Turner Theory

I agree, but why not do using trochoidal waves?

If we want really determine this situation for one specific hull we can use the tank test for determine the real wave profile.

Only with search that we can go ahead and know something more.

In this case the problem is:

If we use the real dynamic stations form with Turner's theory does it functionate?

To the others " the effect of sails, keel and rudder" comes after. First is determine if the theory is valid for dynamical waterline.

 

Well, if one is to make the calculation manually for each speed and heel angle, he would become old before seeing that boat in the water!
But if you can modify your software so that it performs automatic calculation with trochoidal waves and for various speeds/heel angles, it could become viable, imho. It will still be just an approximation of the real hull behaviour, but surely better then Turner's method based on static waterlines.

Tank testing implies that you have already designed the hull, it should come at the end of the process, imho.

Cheers!

 

I'm going to make available the program here. Is not a software, is a simple program tool that use Autocad to do the calculations. It is a open text .lsp, that either one can see the commands and change them.

Use this program with trochoidal waves seemed me easy, it need only the dynamic draughts.

If all inclined hulls tested up today in tank tests, had marks in hull side to do a photo to establish the wave profile and do the Curve of Moments and compare with the tank test results, today, we already had the conclusions as a sub product from tests.

 

The program is annex.

Read Readme.txt

Good luck

ATTENTION !!! The upload do not accept .lsp so I change Turner.lsp in Turner.txt but you need change to .lsp

Cheers

fred

 

Something interesting.

In red - reference boat, 10 m LWL

In blue, project under study.

In green, the same project after adjusted to comply with Turner's Moments of Area.

interesting, no?

Resistance by Michlet.

 

Balanced hull design

I think I may have another avenue for exploring balanced hull design that may lend some validity to Turner’s theory. After reading the comments on the forum discussing what makes a balanced hull design and how Turner’s theory “fails” because it only considers hydrostatic forces, I had a thought that maybe the approach to the problem should be from a consideration that the hull is always oscillating about a point. The hull is constantly oscillating in all axis and motions (x, y, z, roll, yaw, and pitch). Turner only considers the moments on the hull at one angle of heel and in only one plane. I think one should look at the surface of flotation and the surface of buoyancy (ie. the change in the location of the centres of flotation and buoyancy at all angles of pitch and roll) to understand hull balance. When the hull is oscillating it will be the relationship between these two surfaces and the distribution of weight that will produce a yaw. What made me think of this was the toy called a rattleback. Here are some examples that should make this clear:

http://www1.teachertube.com/viewVideo.php?video_id=129635
or
http://www.youtube.com/watch?v=rUwKXujLAyk
or
http://www.youtube.com/watch?v=AcQMoZr_x7Q (at 1 min)

The first video is the most informative. The rattleback is essentially a simple semi-ellipsoid with the distribution of weight skewed. (A rattleback came also be made by an ellipsoid but cut slightly obliquely.) This ‘skewed’ distribution of weight changes the principle axis of inertia. (It is this 'principle axis of inertia' that makes a spinning top spin in a preferred orientation - the spinning top is most stable if it spin about that principle axis.) When the rattleback rests still there is no yaw (ie. hydrostatics), but if the rattleback oscillates in a pitching or rolling motion then it will convert the pitching motion into a yaw. The rattleback is so highly directional that if you spin it in the unstable direction it will convert that force into a pitching motion and then switch the spin direction.

It is my assertion that when an unbalanced hull is heeled and oscillates about that point, it acts like a rattleback. I’m not 100% sure of the mathematics, but here is a possible explanation:

- My initial impression is that one has to evaluate the surface of flotation, as it is responsible for the trim of the hull and is likely where the hull will oscillate. (It may in fact be the surface of buoyancy, or a point in between.) For every surface there is going to be a maximum curvature in one direction and a minimum curvature that is perpendicular to it for every location on the surface. If you imagine an arbitrary point on the surface of an ellipsoid, there will be a different curvature of the surface depending on which direction through that point one is looking at. If you look at the curvature that is parallel to the major axis of the ellipsoid the curvature at that point will be a minimum. Perpendicular to this line the curvature will be a maximum.

