# Why aren't sailboat hulls designed as airfoils?

Discussion in 'Sailboats' started by kerinin, Jun 12, 2009.

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### kerininJunior Member

I'm in the process of designing a boat and learning the basics of sailboat physics and design, and I've come across something that has me stumped.

I've finished designing my hull and started researching daggerboards, so I've been doing a lot of reading about the role of keels/daggerboards. As I understand it, when sailing upwind the daggerboard exists to provide an opposing force to the lift produce by the sails, and as such modern daggerboards are designed as airfoils in the same way as ailerons on airplanes.

In general, it seems that sailboats hulls are designed as two distinct pieces; the hull which provides buoyancy and the keel/daggerboard which provides lift to counteract the sails. My question is: why aren't hulls designed to do both at the same time? In order for the daggerboard to provide lift, it must be at an 'angle of attack' relative to the water; the boat hull must be angled away from the direction of motion (leeway). In this scenario the drag induced by the hull provides no useful work. It would seem that if the same airfoil profile used for the daggerboard were applied to the entire hull, the hull would be able to provide both buoyancy and lift to counteract the sails. The benefit to this approach would seem to be that whatever drag is produced by the hull would be part of the dynamics of the airfoil, rather than simply parasitic.

One of the results of taking this approach would be that the hull's leading edge would be rounded, which brings up another question I have; if sailboats are always oriented at an angle relative to their motion through the water, why are their sterns always sharp angles? This would seem to be less efficient than using a more rounded, aerodynamic shape.

Just to throw out some of the problems with this approach I can see; it would seem like there is are two main reasons that we have keels/daggerboards. The first is that in a typical monohull sailboat the design must take heeling into account, and by pushing the center of lift of the keel farther from the center of mass, the keel can produce a stronger righting moment due to the longer lever arm (I'm designing a catamaran so this isn't as much of a concern). The second is that unlike airplanes (whose physics I'm more familiar with), sailboats must take the boundary between the air and water into account. It may be that issues such as wave drag dominate boat engineering (although I don't exactly understand how wave drag can be separated from the surface pressures created on the hull). I could also see this boundary layer impacting the trailing edge; sailboats can be abruptly cut off at the end; the boundary between the water and the air means that the cut-off portion of the hull only 'feels' the low pressure induced by the air (rather than the water as would be the case for a submarine).

Any insights would be greatly appreciated.

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### Doug LordFlight Ready

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### C 249Junior Member

I'm far from a techno guru in any way, but I think you'll find several reasons that boats are no longer foil-shaped (like many were years ago, when speeds were much slower).

One is that, as you said, they operate on an interface. Wavemaking drag is normally divided into pressure drag (IIRC) and gravity drag. A blunt bow means (AIUI) that the only wave to really move enough water away quickly enough is to pile it up above the water's surface into gravity waves. To do that, of course, takes a lot of force and (IIRC) this force rises by the square as the "angle of attack" of the bow increases and its speed increase.

As I understand it, that's why fine bows without distortions are generally good (all else being equal); because of the squaring effect you are better off having the gravity waves being created at a lower angle of attack over a longer distance, than having them created by meeting a fat bow at a wide angle of attack, then running alongside a hull section with no angle of attack. However, this may be totally wrong! Any experts out there?

In foils, where gravity drag is not an issue, other areas dominate.

Secondly, because a hull has a very low aspect ratio (in terms of resistance to leeway) it's very innefficient at countering sideforce. You're much better off minimising hull drag and just putting on better foils. And (as I understand it) you will also have the problem that the low-aspect "foil" of the hull generates its best L/D figures at very different angles of attack than the optimum angle of attack for the high-aspect foils. So if the foils are working well the hull isn't, and vice-versa.

Just to emphasise it. I'm not an expert in any way; just trying to throw some ideas from books around. I may well be utterly wrong!

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### balsaboatmodelsJunior Member

Hi; I'm waaaaaay far from being a boat designer, but do come from an aviation family. Here's some thoughts - force of sail on mast is applied as a lever to the hull. As that force changes, oh, I guess you'd say its vector, in relation to the hull's heeling angle, and the heading of the boat through the water, so many forces are going in so many directions, and changing all the time, - in which direction do you orient the airfoil cross section?

