(Airfoil) Why do boats have a pointy bow?

[petterg]

A (displacement) boat hull has a design goal of making as little water resistance as possible with the required volume for load carrying capability. To achieve this the hull is given the shape similar to a vertical symetrical airfoil. But as the leading edge of a boat (it's bow) seems to be made as sharp as possible, the leading edge of airfoils are round. Why aren't boats bow round as the airfoil?

 

[russ_watters]

A (displacement) boat hull has a design goal of making as little water resistance as possible with the required volume for load carrying capability. To achieve this the hull is given the shape similar to a vertical symetrical airfoil. But as the leading edge of a boat (it's bow) seems to be made as sharp as possible, the leading edge of airfoils are round. Why aren't boats bow round as the airfoil?

If you note that submarines do tend to have blunt/round noses and overall look a lot like airplane fuselages, what does that tell you...?

 

[petterg]

@A.T. : I think you're the first to compare a supersonic fighterjet with a supertanker! I'm talking about symmetrical air foils. There's not many of such in applications where lift is wanted. Symmetrical air foils are mostly used to cover objects that are pushed through a fluid (including air) to reduce drag (and for airplanes that are designed to fly upside down). Let's stick to symmetrical air foils. I don't think a sailboat hull is designed to provide lift when leaning either.

@russ_watters : You're right. Submarines are more airfoil-shaped than boats. (I once towed a submarine into dock using a 22' rib so I had the chance to see the bow close up. What does that tell me? Maybe that airfoil-shape has major disadvantages when operating near the water surface?

 

[russ_watters]

Several off topic posts and responses have been deleted. Let's stick to answering the question being asked please.

 

[russ_watters]

@russ_watters : You're right. Submarines are more airfoil-shaped than boats. (I once towed a submarine into dock using a 22' rib so I had the chance to see the bow close up. What does that tell me? Maybe that airfoil-shape has major disadvantages when operating near the water surface?

Yes. The hull shape difference is related to the fact that the hull breaks the surface. Water can "pile up" in front of a round hull.

 

[petterg]

So a bow should be sharp above the surface to "split the pile", and round under the surface to be most like a foil?

 

[russ_watters]

So a bow should be sharp above the surface to "split the pile", and round under the surface to be most like a foil?

Pretty much. Note that many also have under-surgace bulges, so you have a bit of both:
https://en.m.wikipedia.org/wiki/Bulbous_bow

Part of the purpose is starting and shaping the bow wake. At cruising speed, ships can sit in a trough between the bow and stern waves.

 

[Lnewqban]

Then, you have the bulbous bow of big ships.

Copied from:
https://en.wikipedia.org/wiki/Bulbous_bow

"A bulbous bow is a protruding bulb at the bow (or front) of a ship just below the waterline. The bulb modifies the way the water flows around the hull, reducing drag and thus increasing speed, range, fuel efficiency, and stability. Large ships with bulbous bows generally have twelve to fifteen percent better fuel efficiency than similar vessels without them."

(Oops, cross-posted)

 

[Cutter Ketch]

The bow of most ships are actually bulbous beneath the water line and sharp at the water line. The sharp part of the bow is about slicing the surface and folding the water to the side.

The surface presents an opportunity that doesn’t exist when you are immersed in the fluid. There is a place for water to be pushed aside where there isn’t already more water in the way. This is very different than being surrounded by fluid.

Another important difference is that water is essentially incompressible while air is very compressible. This makes it much harder to move through water as the water has to have somewhere to go. It can’t just be squeezed. Folding it aside at the surface is even more important.

Even at the surface you can’t just think of pushing the water aside. Doing so produces surface waves, and how you produce those waves can take more or less energy. Ships and boats tend to move faster than the speed of the surface waves, so the bow wave is a shock wave. The wave makes an angle with the bow dictated by the speed of the boat and the speed of the wave. The sharpness of the bow is in relation to the angle of the bow wave and the faster the ship the sharper the bow at the waterline. This is comparable to the shape of the nose or the leading edge of the wings of supersonic jets which are sharp.

 

[petterg]

I can't explain why airfoils are round. It's written that the round shape makes the fluid follow the foils surface better than with a sharp front, but I don't understand why it is like that in applications where you want the flow on both sides of the foil to be equal (lift = 0). Maybe the pointy bow also helps to increase directional stability (much wanted when rowing)?

