Atkin tunnel revisited

"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 envision the results of the inverted deadrise exactally reversed

The "suction" that sinks a std stern is simply a response to the bow starting to climb the bow wave. The further it climbs , the higher the deck angle the further the stern is stuffed/tipped into the water. No suction involved.

The pressurized reverse deadrise area simply keeps the stern FROM going down , keeping the boat level it higher semidisplacement speeds.

The more power applied , the higher the stern lifts , which caused the problem of Bow Steering in one Atkin that was repowered with a far bigger engine.

Atkins simple solution was bow strakes , to have the bow lift at the higher speeds.

A conventional outboard will go faster with any reduction in resistance , so venting the bottom , as a step hull does is fine.

Venting the reverse deadrise lifting area might not prove usefull.

FF

 

Well, at least we agree on this point Fred ...

The reason why I say there's a suction effect is because when the tunnel pocket is full of water, this 'tunnel pocket water' is moving faster than the water below it -- because it must take a curved path into and out of the tunnel pocket -- the classic venturi effect:

http://en.wikipedia.org/wiki/Venturi_effect

Since the Atkin tunnel pocket induces water to move faster within the pocket than the water is moving below, the pressure is reduced in the tunnel pocket forward of the propeller. I think this is the 'suction' that pulls the hull downward and hold it tightly to the surface.

The reason the boat doesn't squat under this suction is because there's another slightly more powerful counteracting effect going on here:

The downward hook in the tunnel aft of the propeller forces the exiting water down which pushes the aft end of the boat upward, counteracting the weaker suction effect in the tunnel pocket. This downward thrust lifts the aft end of the boat and keeps it from squatting -- while at the same time the suction in the tunnel pocket holds or 'suctions' the hull tightly to the surface.

This seems to be a balancing act:

If the hook in the hull bottom aft of the propeller were too great, the downward thrust would be so powerful that it would lift the aft end of the boat so much that air would be able to get into the tunnel pocket. Then the tunnel pocket will no longer be filled with water, its suction will be lost, the prop will ventilate, thrust will be lost, and the boat will no longer be suctioned to the surface -- so it will no longer feel like it "weighs 10,000 pounds" as Robb White described.

This is just my theory of course. Scientifically it makes sense to me, regardless of whether I can explain it or not. No one ever accused me of being a good teacher ...

 

Any "suction " in front of the prop is NOT what caused the boat to go forward , just as the "suction" in front of an aircraft prop does not cause thrust. True enough a stopped jet engine has enough suction to pull in an unwary lineman , but its NOT what causes powered flight.

Since the end of box keel is the prop mount , IF we believe the accelerated boundary layer is part of the CAUSE of the Atkin efficency the water being fed to the prop MUST be accelerated water , so there will be little suction pulling the boat forward . and none downward,. But lots of energy for the prop to further accelerate the boat uses a thrust and as lift from the aft cavity shape.

Since the RD is fulll width of the boat , I do not believe it is a tunnel , as tunnels are seldome if ever lifting surfaces.

The balancing act would seem to be the bow lifting on its bow wave , and the stern keeping the hull level , so the bow wave is much smaller and doesnt need to be climbed at a steep angle.

A probable cause for the reduced bow wave is much (75%) of the displacement is underwater creating no surface wave , and EZ to push with a 6 or better LWL /BWL ratio.

The probable reason for the stability is that the FULL aft beam of the vessel is on accelerated water , solid as a set of saw horses at speed.

FF

 

40 ft version??

Hey guys, what do you think about a scaled up 40' version of Recue Minor or similar design. Built in modern lightweight sandwiched composite.

A real cruiser with live aboard accommadation.

Thanks for the feed back.

James Mills

 

James, Atkin produced progressively larger boats up to the 30'ish range. Following on that, Dave Gerr produced subsequently larger tunnel stern boats in the range you speak of. A few are on his website.

