Hull Design

Hello,
I am writing my Extended Essay (required by the International Baccalaureate) in physics on the design of sailing boat hulls.
I would like to know if anyone could help me about this since it seems to be one of this forum's specialities. In fact, I am researching and prooving what the most efficient hull would be. Is there a site or a book that would particularily help me for this topic?
Also, I would like to know if anyone would have an idea about designing the experiment that is required with the essay, to prove the hypothesis?
Thank you for your help

Pierre Badin

[bobber]

this is probably one of the best sites for help on it, wide range of experts on here who will answer any questions im sure. sail boats are not my thing, although i have touched on them, i dont really know enough on them to help you, but i do know there are lots of people around on here that could.
efficiency is somewhat of a gold dust in hull design though. i think you will be on your own in terms of finding the most efficient hull form, as anyone who makes money from boats isnt going to want to share that sort of secret, but lots of people who know enough about them to help im sure

as for the experiment, i guess the best experiment would be a scale model of whatever hull you come up with, in a testing tank, if you have access to one, and compare that to other hull forms already in use. Often universities will have scale models already of various boats which you can compare against, but i dont know where you are, or if any universities around you have such facilities.

failing that, there is always computer models, software these days that can take a hull form and give out all sorts of figures on its performance and efficiency, but obviously this is going to require knowledge of the software and getting a hold of it.

other than those 2, i cant think of any other physical way of proving it, short of building the boat.

[Mario]

Sailing Hull Design Method -SHDM is a software for experimenting with the sailing hull. You can download it at site www.kus.hr

[grob]

Pierre,

I think like all physics assignments you are going to have to set some boundaries and define your terms. What do you mean by most efficient? is it simply least drag for a given stability. Are we talking Multihull or monohulls. Usualy lowest drag is longer thinner hulls, which does not help in a monohull which also needs stability to enable it to carry the sail power. Are you going to include the rudder and keel, As these are perhaps the most important elements and could form a thesis onthier own!

Now I have possibly made it harder I will try and help by suggesting you get hold of a copy of Principles of Yacht design by Larsson and Eliasson which as well as being a goood text on hull design also has a chapter on Design Evaluation inluding Tank testing.

All the best

Gareth

Firstly, thank you very much for those who answered so fast.

After having narrowed the topic down, I have chosen to look only at monohulls, including the keel but not at the rudder (It would be too long to investigate the compromise between less water drag and efficiency in turning, and would require an EE of its own.)

I'll probably firstly investigate the shape using the software that Mario advised, and to prove the result, i'll make a prototype of it.

I heard about an equation which would determine the maximum possible speed of the hull (past a certain speed, the water makes a wave that would prevent the boat from going faster) They say that the longer the hull is, the faster it can go, because this wave has less impact on it.
Do you have any information on this?

Thank you again,
Pierre

Originally Posted by Mario

Sailing Hull Design Method -SHDM is a software for experimenting with the sailing hull. You can download it at site www.kus.hr

Thank you, it looks like a useful product on its website, however it requires a program called autoCAD, which is not given with it... Do you have any idea?

[sorenfdk]

The equation You've heard about is this: Max. speed = 1.34 times the square root of LWL.

This formula used to be considered the truth, but modern, light-weight yachts frequently sail faster than this.

Another, perhaps better, approximation could be this: Max. speed = 0.553 times the square root of (LWL lifted to the power of three) divided by the cubic root of the displacement. Don't ask me where I got this from, because I don't remember...

I have no experience with Marios software. I'm sure that it is good at what it does (and I'm going to have a closer look at it when I get the time), but I'm also sure that there's more to max. attainable speed than having symmetric waterlines.

Anyway, You might want to have a look at a paper by J.A. Keuning et al titled "The Bare Hull resistance of the Delft Systematic Yacht Hull Series". You can find it here: http://www.sailboat-technology.com (go to "Online articles").

Best regards,
Søren

[bobber]

displacement hull speed seems to be the topic of many debates. some claim it does exist, others claim it doesnt.
especially when you get into the sail boat side of things, which seem to always break any written rules on boat speed anyway.
as far as i am concerned, the 1.34 rule is more a rule of thumb now, than an accurate way to determin max hull speed. its fine if you just want a rough approximation on the boats max speed. for buying comparisons etc, but if you are wanting an accurate number, as i assume you are, then this rule is no longer valid. Others may say different, thats just my opinion.

