Hull Speed

[Tom23400]

They say you can determine your max hull speed by knowing
a sail boats length in the water. If that's the case why is a
racer faster than a cruiser with the same water line length?

 

[Grizz]

Besides...

...displacement, hull design, hull prep, folding vs. fixed prop, rig & sail configuration, racing vs. cruising sails, fin vs. shoal keel, crew capabilities and difference and improvements due to age/design?

A brand new Mumm 30 vs. a mid-70's Pearson is like comparing a 6-cylinder Gremlin to a 6-cylinder Escape,or something like that. One's a rocket and one's, well, not a rocket. Both will get you from point A to Point B, one just a bit quicker.

Of course, one will also cost a heck of a lot more, which oftentimes becomes the deciding factor.

 

[RichH]

Because hull speed is not the speed of the boat .......

but the **speed of the waves** that support it in a quasi-level angle of trim --- one wave under the bow and one wave under the stern and both equally supporting the hull in a ~level attitude of trim. By 'correlation', a boats hull speed is the maximum 'speed' while the boat is in 'displacement mode'. Many boats regularly exceed 'displacement mode'. ie. plane, surf, semi-plane, etc.
:-o

The cruiser is usually 'deeper' in the water, the racer ON the water;; hence, less parasitic and form/shape drag. The racer has an 'optimized' shape and wetted surface area, etc. to lessen friction (viscous shear) .

 

[tommays]

But

A 20' water line will usally allways have the same hull speed BUT the more modern ones will achieve it with much less wind and will generaly point much higher


Buy pointing higher the "race" boat will generaly have a much better VMG (velocity made good)and will get to a place up wind faster because it has to cover less distance


In a recent 34 mile JAM race i raced in my J24 a 35' cat boat killed me on the downwind leg BUT there was a 17 mile return dead upwind and i gained it all back +20 min.


Tommays

 

[Warren Milberg]

The formula for "hull speed"

is typically 1.34 x the sq root of the waterline length. The constant 1.34 must be adjusted for all the factors noted by Grizz below in his post on this subject. It should be considered no more than an "adjustable yardstick."

 

[sailortonyb Allied Mis]

To a noob,

without a math background, or a calculator handy, this info is useless.
I will try to simplify this with probably some error which will quickly be brought to light.
Your hull speed is sometimes referred to as a theoretical hull speed. I think its based on flat water. When you are moving forward, your bow creates a wake or wave.
As your speed increases, so does the wave in both height and length. When this wave becomes long enough say from top of wave to top of wave, when the tops are one on the bow end and one on the stern end, at this speed is your theoretical hull speed. If you exceed (and you can) the theoretical hull speed, the top of the wave in the stern becomes further apart from the top of wave on the bow. This will move your stern down, which in turn will make your bow rise. This squatting position will take lots more power to move the boat. If you are being towed faster than your theoretical hull speed, the tow line will yank down on your raised bow, pull you in front of your own wave and now bury your bow. This cycle will keep repeating itself and thus the term porpoising.
For all intents and purposes, all a new-be has to know is that on an average 25 footer, the theoretical hull speed will be around 5.5 knots. On a 30 footer it will be around 6.0 to 6.3 knots.
The reason that a race boat is faster than a average boat is that the shape of the hull is more streamlined and the boat is lighter and that means the boat can reach theoretical hull speed FASTER, but not necessarily be any faster over all.
There are lots of other variables, but this is the simple version.
I hope when others correct me, they can do it in english that a noob can understand.
Thanks

Tony B

 

[Joe]

if water line length were the only....

...factor in determining a boat's speed through the water... the entire boat design industry would be reduced to a couple of geeks with a simple calculator.

The theoretical hull speed formula was primarily used as a way to determine the most efficient engine size for steam powered vessels. A way to determine their maximum speed by pushing water aside(displacement), rather than lifting up on top of it (planing), because the cost in fuel and horsepower needed to push the hull beyond its theoretical limit was considered technolgicaly and economically prohibitive.

Most modern racing sailboats do not have full displacement, but rather semi-displacement hulls. They are designed to plane under optimum conditions. Their hull designs are vastly different than those of the old steam vessels this formula was applied to. Even before steam, in the great age of sail, hull design was evolving to allow sailing vessels speeds beyond the limitations of a full displacement hull. (Remember the scene in the movie "Master & Commander" where Capt Aubrey was made aware of the new hull design of the ship he was pursuing by one of his crew that had seen the vessel in dry dock?)

So, Tom, the bottom line is that the theoretical hull speed formula is not particularly relevent to the modern racing yacht.

 

[tommays]

PHRF

http://www.ussailing.org/phrf/Tool_%20HI_LO_AVG%20Report.pdf


Here is a 78 page list of the comparitve speed of most every boat you could think of


Tommays

 

[Steve Dion]

here ya go!

[nm]