- Now back to the heeled hull: If the principle axis of inertia (in the heeled hull scenario) is parallel with the minimum curvature of the surface of flotation (in the heeled position), then when the hull oscillates about that point (either pitch or roll), the tendency of the hull to yaw will be a minimum. If the situation is otherwise, then there will be a greater yaw induced motion from the oscillation.

If the heel angle is zero, the axises will be parallel in all floating objects. It is only when one evaluates the hull at an angle of heel that the axises will be ’skewed’ in a so called unbalanced hull. Maybe Turner’s theory produces hulls that minimize the yawing motion because it aligns the principle axis of inertia with the minimum curvature of the surface of flotation (or buoyancy). In theory, one would have to consider the entire yacht to find the principle axis of inertia in any heeled orientation, but maybe one could approximate it by only considering the immersed volume of the hull. This is very similiar to finding the 'metacentric moments' of the hull as Turner describes. If the moment curve is ’neat’ (as in Rainbow or Yankee), then it is my guess that the axises will be close to parallel. If the moment curve is ‘crossed’ (as in Satanita), then it is my guess that the principle axis of inertia (of the immersed volume) and the minimum curvature of the surface of flotation (or buoyancy, in a heeled orientation) are not parallel and the hull will more likely yaw when it oscillates in a heeled orientation.

The original criticism against Turner’s theory is not resolved with this new idea. This model for a ‘balanced' hull still only considers hydrostatic forces and does not consider the waves and wind around the vessel, or the lift and drag that develops from a body in motion. However, I believe it does add to the understanding of hull design. Maybe the final understanding of ‘balanced’ hull design is to compare the principle axis of inertia of the entire yacht in motion (with all of the hydrodynamic and aerodynamic forces) with the surfaces of flotation and buoyancy in a dynamic environment (waves and wind).

Many regards,

Aaron Johnson
Vancouver, Canada

 

Rattlebacks

Thanks Aaron

I myself have been on the phenomenon about rattlebacks but were unable to understand all the parameters. Your connection to boats characteristics seems to be very important. Your suggestions will open up new possibilities for understanding a boats characteristics. Perhaps it could also explain some of the so far unexplained accidents.

js

 

Annex, in the figure, is what I think.

The boat is similar to rattleback in instances, sometimes the force that disturb the equilibrium, sail force, acts during a time, and changes continuously, like the buoyancy motivated by the shape of the hull.

What I see in Turner's theory is that we need have care with hull forms, hence the shapes of sections.

We need do sections forms so they do not introduce excessive lateral moments when heeled so that the rudder can not balance it. By this reason area A need be equal to area C in Turner's graphic. Like rattleback.

If we have trim associated with heel, things get worse.

 

I only came upon this thread today, and some readers may be interested to know that I raced in Fidelis from 1971 to 1978, when Frank Welch sold her. I still treasure my copy of "Faithful Fidelis"

She was a very stable boat to sail, with no vices - preferring a reach in about 10-15 knots, and she wasn't as close-winded as some of the more modern designs. However, we still held our own and had good results up to and including her last year in Frank's ownership, when we were 2nd in division and 3rd overall in the first 5 divisions of the ISC Round-the-Island Race (against about 150 other boats).

On the subject of balance, she always had some feel in the helm, but you never had to fight her. We never broached in my 8 years sailing in her (compare with a modern boat!). However, I question whether the Metacentric Shelf system can be applied satisfactorily to modern lighter displacement hulls (I must try plotting a shelf for my quarter-tonner).

 

How wonderful to hear from someone who sailed on Fidelis! "Faithful Fidelis" certainly needs to be reprinted, it's such a charming book.

If you'd like a copy of my Turner paper, just PM me with your email and I'll send you one.

Cheers,

Earl