Hull Balance

Elsewhere on this site, there is a listing of design software and I just happened to see a product that seems to relate to this thread?:

"SWL method is a method that automatically designs the hull of the sailing yachts that has symmetrical water lines in conditions of different heel angles. "

The website is: http://www.kus.hr/English/metoda.html

 

science fair

I teach 8th grade science in Dublin, Ohio. 2 of my students are doing a project on how the hull of a boat affects the boat's movement through the water. They are having trouble finding people to contact for interviews and/or resources. Help!...any ideas on where to direct them?
Thanks!

 

It seems to me that to do what Wardi is asking for would require new system of measurments and calculation. If I was going for a doctorate in Naval Architecture, I would start by creating a "center of hydrodynamic resistance" this point would be firstly visualized in section view for me, and then represented in the other two views. This point would represent the center of resistance on the skin of the hull as the boat moves through the water. It seems to me that this center of resistance would start out in the foward part of the hull as the boat starts moving, and would move aft as the boat accelerates. If this point stopped at the longitudinal center of the underwater length, then I think it would be safe to say that the hull would be balanced at that given angle of heel and at that velocity. So there we go. Now all we have to do is write a program that will calculate all this for every degree of heel at every possible speed, and viola! -A new way to take the fun out of yacht design!

 

The hull of the boat

I am doing a science fair project on how the hull of the boat affects how well it goes through the water. If you have any information similar or on the subject please post a reply. Thanks!

 

RhinoMarine Metacentric Shelf

I recently purchased a copy of Rhino 4 with the RhinoMarine 4 plugin and am just beginning to learn how to use it. To my surprise and pleasure the Hydrostatics module of RhinoMarine calculates two interesting variables relevant to this topic: Metacentric Shelf Slope, and Metacentric Shelf Intercept (-0.051 and 9.739 respectively, in my case). The RhinoMarine "Help" says that: "Some designers use [these data] as a way to quantify the amount of weather helm that a sailing yacht will develop as it heels. Yachts such as 12 meters have very shallow Metacentric Shelf slopes, and develop very little weather helm even at large heel angles."

Unfortunately, I have no idea what "very shallow" means exactly. A slope of -0.051 seems shallow to me . . . but what's the range here? Does anyone know? Also, does anyone have any experience using these data for the purpose suggested in the "Help"?

BillyDoc

 

The term shallow refers to the graphic view of the figures when plotted against heel angle, the trim angle (nose down) basically shows the same thing.

I attach 3 screen dumps below where I also have added a couple of sample Maxsurf hulls as comparison, AC yacht is a very slender boat, IOR is rather fat, fast yacht is a very beamy twin rudder boat, the Whistler 48 is a Chuck Payne pilot house blue water cruiser. Mikey is my design

Another way to measure balance that I like more is described by Steven Ditmore in this thread - from Cyrus Hamlins' book "Preliminary Design of Boats and Ships", pages 195-197, he recommends that the LCF should shift aft as the boat heels, but the shift should be less than 1% of DWL - I find that high, I have 0.3% on my design

Needless to say, I like slender boats

Mikey

 

Thanks Mikey!

I'm with you totally on slender boats, I like them too! Too much reliance on form stability (beamy boats) makes for an uncomfortable ride. Not to mention a dangerous one sometimes.

Did you use RhinoMarine to develop your data? Can you give us a step-by-step how-to? I've just begun to play with it and am still at that initial stage where I'm more or less lost. I did the hull design using Delftship Pro, I'll attach a screenshot below. Looks like I've got some adjusting to do!

I was actually attempting to do the analysis described by S. Ditmore when I discovered the Metacentric Shelf terms in the calculated output. Now if I can just figure out how to get the LCF at 30 degrees of heel from RhinoMarine I'll be a happy boy indeed!

Thanks again, Mikey

BillyDoc

 

Beautiful Design ! !

I used Maxsurf for the hull development and use RhinoMarine for stability.

I don't know how to do LCF at 30 degrees without re-trim in Rhino but Maxsurf can do it. I am off to a meeting in a few minutes, download the trial version of Maxsurf (or Maxsurf Academic) meanwhile and I will write a manual on it. http://www.formsys.com/

You can search for the link to Maxsurf Academic on BoatDesign, Andrew has posted it somewhere

Mikey

 

mary this subject is a trillion miles in length, but if maybe start googling americas cup, you may find a sort of broad overview people have written thesis on this for 100, years, in fact whole books on just water flow across the rudder top, so good luck!!

 

Thanks again Mikey!

I think I figured out how to do it in Rhino/RhinoMarine as well. Sort of.

The Hydrostatics Calculation page has an option you can check to "Insert surface representing flotation plane." When I check this and use the "Displacement/CG/Trim" option with zero trim the surface appears and remains in the Rhino drawing after leaving RhinoMarine. In the upright case there is no trim and a good surface is obtained. When the heel is set to 30 degrees and the calculation again performed the same thing happens, except that now there is an imposed trim. Bummer!

I then used the "trim" command and the hull surface as the "cutting object" to trim both flotation plane surface to the dimensions of the hull. Then, using the "Analyse, Mass Properties, Area Centroid" commands from Rhino on the two flotation plane surfaces I get something similar to the LCF calculated by RhinoMarine . . . for the upright condition. By something similar, I mean a difference of 19.204 for RhinoMarine vs. 19.202431 for Rhino. But there is still that trim issue for the heeled condition. I noted the centroid coordinates for the heeled condition and tried to correct this problem below.

