Strange motion on sailboat?

[Kim Klaka]

Originally Posted by massnspace

I had an unusual experience on a friend’s boat recently.

The boat in question is a 32’ length, 10’ beam, full keel, 15,000 lb. cutter. I am not going to mention the make as it is irrelevant and don’t want to start a conversation on makes and models.

I had a chance to be on this boat several times: at the dock, motoring and sailing.

I can say that I found the motion to be very “uncomfortable”. It seemed to move in jerky motions, almost throwing me off my feet at times. I felt a bit seasick once even though we were tied to a dock. Again, seemed “jerky”…unexpected motions. I found myself noticing and thinking about the motion a lot, while on other boats I just went about my business and never thought about it at all.

I have owned and been on many FIN KEELED boats my whole life. Both large and small. Never felt anything like this before. The weird thing is I thought the boat would be MUCH SMOOTHER than a fin design as it was way heavier and full keeled.

Is it possible that my brain has been “trained” to feel the motion of a fin keeled boat as “normal”, and that being on this other boat type was then perceived as “abnormal”? Would a person in the opposite position think a fin keeled motion was “unnatural”?

This question had added weight as my wife and I are considering buying an offshore boat to take up to Alaska and down to Mexico, and I was seriously thinking of something with a much fuller keel…..but now I don’t know.

Any ideas or advice welcome.

David
Seattle

Set aside your experience with the vessel docked. As per another post, dock line tensions can create strange jerky motions.
Jerky motion is usually, but not always, due to short period. Which of the 6 motions are jerky? - roll, pitch, yaw, heave,sway or surge? Roll is usually the motion people are most aware of, followed by pitch.
The motion experienced by the boat is strongly dependent on the wave period encountered by the boat. Does the effect occur mostly in a given sea state, or across all sea states? (short chop, sea breeze waves, trade wind waves, long swell etc.)

Roll:
Roll period is dominated by stiffness, also inertia. A cruising boat will always have a high roll inertia (" mass moment of inertia about the roll axis", to be accurate), lengthening the roll period and slowing the motions down. It could be that the boat is very stiff, though that would be a bit unusual in a cruising boat. Does the boat carry full sail OK in more than 20kn true wind? If so, excessive stiffness could be the reason for the motion. The other factor affecting jerkiness is the amplitude of the motion - how far it rolls for a given roll period. The lower the amplitude, the less jerky the motion for a given roll period. The amplitude is governed by damping (for a given wave excitation period). The damping in roll is determined mostly by the underwater profile (this was the topic of my PhD thesis). A long keel usually creates a lot of damping, which should reduce the amplitude of the motion, which is probably not what you are experiencing.
So for rapid roll motion, excessive stiffness is perhaps the most likely explanation.

Pitch:
For pitch, there is not much variability between boats in (longitudinal) stiffness and damping. The two main factors are slamming (bow section shape) and the period of the waves you are sailing in. See earlier comment about sea states.