In aircraft, the wing is at an "angle of incidence" relative to line of flight, meaning if line of flight is flat, wing might be mounted at a small angle up relative to that line; so there's another angle to do the calculus for.

Generally propulsion force on aircraft is applied parallel to line of flight. Mostly.
A sailboat has that force applied at angles ranging all over the map.
In three dimensions.

Did any of that make any sense? I'm wondering now, looking back over it. Oh well, saying it anyway.

Better yet, don't listen to me, get a book like this from someone who knows what he's talking about:
http://www.tillerbooks.com/Cruising_Sailboat_Kinetics.php

Last edited: Jun 13, 2009
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### kerininJunior Member

Thanks for all the great responses!

Doug, thanks for the reference to the Hobie models, it's nice to see that something along these lines has been implemented. It seems like the asymetric hulls have a slightly different goal, although the designers are clearly thinking along the same lines (why not treat the hull as an active component). From what I can tell those hulls are designed to provide some hydroplane-type lift, reducing the wetted area when heeling. Maybe I misunderstand.

C 249, I think I see what you're getting at with the blunt bow - when you say the square of the angle of attack, I assume you're referring to the angle of the bow relative to the boat's centerline. I need to do some research, but i suspect that airfoils designed to operate at low reynolds numbers with high L/D ratios will have sharp noses (and narrow operating ranges) for that very reason.

I'm still trying to understand 'wavemaking drag' in a way that relates to pressures on the hull rather than simply working backwards from the observations that energy is required to create waves. One thing I've considered is the way that the airflow over airplane wings begins to move sideways near the wingtips (rather than parallel to the direction of travel). I think this is caused by vortex formation at the wingtip. It would seem that something similar would happen for boats, except vortices can't be shed at the surface, so waves are formed instead. Maybe a better way to think about it (which doesn't involve lift) is to focus on the high and low pressure systems being set up on a symmetric airfoil w/ no angle of attack: higher at the leading edge and lower along the tail. At the surface, there's no pressure 'pushing back' from the surface at the leading edge, so naturally the fluid moves to that area (creating a bow wave). Logically, a similar situation would set up along the rear in which the low-pressure along the tail creates a 'dip' in the surface. Of course, then issues of gravity, wave propogation, etc would start to come into play. I guess what I'm getting at is that in my mind, it makes more sense to think about the pressure distribution of the hull than to think about bow waves or anything of that nature.

Ragarding the aspect ratio of hulls, I certainly see the challenge here - I'm working on catamaran pontoons so I don't really need any specific aspect ratio.

balsaboatmodels - from my (rather naive) point of view, the forces are actually a bit simpler. It's true that the sails change their orientation relative to the hull, that the wind could be coming from any direction, and that the heeling angle and rudder are all changing the situation, but I think you can simplify all these forces (at any give point in time) into two force vectors; the force of the wind acting on the boat and the force of the water acting on the boat. The wind will always have some forward and sideways component (applied at some distance from the center of mass). The water will always have some rearward and sideways components (also acting with some torque). The challenge is simply to make sure you have control surfaces to balance these two vectors in a useful manner and then to design the surfaces to provide the optimal L/D ratios (in the context of size/maneuverability/stability constraints). The heeling will eventually balance itself between the buoyancy and the vertical lift of the keel, the forward and backward components of the two vectors will eventually balance themselves by accelerating or decelerating the boat in the water, and the rudder position allows you to manually align the moments of the two vectors. Am I underestimating this whole process?

Anyway, thanks again for the comments. I'm sort of thinking out loud here, I hope I'm not rambling too badly...

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### kerininJunior Member

Wow, I couldn't stop thinking about those asymmetric hulls and I realize now that they're actually incredibly clever. I had been ignoring heeling (catamaran again), but obviously even cat's must heel to some extent. The asymmetric hulls allow you to eliminate the daggerboard because as the boat heels, more of the leeward pontoon will be in the water than the windward pontoon, and since they're designed to produce 'lift' towards the center of the boat, the leeward pontoon will provide more 'lift', thus taking the place of the daggerboard.

That's so smart!

