Gurney flaps are small tabs added to the trailing edges of surfaces like sails, keels, and rudders. This document discusses potential uses of Gurney flaps on sailing boats and assesses their effectiveness through theoretical models and experimental data. Gurney flaps may allow hulls, keels, and rudders to generate lift and improve performance at lower speeds. They have been shown to increase maximum lift and stall angle on airfoil-shaped surfaces like rudders. Tests adding Gurney flaps to the mainsail of an Olympic class boat found gains in performance. Overall, the document reviews the hydrodynamic and aerodynamic effects of Gurney flaps and their

1. Gurney flaps on sailing, do they help?
Floriano Bonfigli
About the different uses of Gurney flaps in a sailing boat
A comprehensive study about sailing boat was realized, than hydrodynamic as
aerodynamic point of view was tackled. Then we are four different tracks to join.
We can study if a Gurney flap fitted under hull could really perform surfing as the
theory suggest us. Or we can choose to use it to improve the effectiveness of airfoil
sections that we can find in a boat; that are keel and rudder, exactly symmetrical
NACA sections, or sail that we can model with about ten sections, all of them with
zero thickness but different cambers and twists .
Nomenclature
Cl = lift coefficient
Vmg = velocity made good
Vs = ship velocity on water
α = angle of attack on airfoil
γ’ = angle between true wind direction and boat axis
λ = yaw, angle of deviation from the proper course
Gurney flap below the hull
Gurney flap can help dynamic sustenance of hull in fact it works as a stocky half airfoil
with airflow, that generates lift, only on the lower (pressure) surface . It's been demonstrated that
Gurney flap increases lift-force because it decreases the velocity of the flow on the pressure surface.
So, according to the aerodynamic theory, it might help to surf earlier i.e. at lower velocity ranges
and then with higher increments in velocity. Honestly there are no studies or experimental
measurements that show this phenomenon for a hull in a sort of graph. There are only velocity-drag
graphs for an inclined plat on water surface: when drag that usually increases with velocity begins
to decrease we can fix the point in whom the plate starts to surf. We can imagine a similar
experiment with the plate fitted with Gurney flap; we expect that the modified plat will begin to
decrease its drag at lower velocities.
hull modelled as plate velocity-drag graph, starting point of surfing

2. The hulls of skiffs could be indicated to go deeper in this direction: Australian 18 ft. or International
14’ in particular because open classes. The 18 footer is a triple trapeze development class with over
a century of Australian history. It is purported to be the fastest mono-hull dinghy in the world,
with unlimited sail area on unlimited spars, with the barest of hull restrictions. The International 14
is a high performance double-trapeze dinghy; the class is continually experimenting with new
innovations to improve boat speed or performance, making the I-14 a pioneer in small boat
excellence.
Australian 18 ft. surfing International 14’ surfing
There are also sailing boats that are designed to take a lot of advantage surfing waves in fact they
are crafted to sail around the world. They belong to open 50’ or 60’ mono-hull class. Their skippers
usually utilize the huge waves that form in the Antarctic region and move without stopping and
meeting obstacles from west to east.
Open 60 hull rendering
Gurney flap on keel
It has been established that Gurney flap increases lift on airfoil section; a lot of measurements about
that were obtained by experiments in wind tunnel. Moreover computational results extracted by
softwares based on partial differential equations governing the fluid flow, the Navier-Stokes
equations, can match the facts. Reynolds numbers based on chord of airfoil from 2.4×105 to 2×106
were chosen for all this tests. So, if a typical keel section is a 0009NACA and if we consider a
chord of 1m for it, we will have a water flow velocity from 0.27 to 2.2 ms; it means that we have
strong datas from which we can draw our first remarks.
The first goal of keel is to balance the lateral component of the aerodynamic lift that generates when
wind meets sails especially in clause-hauled conditions; otherwise the boat will lead away in the
water as a balloon is leaded away by wind. For this reason an asymmetry under the water level has
to come up. It is the boat that spins till the angle of attack of the water flow on the keel is what
produces the same hydrodynamic component, in direction and magnitude; the angle λ was born.
Every sailor should know the following equation: Vmg=Vscos(γ’+λ); it’s evident that Vmg goes up
with lower λ. Then we can suppose that a Gurney flap fitted, maybe as a split flap, at the trailing
edge of the keel can perform lift and save on λ.

3. clause-hauled geometry
Connections between λ and vortex drag on keel and canoe body are known: about the keel we can
use the well known equations of the aerodynamic theory, while about the canoe body experimental
measurements show that drag on hull increases by 20% from λ =0° to 7°, the dotted line on the
graph below.
yaw-drag graph for several centre-boards and relative hull
Gurney flap on rudder
A typical airfoil for rudders is a 0012NACA, we can notice an higher thickness in confront of a
keel; in fact it is required that rudder could work with higher α thus with higher angles of stall. It
provides to balance boat as well as keel so the above concepts are still here available. However the
first goal for rudder is to conduct the boat in the wanted direction and, when it is required, to change
it as soon as possible. If the windward tacking anatomy is known it will be clear that the
effectiveness of rudder depends by the available lift that increases with higher α, according to
limits imposed by the stall condition. It has also known that helmsman can improve the efficiency
of his rudder (i.e. obtaining higher maximum Cl and higher angle of stall) ‘pumping’ that means
swinging the rudder. In fact, if we need to have higher rudder angles we can do it oscillating the
tiller in a range of 20÷30° in the proper way that is with the proper frequency. In fact we have to

4. obtain the right von Karman vortex street downstream the rudder. But this is just the same effect
that Gurney flap fitted at the trailing edge of airfoil yields.
static and dynamic α-Cl curves for rudder
effects on swinging airfoil
Gurney flap on sails
There are no difficulties to realize that Gurney flap can perform lift on sails by what we have
already said; in fact we can model sail with at least ten airfoil sections with zero thickness
specifying for all of them chord depth %, maximum draft position, entry and exit angle, and twist.
In addition there were applications in this direction. The mainsail of the Olympic class 470 was
tested and then used in regattas with evident gains in performance. There haven’t been tests on
genoa and jib yet and then how they can interact with mainsail. In theory, on a cat-rigged boat such
as the Finn or the Europe Gurney flap will be even more efficient than on a boat with a sail
upstream of the mast that reduces not a little bit the effectiveness of the mainsail.
genoa-mainsail interaction, pressure distributions