I hear a lot of people on this site talking about how good sharp bows are, aka Fox and 17R, why is this?



If you sharpen an aerofoil over its ideal shape it has more drag not less.



Why is it so different on Cats?

I inquired the same question last week, I am presently restoring an original Hobie. The Bow is sharp to an 1 inch widest point, and hull extreme curved from Bow to stern. This is to provide minimum drag.



I have looked at recent designs and it appears marketing is at work for appearance sake.



It's much like my wife Focus Ford, while areodydamic unstable it is pretty.



Mike

Are they talking about a FINE ENTRY? I am no expert on this, but the leading edge on my previous Hobie 20 hulls compared to my current Nacra 6.0 NA hulls were not appreciably "sharper". However, the battleship bows of my 6.0 are narrower, more vertical and much quieter thru the water.

I can't give you a definative answer as to why some boats have such "sharp" bows. However I can explain a little of the "newer" shapes.



As you go from a sharp shape to a rounded shape (not to be confused with fineness ratio), you go up or down in efficiency. A sharp bow shape is just as inefficient as a very blunt shape. Of course all of this varries with the speed of the hull through the water. The trick is to find a shape that is optimized for a fairly wide range of hull speeds. Now that is a generic answer....



This is too, but....if you want a long one.... go get a book!



The maximum speed for a displacement hull is generally specicied by the fineness ratio (lenght to width) of the hull.



There are several web pages where you can "calculate" a theoretical hull speed for a displacement boat....well if you put in 20', it give you ~6 knots. Now we all have easily broken six knots. So, what gives? Well, they used a simple equation "Hullspeed = SqRt(LWL) X 1.34" Where 1.34 is the accentric factor for that hull. 1.34 is roughly the accentric factor for most monoslugs. A cat would be much higher so, this equation does us no good w/o the graph to get our accentric factor......so....now that I have rambled way outside that answer...



Ok, a destroyer or plumb bow shape allows for the max waterline hence the max speed for a given loa. Also, it allows more volume forward to keep the sails from pushing the bows too far down and reduces diving in waves. Not to mention... it looks cool.



all.... general answers, but if you want to know more... go get a book or ten....



Will_R

One more thing to keep in mind is the Bow wave effect (that Will hinted too). This effect basically means that as the boat pushes through the water, it causes a wave in front of it (i.e. the bow wave). It takes a lot of power to push through this bow wave or you have to have a planning hull to ride up on top of it in order to ever move faster than this wave. HOWEVER, once the hull length to width ratio exeeds roughly 11 (for example 11' long and 1' wide), the bow wave effect is no longer in play because the boat can slice through the wave instead of having to power over it. This has a lot to do with the additional speed potential for cats and tris.

You seem to be asking why a hydrofoil is not penalized by nature for a fine entry, where an airfoil is most efficient with a blunter one.



I believe (corrections welcome) that the shape of an airfoil is designed to avoid turbulence, most often the product of the airstream 'detaching' from the airfoil. Naturally airfoils differ in purpose and so in design, but for a given foil, the 'goal' is to 'hide' as much of your cross section in your given length without inducing turbulence.Detachment is hardest to prevent at the exit, or low-pressure end. So, for maximum efficiency, you put the thickest cross section forward of the center, providing a finer exit than your entry.



Now, air is compressible, which gives you lots of freedom to make the entry blunter than if air weren't compressible - the "bow shock" or bow wave mentioned above is more forgiving in a compressible medium. The "quicker" you get to max cross section without creating turbulance, the finer your exit cdan be. Hence, the more draft you can put in the given length, or the more efficiently you can get your target draft through the air w/out turbulence. So you go as blunt as you can get away with without inducing turbulence.



H2O isn't compressible, at the kinds of forces in play at the surface of the ocean, anyway. So you have less freedom to do a blunt entry, and correspondingly more reward for a fine entry. All the factors about exit and detatchment still apply, you just don't have the freedom to go as blunt in water.



Hope this helps!

Does that mean we should be designing all our foils (rudders and keels) with a finer or sharper nose than the usual NACA aerofoils specify because they are used in water not air?

_

National Advisory Committee for Aeronautics - the pre-cursor to NASA. Many foil shapes, ducts, and other aeronautical thingies were researched and designed under its direction, and are assigned an identification as such.



