How to design keel mass for a given boat?

How to calculate :"How much do I have to put ballast into the keel bulb for a good stability of the boat?". Maybe you can give some equatations for this, or some theorethical feedback on this? What data do I have to assume to calculate this?

 

That's one of the (many) complicated matters in boat design, and so are the equations and formulas multiple..
You got to determine the goals of the design to achieve and thou get some of variables more specific.
Some matters:
-Desingned waterline/displacement?
-Weight of the hull/cargo/passangers and how it's distributed etc
-Sail area/mast height
-Beam/rightening moment
etc

 

Usually you choose the hull shape, keel depth and amount of ballast, and then you can determine the righting moment you can generate. From the max righting moment, all of the rigging forces are determined. So the whole design is done based on the amount of righting moment that you choose. You can not generate any more force on the sails and rigging than you have maximum available righting moment, or you go over and turn turtle.

If you already have a sail plan in mind, it might be possible to back calculate the amount of righting moment you need (which would determine keel depth and amount of ballast). But this would require you to understand the complex aerodynamics of the sail plan and rigging. Traditionally this was always an unknown, hence the design always depended on the amount of ballast chosen by the designer.

Consider too that the size and amount of sail you would put out is based on the available wind. The less wind, the more sail you have up. when the wind picks up, you have to reef or install smaller storm sails. The ability to keep more sail up in any given wind condition is determined by the amount of counterbalance force available.

On small sailboats, the weight of the crew is used as counterbalance, and the larger the boat gets, the more added counterbalance is added to allow more sail area. The real difficult task is to get a lot of righting moment with as little added weight as possible (especially with a racer or a trailerable boat). This means a real deep and long keel (longer lever arm) allows you to have less weight, but it also means you have to be able to retract the keel for port or trailer. Short heavy fixed keels are simpler and give a more comfortable ride on an ocean going yacht, but at the cost of sluggish performance and less responsive steering.

So the amount of ballast and depth of keel is up to the designer and based on the properties the designer wants, or the purpose of the design (racing, cruising, trailering, etc.). There are relatively large racing yachts with less ballast on deep high aspect ratio keels, and there are very heavy and shallow keels on relatively small "pocket cruisers" of only 24 feet long. There are larger dingys in the 16-18 ft length with little to no ballast that use dagger boards and crew weight shifting to limit heel, and small but heavy weighted fix keel day sailors (like a West-Write Potter 15). And of course you have catamarans that have no ballast, but use the wide beam to counter the sail's heeling forces.

So it is up to you to determine what you want in a sailboat.

 

Any ideas of how to aply this to my Palace 80? You can take some data from (at the top of the thread):
http://www.boatdesign.net/forums/boat-design/palace-80-btg-yacht-design-24759.html

What mass would you suggest for her instead of 1500 grams in bulb (306 for hull and misc(so total displacement 1806 grams)).

If I lower the bulb mass to, let's say 1300 grams woluldn't it be wrong?

just suggestion, i know that it should be my own decision

 

That would mean you will carry less sail, or have more heel, in the same wind. If you made the keel deeper you would restore the righting moment AND improve the keels efficiency (pointing ability and lower drag).

Or you could do one even better, drop the ballast even more, and make the keel deeper still. You save weight, lower the keel's drag, and improve the performance and handling. But at the expense of more draft.

 

The drag is not the only problem with extremely long keels on Radio-Controlled yachts. The main feature is that this kind of solution increases the total rolling moment of inertia, together with gyradius. This obviously means a penalty in wave added resistance...

 

The other option if you are worried about drag is to keep the same weight and go to a tungsten keel bulb. At a density of almost twice as high as lead you can carry the same weight but much smaller, reducing the wetted surface. Plus since it is much stronger as a steel alloy, it can be used to make the keel as well as the bulb, further increasing the ballast weight relative to the wetted surface. Though it does cost significantly more than lead...

My first advice is to look at the keel design of the actual Open 70's. Since the designers must have faced this problem as well. Though the balancing act may change somewhat for a scale model.

 

With a MP x10 that of lead and more than 2x that of steel, tungsten would need special facilities for casting, on top of its raw cost, and its a ***** to machine. Fortunately we're talking a model here not a superyacht.

 

Hehe that was why I suggested it. I assume that anyone planning on using tungsten in a large yacht would be familure with the possibility and know the difficulties. But in this small a form you might be able to put it on a metal laith and just accept that you will destroy a lot of blades getting it right.