[fiatracer163]

I would like to know what makes a boat more stable when anchored; hull design or weight. My wife and I would like to do more trips, but we both feel that our current boat (Catalina 27) is not comfortable at anchor. I know that having more weight down low helps the righting moment during sailing, but at anchor will cause the rocking frequency to be higher and rocking angle be less. Almost like rocking a baby in a cradle fast, but not far. I know moving the CG up will that cause slower frequency, but will also make them rock to a further angle. Like rocking the baby slow and farther. Will that mean that a lighter boat having a higher CG and be better at anchor? Or will a wider beam, and flatter hull be more stable regardless of weight? I would like to move up to a larger boat (32-37 ft.) and have been on a few and they all feel like they are more stable. My dream boat will be a good sailor and comfy at anchor. Will weight, or hull design be the deciding factor for comfort at anchor? Boat suggestions welcome, but am looking for more on design and what to look for.

I would like to stay in the 32-34 ft range due to dock fees.
Lake Michigan will be my cursing ground. (Milwaukee will be home port)
Some of the boats I've looked at are the Beneteau First 35 and the Catalina 34, can the same stability be found in something in the 32 ft range?

Thanks.

 

[flyingwelshman]

Lake Michigan will be my cursing ground.

Freudian slip?

I think the issue of stability while at anchor has more to do with your selection of anchorages than the size (weight) of your boat.

I'm sure that there are scientific formulae that can be used to demonstrate that boats with different design characteristics behave differently when anchored - multihulls come immediately to mind - but it seems that we can dictate comfort for ourselves in our 30' boat, by making the right choice when dropping the hook.

We sail on Georgian Bay, which, I imagine, is somewhat similar to Lake Michigan in its shoreline characteristics.

There are many small islands in our area and long fetches on the bay. By keeping an eye on wind direction and speed, and the effect of waves bouncing between islands etc. we can usually find a spot with minimal rocking.

Other things that might help: sleep longitudinally (i.e. lengthwise) in the boat. I generally prefer to have my head towards the bow. It might also help to try and get as low and centred on the boat as possible.

Although all of this is moot for me because I become instantly narcoleptic when I get on the boat.

 

[Alex W]

My Catalina 25 would sail on it's anchor, I wouldn't be surprised if your Catalina 27 does the same. In higher winds (over 10mph) this would be increasingly uncomfortable as the boat would sail up one tack until it stalled, snap back onto the anchor rode, fall off, and do the same on the other side.

An anchor riding sail (doesn't need to be fancy, I made ours out of sunbrella but if I were doing it again today I'd just use polytarp) helped quite a bit.

 

[Barquito]

Look for a boat with a good motion comfort ratio. These tend to be heavy boats with short waterline. The beamier boats may also cause a faster motion. If you get any wave action from the side (if wind and wave don't match), then a narrower boat (less form stability) will be have slower motion. Maybe, like this one: 1976 Bristol 32 Sail Boat For Sale - www.yachtworld.com

OTOH, a bad anchorage will probably be bad in any boat.

BTW, if you haven't already, join milwaukeesailors.com.

 

[PCP]

There are several factors but size of the boat and beam are two of the more important. Nothing like a cat to be stable on an anchorage....well, maybe a trimaran

If that is so important for you there are some gadgets you can buy to improve that. It seems they work. But the more important thing is to get used to it and then you will not notice it anymore.

 

[MarkSF]

Look for a boat with a good motion comfort ratio. These tend to be heavy boats with short waterline. The beamier boats may also cause a faster motion. If you get any wave action from the side (if wind and wave don't match), then a narrower boat (less form stability) will be have slower motion. Maybe, like this one: 1976 Bristol 32 Sail Boat For Sale - www.yachtworld.com

OTOH, a bad anchorage will probably be bad in any boat.

BTW, if you haven't already, join milwaukeesailors.com.

I have one of those boats (shorter waterline, heavy, not beamy, rounder hull form) and the motion comfort upwind and on a beam reach, under sail, is excellent. The trade off is an underdamped motion while motoring, without the steadying effect of the sails. I would have thought this underdamped motion (they call it hobby-horsing when going upwind) would be an issue at anchor.

They do sell various accessories to improve behaviour at anchor, maybe worth trying?

 

[nolatom]

You say "rocking", but do you mean rolling? I think of rocking as pitching, like a rocking horse--fore and aft. Rolling is side to side.

