Shallow fin keel ?

In adding a shallow fin keel to a hull what should I think about in terms of area?
I know that it must be big enough to encompass the ballast , but are there any general guide line ? Shape ?

I know this is a complex question .

Thanks ,Frank

 

Completely depends on sail plan, expected upwind performance and draft restrictions / local conditions. Have you got drawings or pictures of the boat? Can you better explain usage and expectations?

Keel shapes vary almost infinitely, and the amount of lateral area projected again varies quite a bit.

I'd be surprised if you could not find out the designer's intentions by a little research looking for other examples of the same hull. If you can identify the boat, let people know and I'd bet within hours you'd have an answer.

--
CutOnce

 

This if one of many iterations . 26 LOA , draft 3.5 . It is intended to carry a junk rig , with transom mounted rudder on a skeg.
It is of the ubiquitous multi chine form.

thanks f

 

I take it you are using some computer program to draw that. The main disadvantage of those programs, is that they produce pretty drawings without the need of any understanding of design or boats. You need to go back and study the basics.

 

Gee Frank, thats a broad question. How much keel ? The keel...foil... generates lateral resistance, the ballast generates righting moment. How much lateral resistance do you need ? Again, broad question.

Take off the keel...foil.... ,flip it horizontal and turn it into a water ski. When you first start off with the waterski keel you cant stand up..go a little faster and presto...its planning...now jump off your ski and you can ski on your bare feet. .

With a keel if you go slow, you need a big surface...when you go fast a small surface.

Also remember that hull shape, heel angle and rudders influences lateral resistance.

Construction wise...The shorter you make the keel chord, the more engineering the keel and hull structure and the deeper the keel..

This is a classic robust fin keel for an all purpose cruising boat with the terminology like cord depth..

http://www.j105.org/docs/TemplateInstructions.pdf

http://www.marskeel.com/keels/10

 

It is not just a pretty picture , and I drew boat lines long before I knew any thing about computers. It is a well thought out, and researched hull, and the numbers are good .

Now have you any thing positive to add here Gonzo ,or am I going to get the standard routine ?

F

 

Micheal , that is a little help ,thank you. I am about to dive into some books on the subject , but a little insight would be helpful.

I could of course use a low aspect sail plan with a board up front a rely on the rudder for most of the lateral plane . Maybe .

F

 

Frank: A well thought out and researched hull and numbers, by necessity, must include the keel If by "the standard routine" you mean people suggesting boat design is a whole, well, that is the best advise you can get. The design spiral will make the hull change as you add elements to it.

 

I am somewhere in the middle of the design spiral right now . The hull was designed with rig, disp. , use , type of construction , ballast ratio , and a general idea of keel configuration.
The interior details are up in the air right now , but I have a good idea of how it is going to go.

I could just pull the keel dimensions off a similar existing design, but it would not tell me the why for.

 

Designs are based on previous ones. That is what engineering is all about.

 

I agree with you , and this design is not innovative . That isnt much of a picture to go by , But my intention was to present what i thought was needed.

Any way the keel will be of composite construction , or molded fiberglass.
It would be nice to keep the draft as close to 3' as possible .

 

For shallow, long keel, rule of thumb figure would be keel area =7-10% of normal (fore triangle + main triangle for bermudian boat) sail area.
Deep fin keels could be several times less in area; the price for it is necessity to keep boat up to speed all the time.

 

Frank...if you have a good bookstore around "High Performance Sailing" by the Australian designer Frank Bethwiate is a great addition to your bookshelf. Hull, foils, sails all explained in a typical straight forward Australian style.

 

Micheal , Ill check online for the book. My collection includes Marhaj's books
Skenes, Principles of yacht design , and some older design books . looking for a good rig
design book , I am currently studying " Further Offshore ", by John Illingworth , out of date but a wonderful analysis , and a lot of fun.

 

Is your junk rig self standing? Is it single or multiple masts? If a mast is on the eyes of the boat, it needs a rather full bow.