Hull Speed

Sorry Yipster (not you're fault).

This is why patents of complete nonsense are allowed (in US far more than EU). There is no such thing as "hull speed". It is a misnomer from the days when hydrodynamics was not very well understood, yet some "meaning" was need to "why" some things were occurring.

Is the human body going to explode when going beyond speed of sound as also noted years ago, or be unable to swallow and perform simple functions in weightless environment...no. So why is "hull speed" is still being used today, with equals amounts of advancement, as a raison d'etre??!!

If so called "experts" with their "snake oil" patents" started to use correct terminology and sounds science to explain their so-called invention, there would be less nonsense, and less the blind leading the blind.

 

Number, Code name, Nato name, Russian name, and designer?

 

<Thread Cleaned> A post containing insults directed towards another member has been removed. Calling another person "ignorant, and arrogant" is not appropriate for the forum. Please, let's try and keep the forums polite to everyone and stay away from insults. Thank you.

 

Did you all know you can increase hull speed and make rough seas better to sail if you soap it up a little with a oil. I read about in an old book. Probably not a good idea today.

 

That was when the price of oil was low, and the environmental police still to come.

 

I think you’re getting confused.

Any vessels ability to return to its equilibrium position is simply the intact or initial stability. When the vessel is disturbed, it should return to that initial position. This is simply the buoyant force and the gravity force of the vessel floating at ever increasing angles being separated and thus a moment is produced to return the vessel back to its upright (initial) position.

The “disturbance” can be anything from wind, wave, turning, even gun recoiling (if large enough like on the USS Missouri) or simply weights moving about, like cargo shifting. It is ostensibly independent from speed. Speed is just one cause of a disturbance for all vessels, but not the sole one.

If you’re talking about planning craft, as in Blount’s paper, that is subtly different type of stability. Since a planning hull shall also experience hydrodynamic lift, thus a disturbance which only occurs on planing boats. This is speed dependent. The term “Dynamic Stability” in his paper is no difference from the classical definition, save for the fact that he is referring to a type of stability that is unique to planning craft only.

 

This is quite correct; also note that in mostly German and Russian sources we often see 'dynamic stability' of ship as integration of static stability curve. I.e. dynamic stability curve is referred as integral curve to static stability cure, thus showing the areas under GZ curve. Dynamic stability lever d at heel angle Q is area integral of GZ curve from 0 to Q, m*rad.

Actually to me there is not much sense in such integration as damping is not taken into effect. Ideally, dynamic stability curve is used to assess the work required to heel the ship. One will also see such dynamic curves in FreeShip.

On picture enclosed, d (pink) is dynamic stability lever.

So it is all about definitions what dynamic stability is!

 

And of course, not forgetting directional stability, or sometimes also called "dynamic stability"

 

I understand that DCockey, just like me, comes from the general engineering field where a difference between static and dynamic stability essentially depends on whether time is considered or not.

In that context, a statically stable object will tend to return to it's initial position after it was disturbed.
A statically unstable object will have a tendency to diverge from the initial position and (hopefully) move into another stable position.

On the other side (still from the general engineering standpoint), dynamic stability is what happens in the real world.

A dynamically stable object will also tend to get back into the initial position (or path, if it was in a steady motion), but will do it through a time-dependent motion pattern. For example, it might get back into position with a simple over-damped movement, or might do it through a series of oscillations with decreasing amplitude.
The same motion pattern might lead to a departure of the object from the initial position or path - in which case it is said to be dynamically unstable.

I do realize however that in the moment we, the outsiders (or parachutists as they are called in my mother-tongue slang ) have entered the realm of Naval Architecture, we should learn and use the appropriate terminology, which I am trying to do.

Cheers!

 

daiquiri we are not parachutist

 

Darn, it must be hard to understand all that stuff. lol. Are you all sure you studied your chaos theory and hull speed phenomenons books.

All this knowledge, and how the science has progress; and still every time I buy a boat, I have to change the propeller.

The point to Mr. Jansen and everyone here is that designing boats/ships is a science, and it is an art. There are no magic bullets, because for every ying there is a yang.

Years ago, I worked with a guy that had invent a boat hull that had the skin like a golf ball. After spending years building his fullsize hull, he discovered that the benefit wasn't worth all the work. We couldn't keep the boat clean. We couldn't tell the difference in the full size. The test RC boats ran great, but it didn't scale. He then had an idea on injecting air through holes in hull. When we looked at building such an animal we gave up. The added weight of the system would actually make for a slower boat. After that he went to a Teflon/epoxy coated bottom, which actually worked pretty good. But really not much better than a waxed hull.

So far the L/B ratio is the way to fuel economy and speed, but then people have known that for 100 years. Catamarans use that idea by having two narrow hulls.

That is why I designed my boat; 65' long and 14' beam at waterline. Modern boats of this size are typical in the 20' beam range. My boat has double the range, and use a third the fuel, and much smaller engines.

 

Hi D..

Debating in a language that is not your mother tongue, and in technical English too, you have my upmost respect and admiration. I struggle with just basic Japanese!

No, time has nothing to do with it. It is not a function of time. That may be the case in “general engineering”, but not so in naval architecture with regards to stability. Time does not come into it; it is geometry related, that is all.

DC stated :

So.. will it stay upright..this is simple hydrostatics (does it float), …and on the surface, is initial/statical stability. That is all, nothing else.

A vessel when floating at rest can only be:

i) In stable equilibrium…ie ‘stable’, and returns to it original position following a disturbance.
ii) Unstable equilibrium..ie unstable..it does not return to its original position following a disturbance
iii) neutral equilibrium….a rare condition ( and not angle of lol too)..usually confined to submarines stability.

Time is not an issue….neither is “dynamic stability” in the sense, does it stay upright and on the surface. One could argue that after a certain period of time, a vessel returns on its original course when discussing directional stability, but this is as a result of the vessels geometry and its “neutral point” in relation to the stability index. For example, in statical stability, if the vessel returns to its original position, whether it takes 1 sec or 1 year, it is stable, and is solely a function of its geometry (CoB & CoG), not time dependency.

But we digress.

 

Thank you, it was a good thing to do.
I am the culprit of these name calling, and I apologize for my rant to the moderator the forum and Robert.
Lister

 

thanks mentioning your slip lister as i missed to see what wasnt my fault and it was me that had ofset DCockey
thanks saying it wasnt my fault AdHoc, you probably ment me using the word "hullspeed"
reason i wrote "hullspeed" quoted however was exactly as you explained

 

REVISED

No, I'm not confused.

True if the vessel is at rest and the only forces acting on it a buoyancy and gravity. Close enough for most situations when a vessel is underway.

But when a vessel is underway the pressure acting on the hull isn't simply hydrostatic. There is also a dynamic pressure due to the deflection of the water by the hull as it flows past the hull. This dynamic pressure changes as the attitude of the hull to water changes. In some cases a disturbance to the hull attitude (heel, trim, yaw, draft, etc) will cause a change in dynamic pressure distribution which in turn increases the deviation of the attitude of the hull, which in turn causes an even larger change in dynamic pressure which in turn which in turn increases the deviation of the attitude of the hull and so forth. That situation when due to the changes in dynamic pressure is a dynamic instability

Speed is not considered as a "disturbance" in "high speed" dynamic stability.

In a very broad sense stability is stability. But the fundamental difference with the dynamic instabilities which occur at high speeds is the change in the dynamic pressure distribution in response to changes in hull attitude are not given completely by the hydrostatic pressure relationship. That is the fundamental difference between "high speed" dynamic stability and the classical definition of static stability in naval architecture.