boat to make it economically viable to bother with at all.

I'm beginning to believe that regardless of the fuel cost, American boaters will never 'settle' for 15 knots when they can do 2-3 times that speed with the outboard powered planing boats they've become so accustomed to over the years.

 

Howdy All,

For the discussion

I don't know whether the performance improvement is real or perceived with this type of arrangement.

In general when unusual shapes appear with some suggested performance increase there may be a general trend in that direction - but within a few years they disappear as cleaner, more conventional shapes reappear.

This has happened with the channels and hollows on surfboards and sailboards, keels on yachts, hollows in the back of 18ft skiffs, props set into hollows on the bottom of powerboats.

Perhaps ... this is what happened with the Atkin boats too. If they were significantly more efficient then Mr Atkin himself would have been aware of it and it would be much more commonplace in his designs.

Anything that increases efficiency will reduce fuel consumption, increase speed and increase cruising range and will be rapidly adopted by professional designers like Mr A.

I don't know about Ken's suggestions about boundary layers - it would take a higher level of physics than I have - but it would seem difficult to get energy out of a boundary layer in this way.

1/ It is very thin - probably the average propellor tip clearance would put it outside the boundary layer.

2/ Thickness of the boundary layer doesn't matter - it is the behaviour of it that is important - whether it is laminar or turbulent flow. Laminar is low drag and requires a very smooth surface and can only exist over a small forward part of the hull in general. Having a prop buzzing away is unlikely to create laminar flow in that area.

There is a tiny chance of increasing the laminar flow of the forward end of the boat by having a prop placed at the right place at the back - but that is another story.

3/ The boundary layer moves along with the boat to some extent. So a prop blade entering this area from the main body of water that is not moving is going to suddenly change its angle off attack to the water and will probably stall - reducing efficiency rather than increasing it.

So while I have NO EVIDENCE AT ALL ... that would be my feeling about the boundary layer effect so is hardly going to be conclusive! I'd be interested to hear from someone with more Physics than me!!!!

Best wishes

 

Hi Michael,

This shape appeared as a later modification of the Jersey Skiff -- a boat that needed to sail, then later power, through surf and onto the beach since there were no docks or harbors in the area. I think the design served a particular purpose that had little to do with efficiency at the time. It seems this efficiency issue has come up far more recently -- more like a "discovery" about the design rather than a goal or reason for it. Then again, any designer who learns that his design is more efficient that others will probably boast about it, even if it's only a little bit more efficient.

I really don't think efficiency was much fo a concern back in Atkin's time, so it probably had little to do with the marketing of this boat. Gas was super cheap back then, and anyone who could afford an engine was more than likely not bothered by fuel costs.

But like any designer I'm sure Atkin had a goal throughout the years to study the performance of these boats and try to make them more efficient and handle better as he went along. He was apparently also very proud of this hull concept because he called it his "unique contribution to the world of naval architecture" or something to this effect.

Not really. All it takes is to have the prop pushing on the moving boundary layer water instead of on the still water below. Think about motoring in a 'normal' boat on a still body of water and your boat does 15 mph flat out. Now put it in a river that's flowing at 4 MPH and point it downstream and wow, you're going 19 mph -- because the boat's prop is pushing on moving water.

Yes I know this is not an accurate analogy since there are other issues in this example, but I'm just trying to provide a simplified example to illustrate how it *might* work in the Atkin design. I think part of the reason these boats are claimed to be more efficient is because of this effect -- which the boat creates itself -- by trapping the moving boundary layer water and directing it from the two side tunnels toward the propeller in the center of the boat, rather than pushing it away from the hull like most other boats do.

When the propeller can push on accelerated water that's going in the same direction as the boat, it simply must go faster with the same amount of power. This seems to be an easy way to take advantage of the energy that's already been put into the water.

I believe the boundary layer water is funneled from the boat's flat bottom and from the two side tunnels toward the center where the propeller lies waiting for it. I also think the large center tunnel aft -- and the entire aft bottom of the boat -- tends to help 'thicken' the boundary layer water by squeezing it together into a narrower but deeper center channel which feeds directly into the tunnel pocket and therefore the propeller.

If the boundary layer water is only 2-3 inches thick but 5 feet wide in the boat's mid-section, it gets 'squeezed' or 'channeled' together by the time it reaches the inverted vee tunnel section aft -- so that it is now only 1-2 feet wide but 6-12 inches deep at the propeller. This is where the Atkin bottom design seems to be 'genius' ...

