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looking back on importance of aerodynamics

Tom v.d Brink


well no questions or comments yet apparently so we shall continue.

In the mean time i upgraded my computer system and rechecked some things and I got to a point that i think i can

make a better claim about this comment I made earlier.

The amount total aerodynamic drag is as many have already claimed not as high as a potential hydrodynamic drag.


many people have this impression and i to have this impression but is it true?

well to find out we first have to quantify all major drag components of our boat.

to do this the famous aussie Ken Warby, holder of the current water speed record at 317.596 mph,

gave us this example Mr Warby's calculation from his website

of what is suppose the calculation of a boat similar to his boat just under the current record.

well from this if we take the ratio of each drag component (combined by source, i.e planning surface, rudder and aerodynamic) over the total drag we find:

planning surface 14.0% of total drag

rudder surface 51.8% of total drag

aerodynamics 34.2% of total drag


So for this case the claim still appears true.

now eventhough Mr Warby presents a clean looking calculation when I wanted to do a similar calculation for our hulls i found i needed to change the input a bit.

for instance the planning surface area ratio is constant according to this calculation therefore the lift coefficient remains constant and thus the lift increases with change in speed. right you might say? well no in the case of a boat planning at speed lift force can change but in turn this will mean the boat will come up higher out of the water (considering no sudden weight change and no aerodynamic lift). this means the planning surface area changes, thus the lift coefficient changes etc.

well to keep things simple there will be a point where lift force equals the lift coefficient belonging to a certain surface area ratio. since the width of a sponson does not change just the wetted length changes and this equilibrium can be found iteratively.


the other thing i changed to do the calculation was the aerodynamic drag coefficient. Ken claims a Cd 0.2

but for a small boat with big round surfaces (tubes, engine etc) this is highly optimistic. from my 1/2model simulation i found a value for Cd of about 0.47. that's about 2.3 times more.

the rudder i kept the same expect for filling in the proper values,

the result was surprising.


planning surface 31.8% of total drag

rudder surface 10.5% of total drag

aerodynamics 57.7% of total drag


now in my simulations an improved model gave me a new Cd of around 0.37

and since power is equal to the total drag times the speed this meant the boat needed 10.6% less power to sustain the 100mph speed.

in my opinion 10.6% is a great improvement.

and suddenly it doesn't seem half bad to play around with aerodynamic improvements although they are still more difficult to solve

then for instance gaining 3% less power taken by cutting about 10mm length from your rudder.


right so next time as i promised before a bit more on round shape drag. but i forced myself to go back over the calculations and clean improve there validity to present them better.


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