(Airfoil) Why do boats have a pointy bow?

[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.

 

[anorlunda]

the hull stem

I never heard that phrase. What does it mean?

 

[Paulus Suluap]

Forward vertical edge of a hull.

 

[anorlunda]

Forward vertical edge of a hull.

Thanks. I forgot the phrase, "from stem to stern."

 

[tech99]

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.

Slocum's "Spray" had the bulbous head of a Cod and the thin tail of a Mackerel. The clipper-like bow seen in pictures is created by a decorative flat sheet of wood called a cut-water, located beneath the bowsprit and above the water line.

 

[pbuk]

Slocum's "Spray" had the bulbous head of a Cod and the thin tail of a Mackerel. The clipper-like bow seen in pictures is created by a decorative flat sheet of wood called a cut-water, located beneath the bowsprit and above the water line.

A cod's head maybe, but far from the "mackerel" lines of a canoe or counter stern, Spray had a transom.

 

[anorlunda]

Slocum's "Spray" had the bulbous head of a Cod and the thin tail of a Mackerel. The clipper-like bow seen in pictures is created by a decorative flat sheet of wood called a cut-water, located beneath the bowsprit and above the water line.

That's interesting. Do you have a link to a source?

I found http://www.ibiblio.org/eldritch/js/a22.htm which has the drawing of Spray below. I find that hard to match with "the bulbous head of a Cod and the thin tail of a Mackerel" It looks like a pretty standard full keel.

Edit: I see @pbuk beat me to it.