E

 

"Hey guys, what do you think about a scaled up 40' version of Recue Minor or similar design. Built in modern lightweight sandwiched composite."

I am attempting a similar design , but it will be 39 loa by 7ft 6 beam , in order to be shippable worldwide inside a 40ft shipping container.

Fly to the destination , unload the boat on simple pipe rollers , and its VACATION time!

The boat will have to be lightweight , under 8800 lbs , as the Euro' have many burorats .

The "hope" is 18K cruise at 5nm/US Gallon. Slim is in in terms of efficiency as well as seaworthy /seakindly , tho the boat is in no way concieved as an ocean crossing vessel. Thats for the box.

In terms of profile , search "STROLLER" a Herrishoff commuter style that has to my eyes a better profile, and a bow cockpit that should wotk , even in a 7.6ft beam skinny boat.

Engine of choice now is a Fiat, Evico diesel, 150hp that meets Tier 2 and is totally mechanical, so may be maintained and repaired by me.Hundstat CCP prop will be required too.

FF

 

I think this discussion has mostly mentioned the tunnel sterns developed by Billy and later refined by John Atkins, but some mention of the box keel shapes has crept in. The two types are different beasts, though in Billy's era they shared similar elements.

Billy originally attempted to correct difficulties encountered when power came aboard the Sea Bright model (and other types). In his day 100 pounds per HP were not uncommon. Towards the end of his development of the type, 50 pounds per HP were possible. Given this amount of loading per available power, he needed to increase efficiency with as many "tricks" as possible.

His thinking was to use a box (where the drive would live), which got the shaft angle (everything was straight shaft then) parallel or nearly so to the LWL. Shape the box so water flow would be not only less disturbed, but flow cleanly toward the prop. He also incorporated a hook, further reducing draft and helping buoy the stern after passing S/L of 1.4. The box keel boats didn't have a substantial tunnel at all, just a modest hook in the garboard. They were swinging big wheels, slowly, trying to get the most from the limited power available. Billy also shaped his box keel hull type to the "wave form" theory of the era, with his target S/L being the point of ideal yacht "attitude". John picked up the "charge" for the type and continued the work.

The last box keel John designed (the boat that I've worked on for several years) has no reverse in the V at all. The chines are nearly dead flat from 2/3's aft of the bow and further aft, plus the deadrise is just about flat. He considered this the refinement of the type (Fred, this was the 39' boat you were on last year, with me). It was done in 1960.

The tunnel hulls where a different attempt at extreme shoal abilities. Progressively, John decreased the amount of reverse V, in favor of a better tunnel shape, lift strakes and cleaner entry. Much refinement can be seen when comparing the tunnels on the Billy's early designs to the later designs of John. At the end of John's era a big outboard was 25 HP, plus 3 - 5 pounds per HP were possible from an inboard and the weight to HP ratios dramatically reduced over Billy's era. Faster turning, much lighter power was available, so much so that reduction gears were necessary. Smaller props, spinning at unheard of speeds (compared to Billy's day) made tunnel design critical, particularly with the lighter bottom loading possible with newer techniques, power plants and materials available in the late 50's and early 60's.

I believe what John found was in larger sizes, the reduction of drag by removing the reverse V was more beneficial then the modest amount of buoy he got. In small, lightly loaded bottom craft, like the skiffs, the reverse actually did keep water flow "channeled" aft rather then deflecting sideways, like on a conventional shallow V form. There's a distinct difference in the tunnel shapes between the heavier, larger tunnel models then the lighter, smaller ones.