 

[anorlunda]

Resistance is not the only issue in a boat hull. Lateral stability is also important. That has to do with the forces tending to keep the hull moving in a straight line without use of the rudder.

A surface vessel is more subject to wind and waves than a deeply submerged submarine. Wind and waves act to deflect the hull from the desired course.

My sailboat had a full keel and a double-ended hull (pointed at both ends). Both features aid lateral stability. Joshua Slocum bragged that he sailed his boat, Spray (double-ended, full keel), from Perth Australia to the desired port in Africa without ever touching the steering wheel. I think he exaggerated, but nevertheless his feat was remarkable.

WWII submarines had a more pointed bow than today's nuclear subs, because they spent a lot of time on the surface, or just below the surface.

Aircraft also have several axes of stability to worry about.

 

[russ_watters]

I can't explain why airfoils are round. It's written that the round shape makes the fluid follow the foils surface better than with a sharp front, but I don't understand why it is like that in applications where you want the flow on both sides of the foil to be equal (lift = 0).

Cross sectional area and volume enclosed are part of this. A wing needs to maximize lift while minimizing drag, and the shape isn't otherwise that important for the functionality of the plane, whereas the fuselage of a plane needs to have enough volume to hold its cargo while minimizing drag. Air needs to be smoothly directed around the fuselage, but having a sharp point (on a subsonic plane anyway) doesn't really help that. But on the back end, the air has to come back together smoothly; a blunt back end would have the air converging faster than it should, creating a low-pressure area at the back.

It may help to look at a pressure profile, though I'm not sure they are easy to find for a sharp nose/leading edge (they are for normal profiles).

 

[boneh3ad]

Apparently @A.T. posted about supersonic airfoils and got their comment deleted. That's unfortunate because it is an excellent analogy. @Cutter Ketch also mentions this at the end, which is nice.

Submarines have blunt tips and look more "airfoil-ish" because they operate underwater where the distinction between moving through air and water is minimal. There are therefore many parallels between the external aerodynamics of a wing and the external hydrodynamics of a submarine. As @Cutter Ketch mentioned, the bow of a surface ship that remains under the water often looks like this, too.

In contrast, surface ships move along the surface of the water and generate surface waves in the process. The physics of surface waves and surface hydrodynamics are actually intimately related to supersonic aerodynamics. The bow waves are similar to shock waves on a supersonic aircraft, and a blunt tip is going to cause a large bow wave (analogous to a supersonic bow shock) that greatly increases drag. A sharp edge is going to do a better job of "cutting" through the water and form an attacked wave (similar to an attacked oblique shock) that has less drag.

In other words, the body plan of a ship hull will have features such as a sharp tip that are more analogous to supersonic airfoils than to subsonic airfoils. In contrast, however, supersonic airfoils are necessarily as thin as reasonably possible, but the body plan of a ship stays pretty fat since it needs lateral stability and actual cargo space.

 

[DaveE]

In the 1980's Francis Clauser, a Caltech physicists that specialized in aerodynamics, among other things, was hired to consult with a team designing a new America's Cup yacht. One of the hull designs from a traditional designer had the typical fine bow and rounded stern that was common in the past. Clauser saw it and bet the designer that in wave tank testing the hull would be faster backwards than forwards. Of course he was correct.

The Boat That Almost Was

 

[cjl]

Another important difference is that water is essentially incompressible while air is very compressible. This makes it much harder to move through water as the water has to have somewhere to go. It can’t just be squeezed. Folding it aside at the surface is even more important.

This actually isn't true, at least not at the speeds that conventional airfoils operate. Up to around mach 0.3, air can be treated effectively as completely incompressible and you'll end up getting basically the right result (within a few percent error). Compressibility of air only really starts to impact its behavior if you start getting up to a significant fraction of the speed of sound.

 

[petterg]

Thanks for all the answers. I feel like I've learned more than I was asking.
This also made me wonder on more thing:
Assumed a symetric vertical foil moving horizontally with 0 angle of attack. Will it create more drag if it is half submerged than if it is fully submerged?
How will a plot of drag vs height above/partly submerged/under the surface look? As airplanes has less drag when flying high, I assume the drag in the plot will be minimum high up in the air, then drag increases as the height comes down towards sea level. When it touches the water surface the drag increases significantly. As the foil is more submerged the drag increases. But as the top of foil gets below the surface I don't know what happens. Does the drag jump down and then increase with further increased depth?

