Figure 1: Dragon Boats exceeding hull speed. Image supplied: http://www.ottawadentalcare.com/2016/05/
Is there a maximum speed for a dragon boat? Has it anything to do with “hull speed”? Can we answer those questions of whether a 10 paddler boat should be faster than a 20 paddler boat?
You would think that some clever person should be able to work out how fast their particular crew should be able to paddle by measuring enough things (crew weight, crew power, how smooth the hull is, how deep the boat sinks into the water, the density and temperature of the water, etc. etc.)
And maybe they can, but I haven’t seen it.
What I HAVE found is this notion of “Hull speed” which is NOT a limit on how fast the boat can go, it just indicates that to get past this speed you have to inject a hell of a lot more power.
In this post we will calculate the hull speed of a dragon boat.
What is Hull Speed?
When a dragon boat moves ahead, the water is parted to allow the hull to move through it. A bow wave forms at the forward end of the hull (bow) and a similar one forms at the back (stern).
The waves produced are forced to travel at a certain speed (the speed of the boat) and have a length appropriate for that speed.
Figure 2: A Dragon Boat moving slowly
As the boat speeds up, the waves get longer. Eventually, we reach a speed where the waves created near the bow have their second crest a bit ahead of the stern. The boat is then effectively sitting on a wave at the bow and at the stern. The longer the boat, the higher the speed at which this occurs. (this is important)
Figure 3: A Dragon Boat at Hull Speed
Once we go over that speed, the second crest of the bow wave is now behind the boat and the stern drops into the trough.
As you can see, for the boat to go any faster it has to go up and over the bow wave which requires much more power.
Figure 4: A Dragon Boat exceeding Hull Speed
If you look at this video you will see the back end of the faster boats in the trough as they exceed hull speed. (This post's featured image is also a good example)
How do we calculate hull speed?
William Froude (1810 to 1869), a British engineer who had a special fascination with the sea and ships carried out some experiments and determined that there was a relationship between hull speed and waterline length (LWL).
Hull Speed (in m/sec) = 1.25 x the square root of the waterline length (in metres)
What is the Hull Speed of a Dragon Boat?
The waterline length of a fully loaded (1600kg) Champion Standard Dragon boat is 10.5 metres.
Therefore the Hull Speed of a Dragon Boat is
= 1.25 x √ LWL
= 1.25 x √ (10.5)
= 4.05 m/sec => 14.6 km/hr
To go over 14.6km/hr in a dragon boat the crew needs to provide a massive amount of power.
Let’s answer a few questions
These questions have been asked in many forums. So using our new found knowledge, let’s see who would win the following races:
10 paddlers in a small boat. 10 paddlers in a standard boat.
The 10 paddler standard boat will win because the standard boat has a greater hull speed.
10 paddlers in a small boat. 20 paddlers in a standard boat.
The 20 paddler standard boat will win because the standard boat has a greater hull speed and greater power in spite of the greater mass and wetted area (and hence drag)
10 paddlers in a standard boat. 20 paddlers in a standard boat.
The 20 paddler standard boat will win because the standard boat has a greater hull speed (because it sits lower into the water) and greater power in spite of the greater mass and wetted area (and hence drag)