# How much less efficient would a planing hull sea kayak be?

Discussion in 'Hydrodynamics and Aerodynamics' started by mitchgrunes, Jul 11, 2020.

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### mitchgrunesSenior Member

How much less efficient would a planing hull sea kayak be?

Hi. I hope I'm not out of place posting here. I admit I don't understand much about fluid dynamics.

Some kayaks (e.g. race boats, sea kayaks, touring kayaks, and recreational boats) are said to be displacement hulls. This is probably done because displacement hulls are said to be faster at the power levels a human can provide over for long distances.

Others (especially whitewater playboats and ocean surf boats) are considered to be planing hulls.

I think a few kayaks lie in between, but most are definitely one or the other. (By the way, why is that?)

Unlike a speed boat, for the most part, planing kayaks don't actually leave the water, except in playboating maneuvers, or large-wave-forced-motions, both of which are possible with both types of hull. But planing kayaks, when moving forwards, do sit a little higher in the water at the front (bow) than at the back (stern). In particular, as best I understand it, water at the bow is pushed downwards, generating some lift, and tends to drag the surface current at the front inwards towards the bow.

Displacement hull kayaks work differently. They push the water out to the sides, and, if my understanding is correct, this possibly drags some of the water from underneath along with it. So the surface current at the front moves outwards.

As best I understand it, maneuvering is dominated by what happens at the bow, because the relative speed of the current to the boat is greatest there.

Planing hull kayaks turn most efficiently (losing the least speed) on an inside lean, like a bicycle. As best I understand it, this is because the boat surface that effectively pulls the surface water towards the boat on the side outside the turn is out of the water and is therefore ineffective. The pull on the opposite side, inside the turn, helps create the turn. Once the turn is started by a prying stroke outside the turn, the turn is continued by drawing the paddle in towards the boat on the inside of the turn. This makes easy to control, because after the initial pry, the paddle stays on the same side that your weight is leaning towards. If you lean too far, you can push against the water with your paddle.

Displacement hull kayaks usually turn most efficiently on an outside lean. As best I understand it, that’s because the pushing surface of the boat on the inside of the turn is lifted out of the water and is therefore ineffective. This means they are harder to stabilize than planing hulls, because it is harder to pull back hard downwards with the paddle than to push, and because an excessive lean may actually pull the paddle (on the outside of the lean) completely out of the water. (Note: slalom racers do sometimes lean into turns, even though they are displacement hulls. I don’t know all the reasons. Perhaps because you can draw very strongly??)

If you ice skate, this is somewhat like inside and outside edges on skates. Skating on one foot on an inside edge, the other foot is ready to stop your fall if you lean too far, and that feels secure. Skating on one foot on an outside edge, there is no other foot on the outside to stop a fall, so beginners, as well as hockey players who worry about being pushed, often feel unsafe on outside edges.

Let me define a sea kayak in a narrower sense than many people use, as a paddle powered kayak which has a length at waterline that is an order of magnitude or more greater than its width at waterline. They are designed to let you paddle relatively fast for long periods of time. Paddled by a strong kayaker, they sometimes go 1.5 or even 2 times "hull speed", because they are thin. (Planing hull kayaks can also go fast for long periods of time - but only when driven by gravity down the forward face of a wave they are surfing, which displacement hull kayaks can do too. Human power alone simply isn't strong enough to make a kayak plane for very long, though a few hydrofoil planing boats were designed for short sprints and extremely fit athletes.)

I know of one experiment to try to design a reasonably fast paddled planing hull kayak. In particular I think David Knight and Eric Jackson came out with a planing hull slalom race boat. I don't remember whether it was marketed by Wavesport or Jackson Kayak, but the design never became popular.

But the advantages of controlled stability are so obvious, especially in rough sea conditions, or for less skilled paddlers, that I wonder whether how much efficiency would be lost by a planing hull - or at least a neutral hull - sea kayak, that could turn efficiently on inside leans.

Can any of you answer this question, or know whether it has been tried?