As to the seemly outrageous efficiency remarks made about the types, well, I turned around the old girl the other day with an outboard skiff. This, quite heavy 39' motor yacht makes no wake at a couple of MPH. A canoe and paddler make much more wake then it does. With a single blast from the outboard, it takes about 2 seconds for her to gather way, but the cool part is she doesn't want to stop, she just glides along. Several tons of potential energy has a bit to do with this, but, I've moved many a boat like this and no other has exhibited this quality. She tracks like a freight train, backs down like a pig (common of the type), uses a fraction of the HP other motor yachts of general size and displacement and sips fuel. She also has a wonderfully soft motion in a confused sea way. Originally the model was for "bill fishing" in the north Atlantic. Standing at the end of a 15' bow sprit, with a spear, well away from the center of rotation, she had to have reasonable pitch stability or she'd wipe off the fisherman on each passing wave. She doesn't roll excessively and her firm flat aft bilges suggest such.

 

Fred,

What is a Hundstat CCP prop ???

Approximately, what do you figure the cost of the motor, transmission, shaft and prop??

I'm a few years from taking action, but I may, just maybe, build a cruiser of similar design.

James

 

CPP Im guessing, controllable pitch propeller

E

 

Hi PAR,

What is the design name of this Atkin boat?

 

Namaki is her christened name, Ken.

 

Heh ... I was hoping for the design name, not the christened name, since the christened name will not appear in the list of available plans on the atkinboatplans.com website ...

The only two 29' cruisers I can find there tare Neap Tide and Salimina:

http://www.boat-links.com/Atkinco/Cruisers/NeapTide.html
http://www.boat-links.com/Atkinco/Cruisers/Salimina.html

Any chance you'd be willing to tell us which of these it is? Or is it another boat and not one of these? Or is the design name a secret?

 

"Namaki is her christened name, Ken."

Possibly a typo? Namaka perhaps.

I can't possibly overstate how pleased I am with the information I've been able to gather from this thread. Thanks to all who have posted.

My fantasy boat would be a fiberglass or aluminum hull 28 to 30 ft. long of moderate displacement. Enough to carry a moderate amount of gear without putting her to far below her lines. Running a 40 to 80 horse diesel spinning a variable pitch wheel to perfectly tweak my motor's torque curve. Hitting 15 to 18 knots burning 2 to 3 gallons of oil an hour. Dry stacked and keel cooled, nice oversized diesel cookstove, hell for stout rubrail....

 

TollyWally,

I believe the christened name is just a name given to the boat by the owner. Usuaully it has no relation to the design name, which means it is pretty much useless when doing a google search to find more online info about the design.

I reviewed an earlier thread and learned that PAR's boat is not included in the boats on the atkinboatplans.com web site, so it looks like we couldn't buy the plans even if we wanted to -- unless they are available elsewhere. I was mostly intrigued by this comment which made me want to buy the plans so I could study them in greater detail:

 

"I believe what John found was in larger sizes, the reduction of drag by removing the reverse V was more beneficial then the modest amount of buoy he got. In small, lightly loaded bottom craft, like the skiffs, the reverse actually did keep water flow "channeled" aft rather then deflecting sideways, like on a conventional shallow V form. There's a distinct difference in the tunnel shapes between the heavier, larger tunnel models then the lighter, smaller ones."

This goes along with my speculation .

The conventional heavy boats seldom have enough power to get much above SL 1.34 , yet the flat aft does help keep the stern from sinking , so doesn't drag the huge "stern wave" and can go somewhat faster on the same fuel burn.

However once the boat is a "lightweight" with a D/L ratio of 70 or so the ability of not sinking the stern is usefull up to SL 2.7 or 3 .

So for a light enough boat the ability to go fast , cheap fuel burn is a big lure.

The box keel taking much of the displacement should allow a smoother ride in rough stuff , as well as making the displacement easier to push thru , not over, the water.

I love the concept that the vessel can be run aground with ease.
Both to stuff her in a box , and for inshore cruising it would seem a huge advantage.

In the North East US the locals grab any anchoring area , and claim its a "Mooring Field", and prohibit (illegally) any anchoring in "their field".

Would be really nice to be able to anchor Really close to shore in tidal waters (to shorewards of "their field" ) and simply take the ground at times.

Sounds like a Win Win for my style of cruising.

FF