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My fellow sailor said that an ideal sailing boat should turn against the direction of waves when losing control. Then he said that ice sailing boats cannot do this. This is apparently because ice sailing boats do not have a keel. Now
How/why does the ideal sailing boat stop when losing control? Why is this not possible with ice sailing boat without keel?
It depends what you mean by "losing control" and by "ideal sailing boat".
Some boats are designed for cruising, some for off-shore, some for racing. They each make compromises when out of their ideal conditions. For example a racing boat may have a lighter keel which needs to be longer to generate sufficient righting moment, but this would prevent it from entering shallow bays. Meanwhile a cruising boat carries an engine, fuel, cooker, fridge and large compact keel which means it won't accelerate like a racing boat.
If losing control means being overcome by excessive wind or waves, then to some extent the sail plan can be chosen to encourage the boat to head up into the wind as it heels over and most boats will do this anyway. In the extreme case, boats can be designed to be self-righting.
If losing control means that the crew are incapacitated or washed overboard, then it would be helpful if the boat would stop or at least slow down a lot. In reality this will depend on the point of sail they are on (reach, run, beat, etc) and whether the controls are being held by the crew, as is often the case in a dinghy, or cleated off as is more usual in yachts due to the forces involved. In the latter case there would need to be some way for the boat/system to detect that the crew were not in control. Maybe there's a short-range wristwatch transmitter that would trigger the boat to stop if it went out of range?
Perhaps an "ideal" sailing boat would automatically heave-to or something similar when controls are released.
I don't know much about ice-sailing boats, but I imagine they have some way of counter-balancing the heeling caused by the wind, presumably they are catamarans. In theory they could also be designed to head up into the wind when over-powered.
Most sailboats will turn into the wind and drift downwind (and down current, if any) if the helmsman were to let go of the tiller or wheel steering the boat. If the sailor also released the sails' sheets the vessel would drift slowly downwind.
The reason most sailboats turn into the wind when steering control is released, is that most are designed with a center of effort (from the sails) slightly aft of the center of lateral resistance (from the keel and the rest of the profile below the waterline). One can design a boat, or adjust the sails of the boat, to have more or less weather helm (tendency to steer itself into the wind) or lee helm (tendency to steer off the wind).
Although I have no personal experience with iceboats, the physics are similar and certainly letting go of the sheet will drop most of the force giving it speed.
