Sailboat controllability critetia

[Alik]

Hi, here is some info on sailboat controllability criteria proposed.
Your professional comments, please.

[jonathan]

Just one question from me: why are you using the boat's midpoint as the center of the steering moment, and not the CLR?

[Alik]

1. It is standard approach in IMO Res.
2. Geometrical CLR has little common with real underwater CE, it is located much forward.

[gonzo]

How does it work on boat that change trim when heeld?

[Alik]

Thanks,
This trim has no signifacant effect on hydrodynamic yawing monent. Suppose, this only effects rudder aeration then its upper part comes out of water. The latter can be taken into account by coefficients introduced.

[gonzo]

I don't know what the formula tells you, but sail an old IOR boat with narrow water entry and fat stern. It will show you how controlability changes. On the other side of the expectrum, try an old meter style boat with very long boom. They sink the transom when heeled. It also affects controlability.

[Alik]

That is right. But the order of hull assymetry yawing moment is much smaller than that of moving aerodynamic driving force on sails outboard.

[gonzo]

The difference is boat designs change it by quite a lot. The IOR types move the CE of the sails forward when heeled, while the older type with large booms move aft.

[Ian Ward]

I think you are wrong about the immense steering effect of hull misbalance. It is significantly greater than the force of the rig, but the two together can be disastrous. I regularly see boats "loose control" in strong gusts, round up head to wind even with the helm hard across the boat and even tack unintentionally.

Fully balanced hulls show no such tendency and the effect of th erig to leeward give only a slight effect. Try sailing a J24 and comparing it with a Dragon in a good breeze.