Yes, rolling does depend on metacentric height. Greater height means the boat is "stiff", hard to heel and snaps back up quickly if heeled. this 'snappish" motion is hard on cargo, and can be hard on crew, in a seaway. At anchor, usually you are somewhere sheltered.

Less metacentric height (aka "GM") means she is easier to roll, and takes longer to roll back to horizontal. Easier on cargo. Easier on crew too since it's a slower roll, except the increased heel which makes it a deeper roll.

There are some anti-roll devices you might consider using, they're pretty simple:

Stabilizer

 

[rikhall]

I think the issue of stability while at anchor has more to do with your selection of anchorages than the size (weight) of your boat.

I agree. If you anchor in a leeward little cove rather than in Rockport ME with the wind blowing from the south, you will rock and roll a lot less.

Then again, if you are sleeping on a 27 foot boat that weighs 5,000 pounds it might rock and roll more than on a 36 foot boat weighing 26,000 pounds.

Rik

 

[barefootnavigator]

Design is everything. I have a 22' boat although it does weigh more than your C27. Boats with too much ballast for instance will have a snappy motion. Boats with too much freeboard which is every boat designed after 1980 will sail at anchor. We spent over a hundred nights on the hook this year and rode out some pretty big winds but were never uncomfortable. our ballast ration is about 33% that with a brilliant design and proper weight distribution and we have a very comfortable ride. something you may be able to fix is that if you have too much weight fore or aft in such a small boat or above the water line you will be inducing pitch and roll. Try making a small riding sail, lighten your ends and get or make a flopper stopper. Having 100' of 5/16 might help also without adding too much in your bow.

 

[okawbow]

I think the "motion comfort" rating for boats is most accurate when the boat is at anchor. The type and location of the ballast makes a difference in that rating. Also, a sailboat that will go upwind well and is very stiff under sail, will have a snappier motion. A sailboat like my old Cheoy Lee 31, with it's concrete and steel ballast, and round bilges, had a motion comfort of 35, compared to the Catalina 27, at 26. The CL 31's motion at anchor and underway is easy, slow, and not likely to cause seasickness. I never once got sick in 3500 miles of coastal and offshore cruising over 3 months. I used to get queasy immediately when things got choppy in my Bristol 24 with a motion comfort of 28.

 

[barefootnavigator]

here is a comparison between my 22'er and a 35' C&C landfall you might be surprised.
Performance Comparison
Performance Comparison
LOA Falmouth Cutter 22
22
C&C Landfall 35
35
LWL Falmouth Cutter 22
20.833
C&C Landfall 35
26.75
Beam Falmouth Cutter 22
8
C&C Landfall 35
10.66
Displacement Falmouth Cutter 22
7400
C&C Landfall 35
13000
Sail Area Falmouth Cutter 22
403
C&C Landfall 35
514.2
Capsize Ratio Falmouth Cutter 22
1.64
C&C Landfall 35
1.81
Hull Speed Falmouth Cutter 22
6.12
C&C Landfall 35
6.93
Sail Area to Displacement Falmouth Cutter 22
16.98
C&C Landfall 35
14.88
Displacement to LWL Falmouth Cutter 22
365
C&C Landfall 35
303
LWL to Beam Falmouth Cutter 22
2.6
C&C Landfall 35
2.51
Motion Comfort Falmouth Cutter 22
33.59
C&C Landfall 35
29.17
Pounds/Inch Falmouth Cutter 22
596
C&C Landfall 35
1019

 

[Jeff_H]

Look for a boat with a good motion comfort ratio. These tend to be heavy boats with short waterline. The beamier boats may also cause a faster motion. If you get any wave action from the side (if wind and wave don't match), then a narrower boat (less form stability) will be have slower motion. Maybe, like this one: 1976 Bristol 32 Sail Boat For Sale - www.yachtworld.com

OTOH, a bad anchorage will probably be bad in any boat.

BTW, if you haven't already, join milwaukeesailors.com.

With all due respect, I suggest that you have this 100% backwards. Further, I would respectfully suggest that the motion comfort ratio will tell you absolutely nothing useful about the motion of a boat underway, let alone how a boat is likely to ride on the anchor.

