Factors That Affect Sailboat Speed
Sailboats come in a number of sizes. While some might think that larger sailboats will go faster than their smaller counterparts simply because of their large sails, that is not entirely true. A number of factors come into play when it comes to a sailboat's speed, such as the waterline length and shape of the hull, and not to forget the weight of the sailboat. Here, we are going to take a look at all of the various factors that have an impact on the speed of a sailboat.
Size: Is Bigger Really Better?
Ask any experienced sailor, and they will be quick to point out that the hull speed is a major factor that influences how fast you can sail on the open water. This basically means the waterline length, which determines the total length of the wave that's generated. Sailing enthusiasts often refer to the length of the wave that's generated at either side of the boat to get an idea of speed. For instance, once the sailboat reaches the hull speed, a wave crest appears at the bow, a wave crest at the stern, and a trough in the center.
The boat's speed creates this wave, which means the longer the wave, in comparison to the boat's own water length, the faster the sailboat is traveling. Once the hull speed has been reached, it is considered the theoretical maximum that the sailboat can attain. This used to be true – until modern sailboats began to get over the waves and use advanced maneuvering techniques, such as planing.
However, the hull speed is still considered in naval design and is used in the speed to length ratio. For instance, the hull speed is typically the square root of 1.5 times the length of the hull, which is 1.34. Keeping that in mind, a waterline of 20 feet long that generates a 20 feet wavelength typically has a hull speed of around 6 knots, while a larger waterline would translate into a hull speed of 9 knots in the right conditions. That being said, the wind and currents, along with other factors, also have an effect on the boat's speed.
Since the sails require lots of power to get over the hump in the resistance curve, the longer the sailboat is, the better it is said to be at reaching those higher speeds compared to the shorter sailboats. This is mainly because the longer sailboat will create longer waves across the hull, enabling it to move faster. In comparison, smaller sailboats can only generate shorter waves, which results in a reduced speed. It is also important to note here that a sailboat cannot travel faster if the wave it creates is longer than the sailboat itself.
To break things down a bit, here's a simple formula that is used by experts. Both the stern and the bow wave travel at the phase velocity c = Square Root (gL/2Pi), where g is the gravitational constant, Pi is 3.14, and L is the wavelength of the waves. So the velocity is proportional to the square root of L (which equals LWL at hull speed), and longer waves – and longer boats, other things being equal – travel faster.
Weight of the Sailboat
This is one of the reasons why modern boat designers tend to make sailboats that are longer. It is so that the sailboat can get over the wave's mound easily and with minimal resistance. One of the most efficient ways of achieving this is by designing lightweight sailboats. However, there is a downside to using lightweight sailboats. A sailboat that is too light could have stability issues, especially during heavy winds that can easily cause the sailboat to get pushed around and even capsize.
Also, extremely lightweight sailboats might not have the stability required to get the power needed to reach high speeds. This is why most lightweight sailboats today are built with double hulls to compensate for the lack of stability during high winds. While large sailboats of up to 40-feet can weigh around 12,000 lbs., small sailboats that usually make the 15 to 20-feet category can weigh anywhere between 170 to 1,000 lbs. The fastest sailboats use carbon rigs and get away with a smaller ballast ratio by having lots of form stability.
Hull Shape
If you are a sailing enthusiast, then you're already familiar with the hull of a sailboat. This is the part that supports the rigging and, of course, carries the passengers. The hull is also connected to the centerboard at the bottom, which keeps the sailboat from sliding sideways. Few people know this, but the hull's shape also plays a vital role in the speed that the boat can achieve. While some hulls can drag the boat, others are more efficient in creating the appropriate amount of wave resistance. As a rule of thumb, one should always choose a sailboat with a slim and well-built hull. This is mainly because sailboats built with a fat tub-shaped hull are unable to achieve a lot of speed compared to slim hulls.
So, how is one to know if the hull is the culprit when it comes to keeping the sailboat from reaching high speeds? One way that seasoned sailors identify the right size and shaped hull is to focus on the aft or buttock lines. If you notice that from the lowest point of the hull to the transom, there is a straight line and that the transom is built wider, the sailboat will have a good chance of reaching high speeds.
On the flip side, if the hull's shape curves while having a narrow stern, then that is an indicator that the boat will not be able to go faster. This difference is significant since the hull has to cut through the water as it's pushed forward by the wind and sails.
