Myth: Hull Speed

[GordMay]

Originally Posted by nautical62

Charles Kanter writes about hull speed and the basic formula quite a bit in his book: "Cruising in Catamarans" A point he makes is that the formula isn't really 1.34 times the square root of the waterline length, but rather a K value for that hull ...

See Chris Price’s interesting discussion about the Form Factor Constant (K or C):
☞ Data - Marine Formulae #1

More good stuff from Chris Price:
☞ Sailboat Design Data

[Robert Jansen]

Hull speed is a myth, but not for the reasons typically set forth in the various arguments. Let me offer a different perspective.

Beginning with the accepted definition of hull speed for a displacement monohull as being the velocity at which the wavelength of the bow wave couples with the waterline length, there is an unstated underlying assumption which has not been addressed until quite recently. To identify this assumption, one must recognize that from the perspective of system engineering and more specifically system hydrodynamics, a conventional displacement monohull under weigh is an energetically open and dissipative system. By this I mean that the energy utilized to generate the bow wave is lost through the propagation of the bow wave away from the vessel, and cannot be recovered. Hull speed, per se, is an artifact of conventional system dynamics.

That much being stated, ask the following question (as I did some time back: what would the configuration of a displacement (powered) monohull vessel be if, rather than the system dynamics being open and dissipative, they were (asymptotically, at least) closed and regenerative?

I have devised a solution to the problem, and for my efforts, I have been recently been awarded US patent #8,991,326 (for the curious, go to uspto.gov and perform a patent number search).

An (asymptotically) closed dynamic system views the hull, the water surrounding it, and the propulsion system as an integrated whole, to be considered in its entirety.

Most importantly, a dynamically closed and regenerative system does not generate a bow wave, so the issue of hull speed never arises.

It should be obvious that the solution only applies to powered hulls. My apologies to those who rely on the wind for their energy.

I am in the process of completing the scale model experimental technology demonstrator. Results of the first tests are forthcoming in the very near future (within the next 30 days).

[GordMay]

Greetings and welcome aboard the CF, Robert.

➥ United States Patent: 8991326

[Ericson38]

Originally Posted by Randyonr3

Here again, you are try to explain how the formula works and the boat "MUST" be planing, or simi planing...
What has been said by not only myself but others as they come forward, that they have exceeded the numbers in a broad reach under working sails.. The bottom may be flat, but in a broad reach, your side is in the water and the side is not flat.. The waterline may infact be longer but not to the extent to offset the difference in speed gained..
Planing by deffinition is when a boat breaks lose from the waters surface and rides above the water as a powerboat does..
A modern sailboat is not planing when heeled at 12 degrees....

some historical perspective-

History of the planing dinghy

You are planning at 15, but there is nothing wrong with that !

Your stern wave is behind you at 15 kts, not meeting the boat at the transom. With the stern wave well behind you, your boat is past the point of going uphill, which happens when a displacement boat is towed (or powered) in calm water at sppeds in kts of 1.5 and higher times the sq root of the water line length, in feet.

[gordon.livey]

Here is one thing that is not mentioned in this forum.
If a displacement hull has a ratio of the waterline length/width at waterline greater than 7:1, the resistance curve does not go straight up at hull speed as it usually does. Blow 7:1 you can put in as much power as you want, it will not go faster unless it starts planing. Planing takes a lot of power.
The resistance curve goes up at an angle, so if you apply more power, it can exceed hull speed.
As the boat becomes more slender, i.e its length to width ratio goes up, the angle will become less steep. That means that it takes less power to increase the speed above hull speed.
Ships that break the 7:1 ratio are destroyers - if you look at a destroyer from the front you will be amazed at how skinny it looks at the waterline. That's why a destroyer can't fire all its guns off one side at the same time, it may fall over! I think aircraft carriers are also very skinny at the waterline.
Boats that break the rule are rowing shells, catamarans, trimarans, and proas. Please note that a rowing shell is extremely tippy, and the rest have more than one hull, that's why they don't fall over!

Regards,

Gordon Livey P.Eng.

[Adelie]

Originally Posted by gordon.livey

Here is one thing that is not mentioned in this forum.

If a displacement hull has a ratio of the waterline length/width at waterline greater than 7:1, the resistance curve does not go straight up at hull speed as it usually does. Blow 7:1 you can put in as much power as you want, it will not go faster unless it starts planing. Planing takes a lot of power.

The resistance curve goes up at an angle, so if you apply more power, it can exceed hull speed.

As the boat becomes more slender, i.e its length to width ratio goes up, the angle will become less steep. That means that it takes less power to increase the speed above hull speed.

Ships that break the 7:1 ratio are destroyers - if you look at a destroyer from the front you will be amazed at how skinny it looks at the waterline. That's why a destroyer can't fire all its guns off one side at the same time, it may fall over! I think aircraft carriers are also very skinny at the waterline.

Boats that break the rule are rowing shells, catamarans, trimarans, and proas. Please note that a rowing shell is extremely tippy, and the rest have more than one hull, that's why they don't fall over!