As an aside - there's an official document called the "Spinoff". It details the technology transfers from NASA development programs to the private sector - anything from remote sensing to medical devices to computer technology and beyond. One of my favorites was seeing that a manufacturer of windsurfers had made use of a NASA foil and wing refinement effort to make better fins for their boards.

it has a lot to do with stability of the foil too - not just efficiency. Think how unforgiving a sharp/thin nosed foil would be to changes in direction.

Now, one thing that has not been stated is the fact that, we are not trying to get lift off of the hulls (unless you sail a 16), so there is no need to produce lift.



I have crunched a few numbers (Reynolds, viscosity...etc), but do not have time to finish the stuff tonight... probably tomorrow....



In short, a sharply pointed bow is LESS efficient than a wider one... (Hobie 20 vs..say Inter 20)... explaination manyana



Will R

Rudders, keels and airplane wings are foils because they are designed to create lift when operated at various angles of attack to the fluid flow. A hull is usually not designed as a foil if it has dagger boards or a keel. A sharper entry on a hull will reduce drag as long as the boat isn't sliding sideways a significant amount.

The sail design book I have, shows a graph of Cd (drag) against angle of attack for three different foils, an ideal aerofoil, one too sharp and one too blunt.



Any deviation from the ideal, sharp or blunt shows drag increasing regardless of the angle of attack. Indeed it is most pronounced at 0 angle of attack (or no side slipping).

Guys please make a distinction between a sharp leading edge and a fine entry or, less-correctly, a sharp bow.



These two things are independent of eachother a blunt bow can still have a sharp leading edge which according to that book can be draggy and a sharp bow can still have a blunt leading edge.



I think the thread started with a question regarding the talk of sharp bows. Like the change from the plumb Inter 18 to the fine entry bow of the Nacra F18. The last is a better cutter and performs better in chop. It also is intended to create a smaller bow wave. The first makes the boat less sensitive to weight placing and helps to reduce pitchpole tendency in the first stages of a dive. The fine entry also caused less rocking of the boat in waves; also indentified by confusion describtion of "wave piercing" bow.



With regard to leading edges to comments about too blunt or to sharp are correct although a hull is nowhere near to a NACA shape and will behave differently as a result.



Wouter

I've only skimmed through the post so maybe I've missed something. The way I see it you can't really compare a bow shape with a foil shape because the objectives are different.

The foils main aim in life is to generate lift and with lift comes drag. Obviously the lower the drag the better but if it generates lift it also generates drag,

The bow is not trying to provide lift and if the designer is on the ball among other objectives he'll look at minimising minimise drag.

Just to clarify, the reason I am asking the question is that I am building a folding Multihull, and I am using a hull design which is a NACA shape. (65012)



I am intending for the hull to contribute to the lift and so perhaps not need a keel at all.



see www.fourhulls.com



I am not sure I understand the difference between fine entry and sharp leading edge, am I right in saying a fine entry is a shape with the thickest part of the section further back.

I think you can look at the shapes of ship bows for your answer. A submarine is built for low resistance under the water and has a blunt entry. They can travel at speeds far greater than surface ships with less relative horsepower. This is because much of the drag on surface craft is due to energy spent creating waves at the water/air interface. Modern surface craft have a blunt entry shape below the surface that looks like a bullet (bulge keel). However, as you get to the surface, the bow will get finer until it is a relative knife edge at the surface as to minimize disturbance of the water/air interface (make waves). Since you generally want to keep your catamaran sailing on the surface, the fine entry is the way to go. The downside to the fine entry bow is that you lose displacement up forward and thus limit the amount of power you can carry in your rig with out turning your cat into a submarine (pitchpole).

The bulb that you are refering to works for different reasons.



The bulb that is underwater creates a counterwave system that is the inverse of the wavesystem that would be caused by the shipshull is the bulb wasn't there. And these two wavesystems that are the inverse (reciprokal) of eachother cancel one another out. Therefor the wave system that is left is much reduces in magnitude and accounts for much less drag.



Also anything that is less tahn say 5 mtr. below the surface doesn't behave as if it were really submerged like a submeraine. The surface is just to near and water and the surface will be disturbed.



And the bulb you see on navy ships near the bow is the sonar housing.



Can these bulbs be used on sailboats ? Yes, but it will only be effective at a certain speed. This is ofcourse no problem for motor ships with a cruise speed but sailboats typcially sail at different speeds depending on the windconditions. Therefor is unlikely that such a bulb is of real interest to sailboat designers. A better solution (compromise) is to have a fine wedge as a bow and less displacement.hence the trend towards sharp bowed monohulls and catamarans.



Wouter