Critical speeds for Semi-Planing

Ken,
I'm extremely interested in the whole Atkin hull idea. I know you have given the subject even more thought than I have. But doesn't the Rescue hull you mentioned raise up in the aft end at speed? Up indicates lift to me.

Do you still have that Tolman hull you built? If so, put an engine in that bugger and give us a report!

 

Lift and suction can happen at the same time, they are not mutually exclusive ...

Suction comes from the prop moving water into and out of the concave tunnel pocket. Lower pressure (what I call suction) is created in this pocket because the water moves faster into and out of the tunnel pocket than it does when moving in a straighter path below the tunnel pocket. As we all know, when a fluid moves faster here than there, it creates relatively lower pressure here and higher pressure there. And since water is essentially an incompressible fluid, the resulting 'suction' holds the boat tight to the water's surface and makes it feel "like it weighs 10,000 pounds".

Lift comes from a much simpler to understand mechanism -- hook in the aft tunnel section at the transom. This hook forces the propelled water downward, and the result is an 'equal but opposite' reaction which pushes the aft hull bottom upwards.

In other words, the effects of suction and lift are occuring in different locations on the hull ... and they have different observable effects. Suction occurs in the tunnel pocket which is well forward of the transom, and its effect is to lift water into the pocket and hold the boat tightly to the surface. Lift occurs at or near the transom, and its effect is to raise the aft end of the hull which keeps it from squatting.

Ideally the tunnel pocket will be as shallow as possible so it doesn't have to waste any more power than necessary to lift water into it. After all, power used to lift water is power unavailable for forward propulsion. Similarly, the hook in the bottom at the transom should also be as little as needed to counteract squat without lifting the stern so much that it forces the bow to dive, thus creating high-speed handling issues.

Yes.

Okay, just send me about $7000 and I'll get this done for you in no time ...

 

Agreed. Place a virtual surface along a streamline, and you don't disturb that streamline. Thus, the virtual surface has no net effect. It does, however, bring about the irritating problem of now having to deal with a mathematical hull form whose geometry is speed-dependent, somewhat detracting from the elegance of the idea (and the link to its physical basis).

"Tough" might be an understatement, Leo, unless you've come across some new formula that you're keeping secret Of course, from my perspective (more on the engineering side than the math side) this problem can be somewhat simpler- ignore/guess. To get an accurate model of wave breaking, though, would be difficult indeed... and would likely end up taking a fair chunk of computational power, again detracting from algorithmic elegance (and reasonable computation times).

 

Howdy Marshmat,

Thanks for that clarification about streamlines (doh!) ... I just had forgotten the language!!!

The point I was trying to make is maybe in a way it makes sense to forget about where the hull ends in reality and look at the pressure or energy or momentum system instead.

But as a tool for thought ... whatever the length of the hull we put in the depression the pressure distribution remains the same with the same amounts of positive and negative (vertical) lift.

So if it is as short as the original boat it will look like it is "planing" - but if it is extended to fill the whole cavity then it would be at "hull speed" and would only be "displacing"

Again this shows me just how arbitrary the definitions of "planing" and "displacing" are. They have no relevance to the physics at all (or little).

This example seems to show it is not possible to define "planing" "displacing" or "semi planing" in terms of energy, momentum, wavemaking or pressure distribution.

They are historical terms that seem to me to have been misapplied to the physics of the situation.

At the best they simply define some optimal hullshapes for specific conditions or are useful at a conversational level and are by no means rigorous.

Best wishes
Michael

(neglecting boundary layer and viscous drag effects of the different hull lengths for the purpose of the example)

 

... so to answer Leo's question

There are none - ever - period

However anyone is at liberty to create an arbitrary value (or range of values) for the ratio of lift in proportion to the rest displacement of the boat and define it as semiplaning.

But it would be a - kludge (as the computer guys call an inelegant solution that kinda works).

Same for "modes" of "planing" and "displacing"

Best wishes
Michael Storer

(I am being particularly forthright and one eyed on this as a challenge for someone to show the the different modes actually exist in some sort of coherent way)

 

I am certain the Atkin hulls do not follow the same "rules" as a nice standard displacement or plaining hull.

Instead of a std bottom at least half and perhaps as much as 80%of the vessels displacement is in the box keel, so that resistance would be more in line with a submerged body.
Although the flat bottom may have a good deal of lift as would a water ski.

Atkins unique underbody is claimed to be quite efficient , but at modest speeds.