It seems to me that he is using the twisted plywood bottom panels to direct as much of the water from beneath the hull as possible toward the propeller, instead of letting it get pushed aside or slip past the propeller. I'm pretty sure the tunnel pocket acts like a suction device too ... along with the prop it helps to suck this water toward the middle of the boat so it can be acted on by the propeller instead of just flowing past it like it does with most flat bottom or normal vee bottom hulls.

Sorry but I have to disagree with this. I think it *does* make a difference how thick the boundary layer water is. If the boundary layer water is too thin maybe there won't be enough of it to be built up to the same thickness as the diameter of the propeller by the time it gets there. Nevertheless, no matter how thick the boundary layer water is at the prop, there's still more of it going through the prop in a hull like this than on most othe boats, and this suggests that there is a good reason for the reported efficiency of these boats.

My understanding is that while under way the prop is always completely surrounded by the moving boundary layer water on these boats. The unusual bottom is designed specifically to direct this water into the prop, so any time the boat is moving the prop should have most of the boat's boundary layer water funneled into it.

Another efficiency increasing feature is that the prop shaft is practically horizontal, not tilted up to 15 degrees like an outboard or most other inboards. This alone is likely to substantially increase the prop's efficiency. This could very well be the cause of most of the improved efficiency reported in these boats for all we know. There certainly are not many other semi-planing power boats that have such a flat shaft angle.

I guess I should not say this because the surface drive folks produce drive systems with essentially zero shaft angle too. But these drives are very new (relatively speaking) and I haven't seen one used on a small semi-planing power boat yet. I have my own boat design that employs a simplified version of one of these drive systems. It's not a tunnel-stern Seabright hull but the drive system might adapt to one of these hulls -- with a lot of work ...

 

"Anything that increases efficiency will reduce fuel consumption, increase speed and increase cruising range and will be rapidly adopted by professional designers like Mr A."

No they will claim to have "invented" it rather than adopt it.

Look at the many links to websites on the box keel threads , the Austrains made great claim for the low wake and efficiency of their box keel.

AS does a US boat builder that also claims to have invented the concept , with a "special" computer program..

IN a 40 ft boat the boundary layer will be about 4 inches thick , thats a lot of water energy to recapture.

The shrouding of the prop by the hull above and alongside probably helps efficiency a bit , as a Kort nozel would.

"Think about motoring in a 'normal' boat on a still body of water and your boat does 15 mph flat out. Now put it in a river that's flowing at 4 MPH and point it downstream and wow, you're going 19 mph -- because the boat's prop is pushing on moving water."


NOPE , in a 4 K current the boat will move at 4K with no motor installed , just drifting.AS will an empty beer bottle.

Part of the claims probably come from the box keel's displacement ,as it is a nice way to lower the beam to lwl that is actually floating the weight.

The canoe under the visible hull is 6-1 L/B or better , like a multihull , easier to stuff thru the water at moderate speeds.

There also should be "some" lift at moderate speeds from the flat bottom of the box keel body .

Add ALL the small gains up and the 20% to 50% gains become more believable.

Seems to run out of gains at over 20K , where plaining takes over , but it may be fine for folks that are willing to cruise at -20K at 5nmpg , rather than at 30K at .5nmpg

FF

 

FastFred,

"Seems to run out of gains at over 20K , where plaining takes over , but it may be fine for folks that are willing to cruise at -20K at 5nmpg , rather than at 30K at .5nmpg"

LOL, My fondest desire!

"Part of the claims probably come from the box keel's displacement ,as it is a nice way to lower the beam to lwl that is actually floating the weight.

The canoe under the visible hull is 6-1 L/B or better , like a multihull , easier to stuff thru the water at moderate speeds."

I've often wondered if it might work something like that. A long skinny displacement hull being pushed past normal displacement speed but being kept from excessive squatting by the flotation of the upper hull surfaces.

What I don't know about is the interplay from excessive friction and drag from the upper hull surfaces. On the other hand the flat bottom of the keel and relatively flat surfaces of the upper hull might sort of plane providing lift.

As Ken mentioned straightening out the shaft angle is going to help. I also keep going back to the idea of optimal matching of the wheel and shaft speed. Atkin was fairly specific regarding these items.

"Add ALL the small gains up and the 20% to 50% gains become more believable."

Contrasting that with adding up small losses, ...too big a motor, the wrong prop spinning too fast at an excessive angle, excessive drag from trimtabs overcoming poor trim. etc. etc. The peak to valley differences...I sure would like a hull of the former pursuasion!

