Hull shape - hull speed?

Maybe a dumb question but what better place to ask a dumb question than a room full of smart people!?!

As I understand it, the formula for hull speed presupposes a more or less "conventional" shape. My question has three parts.

1. Is a narrow hull with a fine and low angle entry able to achieve higher speeds than a wider blunter hull?

2. Is there a relationship between the actual measured length of the hull at the waterline and hull speed that is different than the relationship between the straightline measurement from lines extended from entry and exit?(did I mangle that description?)

3. Is there a site or book anyone could recomend to a lay design enthusiast to help me understand such things?

Thanks In advance!

Paul

 

In short, a displacement hull speed is rooted in the waterline. A simple thumb rule is that the boat would do about 1.3 times theorotical hull speed. However, because of different hull shapes, this is not always true - therefor actual hull speed can varies from about 1 - 1.6 times approx theorotical hull speed.

yotphix, there are a lot of issues that have an influence on hull speed, but in essence friction and drag are the main critters slowing a hull down and this is directly a result of wetted surface area. Another issue that will make a hull fast or slow is the foil shapes of keel and sailplan. High aspect ratio's generates more lift that low aspect ratio's - ever wondered why a glider's wing is so narrow and long? I believe they would make it narrower still if they can find a way of attaching them safely to the fusalage! Another indicater of hull speed is the ability of the hull to carry it's sail, and so we can go on and on....

There are some good books available; check out the book store on the top of this page

 

Thanks Wynand, I hadn't heard that 1.3 figure before.

Am I right to think that the function normally referred to as hull speed is related to the length of the bow wave - a wave's speed being a function of it's length? Is my perception that a dispacement hull has begun to exceed this speed when it begins to sit lower in the water and the bow wave begins to ride higher and higher on the hull?

It seems to me that with adequate power (strong breeze, large sail area) all of the friction or wetted surface issues can be overcome but that the length of that wave is the true limiting factor. Unless maybe a finer entry with a long taper (like the Dashew boats) creates a bow wave differently?

I have also seen the size of a wake referenced as an indication of the "efficiency" of a hull. (Albeit in a lesser forum! Who knows what value?) Any thoughts?

On the topic of high aspect ratios generating more lift, why then would a plane like a DC3 or a deHavilland Beaver, two real workhorses with great load carrying capacity at low speeds have such low aspect ratio wing designs relative to a glider? Is that perhaps because the advantages of high aspect ratios where poorly understood 50 years ago?

I have read a bit about the Dashew boats recently and have been moored in Newport Beach California for a couple of months where I have seen Deerfoot, some sundeers and a number of Macgregor 65s. It has made me quite curious.

Thanks again!

 

The nature of the wave system generated around the hull is the stranglehold which inhibits planing and is an important factor affecting the performance of displacement craft.
As it moves through the water, a boat's hull produces both transverse and divergent wave system, although only the former appears to affect the resistance of the hull. The transverse wave system results from the interaction of the bow and stern wave systems, and varies with the boat's velocity. At maximum displacement speed, the hollow of the stern wave is reinforced, causing the boat to trim up the bow (squatting the stern), a state which carries a lot of resistance.
A boat's velocity with reference to its wave system and lenght waterline is judged by a Froude number. Maximum displacement speed is indicated by a Froude number of about 0.4 depending on the hull's displacement length to be more accurate than the rule of thumb method of approx 1.3 LWL.
However, take note that wavelength is determined by velocity, and not the hull's lenght.

The quest for good performance starts with the shape of the hull, particular below the waterline. It pays to maximize displaced LWL, thus enhancing the hull's capability of generating a longer wave system by a bow wave which forms earlier and a stern wave which extents as far aft as possible.
Overhang at the bow causes the bow wave to creeps forward, and because the of the sectional form usually produced by the overhang, the displaced lenght is increased substantionally when heeling.
A full bow generates an earlier bow wave, though the same effect would result from a deep forefoot which retains reasonable waterlines forward. In general, this is achieved by an increase in volume of displacement forward. if taken to any kind of extreme, however, additional drag would result owning to the size of the bow wave formed; a problem usually encountered with full bows.

To achieve a bow wave shift, whilst at the same time bringing about a possible reduction in the height of the bow wave, lies in the use of the bulbous bow, but that is another story....

BTW. I have some photos somewhere that shows a model of a displacement hull and its wave system created from low speed to max disp speeds. If I can lay hands on it, I will post it.

 

yotphix, here are the photos to demostrate the wave system created by the velocity of a displacement hull.

 

To make sure that I understand correctly, Displacement Length is the actual or perhaps, DEVELOPED distance from stem to stern at the waterline?

The pictures are great. It looks to me as though the second last photo shows one complete sine wave whose length is one boatlength. The last photo the boat is doing what I asked about previously; sitting lower in the water and not bow up stern down but actually lower fore and aft. Perhaps this is the effect of the overhang you were refering to? the boat is actually getting longer as it puts the wave higher on the bow? Something like that.