P.S. I ignore here those sea kayaks, and surf skis (which some people consider to be kayaks), that have rudders. They are sometimes designed to turn most efficiently using the rudder, with little or no lean. For some reason, they are the fastest paddle powered boats, though rudders tend to be fragile and often fail.

P.P.S. If any of my explanations are incorrect, feel free to correct me.

Thanks!

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### Mr EfficiencySenior Member

No, humans are not capable of powering anything to plane, kayaks can certainly "surf" under the right conditions, and that extra motive power of the paddler might be useful to start it surfing, but for all practical purposes, you don't want a submerged "transom", that a planing hull implies.

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### mitchgrunesSenior Member

I could tell you to look up the Flyak, and similar boats, but the question of whether you consider it to truly be "planing" is irrelevant to my question. For the purpose of this discussion, most kayakers say a kayak "planes" if it generates enough lift that it tends to turn into leans, and my question was about how much efficiency a sea kayak designed that way would lose. Perhaps some people don't consider such a boat (except Flyak, et al) to "truly plane", but the term has now been used the way I used it, in connection with surfboards, kayaks and canoes, for decades.

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### Mr EfficiencySenior Member

On flat water ? Or surfing waves ?

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### BlueBell. . . . .

Mitch,

Welcome.
I think you need to do some research.
Leaning vs edging for example.
Direction of lean (edging in this case) is opposite to the direction you want to turn.
With half a horsepower, maximum, 1/6 HP sustained, there is no planing in kayaking.
Terminology issues perhaps.
More than you want it to be and for zero gain.

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### gonzoSenior Member

That is not correct. You are either planing or not. Each mode has different characteristics, so you need to be specific about which is it. Otherwise, if you call an apple a potato, the discussion makes no sense.

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### mitchgrunesSenior Member

As I said, I'm not certain the definitions of terms used in ship design are the same as those used in board and kayak designs.

Not on a whitewater playboat, or on some of the most playful surfboards.

Good point about leaning and edging. I hadn't thought of edging and leaning being different. I guess the direction and extent of chine curvature affect the way the boat turns if you lean it enough. But some of the fastest sea kayaks have virtually no clear chine or edge (alas, they tend to be rather unstable, and I don't love them), so my explanation of turns in terms of pushing and pulling boat edges doesn't apply to them.

Another explanation, which might also apply regardless of whether there is a clear edge, lies in pushing or pulling the water up or down at the bow. A "planing hull" board or kayak thrusts water downwards at the bow as it moves forwards, which by Newton's third law pushes the boat upwards - often a desired effect in a hull designed to play with waves, because skimming across top the waves makes rotating and moving over them easier. When you lean the boat, in addition to creating effects from edges, I presume that thrust direction is also leaned, giving it a sideways component, that pushes the hull into the lean. In contrast a "displacement hull" board or kayak effectively pulls the water under the boat upwards (then pushes it outwards), pulling the boat downwards. I think that pull too is leaned when you lean the boat, so it pulls the hull outside the lean.

I don't know who originated the idea of "planing hull surfboards". I think the idea was copied to kayaks and canoes by people like David Knight, who BTW has excellent academic and professional credentials in ship design, though I'm not sure if he was the first to do so.

You can see discussions of some of the benefits of "planing hull" surfboards and whitewater playboats at

Tomo Surfboards: [MPH] MODERN PLANING HULL 2011 http://dantomo.blogspot.com/2010/10/mph-modern-plaining-hull-2011.html

and

Displacement and Planing Hulls for Creeking | Tri-Cities Whitewater - A World Kayak Region https://www.worldkayak.com/tri-cities/2008/01/22/displacement-and-planing-hulls-for-creeking

The latter link also discusses a "creek boat", which is not a planing hull, and whose major goal is to rush down very steep creaks, with lots of waterfalls very quickly. When I mentioned whitewater playboats, I was thinking of the boats that were designed to play in waves and holes (i.e., hydraulics, where the surface flow is upstream).

(BTW, there is at least one other whitewater category - squirt boats - which are designed to play underwater in the currents that exist there. I have no idea how they are designed.)

>More than you want it to be and for zero gain.

