Critical speeds for Semi-Planing

I have been fascinated with Atkins body of work for years. I believe Fred's views may be right on the money. A few points I've seen repeated over and over in my examination of Atkin's box keel designs;

The tunnel isn't really part of the "magic” It is used to reduce draft when necessary. Both earlier and later designs do not use the tunnel if the design is not constrained by draft. Meaning no denigration of Ken's theories about the tracking ability and the "rail like" ride of Robb White's boat but one of the definitive feature of these boats is the fact they have a mighty big keel, tunnel or not.

He did indeed seem to be optimizing for a speed of around 15 knots. Over and over again the claimed top cruising speed is 17 to 18 mph.

He put a great deal of effort into shaping the box keel for a smooth flow of water to the prop. He remarks upon that often.

He repeatedly remarks about extending the shaft to get the prop in clean water behind the keel. He cautions against putting the prop tucked into a more conventional location.

He is very specific about the shaft revolutions, horsepower, and propeller, often even as to a specific brand of propeller.

He would often make remarks on the benefits of a flat shaft angle and often skewed the shaft a few degrees off centerline to minimize the effects of the propeller's tendency to favor one side in a single screw installation. He never remarked on this shaft offset as a way to make things easier for the builder, always about removing crabbing tendencies.

Light weight construction didn't appear to be a big part of it. He didn't make a big deal about working to keep weight down. Top hamper yes, but not weight so much. In fact one 28' model need 2000# of ballast. I would think adding that much weight would have been the subject of more pointed remarks if overall light weight were an essential part of the design.

He is very specific about the builder not modifying his designs. As many more folks were way back when, he was conservative about excessive top hamper. But besides the personal vanity of not wanting his designs modified perhaps the specifics of the underhull design were optimized enough that unknowingly changing some things would degrade performance enough to render the design a failure.

In short I believe that perhaps the "magic" comes from getting many smaller details right. The genius was in understanding how to do that.

Fred's observations about the marketplace leaving 15 knot boats behind as the technology delivered more powerful motors makes a lot of sense to me.

 

There is no planing at all! Some stupid questions:
1.
According to Savitsky, planing lift force is generated by bottom trim angle (4..5 deg typically). Here in Leo boat, there is a small trim angle at 12 to 24kn, but maximally only 0.6 degrees at 13 kn.
2.
Where comes the 10000N lift force at 24 kn, as the trim at 24 kn is 0 deg? Not a Savitsky type planing lift!
3.
The max sinkage force is 2000N at 12 kn. This is almost nothing if compared to hull total load = 9375kg = 92000N (46:1).

If one scales speed by 1:1.7, lenght by 1:3, area by 1:9, volume by 1:27, one gets data for Tornado cat size hull sailed on a single hull. Interesting. Why this types of hulls are not used in Tornadoes, if there is this sort of a lift force and wetted area benefit without any trim angle?

stunned,
redu

 

"I find it very hard to imagine a design field that is forever in search of a 3% performance improvement would miss a 15% one."

Maybe the combination that makes the concept less attractive is the boats where the performance was reported needed to be kept light and narrow so had a limited payload - which limits applications."

I think by reversing the paragraphs you have solved your own question.

Todays boat buyer seems to "need" a house afloat , a roomaran that can be used as a cottage.

There are far more boat cottagers than cruisers so the designers and boat assembelers give them what they want.

Even a rag bagger on a world cruise will spend 65% to 90% of the time not underway .

With marina slips being priced by the length, a cube of a vessel is the cheapest house afloat to maintain.

Would the builders create or at least outfit boats that can live full time on a mooring this sillyness might go away.

The tech is Off The Shelf , but there is little demand for it , so far.

FF

 

I would be comfortable stating that as soon as a hull, while accelerating, stops sinking and starts to rise due to dynamic force, it is semi-planing. This is often stated in terms of VCG. As from standstill the hull accelerates and VCG drops is termed full displacement mode, as VCG rises again semi-displacement or semi-planing, and as VCG passes above static level the hull becomes full planing.