There was a post on here somewhere explaining hull speed very well, dismissing the 1.34 rule too. but i cant remember where it is. anyone else know what im talking about? he was on about going through websites and all of them saying about the 1.34. finding that post would be wise i think.
im not going to even attempt to explain it myself, as i am no good at that and will just end up confusing you, and more likely myself.

[sorenfdk]

Search these forums for "1.34" and a lot of interesting stuff comes up!

Best regards,
Søren

[Mario]

This version of SHDM program really requires some kind AutoCAD (version 2000 or newer one), that is product of AutoDesk. You should have to provide it, but the problem after that is using AutoCAD because you are probablly beginner with it.
What kind of software you use at the moment?

[bobber]

http://boatdesign.net/forums/showthread.php?t=1220

found it.

[Stephen Ditmore]

I'm surprised that Soren is using the imperial form of speed / length ratio, which correllates length in feet to speed in knots. In a phyisics curriculum you should be using Froude number, which is unitless when the cancellation is done. Fn=0.4 is the equivalent of S/L=1.34, the conversion factor being 0.298.

Both S/L=1.34 and Fn=0.4 represent the speed of advance of a wave train relative to wavelength. It's an important concept for scaling, and for predicting where a boat's resistance "hump" is likely to occur, but it's not an absolute limit.

Froude Number is named for William Froude, and he would be a good place to begin the historical part of your paper.

Something you'll need to find a good source on is the different forms of drag and what they mean. They have formal names like residual, viscous, & parasitic that I can never keep track of. Basically, one is wavemaking, one is friction, and one is the drag that increases with increased angle of incidence as "lift" from the keel increases, and correlates with the angular momentum of the vortices in the water behind the boat. Each of the forms of drag scales according to different rules. When you tank test a model you have to break the drag down and scale each type independently, then add them again to get the prediction for the full scale hull.

Once you have lift and drag information this is combined with sail force information, windage, stability (center of gravity) information, and perhaps info on weight distribution as it effenct pitching in waves (gyradius) in a Velocity Prediction Program (VPP). You will find many discussions of VPPs in these forums such as Data for a VPP?

You might look for the book "Yacht and Small Craft Design, From Principles to Practice" by Gordon Trower.

Good Luck!

[sorenfdk]

I must have been very tired when i wrote my first posting, because I forgot to mention the units.

In the first formula, the units are imperial (S in knots, LWL in ft). In the second, the units are mixed (LWL in m, D in t and S in knots).

I use the first formula for optimizing foil sections for keels and rudders of existing boats, but I use a factor of 1.1 instead of 1.34 to come up with an average speed. This I use to calculate Reynold's Number, which is important when comparing different foil sections. I guess I could use Froude's Number to get the speed, but in this case, it doesn't really matter...

There's one thing we all seem to have forgotten: These formulas are all more or less theory. Pierre is searching for the most efficient hullform, and if his research has to have any practical meaning/use, it has to take things like waves into consideration (not the waves generated by the sailing hull, but the waves that are already there!)

Best regards,
Søren

[bobber]

its been a few years since i undertook a physic project must confess, but i would say that the natural waves are a variable you cant really control. Id say first thing is set parameters. making all the unknowns set. natural waves, for instance, you wuold need to either discount and base your results on flat water, or come up with a common, typical natural wave height, length etc, and use that through out the experiment. otherwise your going to have results for pages and pages and its going to get complicated.
theres also other variables which depending on just how accurate it needs to get would need setting. Salt or fresh water, depth of water, is it just the efficiency below the waterline, or above the waterline too? because obviously then you have many other natural forces to account for such as wind. this could get very indepth, or could be kept simple, depending on what the final conclusion really needs to show, but definitely parameters need defining. and setting out before you start getting into the calculations etc.
for my projects and so forth on hulls, i always based the results on "clear water" or mill pond as my naval arch tutor would say.

[sorenfdk]

...and in the end, you'd probably come up with the answer that there is no such thing as "the most efficient" hull. There are probably many, depending on whether you're going up- or downwind, sea state, weather conditions etc.

But nevertheless it's an interesting project, and we would all like to hear more about it, I'm sure!

Bets regards,
Søren