Strange motion on sailboat?

[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

[Fintry]

The motion you describe is typical of a sailboat without its mast -- such a boat is very much too stiff and will jerk your stomach out. However, you say that you have been aboard while sailing, so that theory is out.


In my experience of boats ranging from full keel with attached rudder to fairly extreme fins, I can't say that the underwater profile has affected the motion noticeably over other factors.



You might look at sailboatdata.com for the ballast/displacement ratio. If it is very high (over 50) I would expect the symptoms you describe.


As for picking a boat for offshore work, there is no particular reason to choose a full keel boat. We did our circumnav on a Swan 57, which has a less than full keel, although not quite a fin. Such a boat will not track as well as afull keel, so you will use more autopilot, but it has less wetted surface, so it will go farther on a gallon of fuel under power and, for a given SAD, will sail faster.

[Fishzine]

First thing I thought of when reading this was "Jerky" and tied to dock. IN the right (or wrong) seastate any boat is going to be 'jerky' when there isn't much stretch in docklines. What were are the docklines like? fixed or floating dock? lenght of lines, etc....

[massnspace]

Yes, rig is on, and I forget to mention the ballast/displacement ratio is 43.

Also, I have been on boats tied to docks many times….never felt this uncomfortable.

[Kettlewell]

I've owned and sailed a bunch of different types of cruising boats, including full keels, fin keels, cats, and now a long fin. There is a lot more to the motion than just the keel shape. For example, we once owned a small, full keel design with pretty overhangs, but we quickly learned that the boat had a terrible pitching motion. She would just hobbyhorse up and down constantly with a minor chop. I had a fin keel boat that was also beamy and was extremely stiff. You couldn't put the rail under, but the motion was extremely jerky from side to side. She was fantastic off the wind or heading into the wind, but in a beam sea very jerky. Another heavy double-ender with a full keel had relatively slack bilges and was extremely comfy offshore. She neither pitched or jerked, but off the wind she had a tendency to roll a bit. I too have stepped aboard someone else's boat at a dock and wondered about the motion, but OTOH the true test is what the motion is like offshore where you will be sailing.

[GordMay]

Originally Posted by Fintry

The motion you describe is typical of a sailboat without its mast ...

Or a boat with very light ends, as is often recommended.
Heavier ends, often discouraged, tend to produce a greater amplitude, but much lower frequency [slower acceleration] of pitching moment.

A boat at sea moves in six degrees of motion: heave, sway, surge, roll, pitch and yaw.
The first three are linear motions.
Heaving is the linear motion along the vertical Z-axis,
Swaying is the motion along the transverse Y-axis,
Surging is the motion along the longitudinal X-axis.
Rolling is a rotation around a longitudinal axis.
Yawing is a rotation around the vertical axis.
Pitching is a rotation around the transverse Y axis

Pitch describes the up and down motion of a vessel, characterized by the rising and falling of the bow and stern, in much the same way as a teeter-totter moves up and down.
Heave defines the up and down motion of a ship, as large swells heave her vertically [up & down ↕], on the crests and troughs of waves.

[Fishzine]

Originally Posted by massnspace

Also, I have been on boats tied to docks many times….never felt this uncomfortable.

My point was more specific the this boat and how the boat was tied up . How you tie up a boat at the dock matters greatly to how it moves and behaves at the dock. Short dockline with no stretch will cause jerkiness

[Kettlewell]

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.

IMHO length makes more of a difference than keel type or even weight for offshore comfort. But, each boat is the sum of many compromises and will feel different in different conditions.

[Cheechako]

That would be strange for a long keeled boat in my experience, they are usually "slow rollers" rather than jerky. A Catamaran is the worse case scenario for "jerky" , not sure I ever got used to the quick jerk on mine.

I can assure you it has nothing to do with a full keel and must be other things on the "mystery" boat.
I'm unclear if this is only at a dock or...?

But boats can do strange things at a dock, I think the sea action or even tide current interact with dock structures and cause it.
I once watched a highly touted modern cruising boat (which had plenty of draft) from above the marina roll wildly from side to side (mast swinging port and starboard). None of the adjacent boats were rolling near as much as this one example and I thought it might even clash with the adjacent boat's masts!
Was it something about that one slip location? Or was it the boat? There was a decent breeze but only maybe 12-15 knots.

[Knotical]

I wouldn’t think the type of keel should matter on a docked boat; the jerky motion could be from a variety of factors like the wind and current state, any fetch in the water, wake from passing boats, people walking on decks, lines improperly tied, etc.

[boatpoker]

My wife and son are veteran cruisers. We've done may deliveries on various types of boats over the years and none of us has ever been seasick until ... 15 minutes into a 50' catamaran tour boat cruise in St.Maarten they were both violently seasick. They could not handle the motion of a cat under sail. This is too subjective a thing to come up with a specific answer.

[Don C L]

My boat is pretty tender, but I am used to it and I don’t get get sick. But when I sail with my friend with a Catalina 27, that boat is more “jerky” and I’ll feel queasy, but it is a flatter hull and beamier and more responsive to small wave action. This boat you describe , 32’ and 15,000 lbs sounds like it does not have a flatter hull unless it carries that 10’ beam pretty far aft. Is this a design like a catboat?

[thomm225]

Looks like your present boat is a catamaran which has probably spoiled you.

I raced beach cats from 16'- 20' for 15 years and never got close to being seasick even in offshore races, but after I started sailing my old Bristol 27 it took a while to overcome the sea sickness.

1/2 a Dramamine on day one usually took care of the problem for the entire time I was on the boat usually 6 days or so in the Bay and Atlantic Ocean also in the area of mixed Bay/Ocean waves which are the worst.

[gs41escapade]

My thought would be to check the bilge for excess water, or even more likely, fluid tanks where the baffles are either missing or have broken loose. Small lateral motions could cause large and rapid fluid motions which would then feel like the jerky motions you describe.

[gsuescum]

Ugh. I know how you feel. It might just be a weird wave pattern that’s at just the wrong frequency for that particular boat as it’s oriented tot he waves and tied.

I live on a cat and we’ve been in an anchorage for a few days that has small waves hitting at just the wrong frequency close to the beam that interacts with the boats natural frequency. During that time we’ve been bouncing uncomfortably. I’ve been banging into doorways and counters because of the odd lurching. It will be nice to leave.

You should try moving the boat, loosening the lines, changing its orientation to the waves/wind and see it still makes the motion that bothers you.