Magnet orientation is a big factor. If the poles are oriented vetically the influence is dramatically reduced. I have fourteen pairs holding on my back hatch and testing compass interference shows no infuence outside of 18 inches. In fact, I get more deviation from the rebar in my concrete floor that I do from the magnets.

I'm not sure what a magnet in a free swinging toggle might do to a compass. I suspect there would not be a problem due the distance and small magnet size required. Sounds like a little testing is in order. Perhaps an iron target in the toggle and a magnet on the boat would be the best arrangement.

Ted

: The problem is that once you put rare-earth magnets on your boat, you can't
: use a compass any longer, as it will not read reliably.