Don Blount uses simple Volume Froud numbers, <1.3 Displacement, 1.0<3.0 Semi-planing, >2.3 Planing. As can be seen there are no hard barriers.

Odd Faltinsen mentions Fn <0.4 as being displacement, Fn >1.0-1.2 as planing, and 0.4-0.5 <Fn<1.0-1.2 are called semi-displacement vessels.

Will every thread on this board eventually devolve into a debate on the merits of Akin's Seabright hull? Oh how I wish someone would build a couple of models and do some solid research.

 

"Will every thread on this board eventually devolve into a debate on the merits of Akin's Seabright hull? Oh how I wish someone would build a couple of models and do some solid research."

Only the discussions with folks that can accept the speed ranges where the hull shape is perhaps more efficient than the standard models.

"I would be comfortable stating that as soon as a hull, while accelerating, stops sinking and starts to rise due to dynamic force, it is semi-planing."

The Atkin would by your definition be the first to be semi plaining , as the stern rises at very low speeds.

If you would design a "box boat" on the Atkin principals , as part of the experiment , I''l try and get it built.

Since its boxable , inexpensive labor and welding skills (if aluminum) or abilities with foam core are more important than location.

Since its an experiment I would get the hull and deck built and install a used car auto gas conversion . At only 50 to 150 hp required , any old car engine would do fine just to take measurements.

If the boat is not successfull it could always be used as a fish boat , the shoal draft & beachability would sell it.

Should it be successful a real interior and a 150hp Iveco diesel would get installed as the boat is finished.
It could easily be shipped to be finished , by a shop that understands the horrors of weight.

The design hassle is the boat would be shipped to the EU , so would have to meet the EZ specks.

IDEAS???

FF

 

Tad,

There is no consensus on what constitutes semi-planing or full-planing and it makes no difference unless a person is a slave to such definitions anyway. A soon as someone accepts a definition, someone else will show an example that conflicts with it. I would agree that your first sentence is as good as any definition if one is needed. The second and third sentences are questionable to me.

If you look at the sequence of photos of one of my boats, the VCG appears to remain at a constant level or rises above static. http://www.bluejacketboats.com/Bluejacket 24 photos.htm (near the bottom of the page)The maximum trim angle is 2 degrees which occurs at top speed. This is intentional and a result of a design optimized for planing at low speed. There is a consequent loss of a bit of top speed due to a trim angle less than the optimum 4 to 5 degrees. There is more than the usual wetted surface and less wave making throughout the speed range. Higher than the usual wetted surface due to lower than optimum trim angle results in the loss of top end speed.

That is my analysis of what is going on here and perhaps it will give something to discuss other than the Atkin tunnel. I like the Atkin tunnel and think the box keel concept has a lot to offer but don't have the wherewithall to tackle it.

 

Thread Questions:

Hi Leo,

I very quickly & roughly modelled a hull form to match yours for discussion purpose. I have used Molland's method for slender vessel resistance. For my calculations I have mirrored the hull so the sponson CL are at 3.2m apart. The calculations do not factor in wave resistance between the hulls.

I would not try and incorporate Stravinsky's method of calculating planing in this case.

From Molland's computational results, the hulls are operating in "Displacement" mode from 0 knots to 8.7 knots.

After 8.8 knots up to 13 knots the hulls are operating in "Semi-Displacement" Mode with a Froude number of 0.3 up to 0.5

After 13.5 knots the hulls "Semi-Displaning" with a Froude numbers higher than 0.5

Although the term "Semi-Planing" is widely used in the profession, I personally would not categorise this hull form under that term.


All the best,
Noah

 

Fred,

One boat testing would be (to my mind) useless. It only provides opportunity for various unsubstantiated claims. At the bare minimum I would have two hulls of the same length, beam, and displacement, running at the same speed side by side. Perhaps on a yoke. Ideally one would have a family of various forms, vee bottoms, round bottoms, and Atkin tunnels and maybe a pod/box keel or two.