And before you recommend a boat as being a comfortable on the anchor, you probably should spend a night or two or several weeks delivering one in a range of conditions like the short chop in the Georgia coast. The Bristol 32 does not offer a comfortable motion on the anchor by any remote stretch of the imagination. They pitch and roll miserably on the anchor.

But the factors which impact how a boat rides on the anchor are complex and not easy to boil into simple terms. So it is that I once again need to apologize that this is quite long. In and of itself, weight does nothing good or bad for motion at anchor, and certainly does not impact motion as much as it does underway. Neither does ballast stability, at least not as much as underway.

What is by far the most critical determinant of motion on the anchor is hull form with weight distribution a very close second. All things being approximately similar, nothing makes for a more comfortable ride on the anchor than a longer waterline length but we will get to that.

Probably the most critical motion at anchor is pitch since the bow is normally generally pointed into the wave train. If a boat has a tendancy to pitch through large angles, it will fetch up harshly on its rode, the counter will slap, and it pitch through a wider arc whipping the inhabitants who are near the ends of the boat more mercilessly.

The single most significant factor which causes excessive pitch angles is a short waterline relative to the boat's overhangs. A longer water line will spread the bearing of the boat over a larger section of the wave and may bridge several smaller waves at once effectively reducing motion. A longer waterline allows the wave to be felt more progressively rather than at once and so reduces the wave impact forces as well.

Pitching motion can be exacerbated by carrying a lot of weight in the ends of the boat or having heavy spars. This impacts the motion in a number of ways. In theory, a lot of weight in the ends of the boat or having heavy spars produces a large pitch moment of interia which generally slows the accelleration of the boat at either end of the swing while increasing pitch angle. The potential for slower accellerations is often offset negatively because a large pitch moment of interia also tends to result in the boat being out of sync with the wave train making force of the the impact harder at each end of the pitch. Depending on the frequency of the wave train, this can mean even larger rotation angles and in some circumstances harsher accellerations.

But pitch moment of intertia is definately a Goldilocks kind of thing. Carrying weight too centrally and having too little pitch moment of inertia, while good for speed upwind, will result in rapid changes in direction and also a miserable motion.

Full ends (especially the bow) on a boat can also increase the apparent accellerations. A finer bow increases in buoyancy incrementally, dampening the transition in direction and minimizing the impact force of the wave. While the sudden increase in buoyancy of a fuller bow tends to snap the bow in the opposite direction more forcefully. (Cat 27's are notorious for that motion)

U-shaped bottom sections, or flat bottomed sections at the bow, can be fast, but they can be very jarring as the come down of the trough in a choppy anchorage. Again, they build buoyancy very quickly so rather than softening the blow, they can come to a sudden and noisy halt.

In terms of roll, this is another one of those 'everything in moderation' cases. Boats with extreme form stability will attempt to change direction with every wave. On a comparatively light boat with a small roll moment of interia, it will literally follow the wave face, which can be a harsh ride in a short cross chop such as from power boat wakes.

But a boat with too little form stability, will continue roll past the wave face on either side of the roll and again be very uncomfortable rolling through huge roll angles. (As was my experience with the Bristol 32). Like a boat with excessive moments of intertia, this can also take the boat out of sync with the wave making the force of the impact of each wave a bit larger as well.

Boats with too large a roll moment of inertia also tend to roll through excessive roll angles and get out of sync with the waves resulting in harsher collisions with each wave. The worst is a boat with too little form stability and too much roll moment of intertia, closely followed by a boat with too much form stability, and too little roll moment of intertia.

Its all about, "just right". In other words, some form stability, without being excessive, acts to dampen roll and so can actually reduce roll angle some while allowing the boat to stay closer to in sync with the wave face and reduce impact forces as well. Some roll moment of inertia can keep the accellerations more moderate.

A deep keel also can help by dampening the roll as it rotates sidewards through the water slowing the roll rate and thereby reducing the stored energy that might contribute to a larger roll angle. That is a win-win all around.

A boat with a lower VCG can to a lesser extent impact roll rates by resisting the roll on each side of the swing, but because roll angles are comparatively small on the anchor, ballasted stability plays less of a roll than form stability in the motion of a boat on the anchor.

That is about it except for kiting, and that is about the relative position of the wind resistance on a boat vs its keel, rudder and so on. Fractional rigs since their masts are far forward, masthead rigged boats with large rolled up headsails on furlers and the like will kite more than other rigs. Full keels tend to kite less than fin keels with detached rudders, which in turn kite less than moderate length keels with attached rudders, which are the worst of all. Long water line boats, kite less than short waterline boats. And kiting often puts the boat further across the wave train making the motion less comfortable.

