Talk:Hull (watercraft)

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[edit] Most central concept?

"single most central" concept? I think that phrase needs attention, but I don't know what to change it to. --KQ

[edit] Rafts and logs

Do rafts and single logs not constitute hulls? Even if not, the distinction is artificial, and they deserve mention IMO.

Hmm, swamps, roaring streams, and even calm streams where sharp objects and ankle-twisting holes are hard to spot, are more safely crossed by walking on logs. So as long as water levels vary enough, you don't need be swept into a river and save yourself by grabbing a floating log to think of a floating log as something for a person to float on. (Perhaps floating a carcass across a water barrier on a log or raft is more innovative than personal transport by water.)
--Jerzy(t) 16:29, 2005 Feb 14 (UTC)

"Hull" is related to hell and concepts of hiding, and also means the outer layer of something. "Hollow" relates to "hole". So it is probably a linguistic coincidence that hulls, as floating structures, have to be hollow (to "trap ... air" -- not a very clearly stated concept, BTW, in the case of open boats) when made of heavier-than-water materials.

As to logs and rafts, the only reason that wood and foam are significantly lighter than water is is that both of them trap air in sealed capillaries or crevices, and in bubbles. They aren't "hulls" in being necessarily shell- or cowl-shaped, but they are hollow at a micro-scale.
--Jerzy(t) 16:45, 2005 Feb 14 (UTC)

[edit] Mergefrom Displacement hull

Totally agree! P199 14:07, 20 March 2006 (UTC)

Merged. CP/M 02:05, 7 April 2006 (UTC)
Thanks! Ewlyahoocom 06:07, 7 April 2006 (UTC)


[edit] This Article Attempts Too Much

I'm not really sure what is intended by the discussion on "the class of shapes" ... hulls come in many varieties but are grouped primarily as follows:

  • Moulded, round bilged or soft-chined
  • Chined or Hard-chined

After this they can be categorized as:

  • Displacement
  • Semi-Planing
  • Planing

The discussion on bulbs (note that there are bow and stern bulbs)has merit and should be retained as it is a prominent feature of modern ship forms.

The section on hull metrics needs to be merged completed and clarified. So far only Block & Prismatic (Cb & Cp) are covered.

  • Block Measures (LOA, LWL, Bmax, Mmld, d or T and D)
  • Form derivatives (LCB, VCB, LCF, TPI, MCT, KMt KMl)
  • Coefficients (Cb, Cm or Cx & Cp)

The section on bouyancy should be retained (it is afterall, the main reason for the hull) The historical bits should be collected into and Origins section.

A series of images would help this article a lot.

Please advise if there is any objection to re-working this article as noted.

Jmvolc 00:49, 18 May 2006 (UTC)

It seems just the right way for the article, so reworking would be really useful. CP/M 22:28, 21 May 2006 (UTC)

Please don't forget that we have a lot of "general" readers, and that not everone who is going to be reading this article is a shipbuilder, a sailor, or even have ever been on a ship. Ewlyahoocom 23:37, 21 May 2006 (UTC)

The current definition of block coefficient is more confusing than the one on the previous page. Take for example:
" Block Coefficient (Cb) is the volume (&nabla) divided by the LWL x BWL x T."
What does this mean? Kablammo 23:56, 24 May 2006 (UTC)
Added a simpler explanation, and restored the old section. The new part is good, but it's quite technical and requires attention to comprehend. CP/M 00:05, 25 May 2006 (UTC)

The subjects addressed in this article are by their very nature quite techincal. The information provided is techically correct but may not be descriptive enough for the "general reader". Should more text/digrams be added to improve enhance comprehension?

My apologies for the &nabla term - I read that this in the Wikipedia Tutorial that it was to display the appropriate Greek character for Volume but alas, it did not. As described, Block Coeff is the displaced volume defined by the hull divided by the ecompassing block (LWL x BWL x draft). I could provide a diagram that may help, please advise if this should be added.

I have delete a significant portion of the Features section as the descriptions were at best over-generalizations and at worst, just wrong. Hopefully I have not offended. I can provide a more detailed explanation if required. Jmvolc 01:36, 27 May 2006 (UTC)

I agree, this article should be aimed primarily on technical details, but they still may be described in a simpler way along with the technical one. Coefficients can be provided with a direct description (for instance, the second external link has some). I've just added descriptions there, along with some correction, and will add now to some others, but adding some diagrams would be nice.