Regards,



Gordon Livey P.Eng.

Welcome to the forum.

Starting by resurrecting threads that have been dead for over a year I see. Well I've seen 6 yr old threads make a comeback so this isn't so bad.

Even below 7:1 you can exceed hull speed with enough power, there isn't a discontinuity in speed to drag curves when you get to 7:1.


A house is but a boat so poorly built and so firmly run aground you would never try to refloat it.

[boatman61]

I've found traditional designs that exceed hull speed under sail tend to suffer a reduction of freeboard at the bow.. in one case on a boat of mine it was down to 6 inches..

[chala]

Originally Posted by GordMay

Quoting Wiki is for amateurs.

So which one would you prefer to use?

Originally Posted by StuM

NAMES AND ABBREVIATIONS

Unit names are frequently abbreviated. It is a convention that where the name of a unit comes from the name of a person, the name or abbreviation is capitalised, all other units are not.
So we have Ampere, Volt, Watt and their abbreviations Amp or just A, Volt or just V and Watt or W, but meters (or m), seconds (sec or s), kilograms (kg), gallons (gal or g) etc

As mentioned previously derived units comprise two or more units which are multiplied or divided by each other. A great deal of confusing arises where these operations are not expressed properly.
When two units are multiplied together, the resultant unit name is written as the basic unit names without any intervening mathematical or other symbol.
(i.e The commonly used unit of "energy" stored or used in an electrical system should always be written as simply Ampere hour (or Amp Hour, Ah).
When one unit is divided by another , the two words are separated by a slash (divide sign "/") or the word "per" (or just p)
(ie gallons/hour, gallons per hour or gph).

Unit names[edit]
Names of units follow the grammatical rules associated with common nouns: in English and in French they start with a lowercase letter (e.g., newton, hertz, pascal), even when the symbol for the unit begins with a capital letter. This also applies to "degrees Celsius", since "degree" is the unit.[45][46] In German, however, the names of units, as with all German nouns, start with capital letters.[47] The spelling of unit names is a matter for the guardians[Note 8] of the language concerned – the official British and American spellings for certain SI units differ – British English, as well as Australian, Canadian and New Zealand English, uses the spelling deca-, metre, and litre whereas American English uses the spelling deka-, meter, andliter, respectively.[48]
Likewise, the plural forms of units follow the grammar of the language concerned: in English, the normal rules of English grammar are used, e.g. "henries" is the plural of "henry".[49][34]:31 However, the units lux, hertz, and siemens have irregular plurals in that they remain the same in both their singular and plural form.
In English, when unit names are combined to denote multiplication of the units concerned, they are separated with a hyphenor a space (e.g. newton-metre or newton metre). The plural is formed by converting the last unit name to the plural form (e.g. ten newton-metres).
https://en.wikipedia.org/wiki/Intern...ystem_of_Units

[Adelie]

Why are you trying to revive an argument in a thread from 6yr ago? GordMay's post was from 2010.

And StuM's post is from a different thread!


A house is but a boat so poorly built and so firmly run aground you would never try to refloat it.

[44'cruisingcat]

Originally Posted by gordon.livey

Here is one thing that is not mentioned in this forum.
If a displacement hull has a ratio of the waterline length/width at waterline greater than 7:1, the resistance curve does not go straight up at hull speed as it usually does. Blow 7:1 you can put in as much power as you want, it will not go faster unless it starts planing. Planing takes a lot of power.
The resistance curve goes up at an angle, so if you apply more power, it can exceed hull speed.
As the boat becomes more slender, i.e its length to width ratio goes up, the angle will become less steep. That means that it takes less power to increase the speed above hull speed.
Ships that break the 7:1 ratio are destroyers - if you look at a destroyer from the front you will be amazed at how skinny it looks at the waterline. That's why a destroyer can't fire all its guns off one side at the same time, it may fall over! I think aircraft carriers are also very skinny at the waterline.
Boats that break the rule are rowing shells, catamarans, trimarans, and proas. Please note that a rowing shell is extremely tippy, and the rest have more than one hull, that's why they don't fall over!

Regards,

Gordon Livey P.Eng.

This point was raised YEARS ago! On the first page, post #7.

[barnakiel]

Interestingly, it is not important if something is a myth or a fact.

As soon as any myth gets enough likes, it is as relevant as if it were a fact.

This shows that a strong myth beats a weak fact.

Cheers,
b.

[44'cruisingcat]

Originally Posted by barnakiel

Interestingly, it is not important if something is a myth or a fact.

As soon as any myth gets enough likes, it is as relevant as if it were a fact.

This shows that a strong myth beats a weak fact.

Cheers,
b.

Is that a fact?

[Stumble]

Originally Posted by 44'cruisingcat

Is that a fact?

Yes and I have the statistics.

[Jim Cate]

Originally Posted by Stumble

Yes and I have the statistics.

Which, like 87.4 of all statistics, were made up on the spot...

Jim

PS: IIRC, Randy went cruising post 2010. I wonder if he did any 300 mile days?

[barnakiel]

He he he ...

Love,
b.