Jeff

 

[PCP]

...

Probably the most critical motion at anchor is pitch since the bow is normally generally pointed into the wave train. If a boat has a tendancy to pitch through large angles, it will fetch up harshly on its rode, the counter will slap, and it pitch through a wider arc whipping the inhabitants who are near the ends of the boat more mercilessly.

The single most significant factor which causes excessive pitch angles is a short waterline relative to the boat's overhangs. A longer water line will spread the bearing of the boat over a larger section of the wave and may bridge several smaller waves at once effectively reducing motion. A longer waterline allows the wave to be felt more progressively rather than at once and so reduces the wave impact forces as well.

Hi Jeff,

I agree with that, that's why I was talking about size of the boat, well probably I should have said length but that was what I wanted to mean.

I think that nobody would have an argument about generally a ship to be more comfortable than a 100ft boat on anchorage and that one more comfortable than a 50ft and that one more than a 30ft boat.

But I don't think that pitch is the more uncomfortable movement, at least if the boat is not too small. If the boat has a really bad pitch movement probably it is time to go out of that anchorage because it is not protected anymore anyway the pitch amplitude is normally a lot smaller than roll amplitude for the same size of wave.

For me roll is far worse since you can be on a perfectly sheltered anchorage and the tide or a current can put the boat sideways to the waves and small waves that would not create any significant pitch movement can create an uncomfortable roll movement.

....
In terms of roll, this is another one of those 'everything in moderation' cases. Boats with extreme form stability will attempt to change direction with every wave. On a comparatively light boat with a small roll moment of interia, it will literally follow the wave face, which can be a harsh ride in a short cross chop such as from power boat wakes.
....
Its all about, "just right". In other words, some form stability, without being excessive, acts to dampen roll and so can actually reduce roll angle some while allowing the boat to stay closer to in sync with the wave face and reduce impact forces as well. Some roll moment of inertia can keep the accellerations more moderate.

..

Jeff, here you seem to be talking about an anchorage where waves of significant size can be experienced. I think that most people chose anchorages where there could be some wave motion but always very small waves, otherwise it would be better to leave that anchorage.

In that sense I don't understand why you say that form stability without being excessive is a good thing to prevent roll. I mean the more form stability you have, the better to prevent roll, in the small waves you will find on an anchorage and form stability means beam. That's why in what regards that a cat will be better than a monohul and a beamy boat better than a narrow one, all other factors being the same.

That's what these testers are saying when they show admiration on how comfortable a Lagoon 39 cat is on anchorage compared with a similarly sized monohull:

"It was a delight at anchor, being far less prone to rocking about in the swell all night"

Regards

Paulo

 

[barefootnavigator]

"It was a delight at anchor, being far less prone to rocking about in the swell all night"

Unfortunately sailing a lagoon 39 is allot like sailing a dock. Sorry Paulo but I had to

 

[PCP]

"It was a delight at anchor, being far less prone to rocking about in the swell all night"

Unfortunately sailing a lagoon 39 is allot like sailing a dock. Sorry Paulo but I had to

Well, not my type of boat but that does have nothing to do with the subject, I mean the sailing ability of the Lagoon 49. What is true for the Lagoon in what regards comfort at anchor is true for any other cat with the same beam or bigger and there are some really fast out there:

Regarding the Lagoon its performance in what regards sailing is only not good upwind, as in most of the cats without movable lateral spade foils but it is an honest one in all the other points of sail, as good or better than most modern cruiser monohulls (not performance cruisers) of the same size and better than an old cruising boat.

On the last ARC a Lagoon 44 has done very well and finished among the faster boats, better than many performance cats. You can have more information about the new Lagoon 39 here:

http://www.sailnet.com/forums/boat-review-purchase-forum/62341-interesting-sailboats-565.html

Regards

Paulo

 

[PCP]

.. Rolling is side to side.

Yes, rolling does depend on metacentric height. Greater height means the boat is "stiff", hard to heel and snaps back up quickly if heeled. this 'snappish" motion is hard on cargo, and can be hard on crew, in a seaway. At anchor, usually you are somewhere sheltered.