[edit] Planing

The page currently states that planing results from dynamic lift at speed, and in turn defines dynamic lift as "the upward force generated by the water as the hull moves through it". Is this correct? Why would the water generate any upward force beyond that necessary to float the hull? Or is planing a reaction to downward force exerted by water deflected downward by the shape of the hull? Are there any physicists out there who can clarify this? Kablammo 01:49, 1 June 2006 (UTC)

I'll see about how to explain more clearly in the article, but...
There are two types of force the water can provide. Buoyancy, which is the displaced weight of water, and dynamic lift, which is similar to the lift that you get from moving an airplane wing through air. Buoyancy is always there. Dynamic lift is only there while you're moving, because you're moving. As dynamic lift increases, the boat moves up, and buoyancy decreases (more of the weight is being taken by the planing dynamic lift).
Does this help? Georgewilliamherbert 02:08, 1 June 2006 (UTC)

It does. What I questioned was the statement that the water generates an upward force. I would have thought that the water does nothing beyond float the boat. I'm also not sure about the airplane wing analogy-- I had understood that lift on a wing results from decreased pressure on the top surface of the wing. Does that apply here? Kablammo 02:42, 1 June 2006 (UTC)

Let me slip in a comment here--lift results not from decreased pressure, but rather from the deflection of the fluid through which the foil is passing. Any shape that deflects the fluid, including a flat plate, will generate lift; in the case of a flat plate (fluid flow direction -> \ <- angled plate) the bottom of the plate will push the fluid down, generating lift. The flow over the top of the plate will immediately stall out and swirl, slowing the fluid flow and causing drag; the low pressure on the back side will also cause the flow around the aft edge of the plate to be sucked back up, canceling out a good portion of the lift. The suction does have an impact, but it's as a mechanism for deflecting the air efficiently. The "suction causes lift" argument is hard to counter, since it is so entangled with the airflow deflection, but it's still Newton's third law that's really doing the work.
The lift on a planing hull is a case where the action/reaction explanation of lift works, but the suction explanation does not, since there is fluid flow on only one side of the hull (we'll ignore the air, it's 1000x less dense and has very little impact). A planing hull deflects the water primarily down (again, fluid flow direction-> \ <- angled plate) and this action pushes the hull upwards. Calculate the mass of water displaced by the hull, the acceleration required to move that mass of water out of the way, and you get the amount of lift on the hull (forcer = mass * acceleration). As the hull lifts up, the amount of water displaced is reduced, until an equilibrium between lift and displacement is reached, and then you're planing. Since nearly all hulls (except a submerged submarine's) are going to push more water down than up, all hulls will generate some planing force, but wide, flat bottomed hulls will push the largest fraction of water down, and will thus generate the maximum lift for planing. scot 14:34, 1 June 2006 (UTC)
The dynamic lift works pretty much the same as with aircraft's wing. A hydrostatical or hydrodynamical force applied to surface always acts normally to it. When the ship is moving, the resistance force, oriented horizontally ( -- ), is generated . If the bottom is placed upwards at the bow (like / ), the force acts normally to it ( \ ). It may be divided into backwards and upwards components. The backwards one counters the propulsion force, and the upwards gravity force, thus lifting the ship.
Comparing to the airplane wing: the pressure on top surface there is decreased; however, a ship has the same situation, with almost no upwards pressure at all, as air generates much less dynamic force than water.
CP/M 02:45, 1 June 2006 (UTC)

So if I read this correctly, then the water does not generate a force (as stated in the main page), but is directed by the moving hull to create an upward force? Could I suggest that one of you technical folks clarify the main page here? There is more technical description of planing, not in the currently-linked page on planing, but on the Planing (sailing) page. Perhaps that should be directly linked here. That would leave this article as a good general entry, with the technical explanation in the planing article. Kablammo 14:07, 1 June 2006 (UTC)

You are of course correct. Static water is no more capable of "generating" a dynamic force than it is of dancing a waltz. A more accurate description would be paraphrased from SNAME's "Principles of Naval Architecture" ...

    The Planing Hull form is configured to develop positive dynamic pressure 
    so that its draft decreases with increasing speed.

I have modified the text to suit. Jmvolc 22:26, 5 June 2006 (UTC)

Thank you. I in turn have deleted the definition of semi-planing, as it did not conform to your improved wording and seems self-explanatory anyway. Kablammo 00:05, 6 June 2006 (UTC)

I would like to reinstate the semi-planing definition. It is a recognized category of hull type , I will change the definition to fall in line with the planing definition. Jmvolc 01:36, 6 June 2006 (UTC)

[edit] Wave piercing?

