Tech Article - Hull Speed, Displacement Boats and Planing Boats

Tech Article posts are those that came about because of something I was writing in a more typical blog entry and decided to break out as a reference item.

Hull speed is nautical speak for maximum theoretical speed for a "displacement" boat.

A boat is basically just plopped into a body of water and displaces an amount of water equal to the weight of the boat.  A boat's weight and it's displacement are the same. 

That is true for every boat that is not moving.

Once you add movement, that can change.

A displacement hull is the name used to signify a boat design where the hull just pushes water as the boat moves. The fastest a displacement hull boat can move (Hull Speed) is determined with the formula:

HS = 1.34 * SQRT(LWL)

HS is the Hull Speed in Nautical Miles per Hour, aka knots 

LWL is the Length at the Water Line in ft.

As an example, as 38' monuhull sailboat, may have a 33' LWL.  Therefore, the Hull Speed for this boat will be:

HS = 1.34 * SQRT(33) = 1.34 * 5.74 = 7.7 knts

A Nautical Mile is a little longer than a regular (statute) mile, so 7.7 knts = 8.85mph.

That is the fastest that boat can move, (*kind of, more on that later). 

You may wonder why and the answer is kind of neat.

When a boat is moving through the water, it is moving that water and creating a wave of its own.  That wave starts at the bow (the very front) of the boat and rises up highest at the bow.  From the bow back, the height of the wave falls off and then starts to rise again, like any other wave in a body of water.  When a displacement boat is moving very slowly, there is a wave that starts at the bow and then falls off quickly.  As the boat accelerates, the wave takes longer to fall off, until eventually, it falls off and rises again at a distance that is equal to the boats length at the waterline.  The boat is trapped in the trough of its own wave and has reached Hull Speed!

Taken from a post on Cruiser's Forum

So to become un-trapped, the boat will have to exert considerably more effort so that it can push up against the front of that wave and start to ride on top of it.  If the boat has the power to do that, the boat is now said to be planing, where it is riding on top of the water instead of just sitting in it.  Hull design can make this a lot easier, and while the effort to go into planing is substantial, once the boat is there on top of the water, it doesn't take as much effort to stay there.

That's why a shorter bass boat, which may only have an LWL of 28', can reach a top speed around 60 mph.  At 28' long, it is only operating as a displacement boat until it hits 8mph, at which point and when given enough added horsepower, it becomes a planing boat.

It's also why a 1000' long oil tanker, weighing almost 1 million tons can travel much faster than a relatively light (8 ton) sailboat.

* so I said "kind of" earlier, referring to the maximum speed of the hypothetical 38' sailboat and that's "the fastest that boat can move".  If you add enough power, any hull will become a planing hull, but in reality, you never would and probably never could add enough to a regular sailboat to get it to be truly on plane.