Less metacentric height (aka "GM") means she is easier to roll, and takes longer to roll back to horizontal. Easier on cargo. Easier on crew too since it's a slower roll, except the increased heel which makes it a deeper roll.
...

Interesting comments and I think you are right but you are not taking into consideration beam.

Not for you, that I am sure know all about it, but for the ones that are less technically minded let me explain that GZ relates with an arm created by heel and that the weight of the boat over that arm gives the righting moment of the boat at a given heel angle.

Regarding cargo ships you are absolutely right. The ships have a narrow beam regarding their length and the cargo weight is a significant part of a completely loaded ship and therefore the position of the load is very important in what regards ship safety. Regarding that, contrary of what many would think a too low CG on a cargo ship (by loading all the heavy cargo as down as possible) is not what is best precisely due to increased rolling problems. But on a cargo ship beam is nothing that can be changed on an existent boat so the GZ is altered only in what concerns the position of CG, when you load a ship.

But the GZ is dependent not only of the position of the CG but also dependent of beam. Beam is even the more important factor regarding small angles of heel, the one that you will experience on an anchorage.

The relation between length and beam varies much more in what concerns the design of sailboats then in what concerns the design of ships and a sailboat can be stiff not due to his specially lower CG but due to his bigger beam and in that case what you say does not apply in what regards an increase in rolling but will certainly apply to a narrow but stiff boat that will get that stiffness mainly due to his big ballast ratio or very long draft, meaning, a very low CG.

A paradigmatic case is the one of the multihulls, with a huge GZ (3 times or more bigger than the one of a monohull) but with a high CG compared with a monohull.

Regards

Paulo

 

[Jeff_H]

Hi Jeff,

I agree with that, that's why I was talking about size of the boat, well probably I should have said length but that was what I wanted to mean.

I think that nobody would have an argument about generally a ship to be more comfortable than a 100ft boat on anchorage and that one more comfortable than a 50ft and that one more than a 30ft boat.

But I don't think that pitch is the more uncomfortable movement, at least if the boat is not too small. If the boat has a really bad pitch movement probably it is time to go out of that anchorage because it is not protected anymore anyway the pitch amplitude is normally a lot smaller than roll amplitude for the same size of wave.

For me roll is far worse since you can be on a perfectly sheltered anchorage and the tide or a current can put the boat sideways to the waves and small waves that would not create any significant pitch movement can create an uncomfortable roll movement.

I think that you and I are mostly in agreement and that you raise some valid questions that I probably should have been clearer on. As I was thinking about the original question, I was visualizing boats in the 30 to 42 foot range. Your point is consistent with mine that as boats get longer, their motion comfort on the anchor tends to become more comfortable. But when we begin to look at similar length boats in the lower and middle of this range, there can be big diffences between one boat and another in terms of their motion on the anchor, and that was where I was trying to address my comments.

My comment that "Probably the most critical motion at anchor is pitch since the bow is normally generally pointed into the wave train. If a boat has a tendancy to pitch through large angles, it will fetch up harshly on its rode, the counter will slap, and it pitch through a wider arc whipping the inhabitants who are near the ends of the boat more mercilessly." was based on the idea that on boats of the size I was visualizing, anchoring usually is in reasonably protected rodesteads. For the most part if there is enough wind to produce noticable waves, the boat will normally face into the wind and therefore basically face into the wind driven wave train. And in that condition the prevalent motion tends to be pitch and heave.

Of course there are anchorages where the wave source is left over from earlier winds or where the waves come in through a breakwater or inlet, but where the winds are blocked by obstructions on shore. In those conditions roll would be the more critical motion. But in my experience, those anchorages are less typical than the head to wind and wave circumstance, and that was the basis of my comment.

Jeff, here you seem to be talking about an anchorage where waves of significant size can be experienced. I think that most people chose anchorages where there could be some wave motion but always very small waves, otherwise it would be better to leave that anchorage.

In that sense I don't understand why you say that form stability without being excessive is a good thing to prevent roll. I mean the more form stability you have, the better to prevent roll, in the small waves you will find on an anchorage and form stability means beam. That's why in what regards that a cat will be better than a monohul and a beamy boat better than a narrow one, all other factors being the same.