Wave-piercingwould seem to warrant inclusion. See HSV-2 Swift, Incat and Earthrace

  • Wave piercing hull forms are a distinct variant of the displacement form. I feel it belongs in its own entry as people will inevitably search for that term (ie: “wave piercing”) and that if included here would open the door for the inclusion of many other variants (catamarans, trimarans, sea sled, barge, double-ended, etc.) Jmvolc 12:24, 12 August 2006 (UTC)


[edit] Merge in Draft (nautical)

  • Oppose - I don't think it is appropriate to merge Draft (nautical) in here. The draft may measure the clearance required for the hull, but The air draft is a measure of the clearance required above the waterline (under bridges, through tunnels, etc.). A better option would be to make more of the article, in my opinion. -- Ratarsed 08:38, 28 July 2006 (UTC)
  • Oppose - Draft is a separate term, also per Ratarsed. CP/M (Wikipedia Neutrality Project) 08:42, 28 July 2006 (UTC)
  • Oppose - Draft is already defined here, the Draft (nautical) entry expands on the definition and adds value. I would suggest adding the following to the Draft (nautical) article"
    • Draft Marks
    • an image to show "draft" and one to show "draft marks"
    • text on the difference between max draft (the draft to the lowest point on the ship) and hull draft (usually used to compute hydrostatics.

Jmvolc 12:29, 12 August 2006 (UTC)

[edit] Form Coefficients

Some of the descriptions and formulas given under Coefficients are incorrect. For example, are coefficients are based on measures on the design WL (LWL, BWL). Additionally, the text describing the influence of the various coefficients is probably better suited to be relocated to its own article on ship motions. These changes will be made shortly unless there is any objection. Jmvolc 01:45, 5 April 2007 (UTC)

Objection: I don't know what source you're using, but mine is the late 1980s (latest) edition of "Principles of Naval Architecture", and as I recall it defines the block, prismatic, and waterplane coefficients relating to length between perpendiculars (Lpp) not Lwl. Your source is... ? Georgewilliamherbert 03:14, 5 April 2007 (UTC)
(followup) Yes, I know you're a practicing NA and I'm not, though I have the degree, but can you recheck the PNA first? I will as well. Georgewilliamherbert 03:16, 5 April 2007 (UTC)
I added citation and consulted Rawson&Tupper, and note it specifies LWL for Cwp, LPP for Cp and Cb...I recommend these forms--Billymac00 (talk) 04:12, 16 February 2008 (UTC)

It was not my intention to detract from the contributions you have made. An open discussion of this nature is very worthwhile and key to assuring that the article is based on fact, not opinion. LBP is an arbitrary number as the Aft Perpendicular is alternately defined as the forward edge of center of the rudder stock depending on your reference. Neither of these positions have anything to do with the actual form of the hull.

Principles of Naval Architecture is a good reference. Volume 1, Page 18 of the 1988 edition states that "Some authorities take L as LBP some as LWL, and some as effective length." The authorities referred to are primarily classification societies that also use Lr or Rule Length when calculating form coefficients. Given that the article is about defining the hull in general terms I thought it would be wise to follow the guidelines set out by Saunders in "Hydrodynamics in Ship Design" back in 1957. This definition is still used by the ITTC, etc. for definitions of form. Let me know if this works for you. Jmvolc 12:30, 6 April 2007 (UTC)

[edit] Depth

Metrics includes the following definition of Depth (D):

Depth (D) is the vertical distance from the bottom of the hull to the uppermost edge at the side.

This does not seem to fit well with whaleback ships and may not be applicable to all turret deck ships. Kablammo 16:08, 24 October 2007 (UTC)

That's the way it's defined, though. Georgewilliamherbert 02:04, 25 October 2007 (UTC)
That raises the question of what the uppermost edge at the side is on a turret or whaleback ship, which does not have an outside edge but only a curve where the gunwale normally would be found. Has the definition changed over time? Kablammo 10:02, 25 October 2007 (UTC)
In turret deck ships the practice may have been inconsistent. A little book put out by the National Maritime Museum entitled Steam Tramps and Cargo Liners reproduces drawings from a paper by J. Foster King entitled Structural developments in British merchant ships from Transactions of the Institute of Naval Architects XLIX (1907), which show cross-sections of two turret deck ships. The depth of SS Turret of 1892 apparently was measured from the bottom to the horizontal part of the "side" of the hull, that is, where the side curves to a horizontal plane before arcing back to vertical (there being no "edge" present). SS Queda of 1905 however has two depth figures, one measured as in SS Turret, the other of which appears to be measured further inboard and higher, where the true deck meets the inboard and upper part of the "side" of the hull at right angles (and therefore forms an edge). (Difficult to explain without a drawing.) Perhaps the profession or rating authorities had difficulty applying standard definitions to these anomalous vessels. Kablammo 12:27, 25 October 2007 (UTC)
Now answered, by reference to more refined definitions in 1969 convention. Kablammo 10:30, 27 October 2007 (UTC)