It's not zero gain. While most sea kayakers are basically tourists, and don't want to do the fancy play moves that whitewater playboaters do, and therefore don't need to skim across the surface, or waste the energy needed to raise the boat to do so, many do find it hard to balance with the boat leaned outside the turn. Personally, I also like using a shorter paddle than I need to put the paddle outside the lean direction. My concept then doesn't really need a strong enough downwards thrust to skim freely across the surface - all one might need is to have a less strong upwards pull. I get that the displaced water has to go somewhere - but could some of it be displaced downwards as well as sideways without slowing the boat down much?

I assume "planing hulls" are less efficient because it takes power to keep the hull pushed upwards. But, "displacement hulls" push water upwards, into a bow wake. Why doesn't that take just as much power?

BTW, I sometimes do lean into turns in sea kayaks, because it is so much easier to control the boat that way, I got used to it in whitewater boats, and it makes for faster turns - though fast turns are rarely needed in sea kayaks. (Yes, that is edging - it works best if the hull is tilted almost 90 degrees, and you need a strong bow draw.) But I can feel myself fighting the way the boat "wants" to move, and of course that slows the boat down. It would be nice to have a boat that didn't do that.

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### mitchgrunesSenior Member

I wasn't thinking of turning while surfing - but that doesn't mean the sea is flat. But, good point - if the surf is strong enough better sea kayakers than myself sometimes turn or maintain a desired track direction while surfing simply by dragging the paddle lightly through the surface.

When not surfing, in waves, sea kayakers are taught to turn while they are on top of a wave, because that way the ends don't interact as much. So - good point again. Thrusting the water downwards at the bow doesn't work if the bow isn't in the water.

Some whitewater boats push the entire boat upwards while moving, so I guess it is possible for a "planing" kayak to have thrust that isn't at the bow. Now that I think of it, the upwards curved (concave) part of many whitewater "planing" kayaks in the center of the boat must be there to create a downwards thrust in the rear. Of course, such boats are NOT efficient at going fast, so wouldn't make very good sea kayaks.

That said, most kayakers buy boats that aren't terribly fast. They buy very fat boats, because such boats are stable in flat water and small waves. Maybe they wouldn't notice a little inefficiency? I mostly notice inefficiency because I try to paddle with boaters who are stronger than me.

I haven't used surfboards enough to know what is most common there.

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### gonzoSenior Member

You should probably do some reading about the definition of the random terms you use, so members can know what you are talking about. Also, all the generalization about sea kayakers, etc. should have some references to validate them. In my experience they are not correct.

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### mitchgrunesSenior Member

Which random terms? And which generalizations?

I base my usage on the way people around me have used terms, and most of my generalizations on the people I have paddled with, what I have read or heard. When explanations are lacking, I try to figure things out, and sometimes get them wrong.

I would love to know which of the things I said don't match your own understanding.

My intuition and limited experience aren’t always enough to reach correct conclusions. If I had infinite money to burn, or could easily make my own boats, I would test my musings. But at present, guessing is the best I can do.

In any event, I’ve put more thought into my original question, and I think I've answered it - but not the way I wanted.

I largely limited my discussion to boats I have actually used, which all have a flattened shape: they are wider-than-tall.

There exist racing kayaks, canoes, and surf skis with partially (underwater) cylindrical hulls. I do not know whether they are displacement or planing hulls, or somewhere in between, in the kayak/canoe/surfboard sense of whether water is displaced outwards or downwards when the boat moves forward, though my intuition is that they do both.

Some people say they are the fastest, though perhaps only in calm water, because they reduce the immersed ("wetted surface") area.

To the extent the immersed portion is really cylindrical (most actually narrow more or less to a V for a short length at one or both ends, so that streamlining keeps them stable at high speeds), I think leaning would not directly affect the direction of water displacement, or turning, because the same shape is always in the water, unless you lean past the cylindrical shape.

