I have played around with that idea a fair bit. The big advantage is that the tip of the dagger can be the same shape as the keel and when retracted would effectively seal the hull. (No slot to speak of)

The down side is that you definitely do not want to run into underwater obstructions. That has always in the end torn it for me.

One could use a pull down, spring back, system incorporating a notch, or a ledge, at full deployment, so that the skeg, if it hits something slips off the notch and zips back into the slot. But it probably would not react fast enough to prevent damage, and it would only work if the skeg was fully deployed. No good for partial deployment, as multiple notches would stop the zipping back effect.

One could design it as a breakaway, so that the casing would be safe, but then the skeg may not have enough strength to resist the lateral forces on it by the water.

If you come up with a solution I'm sure we would be very interested.

Et