“Hydrodynamic Aspects of Surfski Drain Scuppers” or “The Science Behind the Bullets”!

Tuesday, 16 June 2009 06:21 | Written by  David Hartwanger, RSA Director for Intelligent Fluid Solutions
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Great shot of the bullet scuppers taken in Durban! Great shot of the bullet scuppers taken in Durban! Credits: www.anthonygrote.com

[Editor: David Hartwanger of Intelligent Fluid Solutions explains how the Red7 Bullet Scuppers were designed.]   


The drain scupper on a surfski is designed to keep the foot well empty of water.  This is important because any water trapped in the boat adds additional mass, making acceleration more difficult and increasing boat resistance due to increased boat displacement.

The foot well drain empties through the bottom of the boat, and therefore under static conditions allows water to enter to a level matching the surrounding water surface.  Under dynamic conditions a local low pressure zone (partial vacuum) is generated at the exit of the drain that reduces the level of water in the foot well, ideally to zero.  This is typically to a level approximately 100mm below the water surface in a single, maybe a bit more in a double.  The deeper the foot well (depth below the water surface), the larger the partial vacuum required to keep the foot well dry.  Therefore, in general drainage in double skis is more challenging because the foot wells are often a bit deeper below the water surface.

The partial vacuum is generated by causing a momentum change in the flow over the hull at the drain exit.  Causing flow to change direction or accelerate or decelerate changes its momentum, and results in pressure gradients.  This can be simply illustrated in the figure below.

Red7 Bullet Scuppers

Figure 1: Pressure gradients developed in curving flow


When flow has to curve around an object local high pressure and low pressure zones are formed.  For a blunt object in uniform flow high pressure is formed upstream and low pressure downstream as the flow curves away and towards the object.  The pressure difference acting over the frontal area of the object causes the drag force.  In the case of a streamlined object like an aircraft wing the drag force is minimal.  Instead the flow curves over the upper and lower surfaces generating local high and low pressures below and above the wing.  The resultant force is predominantly an upward lift force.

The Backward Facing Step

The geometry of the drain cap or scupper is characterised by a relatively streamlined fore-body and blunt after-body.  The local low pressure in the wake of the cap draws out the drainage flow.  The resulting flow pattern, illustrated below, is very similar to the classic backward facing step. 

Red7 bullet scuppers

Figure 2: Flow over a drain scupper illustrating local high and low pressures

The backward facing step has been a well used geometry in fluid dynamic research, as the flow field contains a number of important fluid dynamic phenomena, including:

  • Flow separation
  • Turbulent free shear layer
  • Re-circulating flow
  • Shear layer re-attachment

The typical geometry and flow field are illustrated in figure 3 below.

Red7 Bullet Scuppers

Figure 3: Flow field over a backward facing step

All the flow features illustrated above are also present in the flow over a surfski drain scupper.  The key features to note, relevant to the drainage application are:

  • The zone of re-circulating flow in the separated region
  • The shear layer, or internal mixing layer, in the separated region

Immediately downstream of the step a ‘bubble' of fluid is trapped by a turbulent shear layer separating off the top corner of the step.  The trapped fluid circulates behind the step.  Fluid is transferred across the shear layer by means of turbulent mixing and diffusion. 

The effect on the drainage application is that although there is a low pressure zone at the drain outlet, the local flow is circulating.  In fact, a significant volume of water is flowing towards the drain outlet, effectively causing a ‘blockage'.  The drain flow enters the re-circulating ‘bubble' before crossing the shear layer by means of turbulent mixing.  This can be seen in the image taken from a numerical simulation, where the streamlines exiting the drain circulate before joining the background flow.  In effect, the flow field forms a fluid dynamic blockage.  The rate of drainage is therefore dependent on the strength of turbulent mixing in the shear layer.

Conventional Surfski Scuppers

Figure 4: Re-circulating flow at the drain outlet

The Bullet Effect

The main aims of the bullet design are to:

  • minimise the amount of re-circulating flow
  • force more rapid mixing between the drain flow and background flow

Bullet scupper flow

Figure 5: Reduce re-circulation with bullet in place

The presence of the bullet in the wake of the scupper destroys the bubble of re-circulating flow, as illustrated in figure 5.  Instead the drain flow is directed to contact the free shear layer immediately, and to mix with the shear layer over the length of the bullet.  So in effect the bullet removes the hydrodynamic ‘blockage' from the system.

It is also worth mentioning that the effect of the boundary layer should also be considered.  The boundary layer is the region of slightly slower flow adjacent to the boat.  Due to the skin friction at the hull surface the adjacent water molecules are effectively attached to the boat.  Therefore, there is a layer of water where the flow speed increases rapidly from zero (relative to the boat) to the boat speed some distance from the hull.  For a surfski in the vicinity of the drain hole, the boundary layer thickness is approximately 40mm.  So ensuring that the drain flow exits away from the hull surface means potentially more rapid mixing into the higher velocity flow.  The bullet provides some benefit in this regard.

The specific shape and location of the bullet can be varied almost endlessly, depending on the detailed design constraints and intended operating conditions.  The main objective is that the combination of scupper and bullet are optimised at the intended operating condition.  For the surfski application some of the key design goals included:

  • Achieving the target partial vacuum at the minimum boat speed
  • Optimising drainage flow for boat speeds from 6 - 10 kph.


Surfski Bullets

Figure 6: Drainage Performance Comparison

Figure 6 shows comparative results for a condition where the surfski foot well is half full of water, i.e. approximately 50mm deep.  The results suggest that a conventional scupper would begin to drain at about 8 kph, while adding the bullet would reduce this speed to about 7.2 kph.  Negative drainage means water is flowing into the boat, and positive drainage means the water is flowing out, i.e. the scupper is working.

At 9 kph, where a paddler may typically be trying to accelerate after a ‘mishap' with a cockpit half full of water, the bullet improves performance by 40%.  Of course the sooner the boat is emptied the quicker the paddler can accelerate back to cruising speed.

An added benefit of the bullet is a substantial reduction in drag.  There are various kinds of drag force, e.g. skin friction, pressure drag, wave drag. However, the scupper appendage is characterised by pressure drag as a result of local high and low pressure zones formed around it.  The net effect of the bullet is to create a more streamlined body around which the fluid must pass.  More specifically the impact of the bullet can be simply illustrated in the figure below.

Surfski bullets - drag

Figure 7: Drag reducing effect of the drain bullet

The location of the high and low pressures means the bullet, forming a streamlined after-body, substantially offsets the drag force of the scupper cap.   The reduction can be as much as 60%, although a standard drain scupper may only contribute between 1 - 1.5 % of the total boat resistance.  However, the drag reduction makes it feasible to either mount larger drain scuppers or more of them to increase total drain flow without adding any further hull resistance.


Understanding the flow physics associated with the classic backward facing step is crucial to finding mechanisms for improving and optimising the drain flow appendage on surfskis.  The bullet feature provides a relatively easy ‘fix' to existing drain scupper designs.  No doubt further development and optimisation is still possible, and should be encouraged, to keep your feet dry for longer.

Intelligent Fluid Solutions

David Hartwanger

RSA Director for Intelligent Fluid Solutions


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