 

Check out Bill Allison's flatscat design <www.flatscat.com>. Be sure to check out the performance videos.

There is an internet forum frequented by duck hunters - some of those folks have gotten some of the pocket tunnel flow dynamics figured out. They are well aware of the suction created with pocket tunnels and typically add an air vent to the leading edge of the tunnel. By allowing air into the tunnel, the boat will jump up on plane much quicker and add 3-5mph to the top end speed. People who have done this mod claim that if feels like they pulled anchor when they open the vent. It works so well that some commercial boat mfrs. have started venting their pocket tunnels. Only caveat is that the water is more ventilated, so you will need a prop with a larger blade area and extra cupping to grab the water more efficiently. Search for vented tunnels at:
http://refugeforums.com/refuge/forumdisplay.php?s=&daysprune=1&f=10

 

Now there's a new thought that seems to go against all previous wisdom. Let air into the prop stream? Most outboard users spend effort to prevent that from happening. If a larger area prop with cups results in greater thrust with ventilation, we gotta take a new look at this.

I enjoy the back and forth banter of ideas in this and like threads even if it does seem a bit like the scientists arguing about what we would when we go to the moon. Turns out they were pretty far off base with some of their cogitation. On this forum disagreements are mostly taken in good nature which makes it worthwhile.

 

The attached .txt is from a posting that Robb White made back in June 2004 on the Atkins Boat group. He made several postings but this one describes what actually happened on the first voyage; his impressions and thoughts. Remember that Robb was an experienced small boat builder so his observations and "noodlings" have practical experience behind them.

 

I think the point Fast Fred makes earlier is pretty spot-on.

In effect, esp in Robb White's boat, we are looking at a displacement glider hull. The seabrights were built for a very specific purpose. While horsepower changed the world, it appears that Atkin mereley bastardized (in a good way, mind you) the hullform into something that displaced her water in a way thats easier to get through. When Robb coupled that hullform with his very light weight thanks to the tulip poplar strip planking, he took what little burden was left on the bilges and removed it.

Fred, you really opened my eyes up on this one!

E

 

I tend to think of this approach as being half way between a fully submerged propeller (Atkin tunnel-stern designs) and surface piercing propulsion technology where the top half of the prop is always out of the water.

Yes of course, because the suction that was previously lifting all that water up into the tunnel is eliminated by the introduction of an expandable fluid -- air. Since water does not expand under suction but air does, the boat can no longer lift water into the tunnel with an air tube in the top of the tunnel to keep it ventilated.

Note that the Atkin design appears to require the tunnel to fill with water -- because then this suction becomes a desirable feature of the boat's performance -- unlike in the flats boats where it is an undesirable feature. If you ventilate the tunnel of the Atkin boats I suspect you'll destroy the effect Billy Atkin worked so hard to develop all those years.

No doubt, since opening the vent lets air in, eliminating the suction that was holding up half the water in the tunnel. This lets all that excess water fall out of the tunnel immediately, thus lowering the effective weight of the boat by several hundred pounds. No wonder it feels like they just pulled the anchor free ...

This is why I said earlier that vented tunnel technology is half way between the old fashioned fully submerged propeller designs like Atkin and the most recent surface drive systems being promoted these days. If you take this one step further (I have in one of my designs) you end up with a boat that uses a bottom shape that that traps and funnels the boundary layer water toward the prop, while at the same time using a vented tunnel surface piercing propeller that avoids lifting water into the tunnel pocket in the first place -- all with a nearly horizontal shaft for maximum efficiency.

Tom, I don't think the vented tunnel method tries to introduce air into the prop stream. Of course it happens to a degree, but the real goal here seems to be to allow air into the top of the tunnel so it can displace the water that was previously being lifted into the top of the tunnel by suction. The simple act of eliminating the lifting of this water is what adds to the efficiency, then the saved energy can be used for higher speeds or faster hole shots or whatever.

Here's the part of that write-up that was very 'revealing' to me:

I have always understood Robb's comment here to mean that the suction produced by the water flowing into and out of the tunnel pocket is holding the boat so tightly to the surface that it cannot bank to the inside or the outside in a turn ... and this suction is also what "holds the boat in a tight grip" and makes it feel "like it weights 10,000 pounds".

I'm also pretty sure that if you introduce air into the tunnel pocket of this type of boat without changing the rest of the boat's design, you're going to lose not only the flat turning characteristics but also the feeling that the boat is being held in a tight grip.