I would be happy to provide NA support and make the 3D computer (Rhino) files. We need a sponsor to fund NC machining of the forms, which used to be about $300 for a machined foam blank. I can glass the hulls and do the towing here.

Tom,

I agree that firm dividing points are impossible and useless, too many variables! These days I think in terms of intent, penetrating hulls vs lifting hulls. The penetrating (I think that term is from Tom Fexas) form is much more interesting to me at the moment. Though I am working on a VSV this month.

As far as I know VCG movement (hull sinkage and rising) is only measurable in a lab, not IRL. Theoretical at best!

 

How are you going to replicate the effects of the designed propulsion system with a towed model???

Tank testing towed models will prove absolutely nothing. Wasn't there already a guy who tank tested a tunnel-stern Seabright model one time, only to report that the hull had so much drag that it was like towing a boat with a barrel tied to its underside? What else would you expect when you're dragging the hull instead of pushing it *with a propeller* as it was designed to be driven!

Look ... one of the VERY significant reasons why these Atkin tunnel-stern Seabright hulls are reportedly more efficient than other boats is likely because of the conditions in which they are propelled. If you're not going to bother propelling your test models the same way the full-sized boats are supposed to be propelled, what good will your results be?

If any of you actually think you can get sponsors for some REAL testing -- and by this I mean propelling the boats properly, as they were designed to be propelled -- then I will do some valid and reproducible testing for you. But let's forget the models and the tow testing and take a look at another option ...

I propose to build two half-length boats and power them with real engines and propellers, then drive and test them with real people at the helm -- one a tunnel-stern Seabright skiff and the second another high-efficiency power boat, perhaps Bateau's Nina or an Atkin Ninigret?

You guys select the boats and I will build and test them. Then when I'm done you can use the same boats and run your own tests to either confirm or deny my results if you feel like it!

I have a huge, shallow tidal flat on the Visayan Sea across the road from my shop, and the conditions here are almost always calm, so there is nothing to stop me from averaging a dozen or more hours of testing a week after the boats are built. I would power them with cheap China-made 6.5 HP gas inboard engines, and I already have the first engine. A second one will only cost $100 or so, and I will provide the props, prop shafts, shaft tubes, stuffing boxes, etc. for both boats myself. Then we will have two boats to test side by side for some 'real world' comparisons.

All I ask is that someone else comes up with the money to pay my employees $2 an hour plus materials to build the boats. They won't be 'pretty' but the hulls will be stiff, strong, smooth and accurate -- ideal for running weeks or months (or years?) of comparison testing of any kind you specify.

So let's run the numbers and see how much it might cost:

Second engine like the one I already have = $100
8 sheets of 4mm marine plywood for hulls = $56.
Epoxy and glass for joints and filler putty = $40.
Station mold plywood = $20
Paint and fairing compound (Bondo) = $30
Lacquer thinner, sandpaper and other consumables = $50
--------------------------------------------------------------------------------------
Total $296 so let's round it off to $300 for simplicity's sake.

How long will it take to build each boat ... two weeks? If so, let's figure a month of labor to build both boats. At 200 hours a month (yes, people still work 6-day weeks in this country) and $2 per hour that's $400 for labor.

It looks like I can have two half-sized boats built and ready for testing for a total of only $700 -- and for this we get two hulls capable of being used by full-sized human beings, which means direct comparison testing in real world conditions is not only possible, but also practical and affordable.

So ... if any of you think this option is worth considering, let's figure out a way to come up with the money ...

The guys in the [bolger] Yahoo group challenged themselves to pledge small amounts of money to be paid to the first person who builds a 'SuperBrick', and from what I recall their total pledges amounted to something like $1000. I wonder if there are enough people interested in these tunnel-stern Seabright skiffs to come up with anywhere near the same amount of money via small individual contributions? If not, is there anyone here with connections to a company that might be willing to sponsor such a project?

