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Displacement hull speed

Robin

Well-known member
Joined
30 May 2001
Messages
17,014
Location
back home where democracy needs no guns
I don't know about you but I find multiplyng by 1.5 in my head rather easier than by 1.34, and the difference so slight it hardly matters.
Hence the use of the word "approximately" in describing a rule of thumb where two decimal points of accuracy is frankly pointless.
Agreed but since I need a calculator anyway to do the square root bit, it is just as easy to multiply by 1.34 on that as by 1.5, sorry pedantic not intended.
 

William_H

Well-known member
Joined
28 Jul 2003
Messages
12,239
Location
West Australia
The concept of hull speed is misleading. The formular given is indicative only but does show how water line length can indicate hull speed.
In reality we are talking a graph of horse power versus speed. This graph is not linear but shows that after about square root water line lenght in feet it gets harder to gain more speed with more horsepower getting even harder with higher speed. Most boats can comfortably get to the 1.34 or 1.5 number over square root but a the cost of fuel use. A boat with a bigger engine might push to double the square root but at enormous fuel use in a big engine. So it is all a variable feast. All assuming a true displacement hull (if it starts to plane then higher speeds are more easily achieved) and the effect is far less notieable on catamarans with narrow hull.
 

oldmanofthehills

Well-known member
Joined
13 Aug 2010
Messages
2,822
Location
Bristol / Cornwall
The concept of hull speed is misleading. The formular given is indicative only but does show how water line length can indicate hull speed.
In reality we are talking a graph of horse power versus speed. This graph is not linear but shows that after about square root water line lenght in feet it gets harder to gain more speed with more horsepower getting even harder with higher speed. Most boats can comfortably get to the 1.34 or 1.5 number over square root but a the cost of fuel use. A boat with a bigger engine might push to double the square root but at enormous fuel use in a big engine. So it is all a variable feast. All assuming a true displacement hull (if it starts to plane then higher speeds are more easily achieved) and the effect is far less notieable on catamarans with narrow hull.
That is simplifying. Its like the speed of sound. At around the hull speed the disturbance caused by the bows causes a massive drag at the stern as that equates to a wave length at that speed. In that region the drag rises so sharply that a vast amount of extra power is needed to get through the barrier speed and lower powered or less slippery craft cant get near it. Above it the effect diminishes but resistance still keeps rising as the square of the speed.

Very slippery things such as cats, torpedos and planing craft have enough power/ resistance to break through the barrier
 
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