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Old 11-03-2010, 14:01   #1
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Myth: Hull Speed

I've heard this term "hull speed" come up in a couple dirrerent threads and thought this might be a good oppertunity to "KILL THE MYTH"
a couple days ago I took "R3" out for a sail.. had a guy here on the docks ask if he could go along.. sure, I said as I could always use an extra hand..
Winds were blowing between 15 and 20 so it looked to be a good day..Tides were high and slack so it provided a great sail.. on the way back up the channel, we pulled the kite up..
As I trimmed the kite the speed started to increase until we were looking at 10 and bouncing off of 12 knots from time to time..
And then I was acused of altering all my guages as the guy that we took out with us said its impossible for a boat of my size to go that fast due to "HULL SPEED" I tried to tell him that "Hull Speed" is only a factor in the most triditional style of "Full Displacement" boats and mine surely didnt fall into that catagory...
As he left the boat that afternoon, he still claimed I had altered the GPS in some way..
I didnt dare tell him, we had often done 16 and once I saw 18 surfing off a wave........
Hull speed is only a frame of mind!!!!!!!!!!
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Old 11-03-2010, 14:05   #2
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Sure fire way to tell is to look at the wake just at the stern. Once it no longer boils you've broken into a plane.
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Old 11-03-2010, 14:18   #3
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Randy--

In last year's Isla Race ("Regatta del Sol al Sol") our sister First 42, Ocean Angel, was routinely doing 14-15 knots under Spinnaker on their way to the isalnd and we have routinely done 10+ knots broad reaching with a 155. The boats are very fast and do like wind.

FWIW...
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Old 11-03-2010, 14:18   #4
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Originally Posted by Sailmonkey View Post
Sure fire way to tell is to look at the wake just at the stern. Once it no longer boils you've broken into a plane.
Now that brings up something else.. Is there only two areas of movement over the water..that when you exceed hull speed, you have broken into a plane... I dont think that is right... Hull speed can be exceeded without bouncing over the top of the water.......
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Old 11-03-2010, 14:19   #5
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Hull Speed

You can definitely go faster than hull speed on displacement boats. As you say surfing off a wave. However, Hull speed is definitlely more than a state of mind for all displacement hull boats.
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Old 11-03-2010, 14:33   #6
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For displacement boats, all "hull speed" is the the calculated speed of a wave having a wavelength equal to the boat's waterline. The idea being that it takes a lot more power to climb the crest of the bow wave.
Increase the waterline by heeling, wow you're exceeding hull speed!
Same with surfing down a wave.


Point being, it's just a "rule of thumb" not a hard and fast "law of nature".
Of course when a boat goes on plane, "hull speed" doesn't really mean anything.
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Old 11-03-2010, 14:34   #7
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Nobody is saying you cannot go past hull speed if you have enough propulsion power to do so. In many cases, especially sailboats, this is not possible because so many lack adequate power to do so.

There is a clearly defined technical definition of hull speed.

This speed is where you have a wave at the bow and stern and a big trough amidships. Being at hull speed means it takes a significantly more amount of energy to go beyond this speed. This is pretty much irrespective of your hull shape. Its all proportional to the length of the waterline.

There are plenty of vessels which frequently exceed hull speed without planing. Many swath hulls and plenty of catamarans have enough propulsion power to do this.

I used to have a Hobie 18 that could easily do more than 20 knots. At that speed it clearly was not planing.

Nobody has mentioned it yet, but the formula for hull speed is 1.4 times the square root of the waterline in feet. The units for speed are in knots.
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Old 11-03-2010, 14:43   #8
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Kayakers and rowers regularly exceed hull speed. It just takes a lot of power. Most displacement boats can exceed their hull speed, you just need a lot of power, be it from motor, wind, or waves.
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Old 11-03-2010, 15:39   #9
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Quote:
Originally Posted by Randyonr3 View Post
Now that brings up something else.. Is there only two areas of movement over the water..that when you exceed hull speed, you have broken into a plane... I dont think that is right... Hull speed can be exceeded without bouncing over the top of the water.......
Ok, true. I was using the word plane a little too generously. My racing cat (Inter 17R) can eaisly exceed hull speed, but I am definatly not skipping across the surface of the water. I think I need to find the exact definition of "planing"
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Old 11-03-2010, 16:03   #10
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where's Joli?