If we proceed with this scenario you're all welcome to come over here yourselves and run your own tests after I've finished building the boats. Or come over now and help me build them, then you and I can run our tests side by side. Another option would be to come up with enough extra cash to have me ship the boats to you when I'm done running my first round of tests here. Then you can independently confirm or deny my results by running your own tests on your own backyard, and maybe we can report on this issue -- confidently and conclusively -- once and for all.

Just a thought, for whatever it's worth ...

 

As a person who is at least partially responsible for the Atkin thread devolution I am willing to put my money where my mouth is. I’ll put $100.00 up as a share of the CNC foam blank costs for an Atkin hull if we can come to an amiable decision as to which one. My heart’s desire and mind’s eye selection would be a 28 foot boat capable of overnighting at a speed of 15 knots. Looking at the list of plans available at the Atkin site Jersey Blue, Naiad, or Spermaceti all seem close. The photos show Spermaceti to be a lapstrake hull; perhaps this would be a problem?

Perhaps a fresh thread similar to the “proposal” threads could be started if others desire to bring this project to fruition.

 

I like all three, especially Spermaceti, yet even though they are all Seabright skiffs none are the "tunnel-stern v-bottom" variety ...

If I'm not mistaken, weren't we discussing the testing of a tunnel-stern v-bottom Seabright skiff such as Heron, Nibble, Rescue Minor or Shoals Runner?

I have nothing against the others, in fact I think some of the non-tunnel-stern and/or non-v-bottom Seabright skiffs are better sea boats. But I think it is specifically the tunnel-stern v-bottoms that have gained the reputation we all seek to confirm or deny.

 

Hi Ken,
My perception is that the thread wanders about a bit. Rather than repeat my thoughts on the Atkin designs refer to my previous post # 76.

I do not fully understand all of the implications of the proposed model test but assume that explanations will be forthcoming as something comes close to actually happening. I can imagine the problems of actually comparing apples to apples. It would be my hope that regardless, enough useful data can be developed to make the exercise worthwhile. I am willing to step up to the plate if we can all agree to test something that has relevance to my personal goals.

 

Ken,
Another obvious thought, you have a completed Tolman hull. While I fully appreciate the work and cost to complete this project to finished standards it is most frustrating to be at the edge of my seat waiting for real world data. Even if my theory that a measurable part of the Atkin “magic” consists of skillful optimization is correct your hull should show us something.

Stick an old car engine from a wrecked little Japanese car that makes 30 hp at 1500-2000 rpm in that thing with a direct drive and lets see what it will do

 

With respect.....

Towed model testing is the basis for a great deal of our understanding of hull form characteristics and effects. As far as I know, all professional Naval Architects regard model tests as valid science. Off course there are limits to what can be discovered with model tests. Apparently towed model testing does not support the claims of superiority for the Atkin hull, but that does not invalidate the data.

Towed model testing will provide a baseline resistance comparison. Once we have that you can then run self propelled tests. To be close to accurate this will have to be done with one propulsion unit, motor,shaft, bearings, and prop. Trying to create two identical power units is too difficult. The power unit and the propeller will have to be scaled to fit the hull.

Combining data on bare hull resistance, self propelled data, and open water propeller tests will give us data on wake fraction, relative rotative efficiency, thrust-deduction coefficient, propeller efficiency, and ultimately hull efficiency. This is covered in J.N. Newman's Marine Hydrodynamics, 1977.

 

This is why I suggested a comparison test with two real boats, since it will more than likely provide conclusive results. I am assuming here that we actually want conclusive results ...

I suggested Nina or Ninigret as the second hull in a comparison test because these boats have been reported to be very efficiently driven. If a tunnel-stern v-bottom Seabright skiff can beat the efficiency of a Nina/Ninigret in a direct comparison at 0-15 knots, maybe there really is 'something special' going on here ... but how will we ever know unless we do a direct comparison?

I felt that maybe I could offer something of real value here since I live in a place where materials and labor are very low cost, and I have already built a 22' hull with a Rescue Minor bottom -- not the easiest hull to build by any means, but at least I know what I'm getting into. Then again, if others feel a tank test with a towed model will answer the questions we have about these boats, then let's try that instead.