Good Wynand this helps!
Thanks so much,

Paul

 

Wynand N has provided a good explination on overall hull drag. To answer your specific questions, we need to crawl into the actual mechanism if wavemaking drag force. First, you must recoginze that the wake is nothing more that the physical representation of the pressure that is generated by the hull forcing the water out of the way. Lets say I have a hull in the shape of a brick. I have a very high stagnation pressure on the bow transom. It has to move all that water out of the way to allow the hull to pass. This high pressure generates a large bow wave. Conversely, on the stern, the water must flow back in behind the body as well as it can, this generates a stern wave. Now all the pressure on the body resisting forward travel is concentrated on the bow and stern only. If we compare that to a hull form that is a long wedge, we have a lower pressure (but along a longer length) with the same pressure loss at the stern. To improve this, we taper the stern also, resulting in the classic Wiggley hull form, used to develop wavemaking resistance. In the Wiggley hull form, there are actually 4 generators of pressure, the bow, the forward quarter wher the hull turns aft, the stern quarter where the water begins to close in, and the stern. It is the interaction of these 4 pressure waves that determines the shape wake and the drag on the hull. By judicious choice of the relationships between the location and size of these wake components, you can set the wave drag of the hull.

 

Wynand N and Jhardiman have given a good explanation of the most
important wave-making factors, but there are also viscous effects
that are significant, particulary for small hulls travelling at
Froude numbers below about 0.5.

Here are some scatty notes to show you that you are not alone
in your ignorance. We are all bozos on this boat!

There are three very important viscous factors (along with several
others I won't mention now) that affect wave-making: the stern-wave
detachment point, the boundary layer (BL) separation point, and the
shape of the streamlines near the stern. Havelock, Inui, Maruo, Beck,
and many other notable ship hydrodynamicists have investigated these
aspects, however they are still regarded as very poorly understood.

In an interesting report of experiments performed in Japan during
the 1980s, Doi \cite{Doi80} and Kajitani \cite{Kajitani87}
presented photographs of stern waves produced by some small model
hulls.
Those photographs show that stern waves begin at different places
on the hull (forward of the stern) depending on the Froude number.
At some Froude numbers the starting point is close to the stern, at
others it is further forward.
The situation is, as you can appreciate, more complicated for
multihulls where waves created by one hull can cut across the
other hulls.

The detachment point is not the same as the location on the hull
where the BL separates, rather it should be regarded as a point where
the BL begins to thicken rapidly due to the adverse pressure gradient
as the hull curves inwards near the stern \cite{Maruo76}.
The shape of the streamlines near the detachment point can have a
significant effect on wave-making, but we know very little else.

On well-designed hulls, the BL separation point is close to the
stern, usually within about 5\% of the hull length.
We know almost nothing about the wave-making due to BL separation
near the stern.

Stern waves have been observed to begin as far back from the stern as
20\% of hull length on some small models \cite{Kajitani87},
although it should be said that the S103 Inuid hulls used
in those experiments are not good representatives of real ship hulls.
The ``stern wave starting point" is not necessarily identical
to the detachment point, but it is an indication that viscous effects
are important at that location on the hull surface.
In my program Michlet, I have tried to account for this effect on
hulls with transom sterns below Froude numbers of about 0.35 but I
wouldn't bet a family member on the accuracy. (Ok, maybe the teeenage
male if the odds are good.)

Ship hydrodynamics has advanced very little over the last 40 years so
amateurs shouldn't despair at not understanding the wave-making
problem. In its final report to the 24th ITTC in 2005, the Resistance
Committee, concluded that it could not propose general
guidelines for the prediction of far-field waves and wash effects
because there was not enough experience with the problem.
Numerical methods, it seems, are still evolving, and experimental
data required for validation of predictions is almost non-existant.

So, after millions of dollars and many thousands of man-hours spent
on the problem, we are still a long way from even a reasonable
understanding of the physics involved. CFD is, of course, an enormous
advance because we can now display our almost complete ignorance in full
colour!

\bibitem{Doi80}
Doi, Yasuaki,
``Observation of stern wave generation",
Proc. Continued Workshop on Ship-Wave Resistance Computations,
Izu Shuzenji, Japan, 10-12 Oct. 1980, pp.\ 155--172.

\bibitem{Kajitani87}
Kajitani, H.,
``A wandering in some resistance components and flow",
{\em Schiffstechnik\/},
Vol. 34, 1987, pp.\ 105--131.

\bibitem{Maruo76}
Maruo, H.,
``Ship waves and wave resistance in a viscous fluid",
Int. Seminar on Wave Resistance,
Japan, 1976, pp.\ 217--238.

All the best,
Leo.

 

Wow! I suppose that would be why my local barnes ignoble doesn't have a book on it's shelves titled "Wave Making for Dummies"! Many thanks all for illumination. I have reread all replies twice now and if I do so a few more times I think I might begin to understand the depth of my ignorance!
Really though, a tough topic to explain to an art history major and well done.

Paul

 

Good One Leo!

 

Thanks It's almost as much a firmly held belief as a cheap shot. When I look at the seething mess of splash, spray and white-water around a fast-moving ship, I think of the great simplifications needed to make the problem mathematically tractable. After all the required simplifications and excisions are made, are we really justified (or sensible) in using sophisticated techniques? I dunno. Maybe.

Leo.

 

Is the hull in the picture making things better or worse?