Regards

Paulo

I am not really talking about large waves, but even in a moderately protected anchorage you can have wave trains of a foot or more. And on a boat in the range that I was visualizing this can really result in a lot of motion. I was also thinking primarily of monohulls. With a monohull, as the form stability becomes excessive, the roll angle could be less, but the motion and accellerations can be much harsher. In an extreme case, such as a broad beamed flat bottom boat, the bottom can actually slap on each roll.

I was not addressing Cats in my comments but the extreme beam on a catamaran means that the boat is experiencing comparatively small roll angles and that the motion felt on board is more consistent with heave than roll, but occuring at a fasted rate than is normally associated with heave.

Jeff

 

[PCP]

...
My comment that "Probably the most critical motion at anchor is pitch since the bow is normally generally pointed into the wave train....reasonably protected rodesteads. For the most part if there is enough wind to produce noticable waves, the boat will normally face into the wind and therefore basically face into the wind driven wave train. And in that condition the prevalent motion tends to be pitch and heave. ...

Of course there are anchorages where the wave source is left over from earlier winds or where the waves come in through a breakwater or inlet, but where the winds are blocked by obstructions on shore. In those conditions roll would be the more critical motion. But in my experience, those anchorages are less typical than the head to wind and wave circumstance, and that was the basis of my comment.

Jeff, I anchor more than a hundred times a year always in different anchorages and I can tell you that the ones where the sea current manages to go around a large inlet and cause the waves to be in a different direction then the wind are many and I can tell you that it is very difficult if not impossible to tell the ones where this occur. Sometimes there is no water movement there when you anchor and at the middle of the night the boat starts to roll. In these cases you will have always some roll depending of factors like tide or wind strength or passing ferries. They will not cause a problem to me or my wife but they cause a problem to many. We just get used to it.

I would say that those tend to be far more common then perfect anchorages where no roll will be felt.

Regarding strong winds what I do (and most do) is to approach the boat from the shore as most as I can. Even with 35ft winds I don't have any noticeable pitch in my boat (41ft). I admit that can be different in a smaller boat, specially if it is a lot smaller.

I am not really talking about large waves, but even in a moderately protected anchorage you can have wave trains of a foot or more. And on a boat in the range that I was visualizing this can really result in a lot of motion. I was also thinking primarily of monohulls. With a monohull, as the form stability becomes excessive, the roll angle could be less, but the motion and accellerations can be much harsher. In an extreme case, such as a broad beamed flat bottom boat, the bottom can actually slap on each roll.
...

With lot's of wind needed to create those waves in an anchorage modern light boats just sail at anchor. The movement creates added stability and on those conditions you don't experience roll on beamy boats. With the force of the wind the waves (small) come from forward but the boat moves and sails from one side to the other and just a bit forward, enough for you to see the slack on the chain. Not a violent movement and one that one just gets used to it. Both my boat and my previous boat have done this on winds over 20K.

But probably you are right regarding pitch with small heavy boats with big overhangs even if I have no experience with them.

Regarding slap and noise on some occasions in beamy flat boats you are absolutely right but that is just noise, not movement since the movement stops almost immediately with the slap. Anyway my take on that, at least in what regards me, is that I have not a problem with any noise or movement (unless they are strong ones) that I understand. when I sleep my brain just disregards habitual sounds and movements and I only awake with unusual ones. The same happens with my wife except that she sometimes did not even awake with strange ones

Regards

Paulo

 

[mad_machine]

I have to agree with Paulo here. Unless you are sailing on lakes (great lakes excluded) then there is a very good chance that your local winds do not correspond at all with the waves that were produced 100s or 1000s of miles away.

Here in the Southern Jersey Shore. Early mornings have an off shore wind as generally the water is warmer than the land. By mid-day that will turn 180 degrees into an on shore wind as the land heats up. Unless there is a storm off shore, this is the way the winds are while the waves continue to roll in from East to West

 

[capt vimes]

I agree here with PCP and mad machine...
Last summer we anchored in a pretty sheltered bay off ischia on a salona 41... The sea was flat, well almost, not much wind, just a slight breeze but the salona rolled like hell the whole night through...
And all of this motion was coming from a small wavetrain, not higher than a couple of inches probably which came in abeam for the whole night...
It was really uncomfortable and i think that the frequency of the waves was just wrong and caused a sort of resonance with the boats own roll characteristic...

I also never felt pitching as a major problem at anchor, at least not on 40+ boats and if it starts pitching violently, it is time to get your head out and check anchor, wind and conditions in general...
Just my 2 cents...