Fin Keel

I am trying to understand the fundamentals of sailboat design and have been reading Principles of Yacht Design . My question is how does a boat designer determine the location of a fin keel on a hull? What I have gathered from the book is that its aproximate location is centered on the DWL. Another question I have is in regards to the ballast ratio. According to the text it is keel wt/total wt. By total weight are the authors taking into account the wt of the keel? I apologize if these questions seem fairly simple, but I'm trying to understand the basics. Thanks--

CBrown

 

The location of the keel depends on two things: The Longitudinal Center of Buoyancy (LCB) and the Balance of Helm.
Since the keel constitutes a large part of the total weight (and a small part of the displacement) its longitudinal position is important - you'll want your boat to float on the DWL.
Somewhere around 70% of the total sideforce (that counteracts the sideforce of the sails, so the boat won't travel sideways) comes from the keel and acts on a point on the surface of the keel. If this point is located too far from the center of the sails (seen horizontally), the boat will be unbalanced - you'll have either too much or too little pressure on the rudder. Where to locate the point (which BTW is called the keel's Center of Lateral Resistance or CLR) is described in Ch. 8 in "Principles..." There are other methods than Larsson & Eliassons, but it seems to work quite well (we've used it on a number of designs and never had any complaints...)
And to your second question: Yes - the keel wt is part of the total wt.

 

When I`m looking at the position of a keel, this position is fixed by the weight calculation. Take in mind that weight of the keel is something arround 30 ... up to 50% of the light ship weight.
The problem, balance of helm, is in my oppinion quite an other problem. If your looking at a "modern" fin keel boat, you have 3 "wing" under water, 2 above water. 3 wings under water means hull, keel, rudder. Only looking at rudder and keel and their forces, you can easily find a point of intersection which is far away of the hull and a small variation of the helm will change this point of intersection rapid..... or let`s name this intersection point, Center of Lateral Resistance.
For a long keel boat with ONE wing under water and a flap as rudder, I agree with location CLR to sailarea, for a modern boat it`s not important. Much more important to get a boat in balance to the helm is the efficiency of the rudder itselfe.

Peter

 

I'm sorry, but I disagree - The (horizontal) distance between CLR and the Center of Effort of the sails (AKA the lead) is just as important with modern fin-keeled boats as it is with long-keeled boats.
Most - if not all - sailors I've talked to, want to feel just a little bit of pressure on the helm in order to feel changes in sail trim etc. To avoid excessive drag, the rudder angle should be no more than a few degrees, and to ensure this, you need to know where to put your CLR.
Larsson & Eliassons method is - as I said in my earlier posting - just one way of finding out where. At a meeting in RINA, the late Austin P. Farrar proposed another, which he called "A New Rule of Thump". These two methods differ - Larsson & Eliasson deal with an extended keel, Farrar takes the rudder and the hull into account, too. Each of the two methods produce well-balanced boats, but if you want to be absolutely sure, they can easily be combined.
Unfortunately, I don't have Farrars original paper any more.

 

Soren, perhaps ... or I`m pretty sure, you are able to read German language. The Institut for Nval Architecture in Kiel made some very interesting tank tests, years ago, to find out what`s going on even with heel between rudder, hull, keel and sails. I think two publications are really very good "Kurshaltefähigkeit von Segelyachten" and "Flosse und Ruder von Segelyachten im Verband mit Rumpf und Rigg", both written by Prof. Harro Postel (hydrodynamica). The results are very interesting. Basic was a design of a 1/4 ton which was known as unbalanced. Same result was found out in the tank. Than they modified the parameters like sailforces and position, keel, keel position, rudder. The real interesting thing was, they ended with a smaller fin keel and a bigger rudder.
Same test they`ve made at several designs. Problem is/was that there is no formular available to say that`s good or whatever. One of the guy`s Prof. Wagner (aerodynamics/sailing yacht) tried to collect the founded results and, very interesting, stated that the efficency of the rudder was the real common point on all "good/balanced" designs. He stated a crw-formular with a limit good/bad ... but as said, based on rudder design not on position or keel design.

 

Du hast recht - Ich rede Deutsch! But writing in German is not one of my strong points, so:

The papers you mention sound very interesting - do you have a link, so I can download them?

 

War klar Btw ... my daughter speaks danish, she`s going to the danish school in Flensborg.
I`m sorry, I have no link. But I can copy the paper and send it by post or we meet for a beer in Kobenhavn, I think I will be there in two weeks. Btw, in your profile link to your homepage a www is missing.

Peter

 

Most likely, I won't be in København, but send me a mail when you know more precisely when you will be there, then I'll know where I am.
If you could copy and mail me the paper, I'll be glad (and of course cover your expenses...) You probably have our adress from our homepage (thanks for the hint about the missing "www"). If not, here it is:

CDE Danish Marine Design
Att.: Søren Flening
Wilders Plads 8A
1403 Kopenhagen
Dänemark