I guess if a boat can run SL 1.8 or 2 with the same power it would take to run a std boat at SL 1.2 it is quite a good deal, IF thats the speed range desired.

"Leo - what if we conceptually extend the hull back to fill the hollow without disturbing its shape.

Does that change any of the effects we are talking about?"

Perhaps the prop wash IS the filled in missing piece of the hull?

With the water flow being very accelerated in the last 1/5 or 1/6 of the hull , the stern can not sink as the bow climbs up on its wave, the stern is also lifted , hopefully keeping the hull angle of attack in the proper range



To get beyond the SL that Atkin thought would suffice , a simple trim tab , being lifted to not depress the bow should allow the best angle of attack to be dialed in.

When double power was retrofitted into an existing hull the boat began to get spooky and bow steer. Too much stern lift , the simple solution was a set of running strakes to lift the bow as much as the stern. Perhaps the trim tabs would do as well.

Weather a "normal" hull is plaining or not is for the linguists , but what the accelerated stern wash of the Atkin should be called is ????

Interesting tho that the more efficient boats seem to leave little stern wave , something in common with the Atkins and other experimenters with this hull form.

FF

 

I think you're right on the money, Fred.

Building one of these boats with significantly less hook at the transom -- and adding adjustable trim tabs to replace the missing hook whenever it might be needed -- would allow the trim tabs to control the amount of transom lift, and thus amount of bow dive, without resorting to lifting strakes at the bow.

The ultimate goal here (in terms of maximizing fuel efficiency) is to balance stern lift with bow dive by deflecting the flow of water as little as possible.

The more you deflect the water from a straight line path the more energy it takes away from forward propulsion. This is why you should never make the tunnel pocket any deeper than necessary too. Adjustable trim tabs can dial-in transom lift, but they will never reduce the drag created by a tunnel pocket that is too deep.

 

I've enjoyed reading this thread. I can't say that I would ever be one to try to formulate the math to predict a solution for every possible hull shape and speed. But I can understand why there is so much interest in doing that. In fact, if the magic formula is ever arrived at I would only be too glad to use it.
I would like to just mention a few comments that have come to mind while I have been reading.
One is that no one has mentioned that there is a significant amount of water that moves forward as a hull passes through the water. Is that being accounted for in your math?
About 30 years ago I met a fellow who had been Lindsey Lord's assistant during his research into why there were structural problems with PT boats. He said the first thing they did was to make models for towing tests of every boat they could think of that had a reputation of being a fast boat; this included sailing vessels as well as powered types. He said they were astonished to find two displacement sailing vessel models which did not demonstrate a speed limiting speed/length ratio. One was Herreshoff's Ticonderoga and the other was, if I recall correctly, a model of a sailing vessel of about 140 or so feet length on deck named the Puritan. If the name wasn't Puritan it was Pilgrim. I guess you can see that I wasn't familiar with this boat. But I will bet a nickel that the name was Puritan. So you can plug their lines into your program and see if you get similar results as the towing tests did.
I was much perplexed at my first exposure to Crouch's formula. I thought surely one had to take into account the hull characteristics to have any sort of useful results for predicting the speed of a powerboat with a certain amount of power. But, over time I observed boats whose shapes I intuitively relegated to the really slow category performing much better than my expectations. And surprisingly, Crouch's formula seemed to fit them to a T.
The thought of all this number crunching makes me long for the good old days of empirical design evolution. But I have to admit that I can hardly afford to build a proven model, much less an experimental one.

 

I think you're referring to the water we have previously referred to in other threads as the "accelerated boundary layer water". I don't do math on this topic myself, but I do recognize that the Atkin tunnel-stern skiffs seem to utilize this moving boundary layer water to enhance their fuel efficiency much more than other boats.

These boats use the prop to push on this accelerated water when most other boats prefer to have clean, non-moving water to push on. The tunnel-stern hulls also trap as much of the moving boundary layer water as possible under the hull via the inverted vee aft, thus they can take advantage of this moving water more effectively than other designs.

 

Sorry, kinda off the topick; I'm trying to use Michlet for the first time, are there any tutorials or anything avalable anywhere?

 

Even the Devil has an advocate. With all the talk about the accelerated water from the hull boundary layer being an enhancement to drive force at the propeller, it has taken on the aura of a fact rather than a theory. Where is the proof? Not saying that it isn't true but, it has not been demonstrated as yet. Neither has empirical data been gathered to show that this is the source of enhanced performance of Atkin's hulls nor even if that performance exists.

