Basic hull shape question...

[Keycube]

Okay, this may come off as a silly question, but being that I am not yet that familiar with boats, this issue has always been a mystery to me; so, please humour me. :-)

If there is a standardized ultra-efficient aerodynamic shape (an airfoil), why isn't the wetted area of a boat basically sculpted as such? I always wondered why racing boats weren't essentially an airfoil on the underside, with the keel(s) to balance the boat and essentially make the boat "sailworthy".

Thanks for any help.

Robert

[Stephen Ditmore]

Actually that used to be the prevailing theory - that a hull should be paterned on a "cod's head and a mackeral's tail" - until the 1830's, when finer bowed clipper ships started establishing their superiority. The decisive blow for yachties may have been the wide but fine bowed schooner yacht America's racing success in 1851, when she won the cup that became the "America's Cup" from England.

At low speed/length ratios a full bowed hull can do well, and full bows are still common on tankers and other slow ships. As a vessel nears "hull speed", however, research shows that the center of it's underwater volume wants to be at about 8/15 of the waterline length to minimise the creation of surface waves (and farther aft than that on a planing shape with an immersed transom once it goes fast enough for the water to break clean). It doesn't pay to push up a big bow wave at hull speed.

An airfoil could be designed with its volume aft of 50%, and it would be low drag relative to its thickness if shaped well (esp. if conditions allow for laminar flow), but unless it is thick it will tend to stall at high angles of attack. Keeping the volume farther forward allows the overall thickness to be less, and the keel area to be less (since the foil can operate at higher angles of attack).

One could design a boat on which the immersed waterlines are shaped like airfoils where the forward ends of those immersed waterlines are well aft of the bow of the boat. If designing an old fasioned full keel yacht or (at the other end of the spectrum) a boardless beach catamaran I might be tempted to try this.

[Stephen Ditmore]

It occurs to me that the waterlines between the DWL and the keel on some 12 meter (America's Cup from WWII - 1987) yachts might resemble foils.

[Keycube]

Originally posted by Stephen Ditmore
It occurs to me that the waterlines between the DWL and the keel on some 12 meter (America's Cup from WWII - 1987) yachts might resemble foils.

And why would that be, exactly?

[Stephen Ditmore]

Tom Speer's our aero/hydrodynamics expert - I'm going to give you a less technical answer. Though they continued to build & race them through 1997 a 12 meter is basically a traditional type of yacht. The 12 meter rule pretty much forces a midship section with deadrise (a V shape). Therefore the keel and hull work together to resist leeway, and it would make sense for the lower part of the hull to be shaped a little like a keel if it can be done without pushing the underwater volume too far forward.

[tspeer]

Originally posted by Keycube
...If there is a standardized ultra-efficient aerodynamic shape (an airfoil), why isn't the wetted area of a boat basically sculpted as such? I always wondered why racing boats weren't essentially an airfoil on the underside, with the keel(s) to balance the boat and essentially make the boat "sailworthy". ...

The nice thing about standards is, there's so many to choose from! There's no standard airfoil shape. Today's practice is to design an airfoil specifically for the requirements of the application. Much like boat hulls are designed specifically for their application.

I seems to me classic, full keel race boats are essentially an airfoil on the underside.

Modern boats have a lot less wetted area. But the keel is still an airfoil. And the hulls are sculpted to provide the required form stability with minimum wetted area. Modern boats also have wider flatter runs to provide dynamic lift.

So I guess I'm a little confused as to just what you're driving at.

Cheers,

[Tom Lathrop]

Tom Speer,

I too am wondering what the question really is but will take a stab at it anyway. An airfoil on a plane has one job to do and that is to generate dynamic lift to support the plane in the air. That is the same job of the keel on a sailboat except that the keel must work in both directions. While the plane's wingfoil will work in the inverted position, it is not nearly as efficient in that job and requires a much greater angle of attack to generate enough lift to keep the plane airborne when inverted.

The hull of a sailboat has a quite different job. It must support the weight of the boat both moving and at rest. It also works in the interface between air and water and depends on buoyancy to support the boat rather than dynamic forces. It's shape is determined by the need to have enough buoyancy to offset the weight and generate as low drag as possible. The fact that some hull shapes may look a bit like an airfoil is just coincidence, except for the part that may serve as the keel, of course.

The above does not address the situation of planing sailboats which would be a bit different but still not have anything in common with airfoils.