The best part is that there's still the possibility for slight hydroplaning, since at higher heeling angles the pontoons will produce some element of vertical lift. I've been considering designing the pontoons as deep, narrow symmetric airfoils - now I'm thinking it would be more interesting to design them as asymmetric foils (still deep and narrow), and angle them 30-degrees or so toward the boat center - this way they produce some hydroplane effects at any speed, and as the boat accellerates the heeling angle will increase, causing a greater vertical lift component. They would act similar to the hydrofoils on the Hydroptere basically.

Thanks again for that great link - I had read that thread a couple time already but hadn't really appreciated how interesting it was.

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### gggGuest...

I don't pretend to understand it in detail, but its all about the interface between the incompressible water and the compressible air... Straight aerodynamics won't help you much because its a special case. You don't get wavedrag with a submerged submarine either...

As Chris says, the reason why asymmettric hulls have been almost completely abandoned is because they really don't work very well compared to daggerboards or centreboards. I don't know that you've quite got the point about aspect ratio... As I'm sure you know its all about induced drag and tip losses. So if you consider an asymettric hull you have a wingspan of about 6 inches, and a chord of say 20 feet... So an aspect ratio of, what, is it 0.00625? The lift/drag of that isn't really going to be very impressive...

So that's why the most effective form is to build daggerboards that provide the best possible lift, and a hull that slips through the water as efficiently as possible, and especially slips through waves as efficiently as possible.

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### kerininJunior Member

That's a good point about aspect ratio - I hadn't considered it. I'm currently thinking about a very slightly asymmetrical set of pontoons, nothing as drastic as the link Doug provided. I'm also toying with the idea of using a deep, narrow hull with an elliptical elevation section; essentially two wings in the water w/ the DWL at the ceter line of the ellipse. Similar to this design but thinner.

Now I just need to find some airfoil data at high Reynolds numbers and very low angles of attack on barely cambered sections.

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### Doug LordFlight Ready

Asymetric hulls

I think what you have to consider is how important windward performance and speed are to your on the water experience. The Hobie Cats were extraordinary because for one of the first times in the history of popular small sailboat design someone was able to drastically simplify the sailing experience-all you had to worry about was keeping the lee bow just above the water. No worries about grounding at all.
But the newer boats like the Blade and Falcon F16 and others are way ahead in performance. So, really, its what you want out of the experience that counts-each type has advantages and disadvantages.
Good luck!

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### kerininJunior Member

I see your point regarding the various factors which must be balanced, but it seems to me that at least two design considerations can be addressed by using wing-shaped pontoons.

The first factor is the hull drag. If we assume that the daggerboard *must* produce some amount of lift to balance the sideforce of the sail, and that whatever airfoil is used for the daggerboard will have some component of drag, it seems that treating the entire hull as the keel and using the same airfoil for the hull that would otherwise have been used for the keel means that the hull drag can be completely eliminated. In the first scenario we have the daggerboard drag and the hull drag, while in the second scenario we only have the hull drag (which will be virtually the same as the daggerboard drag). This isn't exactly true, since as gggGuest points out the aspect ratio of the daggerboard will be much higher than the wing-pontoon, so the L/D of the wing-pontoon will likely be lower than a daggerboard with the same airfoil. Another complication arises due to the fact that moving from a daggerboard that's (lets say) has a .25m chord to a pontoon with a 5m chord will increase the reynolds number on the airfoil by a factor of 20, but this would seem to actually work to the benefit of the wing-pontoon, as the L/D ratio of most airfoils increase with reynolds numbers.

The second factor that a wing-pontoon would seem to address well is the pitching moment. Most modern cats (I'm thinking of the Blade as mentioned earlier) are designed with wave-piercing hulls to reduce the buoyancy of the pontoon fore and aft. Using an eliptical 'wing' plan-form will concentrate the pontoon's buoyancy in the center and reduce it at the front and rear.

The fact that the overall draft of the pontoon can be reduced by eliminating daggerboards seems like a nice bonus, but isn't really why I find this idea interesting. In any case, if the pontoon had an elliptic shape, that would certainly end up reducing the damage caused by beaching the boat.

The more I think about this the more curious I become as to why catamarans aren't designed with thinner, deeper hulls. It would seem that even if the pontoon wasn't treated as an airfoil, increasing the aspect ratio of the hull would lead to much faster boats, both due to the friction drag on the pontoon and the wave drag (since each pontoon would have a smaller beam). Maybe the increase in wetted surface area outweights the drag reduction?

