Often on websites you find something like the following …
Displacement hulls – will not exceed hullspeed or not exceed it by much.
Planing hulls – can exceed hullspeed by lifting over the water and planing at speeds well exceeding hull speed.
Semi Displacement hulls – they can exceed hullspeed by some degree but at the cost of some excess power and fuel consumption.
Catamarans and other multihulls are rounded shapes and narrow so they cannot plane like a flat bottomed boat.
There are two modes a boat can have – displacing up to hull speed then planing mode above.
“Hull Speed” is the point where a boat reaches a point of extra drag because of the position of the waves it generates as it travels. It is generally quoted as 1.34 x (square root of waterline length in feet) is the speed in knots.. For a 16ft boat the hull speed is a bit over 5 knots.
These are all demonstrably bunkum. They are illogical and arbitrary explanations of natural phenomena. They belong with “jib slot effect” and “wings develop lift because the air has longer to travel on one side”.
The Questions are formed about planing and displacement.
The normal explanations on the net are cloudy thinking and the more you think about it … the more absurd they become. They just don’t fit the evidence and the explanations get weirder and weirder the more you ask questions.
I finally put this together on a drive with and old school friend, Graham Murray, on a trip to see the Newick designed trimaran he’s been building for a few years. As high school students we had both drafted a letter to Alan Payne, the great Australian yacht and boat designer about becoming boat designers. Well Graham did everything actually. Armed with Alan’s reply I headed off to study Chemistry and Graham became an airline pilot. We ran into each other on the net years later and I went to see the boat he was building up the coast.
Anyway the question that came up in a car about how monohulls have “planing” as a way to exceed this “hull speed, but multihulls have their “slenderness”.
It bugged me really badly … because the natural world should not have TWO explanations for one phenonenon.
The second part that bugged me is that there was on one hand this idea of “displacement sailing” and at some particular speed it became “planing” .. er, except for catamarans and trimarans which “don’t plane”. Why should displacement suddenly turn into planing and ignore multihulls all together?
So … I do what I do and thought about it for a few months. That’s what I do – when I see an illogicity I have to bother it. It took me years to get some basic grasp on Quantum theory 🙂 The result of my thinking is out on the net, but it is down in the corners and not put together in a concrete way. Unlike the crappy normal explanation which is widely repeated with authority.
Ok … I am an iconoclast – on a bad hair day anyway. So the results of my thinking …
Planing and Displacement add together to equal One.
I don’t have any problems with the terms “lift” or “displacement”. They are consistent terms right through the speed range.
But “planing” is something that people say “suddenly appears”. I don’t disagree with the EXPERIENCE we call planing. But Science tells us that things don’t suddenly appear but apply consistently.
And then it is said that some craft such as multihulls are non planing. If they generate lift (which I demonstrate they do … then does “non planing” really exist as a category?
It seems to me a boat … as soon as it is moving is developing some degree of lifting forces … either in a positive or negative direction.
It seems absurd that “planing” forces that “appear” when the boat reaches a certain speed. Surely the logic is that the same rules apply from the very beginning of when the boat starts moving. So logically there are lifting forces as soon as the boat is moving.
For scientific objectivity we can say that their total might be positive or negative. That they will either tend to lift the boat or they might pull it down into the water. The reality is that the lifting forces along a hull tend to be a mix of positive and negative – and at any speed they will add up to make a non zero total … they will have a small influence at low speed and a growing influence as the boat goes faster.
If a boat is said to be 100% planing …. that means it can’t displace anything at all. It actually cannot be touching the water … which means it cannot develop forces to plane at all. It cannot interact with the water at all. It is all mutually contradictory.
I would tend to say that lifting forces … or better word “vertical lift” is made up of two components.
Immersed volume of the hull plus dynamic lift.
When added together they equal the weight of the boat.
Planing lift + water displaced = weight of boat, gear and crew. And that gives us an equilibrium.
So how do you know the boat is generating lift? And how all multihulls probably plane.
Generally there is a depression behind the boat. The weight of the water in that depression equals something the lift from the dynamic lift.
Here it is behind a motor boat.
And just to show that the categorisation of multihulls into non planing is also arbitrary and incorrect … I offer this picture … if you see a smooth hollow behind ANY boat … the force that pushes the water down behind the boat equals the force that pushes the hull up.
That leeward hull is getting significant lifting forces.
Hull speed is not a place where the boat goes from “displacement mode” to “planing mode”.
There is something theoretical called “hull speed”. but every boat can exceed the hull speed by some amount with adequate power. The exception might be a perfect box with the bow end immersed and pushing a big bow wave … but move the weight back a bit so you have a clean entry and …
In the case of a hydrofoil boat there is a hollow in the water surface over the foil/s. Just like if you fly an airplane real low you get a depression in the water underneath it. There is some up force so the water surface makes the compensating down force visible.
But what we have is different boats that can move through the hull speed more or less easily. The limit they are up against is wave drag … as opposed to wetted surface drag and some induced drag.
The wave drag increases quickly as the boat gets to hull speed – so that part is true. But it seldom or never limits the speed to hull speed – most boats can exceed it to some extent
Why do Some Hulls exceed “Hull Speed” more easily. One explanation for all hulls.
There are two traditional methods of beating the wave drag. They are often said to be separate.
One is make the hull very slim with the displacement well spread out. This way the wave drag doesn’t grow as quickly around hull speed. The boat doesn’t “push” the water far at an point as it moves around the hull so no large waves are created.