 

[Lnewqban]

...
This also made me wonder on more thing:
Does the drag jump down and then increase with further increased depth?

I believe it will happen in the way you have described it.
You will mainly have underwater skin friction plus turbulence at the surface.

 

[boneh3ad]

Thanks for all the answers. I feel like I've learned more than I was asking.
This also made me wonder on more thing:
Assumed a symetric vertical foil moving horizontally with 0 angle of attack. Will it create more drag if it is half submerged than if it is fully submerged?
How will a plot of drag vs height above/partly submerged/under the surface look? As airplanes has less drag when flying high, I assume the drag in the plot will be minimum high up in the air, then drag increases as the height comes down towards sea level. When it touches the water surface the drag increases significantly. As the foil is more submerged the drag increases. But as the top of foil gets below the surface I don't know what happens. Does the drag jump down and then increase with further increased depth?

For a partially submerged foil, you will basically have three regions you will care about: the submerge portion dominated by viscous and form drag in water, the exposed part dominated by viscous and form drag in air, and the surface region dominated by wave drag. The surface region producing wave drag will be the biggest driver of drag in this scenario (assuming the foil isn't extremely long), followed by the underwater portion, and then the exposed portion.

So, depending on the length of the foil, there will be an absolute minimum in drag if the whole thing is in air, then an increase as you dip it into the water, and a maximum right before it becomes fully submerged (i.e. the entire surface is generating underwater drag with a small sliver that is still above to generate wave drag). Then it drops off again as it becomes fully submerged.

This is why the above and below-water portions of the keel of a ship are typically dramatically different shapes. The design principles for minimizing drag in each region are different.

You will mainly have underwater skin friction plus turbulence at the surface.

No. "Turbulence" is not a form a drag. Turbulence is a property of the fluctuating nature of a shear flow. Its effects can lead to more (or sometimes less) drag, but it does not directly produce drag.

Underwater and above, the foil is subject to skin friction drag and form drag (which is largely driven by separation in this case). An increase in turbulence in the boundary layers will increase skin friction drag but tend to decrease form drag due to its tendency to delay separation. In a situation like this, I'd imagine separation is minimal and the increase in skin friction drag would be the dominant factor.

At the surface, wave drag is dominant. Sure there might be some turbulence, but the drag created by pushing water out of the way in the form of waves, especially when they become nonlinear and "break," is by far the dominant mechanism for drag in that region. This is one reason some modern ships are starting to use wave-piercing hulls.

 

[anorlunda]

Most posts in this thread discuss drag. Stability was mentioned. I also want to point out that keeping waves off the deck is another important factor. Despite bulbs under the water line, they are flared above the water line to deflect waves to the side.

versus

Of course, modern submarines find it unnecessary to be on the surface in heavy weather.

Come to think of it, in the second picture, the conning tower can be considered the pointy part and the main hull the sub-surface bulbous part. Just broaden the definition of which part is the bow.

 

[Dr. Courtney]

Once you get down to the size of recreational fishing boats (mostly 14-24 feet in length), the priority is more the comfort and safety of the passengers rather than fuel economy. Most recreational fishing boats tend to be over powered and operated at speeds far above optimal fuel economy. The pointy bow is mostly about passenger comfort in rough water and the ability to move faster in rough water without beating up the passengers.

 

[Dr. Courtney]

On the Louisiana gulf coast most shrimp boats like that pictured below also have the pointy bow design. I'm not sure if it is simply tradition or the ability to handle rough water favoring this design, but it is nearly universal in the Louisiana shrimping fleet. Fuel economy IS a big deal to Louisiana shrimpers, as they often plan their routes to maximize their catch relative to their fuel costs rather than per unit time. But fuel costs didn't become a large factor in their decision making until about 20 years ago, and most of their boats are older than 20 years old.

It would be interesting to know the trade-offs in hull design when navigating at angles more oblique to the prevailing current. It would also be interesting to estimate the relative drag of the ship itself compared with the shrimping nets and associated gear. In the last two decades, shrimpers have had an increasing emphasis in catching shrimp as close as possible to their home port. Most boats I see while fishing spend a lot more time under power with their nets down catching shrimp than with the nets up motoring from the port to their shrimping spots. Of what use is a more efficient hull design if most of the fuel is spent dragging the nets around?

