I have to admit I don’t know how to read the graps that accompany a given boat characteristics, optimal paddler’s weight etc…
I hope I’m not in the minority…many would probably benefit from a primer on how to interpret these graphs.
For example the graph at the link below…if I did not read some of the concluded interpretations I would have no idea what to make of it.
http://farm3.static.flickr.com/2660/3968434007_07e005a010_o.jpg
charts
I’m not very knowledgeable about these charts either, but I’m in the market for a new boat and would like a primer as well. That hull shape looks like my Chatham 17 with the flatish bottom and full chines. Looks like it cruises most efficiently at 4-4.5 kts and then the thrust (lbs) needed to go faster goes up exponentially. I’m assuming it’s empty. If it is a chatham I can attest to the bow wake it puts up.
If Im reading it right, the 150 lb person with 100 lb cargo can lean 90 degrees before going over. I guess the cargo really keeps the CG low. This is of course my novice interpretation of this chart and I’ve definitely seen more complicated ones. Wish all companies had a standard chart sys.
it’s something to look at
like catalog photos are something to look at. I think a Youtube clip showing a drummer beating on various parts of a kayak for a drum solo would be equally useful. Seriously.
Chatham 16
The chart is for the Chatham 16. The very top of the curves shows where the boat begins to loose stability and counter acting righting forces are needed. (J lean, or brace). If unchecked the boat will continue to go over and capsize. Most boats are designed to handle about a 25 degree lean before this occurs. In the case of the 150 lb paddler with 100 lbs of gear that seems to improve to 35 degrees. The foot lbs of force indicates how much force is required to get the boat to heel over to i’ts tipping point. The Chatham is a very stable boat according to these charts. You could probably eat lunch in three foot beam waves with impunity.
Well
You probably have issues with the “Righting/Heeling” graph 
Here is one way to look at any of these curves - you put kayak to that angle of heel and measure the force that prevents that kayak from rotating around it’s long axes. Since rotation is involved, torque, or foot-pounds, is used.
If you look at “3” curve the torque goes linearly to ~40 degrees, levels off for a bit, and then drops. So, if you ware sitting in kayak, it would require you to apply increasing degree of force to heel it up to 40 degrees. Then you would have to ease off otherwise you’ll flip.
It is a bit hard to do apples-to-apples comparison with this graph since weights don’t quite add up - “1” is 150lb paddler, “2” 200lb p,“3” 150lb p+ 100lb l, “4” 200lb p+ 100lb l. Additionally, the paddler load is not very well specified. But, some conclusions can still be made. If you are 150lb paddler, adding 100lb load will make your kayak harder to edge, or more stable if you will. Same for 200lb paddler. Both paddlers would probably say that primary stability, whatever that is, goes to probably 20 degrees for unloaded boat. The 200lb paddler might think the boat is wobbly, unstable, or very responsive
The 200lb paddler +100lb load would feel change in primary ~10 degrees earlier than 150lb paddler with 100lb load.
I don’t remember ever seeing a stability graph in SK mag where 150lb paddler + 100lb load was not the most stable, but that might tell more about my memory, than about stability data.
What does it actually mean? If this is the boat you are considering, you should definitely demo it
Read This
I agree with LeeG. However, you can glean some interesting things from such charts. I suggest reviewing the following site for an explanation of what stability curves mean as described by someone whose day job is designing, building, and paddling (very well) kayaks.
http://www.guillemot-kayaks.com/guillemot/information/kayak_design/kayak_stability
the charts are fun
They’re entertaining to read after paddling the boats in question then comparing their charts. The only problem is that it’s only one aspect amongst many that make up the whole. How one boat handles waves off the stern or wind off the beam and at different angles of lean aren’t in the picture but make up a significant part of the boats handling.
Basic chart info
Basically, the higher the curve, the greater the initial stability.
The faster the slope comes down the less is the secondary or… the more the slope goes off to the right as it comes down, the greater the secondary.
A slope that goes way up but right back down would have high primary and low secondary.
Here’s a question: could you have a kayak with low primary and very high secondary? Yes, for example if you had a kayak that was 16" wide in the water but the sides curved out up high above the water about a 16". Generally it never occurs because the sides would have to be higher than the width.
nice info so far
I like Jay’s general explanation.
FYI the chart is of a 17 foot kayak.