But:

1. They are relatively unstable stopped or at normal club paddling speeds. I’ve tried such boats for a few minutes. That isn't enough time to be certain, but I think I would waste more energy keeping them upright than the lower friction would save, especially if the water isn't calm. In the flattened boats I have paddled, it is usually sufficient to loosen my hips so my torso passively bends to keep my weight over the boat, to avoid flipping. In cylindrical hulls, I bet you need a rapid active response. I'm not even certain my reflexes are fast enough. You can also use "support strokes" that keep pushing against the water to stay upright, but that wastes energy – and might require fast reflexes too.

I’m not alone in this conclusion. Epic, a company that makes many racing kayaks and surf skis, claims that boats that are too unstable are effectively slower.

2. In cylindrical hulls, I think there would be little advantage to doing a leaned turn, because the lean would not help create the turn. I guess this is why the boats I've seen with more or less cylindrical hulls have rudders or skegs instead, and why people say you should not lean them.

In a flattened kayak, you can also put the boat on edge, and stretch out to do extended draw strokes and C strokes (look it up), to create strong fast turns, but with decreased stability, I guess you can’t put a boat as far on edge.

3. Cylindrical hulls have a deeper draft than flattened hulls, which makes them less useful for exploring shallow waterways, a frequent club boating activity. Also, taking a rudder or skeg into shallow water is asking for trouble. Sea kayak rudders (and/or their control lines) and skegs often need repair several times per season, which reduces practicality.

4. One might hope that a deeper draft, and a correspondingly lower sitting position, would create a sufficiently lower center of gravity, to stabilize your boat, if your lower body is heavier than your upper body. But my upper body is heavier than my lower. The net center of gravity will always be above the water surface.

(A heavy bottom ballast could change that – but would slow down the boat a lot, because it would sit lower in the water, and make the boat less fun to play with.)

In conclusion: kayaks DO exist with the characteristics I asked for. Their streamlining would not fight leaning into turns, and some people say they are the fastest in calm water. But, as far as I can guess without actual experience, they don't make any sense, at least for me.

Sorry to have wasted your folk's time.

Last edited: Jul 16, 2020
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### Mr EfficiencySenior Member

The most likely way to increase speed, would be to extend length, there is not enough motive power in a human to be talking about 'planing', which by an definition, involves the craft being lifted bodily in the water, by the deflection downward of water, from the forward motion of the boat. At the speeds you are talking about, it is minimal.

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### Tiny TurnipSenior Member

Mitch, your initial post is quite wide ranging. As I understand it, you are asking if it is possible to improve the ease of turning a sea kayak hull. (using an inside lean rather than an outside lean?) Perhaps a discussion using the term 'lifting surfaces' might be more helpful. This does not necessarily mean they are designed to lift the hull out of the water. Commonly, in sailing boats, they are used to control sideways movement of the boat - resisting leeway, or turning.

Such surfaces might be quite modest protruberances, not necessarily long and delicate foils as seen on the IMOCA boats, for example. However, lift generated for any purpose will be greater the faster the boat is moving, and also worth noting that they will generate drag, and have a negative effect on efficiency.

A couple of thoughts shooting from the hip - (I'm no naval architect or kayaker btw )

1) a modest foil (lifting surface) perhaps (but not necessarily?) just above the waterline at the bow and stern. A lean into the turn engages the surfaces in the water. The bow foil generates lift to exaggerate the movement into the turn, the stern foil the other way up to pull the stern round outside the turn.

Sketch below.

2) You are concerned about rudders being vulnerable. I would argue that that comes down to the rudder design to some extent. An ex member here, Rick Willoughby, has done extensive work in pursuit of efficiencies with pedal powered boats using 'dipping rudders' - one on either side, which are only in the water when needed for the turn.

Last edited: Jul 17, 2020
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### Will GilmoreSenior Member

Mitch, welcome to the forums. You pose some interesting questions, you obviously think deeply about these questions and have got some very good responses and suggestions to work with.

I am not a designer and don't know the details around the math or the theories that these guys do. I'm an interested sailor wannabe. What I have to offer is experience as a slalom and whitewater kayak racer from WAY back in the late 70s. My son is s creek boater and my wife and I spend our time in recreational touring family kayaks now.