I do know that a major point of emphasis for both recreational fishers and commercial shrimpers is being able to operate in shallower water. The shrimp boats that can run back and forth in shallow water closer to the beach catch a lot more shrimp per gallon of fuel some months of the year than those forced to operate only in deeper water. Likewise, shrimp boats that can run in many of Louisiana's shallower bayous and inshore waters also have advantages.

Correct me if I'm wrong, but I think the "pointy bow" design can usually operate in shallower water than the bulbous bow designs, for boats of comparable length and capacity.

 

[anorlunda]

Correct me if I'm wrong, but I think the "pointy bow" design can usually operate in shallower water than the bulbous bow designs, for boats of comparable length and capacity.

Maybe yes, but not necessarily yes. The draft of that boat in the picture is likely 5-8 feet with the deepest part mid-ship. That's enough for a bulbous extension that does not increase the draft.

That boat in your picture is almost big enough for a bulbous retrofit IMO. It is an economic trade-off, efficiency versus initial cost. It might be a good business opportunity for an enterprising marine architect down near the gulf to design something economic enough to be attractive to shrimpers. If it could be mass produced and bolted on, the cost could be lowered. Load at the attachment points might be the difficult part, especially for the running aground contingency.

 

[Dr. Courtney]

Maybe yes, but not necessarily yes. The draft of that boat in the picture is likely 5-8 feet with the deepest part mid-ship. That's enough for a bulbous extension that does not increase the draft.

That boat in your picture is almost big enough for a bulbous retrofit IMO. It is an economic trade-off, efficiency versus initial cost. It might be a good business opportunity for an enterprising marine architect down near the gulf to design something economic enough to be attractive to shrimpers. If it could be mass produced and bolted on, the cost could be lowered. Load at the attachment points might be the difficult part, especially for the running aground contingency.

That might make sense for boats and operations on the scale of Omega Protein, but most mom and pop shrimping operations are not going to be able to invest in fuel efficiency improvements that may or may not pay for themselves over a 5-10 year period. Most of the time, they are simply trying to pay the bills this year and still be in business next year. They already have a bunch of "cheap" tricks to reduce fuel costs - like mooring to a fixed object, letting down their nets in the current, and catching shrimp without any fuel in certain tidal conditions.

In contrast, the Omage Protein ships travel much larger distances from port, have ships large enough to show larger benefits, and are owned by a company with the wherewithal to make an investment that might take 5-10 years to pay for itself. Still, the idea may seem like snake oil to them.

 

[sophiecentaur]

Joshua Slocum bragged that he sailed his boat, Spray (double-ended, full keel), from Perth Australia to the desired port in Africa without ever touching the steering wheel. I think he exaggerated, but nevertheless his feat was remarkable.

Joshua Slocum was clearly a great and resourceful sailor. He was also a showman and loved the idea of being a hero figure. His story of 'fixing' the errors in his ephemeris tables and washing out his ship's clock when the sea got into it sound a bit far fetched. i don't think he would have been fun to travel with! Fab book though.

 

[essenmein]

I designed and built my own little 12ft boat, something to keep me sane in the winters here.

Anyway, I like boats, so bow design for me was partially aesthetic.

I did however wade into some of the science behind bows, I quickly discovered there are many schools of thought on the matter!

It also largely depends on the sea conditions around where the boats are built. For example the "Carolina flare":

Cool little vid going over different bow styles and what they are trying to acheive:

 

[essenmein]

The bulbous bow is an interesting thing.

The idea is to create destructive interference of waves, the bulb creates a trough where the bow meets the surface, thereby canceling the bow wave. So there is energy lost by creating the initial trough, but this is offset by the much lower wave resistance along the rest of the hull due to the canceled bow wave.

It is very important to note that the bulb is designed in conjunction with the rest of the hull to do this at one specific speed. Which is why you see it a lot on ocean crossing vessels that steam at essentially constant speed.

So a bulbous bow makes no sense on a boat that changes speed all day (ie most recreational and near shore boats).

