Maximum beam ?

Did you all agree on this point?

 

Slowmo,

It would be nice if you could be constructive. I don't mind an argument, or discussion, I do mind downgrading scarcastic comments!

 

Curmudgeon

2% aft of the center of the waterline is usual. Hull design is complicated, and you can't learn enough via the internet to design a hull. You need to study at least the following text books:

-The Elements of Boat Strength: For Builders, Designers, and Owners by Dave Gerr

-Elements of Yacht Design by Norman L. Skene, Maynard Bray

-Principles of Yacht Design by Lars Larsson, Rolf Eliasson (foam core sloops, mostly)

Old-Salt

 

Old Salt,

Where did you get that Rule-of Thumb? And how do you compare that with a percentage of the waterline lenght?

 

Ah...

Hello...

55-65 percent of the LWL from the stem is a good 'rule of thumb' - unless you prefer metric - rule of Napoleon - but being a percentage I suppose historical differences can be put aside - WHAT...

How many people do you want to float aft in the cockpit...

How much reserve buoyancy do you want in the bow sections so you don't become a submariner...

Also a sailboat hull is not a static foil but a symetrical shape that displaces a rather viscous fluid - and this shape (or waterplane) varies greatly (depending on it's design) as it heels over or as it pitchpoles thru waves - or in my case - soft sand - oops - rethink the transponder location or recalibrate the depthsounder for depth of keel - hard to do basic subtraction whilst pouring another whiskey - sorry what were we talking about....

It's all a matter of what you intend to do...

Go check out Rodney Johnstones J boat plans at the J boat site - forget the rules - they bend the design - I just want to go fast one designers give you a better perspective on what works for overall conditions...

But that is just my opinion - and I am just 'Sean'....

SH.

 

Center of Buoyancy

In Larrson and Eliason, Skenes, Westlawn School of Yacht Design, and many others the CB is referenced to the bow/ waterline intersection(in a percentage of the LWL AFT of the bow /waterline intersection. And in almost EVERY case the CB is located aft of the middle of the LWL. Lars/Eliasson says the optimum CB location is between 52.8% and 53.6% aft for Froude Numbers of .3 to .4 for medium displacement boats(see page81). Correct positioning of the CB is critical as is the Prismatic Coefficient and both are highly dependent on the vessel's designed speed range....
You can take an lite course or a more involved course in sailboat/powerboat design at Westlawn and you can get "Princibles of Yacht Design" by Lars Larsson and Rolf E Eliasson ISBN : 0-07-036492-3 published by International Marine in Camden ,Maine and probably available thru Amazon. Other great sailboat design books are: "Design of Sailing Yachts" by Pierre Gutelle, ISBN 0-948646-54-3 ;"Yacht Design Explained" by Steve Killing , ISBN 0-393-04646-X ; and "Skenes Elements" of Yacht Design by Francis S. Kinney,8th edition, ISBN 0-396-07968-7 plus great books by CA Marchaj like the "Aerohydrodynamics of Sailing" and others, and "High Performance Sailing by Frank Bethwaite.
Read up!

 

Oh, no - I was not saying that the various rating rules haven't influenced the design of production boats - they sure have (in some cases unfortunately!).
I was only referring to the longitudinal location of Bmax, which hasn't changed much, although it seems to have moved a little bit aft in recent years.

BTW - are we talking about Bmax or BWL? Not that it matters much, but often, BWL is located a tiny bit forward of Bmax...

 

I agree on everything you say, but the centre of buoyancy isn't necessarily located at Bmax (or BWL).
Larsson & Eliasson have their numbers from the tests carried out by Gerritsma et al. at The Delft University, but today many yachts have their CB located further aft - up to 56% of DWL. Probably because interiors and stuff have become lighter, but not the crews

 

The J Boats are indeed fast. So are a few of our designs. Funny thing is, they look quite different from the J's!
This just goes to show that there is more than one answer to most problems...

 

CB / speed /Prismatic

Just to rehash: I said that these elements are intrinsically related to the intended speed range of the boat which, of course, is directly related to weight. Larsson /Eliasson point out that there is a relatively high performance penalty for getting it wrong....

 

You are absolutely right (again!)
It's just that the penalty for not having LCG and LCB coincide is even higher

Ouch! My brain hurts - I think I've posted too many replies for one night! Gotta go to bed, but I can't wait to see which direction this thread is taking!

 

LCB /LCG coincide?

Just for the hell of it: not on a Moth foiler, or a tri flying the main hull or a cat flying the windward hull, but I'm digressing-hope not to confuse the "students"....(this is fun though!)

 

Slowmo - Try This...

Hello...

Go to...

http://www.carlsondesign.com/#Fun_Shareware

Download his free software for developable surfaced boats - also some free sail design bits - play around with some of the better hulls or try out your own hand at it - this little program will give you a few design centers and displacement calculations to play with - it is very basic but it works - my friends son has built two Radio Controlled model sail boats using this little application...

Heh - it's free - and so is a library card...

You will need Winzip to extact and use the files....

Best of luck - just don't get caught up looking for hard and fast answers - you design a boat towards its intended use or application - if there was one answer then we would have but one boat to choose from...

Cheers....

SH.

 

Great thread!

I guess that there was no Olympic sailing on television this evening. . .

 

To be honoust Peter, I think these race classes did affect the shape of the yacht design of the last decades. The only hard thing; I heard (do not surely know if it is true) that the ORC (IMS) rules are changed every year. I have no clue if the rule is changed significant, but this would make it harder to affect the yacht design. (I hope you now what I mean)

In my opinion, yachtdesign changed due to several racing classes. If you look one century back at the racing yachts, all yachts looked like them. A long, deep keel, several different sails etc etc. All yachts produced around that time looked that way.
Modern racing yachts (I say Mumm 30's and so) look wide, with a deep short keel, a huge mast and only 2 upwind sails up at a time. If you look at the cruising and semi-cruising yachts build nowadays, I think these Mumm's and so do have affected the modern yachtdesigns.

So: If the designers found something new in the racing classes, the will eventually apply it to the cruiser and semicruising classes.


Sorenfdk, could you please try to tell me what you mean with station 4 and station 6. I am trying to think with you but I do not know what you mean with it.