I shall attempt an expaination.
In the the unschooled terms I understand, by using my two canoes as examples. The Mad River Guide I paddle is of a rounded or vee shaped profile on the bottom. Some think it “tippy”. The Wenonah Vagabond I also paddle is flat bottomed. The Vagabond feels stabile on entry and when my dog moves around in the bow. Not tippy. In rolling waves such as a boat wake the Vagabond becomes a handful as that flat bottom tries to follow and roll with the waves. The Guide on the other hand rolls a little, then it’s secondary stability (increased hull width as the boat leans into the wave) arrests the roll (to a point) and allows me to prepare for the next wave. Much less dramatic reactions to the waves because the rounded/ vee shaped hull lets the wave action roll under it, beneath it, and as we lean a little, our stability (secondary) increases, as the hull width widens. And yes, I am prepared for the well meaning replies to my poorly articulated expose. I am no authority on these matters and realize the Innuits and Greenlanders knew more about this stuff than I ever will. Besides, I paddle a canoe.
I find
it pretty interesting actually; however, it has elevated far above the level of the actual question.
I would agree with the comment above that a beginner will not really be able to fully appreciate the benefits of higher secondary, but disagree strongly with the comment above that secondary does not "kick in" or make a difference in rougher water.
I see the level of primary stability to be a matter of taste. More primary makes some feel more secure and others feel more constrained.
I personally would prefer less primary stability as it makes the boat feel more lively and easier to throw up on edge.
However, I see no downfalls to good secondary stability and think it makes a big difference in rough water. There is no question that secondary stability "kicks in" when in rough water.
In truly rough water, particularly in beam seas, if you keep loose hips as you should the boat will get tossed around quite a bit underneath you (especially if it has lower primary stability). Secondary stability will ensure that although the boat is being moved underneath you that it never reaches the point to where it has exceeded its center of balance far enough to where it is going to want to tip.
Of course the boat can be prevented from tipping with a brace or a hip flick, but having to constantly brace is not necessarily preferable.
Another place where it makes a big difference...if you are paddling in an area where waves suddenly peak up very high without breaking. I was paddling in the Triangle at Tybee Island last summer on an extremely rough day. I got caught and lifted by several of these waves. Problem is that in a boat with less secondary stability is that if you get thrown a bit off balance by one of these sudden waves, you have nothing to brace off of as the backside of the wave is so steep that your paddle can't reach it to brace. Secondary is very nice in this situation so that you don't get off balance in the first place.
Finally I find it nice in situations where you you get side surfaced by a fast moving wave. I find that staying upright in these situations requires a bit less bracing effort, and it is nice when the wave has passed under you and you start to lose the support of your brace as it is easier to stay upright. We've all been in those situations where we do not readjust our lean fast enough, our brace support goes away...and you end up having to roll.
In short...secondary stability is certainly an assest in rough water regardless of your paddling ability. Less secondary stability offers no advantage. Primary stability on the other hand is largely a matter of preference I believe and there are advantages and disadvantages to more or less primary.
Matt
Point Well Taken
If you ask a boat designer to explain “secondary stability” you will get a precise explanation of an objective characteristic of a hull which can be calculated and graphed. If you look at SeaKayaker Magazine or the websites for several of the designers/sellers of kits you will quickly find out what “secondary stability” means from an objective, technical perspective.
If you ask a paddler to explain “secondary stability” you get at best a subjective description of what traits that paddler believes makes a hull better for them in rough water. Not surprisingly, different paddlers like different traits, and the relationship between “secondary stability” as designers use the term and “secondary stability” as paddlers use the term is tenuous at best. For example, the Nord LV and the Explorer are boats which have reputations for being very good in rough water, but their stability curves are very different. You will see reviews of boats saying the boat has very good secondary stability when the stability curve shows that from a technical perspective, it has almost none.
So, yes its a little like Alice in Wonderland and the Queen of Hearts.
