What is hull speed

Until then, they must do whatever is necessary to keep up with the boat. And just how do they manage to keep up with the boat? By changing their wavelength. Our second quirk of physics: that wavelength gets tied to the speed limit for the wave. If the wave wants to go faster, it must stretch out longer, following this simple formula:. Where g is the acceleration due to gravity. Plug in the constants, work out the unit conversions, and you get the formulas in Table 1.

But that formula only describes the speed limit for a wave, not a boat. This is where we get to put the two ideas together. As the boat goes faster, the waves at the bow and stern get longer. When you reach hull speed, the bow wave lines up with the stern wave. The two waves double up on you. Your boat appears to sink down into one big wave trough. If you leave hull speed at this simple theory, you miss all the good parts of the story. So far, hull speed is this impenetrable limit, like the speed of light.

But physicists already imagined theoretical ways to get around lightspeed. And this story has a few good parts that let us get around the hull speed limit. If you push beyond the speed limit, the wavelength gets longer than your boat length. No law against that. At this point, most boats start to surf on their own bow wave; nothing wrong with that. Sure, hull speed is a difficult hump to get over, but the shape of your hull determines the resistance from those waves. Not some magical formula.

Figure Example of Sailboat Surfing [2]. Hull speed is bogus. Hull speed can therefore be said to impose a flat limit on how fast a sailboat can go. Although the physics behind hull speed is sound, it is heavily dependent on the hull's shape. Long and thin hulls with piercing designs can easily break their hull speed without planing. Such hulls are found on:.

A hull's design can enable it to circumvent the workings of hull speed. It is for this reason that hull speed is not used in present-day ship design; naval institutions nowadays favor more modern measurements of speed-to-length ratio, such as the Froude number. The formula for hull speed only needs the length of the vessel's waterline in feet , denoted as L waterline.

Load your boat heavier! If you think about a normal displacement hull, it's usually narrower near the bottom than at the deck. So pushing it down with some weight will lengthen the boat's waterline, and so its hull speed is increased. Of course, heavier boats are harder to move, so while your loaded boat now has a higher hull speed, you would need more power to move it.

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