 

[anorlunda]

Cool little vid going over different bow styles and what they are trying to acheive:

Wow! Thank you for sharing that. That video did such a thorough job, that I would say that it is the definitive answer the title of this thread. It covers the design options, and the pros and cons of each choice. I urge everyone on this thread to watch the video before adding more comments.

 

[trainman2001]

Bulbous bows appear to have started showing up in the late 30s. When looking at dreadnought design at the turn of the last Century they were symmetrically pointed bow and stern. Even battleship designs prior to the Iowa class, the bows were "clipper bows" with mild flares. When the Iowas were designed they took the 600+ feet length of the South Dakota class and lengthened the ship over 200 feet. Most of that 200 was a great increase in both length before getting to turret #1. There was some increase amidships giving room for twin funnels and more AA emplacements. The bow had a modest bulge below the waterline ad a very sharp, almost razor-like stem post. The result: 7 knots from 26 to 33 enabling these 900 foot monsters to keep pace with the fast Essex Class carriers of the day.

 

[essenmein]

Not sure if this is too off topic, it is somewhat related, since the bulbous bow is all about reducing wave making to save energy.

When it comes to displacement hulls, length plays a huge role, longer means faster before the waves you generate rob you of a lot of power:
(source: https://www.usna.edu/NAOE/_files/documents/Courses/EN400/02.07 Chapter 7.pdf page 7-18)

 

[Paulus Suluap]

Wing leading edges generally require a lack of sharpness for increased stability. A BAC 111 jet prototype crashed due to leading edge excess sharpness caused problems. There have been sailboats with rounded off stems which provided advantages according to the designers.

 

[cjl]

Wing leading edges generally require a lack of sharpness for increased stability. A BAC 111 jet prototype crashed due to leading edge excess sharpness caused problems.

It's not for stability, it actually allows for a much higher maximum lift coefficient and reduced drag. An airfoil with a sharp leading edge will stall much earlier.

 

[Paulus Suluap]

It's not for stability, it actually allows for a much higher maximum lift coefficient and reduced drag. An airfoil with a sharp leading edge will stall much earlier.

Thanks for replying. I thought that generally the envelope of inherent stability is increased by a more rounded leading edge, the wing is more tolerant of a wider range of angle of attack & it's stall behaviour is more benign. The BAC 111 crash was caused by an abrupt stall where the recovery time exceeded time to the ground.

 

[tech99]

The air particles approaching the leading edge of a wing can sense that they are about to change direction. The leading edge sends acoustic waves forward which do this. A broad leading edge is possibly a better radiator of acoustic energy. Upstream acoustic waves can be seen, for instance, in a gentle stream of water falling on a knife edge.

 

[Arjan82]

Acoustic waves in water can generally not be 'seen'. Are you sure you don't mean capillary waves here? That's what I expect to see in the situation you describe.

Also, generally airfoil shapes are of no consideration whatsoever when designing ship hulls of any kind. The comparison between te two is completely flawed. Things that are of consideration:

- Wave resistance. For a Froude number of around 0.25 or so you have two wave crests near the shoulders of the ship, which gives you a lot of possibilities to decrease wave resistance by properly shaping the hull. These ships also generally have bulbous bows (often container ships). For other Froude numbers they are not nearly as effective (for some ships (bulkers/tankers) they are used to lengthen the waterline below the water, increasing displacement and thus cargo capacity, but since they are charged in the harbor for length on the waterline, they pay less...)

- Seaway and added resistance due to waves. For fast ships or short ships the bow needs to plaugh through the waves, this increases their resistance. Sharper bows decrease drag, but also decrease displacement and thus cargo capacity.

- Displacement. Ships that aren't to bothered by waves because they are big and slow, generally have very round, almost square bows. In this way they maximise displacement which allows them to carry more cargo (tankers/ bulkers)

- ... many more things

Lastly, airfoils do not have their rounded noses because of resistance. But, as stated earlier, to increase maximum lift and improve stall behavior. It is the pressure recovery part aft of the thickest part of the airfoil that determines a large part of the drag of an airfoil.

 

[Paulus Suluap]

Airfoil shapes are commonly used for the fins on sailboats that are often described as creating lift & as having an angle of attack which counteracts leeway caused by wind on the beam hitting the sails. Those fins will often have somewhat rounded off leading edges for similar reasons that wings have them. In this situation the hull stem will also have a certain angle of attack & might benefit from a less sharp leading edge.