Good explanation NM
Re: Flat Bottomed raft.
Great descripton!!
re: Not more stable
My.02
You need to adapt your abilities to the boats capabilities.
(Nee; Clint Eastwoods famous “Man needs to know his limitations”)
I went from a flat god only knows how wide perception america to a 23" Artic Tern and it took a year of paddling to get totally comfortable (wakes, and chop and quartering winds etc.etc.) “Adjusting my abilities to match the boats capabilities”
stability
well thaks to all for the indepth answers to my questions,i feel that i will be able to make good informed decision on my purchase of a new kayak. since i am sort of a nubee to kayaks i do understand a lot better now on the benifits of primary and secondary stability and feel that i would rather have the benifits of a boat with more secondary than having a boat which would not be able to push the limits of what kayaks were made for.
tim
Yes, boat follows path of H2O Molecule
Yes, a boat and its contents/crew floating on deep water waves experiences the local gravity of one of the wave’s water molecules. A paddlers feeling of “down” is perpendicular to the waves surface.
If you were on a fixed dock looking perpendicular to a wave’s direction (looking lengthwise down a crest) and observed an object floating, you would see the object move in a circular path as each wave passed. The waves move on by, but the floating object travels in a circle and ends up in the same place it began its trip, assuming there are no other forces like wind or current acting on the object. It is this circular motion that creates the centrifugal force which creates the localized gravity.
A person floating on waves does not sense the circular motion, however they will feel an up-and-down motion. In the trough you are at the bottom of the circular path so the angular acceleration combines Earth’s gravity to create a downward force greater than gravity alone. If the waves are large enough you can feel the extra weight in your seat. When on the wave’s crest you will feel lighter as you are at the top of the circular path.
This physics are similar to being on a high speed ferris wheel. At the bottom the local gravity will combine with Earths gravity make you feel heavier in the seat. At the top you will feel lighter and maybe even feel that bottom-dropping out feeling in your belly.
The situation while surfing a wave is different. When surfing, you are moving through the water, but with the wave. One’s motion is on an almost unaccelerated path, so gravity for a surfer is still vertically down.
Quoting Ross Garrett in ‘The Symmetry of Sailing’: “… if the boat is moving throgh the water with the waves on an almost unaccelerated path it will sink slightly into the Low-g water near the crest and float higher in the high-g water of the trough. When a boat starts surfing down the face of a wave near the crest, the water beneath it is effectively lighter, the boat sinks lower and much “light weight” spray is thrown up.” Anyone who has done some surfing has experienced this effect.
comfortable on edge
The important thing is: are you comfortable with your boat on edge?
To determine this, don’t listen to what anyone else says about the boat. You’ll hear lot of platitudes about “well-mannered” this, “reassuring; solid secondary stabilty “ that , etc. If you can, before you buy, try a boat out. See if you fit in the cockpit well, with good contact where you need it (Thighs, hips, butt, feet, not too loose, not too tight ) Then put the boat on edge and keep it there as long as you can. Paddle forwards and backwards keeping it on edge , do a few turns. Some boats will feel more comfy than others while on edge . Don’t listen to what others say. The boat in which you feel comfortable while on edge would be a good boat for you.
Secondary
I don’t quite agree that secondary is something hard to put your finger on. Having owned a lot of different boats…Avocet, Aquanaut, Explorer, Romany, Greenlander Pro, Nordkapp LV and a Chatham 16…I think that secondary stability is very tangible and can be easily and immediately assessed the first time you sit in a boat on flat water.
I think that the thing that may make it different for different people is not so much its subjectivity but rather their physical differences. At 5’8 and 200 pounds a boat’s stability profile will different to me than it will for someone who is 6 foot and 175.
I think it has a lot to do with body weight and distribution of that body weight. What has good secondary to me may have poor secondary for another due to our physical differences.
Matt
Yeah,
I think I need to go paddle to clear my head. Whew!