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### Doug LordFlight Ready

Take to heart what gg said-there is a HUGE difference and its not speculation:the asy hulls have been trounced ,from a performance perspective, by boats designed with the lowest drag for the lift required and for the lowest drag for the speed it is designed to achieve. There is no real debate on the performance issue-just none at all. Other factors could enter into it-but performance is not going to be an asset of an asy hull vs a low drag high lift hull/board combo.

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### C 249Junior Member

The hulls of performance cats are already very slender. But if you make them narrower and deeper, you increase wetted surface area (for the same immersed volume ie the same displacement).

A U-shaped or elliptical hull section has the lowest wetted surface area for a given volume, and that will create a shallower hull than the deep Vee style hull that you want to increase lateral resistance.

The extra volume in a U-shaped hull actually allows you to have a narrower midships beam (for a boat of the same dimensions and displacement) than a V-shaped hull. It also allows you to develop some dynamic lift from the flat area along the centreline of the U. A deep Vee section develops almost no hydrodynamic lift.

I'm 99% sure you'll find that the current cat designs are not so much reducing the hull volume in the ends, but actually moving the volume in the ends low down under the waterline in high-volume U-shape sections. Compare a Viper F16 to the earlier Taipan 4.9 F16, or a Flyer A Class to an older Boyer Auscat, and you'll clearly see the evolution towards higher volume underwater sections in the end. The modern boats have much more bouyancy (volume) there.

The people who have been developing cat hulls are far from silly. Guys like Martin Fischer (who has a PhD in fluid dynamics) know what they are on about, and the success of their shape development is proven every weekend on the water. As Doug says, the Hobies are great for what they are, but their performance is proven by the fact that they rate over 10% slower than comparable modern 16 footers, despite carrying about 10% more sail. Upwind, there's no comparison between a F16/A Class type and the Hobie 16 type.

13. ### Guest625101138Previous Member

Yes. As you push for speed in a slender displacement hull, increasing the block coefficient reduces the drag. Keeping reserve buoyancy in the ends low reduces pitching. Also lowers the risk of pitchpoling because the boat is not slowed by driving into waves as much.

You also see the trend to higher block coefficients in powered cats.

Godzilla produced this hull for a powered boat limited to 18m in length and displacing 5 tonne:
http://www.boatdesign.net/forums/at...power-boats-boston_5t_15kts_18m_linesplan.png
The block coefficient is 0.75.
Getting the hull drag low is the priority for any sailing boat. Windage is important but the water is around 800 times more dense so this is the area that needs the priority attention when considering drag. On the other hand all drag needs to be considered.

The other option with the hull is to go for hulls that plane but then that is what the fast monohull dinghies do.

Rick W

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### grobwww.windknife.com

The G-Cat designs by Hans Geissler do this very well.

Gareth

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### grobwww.windknife.com

I can bring a different perspective to this as someone who has tried what you are talking about, I built a four hulled boat in 2005 that used 6012 aerofoil sections as hulls.

The main problem with these was wave drag, aerofoils work best in the 9-12% aspect ratio range (width/length) this aspect ratio gave very high wave drag and the boat needed a lot of power to get over this drag hump. Once it was over the wave drag hump the hulls seemed to perform very well. It always felt like it was getting up on the plane but it may have been just getting over the wave drag hump.

However as sail boats need to work well over a high speed range I did not consider that it could be a success. Also I thought that there were better ways of achieving very high speed hulls than this hull shape.

The positive side of this is that it was very good at generating lift and did exactly what I hoped as far as resisting leeway, even though the bottom shape was U shaped and it had a very shallow draft, it did not need a deep V, a skeg, or asymmetric hulls to achieve this without boards. The other positive thing about this hull layout was the high stability of the boat as it has its buoyancy at the ends.

What did I learn from this experience? Boat design is about compromise, if performance is the main aim then modern cat designs have it about right, you need a good stable low drag design and relatively high aspect ratio boards (in this case AR is length/depth). I learnt that shorter fatter hulls can generate a lot of lift without boards, in fact my new boat employs this concept it’s just a case of getting the aspect ratio’s right.

My perspective has always been that I don’t believe performance is king I am a big fan of usability.

Gareth

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