The other way is to develop lift forces that reduce the displacement. The displacement is what causes wave drag. So if you reduce it … you reduce the wave drag.
There is also a generality about what sorts of boats can get past hull speed more easily.
Basically what happens is that at any speed there is a wave at the bow and according to Froude there will always be a wave train behind that. The distance between the the peaks of the waves (wavelength) varies with the speed of the boat .. or the speed of the wave train if there is not boat involved.
As the boat goes faster the second wave after the bow wave moves further back.
The maximum drag occurs when the bow wave … is at the bow and the second wave is at the stern. The reason for this extra drag is that the boat now has a trough under most of the hull … it is the also the part of the hull that is supporting the weight. So the boat sinks deeper in the water.
If the boat carries a lot of its weight in the mid section … like a heavy commercial boat or older yacht … it sinks down substantially.
An accurate painting by artist Alexander Creswell makes the point
Or a tug being driven fast.
So why doesn’t a light dinghy or powerboat or multihull bog down and why can it beat this barrier. And can it be explained without breaking monohulls and dinghies into two groups.
Two reasons …
- first of all all these boat types are not that deep so cannot develop a deep wave.
- Second … when the peaks of the waves are at each end of the boat the boat will sink down if it has too much volume in the trough area – so the boat will be trapped in its own wave train. Boats that are slender (multis) or lighter displacement have adequate flotation at the ends, where the wave peaks are, not to drop into the trough significantly.
An example of how a “displacement hull” can go much faster than “hull speed” proves the consistent explanation.
The original theory would say the “DISPLACEMENT” hull can’t go much faster than hullspeed “EVER”.
But here we have an exception.
What about when a truly displacement boat surfs on a big ocean wave.
It can go much faster than its hull speed … why? . What is happening there … in effect the wave the boat is on PLUS the boat’s own wave at hullspeed end up supporting the middle of the boat better so it doesn’t sink down as much. The peak of the ocean wave is holding the boat up and preventing it from sinking. The traditional theory falls apart and the one I am putting forward explains it beautifully!
If it was a matter of “displacement hullshape this just couldn’t happen. But here is a photo from a famous event … to Australians anyhow! Not to mention old timers at the New York Yacht Club.
Photo by Stanley Rosenfield … see many more photos at mystic seaport http://www.rosenfeldcollection.com/inde … rtRow=2661
This is the photo of the Australian 12 metre “Gretel” that took the first race off the American’s in the Americas cup for decades. She caught a monster wave. 12-metres are classic “displacement boats. But here we have a wave preventing the boat from sinking down and she can greatly exceed her normal speed. This photo is a moment of some pride for sailing Australians. I used to slip and unslip that boat years later – it was pretty cool!
The ocean wave cancels out the trough that would normally be under the hull. The interesting thing from the photo and for sailors is that for an old heavy boat like this it will never start planing down the face of the wave like a surfboard. The wave comes up from behind and the boat just ignores the point where a surfboard or racing dinghy would start surfing down the face. The big heavy boat only starts moving fast when the crest of the wave is UNDER THE BODY OF THE HULL. I would expect that happens because the gravity effects of the boat falling down the face are much less than the effect of the crest of the wave supporting the boat.
Remember that the classical theory of displacement hulls cannot explain this behaviour at all – it says REGARDLESS of the amount of power you put in the boat will never go much faster than hull speed. But in reality it happens all the time.
What happens to the stern wave when you are going faster than hull speed?
Lots of sailors know the answer to this one. The reality is that there is a bow wave and then the second wave will be the right distance behind the first according to the equation by Froude.
So at slow speeds the second wave will be a very short distance behind the bow wave and as you go faster the second wave moves further back. At hull speed it is near the stern of the boat. So at higher speeds it is behind the boat.
If the boat is doing around 5 knots the second wave will be about 16ft behind the bow wave. So a 16ft boat would be at hull speed, an 8ft boat would be truly “planing” (it’s ok to use the word descriptively, but not technically) and a 25 footer would be well below hull speed.
Here is a picture of my sailing a Goat Island Skiff at Mooloolaba. See that the stern wave is about 2/3 of a boatlength behind the stern.
So if we do the calculation. 2/3 of the hull is in the water and the second wave is about 2/3 of a boatlength behind the boat. This is about 20ft. Square root of 20 is about 4.47. Multiply by 1.34 (the constant for the dimensions we are using) is 1.34 x 4.47 = 5.99. So the speed here is 6 knots. Not that fast for a Goat! John Goodman and I had his between 9 and 11 for hours each day in the Texas 200!
Summing up about Planing and Displacing
So as you see … the whole thing is not about different types of hull … it is a continuum. Every boat exceeds hull speed to some extent … but lightness and spreading the hull volume more evenly over the length also help.
Every definition of planing, semi planing and displacement is flawed because every boat has lift from buoyancy and dynamic lift from movement (which can be positive or negative – it actually varies through the length of the hull.
Water displaced + dynamic lift = total weight of boat, gear and crew.
planing force + displacement = total weight
So “planing” is something that belongs in descriptive language rather than being a technical term. Like saying the moon is “golden”. So it can still be used for something you experience … but it is something that just can’t exist in a 100% pure form …
… because if it did … the boat would not be touching the water at all … so could not develop “planing lift” from the water. So it always exists in combination with “displacing”.
Instead of planing and displacing being mutually exclusive, from the moment the boat starts moving their sum is equal to the weight of the boat, crew and gear.
More goat pics here … I need to link back to flickr where the images are kept.