Question: If the propeller operates in accelerated flow, does that say that the driving force will be greater than if it operates in static water?

It is clear that the slip, or apparent slip, at the propeller will be different if it operates in water that is flowing at a different speed than the ambient water surrounding the boat. Does this make the driving force greater?

At rest or low speed acceleration, any propeller has a high percentage of slip and driving force is derived from water mass thrust aft following Newton's laws of motion. At high speed, the slip is as low as 5 percent and the propeller is operating more like a screw and the amount of water being thrust aft is relatively lower than at rest. At high speed, the water is acting more like a solid than a liquid so the screw theory seems to be more applicable.

In either case, driving force depends on momentum change (over time) in the water resulting from the spinning propeller. Does this change the way we should look at the Atkin hull shape?

Or maybe I have confused myself.

 

Barend,
there is that michman.html in the package. You'll need all this stuff.
The key file is in.mlt, where all the hull and associated stuff is stored. Text editor is used to modify. Simple typing errors may turn fatal: Michlet wouldn't start at all.
It may be a good idea first to use the mathematical hull types. Easily generated. But your proprietary hull may turn to a hard work, if a math type does not fit.
There is a high learning threshold. For instance varying hull sectional shape, side profile or top view profile is made using cryptic decimal numbers: Learn them or better write your favorites down. Later you'll find Michlet fast, in playing "what if" for instance. Varying draft, load, lenght, sinkage, trim, etc is easy indeed.

Used this tool for 8 years, just great!
reino

 

Tom - I think you are quite correct. That Atkin did not use the tunnel hull on most of his boats seems to indicate something to me.

He was one very smart dude - if he had found something with a significant performance improvement he would have run with it - after all it would have been a revolution in boat design - and what designer doesn't want their name associated with something like that?

There is huge money to be made for such a breakthrough - and he was a commercial designer.

If it even had the SAME performance on a lower draft it becomes a highly useful tool - and so you would expect to see it more repeated in his designs.

I think at the best he had found a way of reducing the negative effects of such a setup - and that is where the efficiency argument is. Not as efficient as conventional, but not too bad either - unlike some other tunnel setups from less intelligent designers.

Best wishes
Michael Storer

 

I guess we should clarify what you mean by "it has not been demonstrated as yet" before we proceed much further here. Robb White seems to have demonstrated -- to himself anyways, if not to others -- that his boat achieved some pretty high mileage numbers. He wrote about it to inform others, but I don't know if he every physically demonstrated this performance to anyone else. Here's what he wrote:

"In case you have never read about the boat, I’ll give you the specs:
It is twenty feet long and 76” wide. It is strip built out of tulip poplar
and is powered by a three cylinder Kubota Diesel tractor engine rated
at 20 hp. It will run 20 knots in six inches of water and gets about
28.6 nautical miles per gallon of Diesel fuel running at its most
economical speed of 10.5 knots. It is not for sale and I don’t ever
intend to build another one."

Google says 28.6 nautical miles per US gallon = 32.9 statute miles per US gallon, so it seems Robb's boat was an exceptionally fuel efficient boat at 10.5 knots (12 mph) ... assuming you believe him of course.

All I have seen so far are reports from people who have used such boats, and this is certainly not the empirical data you're referring to. I don't think Robb White gathered much empirical data, but apparently he did check his mileage at least once.

If his mileage figures are accurate, it makes me wonder if there are other monohull power boats in the same size/weight range that get similar mileage ... or even half this mileage at the same speed? Do boat manufacturers these days actually report mileage numbers like car manufacturers do???

Or perhaps we should question Robb's claim from the start, and not believe it until someone else tests the boat and confirms his reports with their own empirical observations -- possibly also supported by the observations of unbiased independent observers who will confirm that the tester is telling the truth.

Personally I don't see any reason why Robb would make up something like this. He wasn't in the business to build these boats for profit, and he didn't seem to have anything else to gain by reporting such good numbers. Then again, just because I believe Robb's numbers does not make them true ...

 

To save others looking for the detail:
http://www.robbwhite.com/rescue.minor.html

These hulls are basically a canoe with stern overhang. The clever bit is being able to get a big diameter (read efficient) prop to operate in such shallow water without ventilating. The broad stern does not harm stability either.

Fundamentally they are long slender canoe hulls so you would expect good fuel economy.

The same sort of hulls are being used on the efficient powered cats now only I do not believe any are using the tunnel idea to swing bigger props - this will no doubt be the next big step that someone will reinvent.

Rick W.