LCF, LCB moving when heel

[sail-bob]

My first sailboat design version attached.

LOL 5.5m, Beam=2.13m

When heeling 30% the LCF and LCB moves aft quite a lot...

0 degrees: at displacement around 0.5 tns is LCB=2.55 and LCF=2.4
30 degrees: at displacement around 0.5 tns is LCB=2.08 and LCF=2.09

I hoped that this design would be pretty standard for small day crusers
but to my understanding it is bad design when LCB and LCF moves this much.

Does anyone have an explanation?

Cheers

[lewisboats]

It is the amount of flare in the sides aft, increasing the beam and buoyancy as the boat heels. Most sailboats are nearly verticle in flare as the approach the sheer, thereby keeping the Centers fairly close.

Steve

[sail-bob]

Yes,

I agree.
But if force from the sails is considered and assuming that the CE of the sail is behind the Center of Lateral Resistance ( CLR whatever this is ;-), but assuming that keel position is vitally important), this force would actually push the LCG aft and align pretty well to LCB?

I just wonder how these boats sail well (or maybe only fast...):
http://www.backmanboats.com/B21_start.html

Cheers

[Tactic]

I doubt this type of hull form would perform well at 30 degrees heel.
You would need to keep this hull as upright as possible for best results.

see this thread on a r/c sailing site
http://www.rcsailing.net/forum1/showthread.php?t=3436
some disscussion of the subject of hull balance...quite important on models

[lewisboats]

To balance the hull at 30 deg you would have almost no control of the helm. As the boat heels, the CE moves forward and outward, changing the force vectors. The CLR is ahead of the CE in profile at rest but changes in relation as the boat heals. Also, as the hull heels, the hull itself acts somewhat as a foil, twisting the boat even more. Boats with plenty of beam aft and flat buttocks become more difficult to control, and need more TLC than more evenly displaced hulls. If you look at the pictures of the backmans you will not see that much heel, as the boats are designed to be sailed relatively flat.

Steve

[jehardiman]

The answer to the question about why it is bad is that if the LCG does not change, then the trim on the boat will be excessive and direction stability and lateral resistance will be compromised. On the flip side, in a small open skiff, the LCG can change that much. In 470's it is quite common to have the helmsman against the transom with the crew as far and out as the trapeze will allow in heavy winds to counter this type LCB shift.

However, as had been said, with the hull shape you drew, it will be much much faster if sailed flat. And easier to handle too with the chines you have in it. Also, you need to understand the relationships between LCG, CLR and CP before you go off and design a small boat hull. They all fit hand in glove in a good hull form.

[webbwash]

Just as an added note to what Mr JEHardiman has to say ---

Check out the current designs that the International 14, 49er, and many of the Skiff class sailboats have in common -- and the answer is -- SAIL IT FLAT and on a hard reach sail it from the aft end of the boat.

Start from a known and make minor modifications until you have proven your theories and concepts right. Or just go radical and forget about the new car you promised yourself every year for the next century.

[sail-bob]

Thanks for your good replies- it really makes me rethink...

Although LCG can be moved (by myself and guests) I start to see the benefit
and drawback with such a design. The problem is that I have no SW which can calculate dynamic behaviour and I am not skilled enough in math to do it myself.
Since the goal is a relative fast but still "allround" boat, I'll go back to more classic designs (no swing keel in V1 :-)).
B.T.W. If I would build a small scale RC model (e.g. 1:6), would that give a hint on the handling of the real thing?
This would maybe require 2-3 models but hopefully result on a acceptable (but likely not ideal) final full scale boat.

And that's how I am, never buy what you can do yourself - trial and error gives a nice learning curve and is much more fun - And I don't mind being flamed for this attitude.

Cheers