There are other factors which have to be taken into consideration in addition to hull the speed formula (where's Joli?)

The first thing that comes to mind is the fineness ratio of your hull. This one consideration is why catamarans are calculated to have higher theoretical hull speeds than the same LWL mono. A Catamaran might have a ratio of 10 to 1 (length to single hull beam width at the waterline), where a mono will be 3 to 1 or 4 to 1. The old 1.34 times the square root of LWL does not take this into account.

So, a long, narrower hull will have a theoretically faster hull speed than a long fat hull shape.

2nd, any boat can exceed hull speed if it has enough power to climb it's bow wave.

I'm sure there are more factors and I'm certain that our racers will point them out.
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Old 11-03-2010, 16:13   #11
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S&S and DavidM have given good definitions of hull speed. It is related to the waterline being equal to the wavelength. Waves with longer wavelengths travel faster which is the reason why a longer waterline (longer wavelength) translates into a faster hull speed.

A good way to think about hull speed for a lot of people is to think about trying to climb over your bow wave. One way to do this is with the correct hull shape and horsepower getting up onto plane. The other way that things like multihulls use is a combination of horsepower and fineness ratio. Fineness ratio refers to how pointy the bow of a boat is and if it is fine enough, it will pierce through the bow wave.

I agree, calling it hull speed is a little misleading.
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Old 11-03-2010, 16:43   #12
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A good looking boat generally goes faster then an Ugly one...
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Old 11-03-2010, 16:56   #13
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Basic Theory
"Hull Speed" is a broad based method of giving an approx. speed of a boat.
Generally it is the square root of the waterline length X 1.4.
It is very accurate with wide beam, heavy displacement type boats.
A general rule is 5 hp per ton is needed to achieve displacement speed.
For a 30ft , 5 ton boat displacement speed is 7 knots using 25 hp.
This is with no current or wave action.
Extra power will produce a bow wave and stern wave which forces the bow of the boat upwards and the stern down (going uphill) Speed can be increased but large amounts of power will be needed.
For a boat to plane you need a minimum of 50 hp per ton (more like 70 hp) and a hull shape which helps to hydrodynamically lift the hull.

Hull width and hull shape can improve performance.
A narrow boat with a fine bow entry, will delay the formation of the bow and stern wave.
This is called a semi displacement boat.
I once had a 36ft semi displacement power cruiser which weighed 7 ton and a 280 hp diesel power and a maximum speed of 18 knots. (40 hp/ton)
A 36ft planning hull of 8 ton (heavier motors) needed 550 hp but had a higher top speed (25knots)
These are real life examples.

However it is possible defy the "hull speed" rule.
A narrow hull design of 16-1, 40ft X 2ft 6 inches wide does NOT form a bow wave or stern wave at high speeds. (even at 30 knots)
The boat is NOT planning but is considered "Fast Displacement".
Power requirements are 25 Hp per ton, approx 1/3 of a Planning boat.
This is the concept used in Catamaran ferries and the new generation of power catamarans.
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Old 11-03-2010, 16:57   #14
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Quote:
Originally Posted by klem View Post
A good way to think about hull speed for a lot of people is to think about trying to climb over your bow wave. One way to do this is with the correct hull shape and horsepower getting up onto plane. The other way that things like multihulls use is a combination of horsepower and fineness ratio.....
This view was correct 50 years ago, things have changed my friend.

I already mentioned fineness ratio in my post. The next item is

Prismatic coefficient - Cp which is Cp= D / Am× LWL

The next item is

Water plane coefficient -
Cw , which is defined as Cw = Aw / BWL× LWL

And we can't ignore

Fully loaded displacement - mLDC which is defined as mLDC = 2 × BWL x LWL × Tc × Cp × Cm × 1025
mLDC = 7136

My point is it's a very complex subject as there are a lot more hull design considerations than I've put forth here.



If interested, you find them detailed here:


Hull Design Formulas



Yes, it's a catamaran hull design page but many facets of hull design are pertinent, regardless of how many hulls a boat has.



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Old 11-03-2010, 17:20   #15
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Tropic Cat,

Yes, I am well aware of the issues you mention. Maybe I was simplifying too much but I was trying to provide an explanation that made sense to people who haven't studied fluid dynamics. I admit that it isn't entirely accurate but it is a start. Now that I think about it, I haven't studied this stuff in 7 or 8 years and am out of date with what new stuff has been discovered.
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