Apples AND oranges - not either one
J, everything you say about the wave dynamics is so - the circular motion, acceleration, local gravity, whatever. But that’s just part of the picture.
When considering the circular motion and it’s relative gravity - the ability to keep vertical over the kayak is critical to being able to just ride along with the energy in the circular motion as you describe. You still need to keep the total mass centered over that imaginary dot going around the circle.
A log, carcass, or clump of seaweed does as you say, it just rides along. An empty kayak does so too - and will also generally stay oriented with hull to the wave face. A system of two things in the waves - like a kayak with a lot of buoyancy and a kayaker with a higher COG - and the articulation/interaction between them - and it introduces more factors.
When you then add in the kayak/kayaker on the wave face - the hull tilting on the wave face (as it goes up and down around the circle of water motion) ***makes it harder to keep the kayaker’s COG over the kayak’s COB - this is what those drawings are about - NOT wave particle motions!***
The circular acceleration’s “relative gravity” will not hold the kayak to the wave face (if it did you couldn’t surf!) or help the paddler keep their COG over the kayak’s COB. The net effect can be just the opposite.
As the hull tilts with the wave face the kayaker’s mass is going to be shifted over to the side relative to the upward acceleration. In a wider/flatter kayak it is is harder to tilt the kayak back more vertical into the wave and get it’s COB back under the paddler’s COG so both can ride up on that acceleration (or down with deceleration) force together as a single unit. Again, this is what those drawings show.
Too much primary stability can make it harder to “keep it together” on the wave.
Real world my flatter 28" wide Tarpon 160 pitched a lot more and with less ability to do as much about it as I can in my 21" wide Q700. Granted, it could ride out most stuff, and could be leaned to decent enough effect, but when if flipped it flipped hard - in stuff my QCC wouldn’t be close to flipping in as it is easier to edge and keep under me, offering better control. Overall my narrower and rounder overall SOF (even with the hard chines) gets rolled around less still. Easier to stay vertical over the kayak and just go up an down with the energy.
A simpler way to say it: The easier a kayak is for the kayaker to roll at will, the harder it is for a wave to roll the kayaker against their will (also a main reason to learn to roll!).
Sailboats are bad examples as they act as one body on waves, not two (which are much more top heavy even when in sync and acting as one) - and are also designed to maintain a heeled orientation going through waves while maintaining a good degree of equilibrium vs. constantly alternating heel to maintain COG over COB to avoid capsize as a kayaker in waves does.
also Diesel and Meridian 
Matt, I think my first recall was when you were on the Left Coast with your Meridian, before you bought the Aquanaut. Also when I started looking at planing hull boats you told me about your Diesel
Feels to me…
Not being an engineer or master of fluid dynamics or advanced paddler, it seems to me that I can feel the ‘secondary’ of a boat when I edge it and/or paddle it in dimensional water.
My terming it ‘secondary stability’ may be colloquial, but it feels very much to me the way Matt describes it.
Not to beat a dead horse, but
it occurred to me that all the discussion has been about stability in rough water. There is another reason for good secondary stability, and that is edging the boat while turning or moving abeam. Sea kayaks especially are designed to track, which makes them harder to turn. But if you edge the boat, it brings the ends out of the water, shortens the waterline and makes the boat much easier to turn.
When I’m testing a boat, I’ll do a series of turns (high & low brace, duffek/bow rudder) and draws to see what the secondary stability is like. Try this: get the boat moving sideways with a hip or sculling draw, then edge away from the paddle. You will notice a marked increase in sideways motion. Keep going sideways, alternating flat and edged positions. I like this test because you don’t have the support from your paddle like you do in turns, and you have to rely almost totally on the boat’s secondary stability.
Another Lash
This link goes to one of the best descriptions of secondary stability around for paddlers. It combines the technical aspects and the subjective aspects as perceived by paddlers.
http://www.guillemot-kayaks.com/guillemot/KayakStability
Done by someone with a solid reputation as a designer and rough water paddler.