James Theodore of Friday Harbor, WA asks:

How well would the 40 survive a knockdown or even worse a rollover?

Jeff Leishman responds:

You bring up a good question on the basic design parameters of a true ocean going boat. As the chief designer of the Nordhavn 40, and all of the other Nordhavns, it is my job to ensure that these boats can survive a knock down or worse yet a complete 360 deg. roll over. On paper the N40 does show a positive righting moment through 180 deg. So a knock down is something that would not present a problem assuming that a window or windows didn’t get blown out. That is the reason for the storm plates that we suggest for the side salon windows. The Diamond Sea Glaze windows that the N40 comes standard with have welded sockets on the corners of the salon windows, which allow the storm plates to be easily attached when going to sea.

Occasionally we get reports from owners on N46s or other Nordhavn models that claim they took a knock down without any significant damage or trouble. A knock down, while quite scary, is unlikely to be catastrophic if the boat and crew prepared for such an event. A 360 deg. roll over is another matter. Theoretically on paper the N40 would easily right herself in a roll over. However, in real life, a roll over would be a very violent event that would undoubtedly cause some type of damage to the boat and the crew. Unsecured objects get tossed about increasing the likelihood of windows being broken from the inside, which could cause severe flooding. Antennas and rigging would likely be lost or damaged. Provisions and other stores get shifted which cause the center of gravity to shift hindering the ability of the boat to come back up. All of these things would contribute to a very dangerous situation that is difficult to predict. The same waves that will put a boat upside down will also put the boat right side up and the negative righting curve of the N40 is quite small compared to the positive righting curve, meaning that it would take a lot less of a heel angle for the boat to get back to a positive righting moment that it took to get it into the negative realm. A very important element of surviving a roll over would be how well the boat is prepared to handle it. If everything is tightly secured, storm windows are in place and the crew doesn’t get too beat up, then a roll over could be survivable with the boat continuing on.

This subject has so many variables and the sea is such a dynamic force that it makes it almost impossible to predict any outcome.If I had to pick a motor yacht to try a roll over in the N40 would be high on my list. Let’s hope we never know the answer.

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