Going back to Rhino I used the "Undo" to get my two surfaces back to their original form, used the dimension tool to determine the trim angle for the "heeled" surface, and used the "rotate" command to level it back out using the centroid coordinates as the rotation center. Now I have something that I presume is close to the flotation plane for the heeled but untrimmed condition, and can again trim it with the hull and calculate the centroid. Comparing the two centroids (LCFs, I hope!) I have found that in the heeled condition my center of flotation moves forward of the LCF in the upright condition by slightly less than 1%, so I conclude that I need to slightly thin the hull forward and fatten it aft to correct this.

This would be easy if RhinoMarine would do it's thing without adding trim in the heeled condition, does anyone know how to make it do this?

BillyDoc

 

Are you sure Rhino is doing that correctly? That hull looks more like 0.5%, OK your design is classic long keel and mine is fin keel so maybe. Didn't think the keel would make that much of a difference though.

Here's the Maxsurf manual

And here's the web address to the academic version
Andrew Mason
Formation Design Systems
http://www.formsys.com

Maxsurf Academic
http://www.formsys.com/academic/maxsurf/

Mikey

 

Damn Mikey! Have you got an eye!

I screwed up the calculation and took the percentage change as measured from the aft end and forgot totally about the water line length. In fact, I only remembered it when I went over the manual entry you posted. When I recalculated it correctly it was 0.51% just as you said. I guess this confirms the method using Rhino anyway.

I am totally impressed!

BillyDoc

 

Mikey

 

A lines plan is a bible and I like to study them, your design looks well balanced, water would like it

Fundamentals don’t change quickly, Evolve is the word, I like to study how boat design evolves, and how science is applied to it. Science by the way, I see like this; First comes science, then comes the understanding of science, then comes applying the science in a real world environment – because boat design is by definition a compromise –and compromises don’t generally take giant leaps forward – except at the cost of something else

A typical example of the “at the cost of something else” is the Pogo 40 from the seaworthiness thread, great boat, it’s just that crew comfort and to some extent seaworthiness was forgotten on the way…

I heard that the designer of the Pogo 40 is planning to be the first one to sail to the North Pole. The boat is so flat bottomed that it will glide perfectly over the ice and the super thin stick of a keel will easily slice through the ice helping to stabilize the boat

I like Cyrus Hamlin's LCF method, and I have suggested it as a quantitative measure for measuring hull balance as a boat heels, no one picked up on it

Mikey


"Water likes diagonals and buttocks but doesn’t care much for water lines, they don’t mean much for water"

 

Correction to Rhino method for Balance Calculation in #85

After the 17th iteration trying to get my hull to balance while at the same time keeping a reasonable COB, Cp and etc., I finally broke down and bought Cyrus Hamlin's "Preliminary Design of Boats and Ships" and, sure enough, I wasn't doing it correctly! Mr. Hamlin gives a drawing to illustrate his method on page 197 . . . and the thing is used as artwork on the dust cover of the book as well . . . which makes the process much plainer, and much simpler to do. On my 18th try, I nailed it!

So, let me explain this method assuming you have Rhino to use. RhinoMarine is not required at all. Mr. Hamlin argues that hull balance is far more important than sail-plan balance, because you can always tweak the sails and balance them as you go . . . but the hull is a bit more difficult to modify while out on the briny blue. I agree with this point of view completely and have been surprised to find so little information on this subject.

The idea behind Hamlin's method is to determine how much the Center of Flotation (COF) moves fore or aft, relative to the upright COF, when the boat is heeled 30 degrees. No longitudinal movement would mean no effect on the helm, movement forward means lee helm (bad!) and movement aft means weather helm (good! if it's modest). Ideally, you want the COF to move aft, but no more than 1% of the waterline length. Fortunately, Rhino has all the tools needed to do this analysis (and unlike every other CAD program I have ever used, everything actually seems to work, and work very well indeed). So here is my adaptation of Mr. Hamlin's method, with pictures.

First create a horizontal planer surface that intersects your hull at the waterline. Then move to whatever view has you looking directly at the bow, as in the first picture below. Copy your horizontal surface and rotate this copied surface 30 degrees about the intersection of your first horizontal surface and the vertical midline of your boat. Check that your result looks something like the figure. Next use the hull as your "cutter" (even if it only has one headsail) and trim both surfaces.

Change your view to a "plan view" as in the second figure and select the trimmed surfaces one at a time, go to the "Analyse, Mass Properties, Area Centroid" command from Rhino. Note the first number that comes up after the analysis (or whichever dimension represents your longitudinal direction). You should see a "point" come up on your trimmed surface with each analysis, and if the point that appears for the heeled waterplane is slightly aft of the one from the upright waterplane you are in good shape! Using the numbers you noted you can subtract one from the other to get the distance moved, then divide this result by the waterline distance and subtract from one to get the percentage moved. In the example shown, the COF moved aft 0.37% of the waterline length when the hull was heeled 30 degrees. Oh, and the Center of Buoyancy is at 55%, and the Prismatic Coefficient is 0.57. I'm a happy man!

It's also instructive to rotate your heeled waterplane back to horizontal and compare it to the upright waterplane, which is what is shown in the third figure. The top part of the figure shows the two waterplanes as they develop, and you can see that the heeled waterplane is canted to windward a few degrees, which I take to be a good thing. The bottom of the figure shows the two waterplanes aligned, which is a good check for weird asymmetries.

BillyDoc