I don't mean to tell any of these highly skilled and knowledgeable people they are wrong, for the most part, they're are right on, but when you talk about the difference between planing kayaks and displacement kayaks, they are just that.

To get a little into the math and theory here, I'll try to explain what 'Hull Speed' is. Hull Speed is the term used to describe the condition where a body moving through the water has reached its most efficient motion for the energy put into its movement. That condition applies only to displacement speeds. You will often hear it spoken as "theoretical hull speed", but it isn't theory so much as a definition.

When a boat moves through the water, it creates a series of waves by pushing the water aside. These waves have a period and wavelength based upon the speed of the boat moving through the water. As the boat accelerates, the wavelength gets longer to, essentially, preserve the period of the wave. Once the hull is moving fast enough, that wavelength equals the waterline length of the boat. There's a specific formula for calculating Hull Speed.

Hull Speed = 1.34 x √LWL. This is the definition of Hull Speed.

Now, Lots of people act like this is a limit to displacement speed, it is not. It is only the point where the effort to effect begins to slow. As you push the vessel faster, the wavelength continues to get longer until the back end of the boat falls down the stern wave. That is why, as a powerboat gets up onto a plane, it first squats the stern as the stern falls to the bottom of the trough. At this point, the boat needs to exert more and more power, because it is essentially trying to climb its bow wake.

Planing happens when the boat is going fast enough that it has managed to reach the top of its bow wake.

For most boats, Hull Speed is faster than a kayak can go, but not for a kayak. If we do the math for a standard slalom boat, the waterline, underway, will be just shy of its total length. In my day, official length was 4 meters, but I understand it has been reduced to 3.5 meters. That would put Hull Speed at about (1.34 x √11') = 4 knots.
"in the Guinness Book of Records. The endurance athlete Brandon Nelson covered an incredible 244.4 kilometres in just one day in 2013. He reached an average speed of over 10 kilometres per hour (6.2 mph or 5 and a half knots) – and that over a full 24 hours. The average speed also includes the three times Nelson fell into the water during the record attempt."

For short bursts of speed, a play boater with a flat bottom and wide stern that has good exit design, planing should be achievable.

As for the most efficient bottom design, efficiency isn't straight forward. It is a subjective term until parameters are imposed. What's the goal? What are the factors? Efficiency can include use in shallow water or whitewater or surf, it might include carrying camping equipment or accommodating a passenger, maybe price and build materials are a consideration. What is efficient for a slalom boat is not efficient for a whitewater down river boat. Play boats have a lot of rocker designed into them so they can spin on a dime at the crest of a standing wave. How much balance and experience does the paddler have? There are trade-off that help decide what efficiency means.

Whitewater kayakers lean into their turns because peeling in and out of eddies can flip you over if you don't. Slalom racers use a "Dufex" stroke for fast hard turns and that throws your weight forward and on one hip. It drives water under the bow, but it works best with transitional currents such as eddy lines. You have to be moving forward to use it.

My experience is that shallow water slows you down. As the water being pushed under your boat encounters the bottom, your bow and stern waves build much higher and your whole boat appears to squat lower in the water. You end up with a taller steeper bow wake to paddle up. Having a lot of rocker will make this worse. Consider how the bow enters the water and howe the stern lets the water exit. These things will be as big, if not bigger, a factor, than your cross sectional shape.

Remember, there's two factors that come into play when calculating resistance. You have skin friction and you have cross sectional displacement. The latter is complicated by how the water is displaces as well as how much water is displaced.

It is good to have you join us. I look forward to following this discussion. I enjoyed reading your thought processes and ideas, they are very good.

Good luck

-Will (Dragonfly)

Last edited: Jul 17, 2020
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### fallguySenior Member

Here is a highly efficient design. No, not planing. Humans would be able to plane under various methods, but it is not sustainable. So, the next best thing is a high hull speed that is ultralight and ultra low drag.

Many posters come and go on this forum thinking they are onto a new idea and are often upset by the facts.

If there are methods to overcome or nullify drag; you won't be in the water anymore. It is called flying. And so far, we don't have human powered flying machines either.

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Au contraire