Of swings, roundabouts and stability in kayaks.

[John Anderson]

I was out paddling with Phil at Swansea channel and we got to talking about stability and particularly why would you choose a kayak with low stability. It's a very good question and I thought I ought to answer it more fully here, from my rather limited understanding. If someone with a better understanding of the subject would care to comment, that would be most welcome.

Lets start off with the thesis that we want to use edging and leaning to assist in boat control and that these are good things to be able to do in rough water. I don't have any data for the stability of our magnificent local Mirage kayaks, but I do have some for the Valley Aquanaut, which feels similar to me, at least as far as primary stability goes although the overall profile is probably quite different. Lets have a look.

The graphics ( shamelessly copied from Sea Kayaker Mag in the interests of paddler education) show the righting moment against the heeling angle for the Aquanaut, Nordkapp LV and the Tahe Marine Greenland. There are four curves as the stability profile depends on the load, which in turn has an effect on the waterline height, centre of buoyancy and waterline length and beam.

The higher the curve goes, the more force is required to tip the boat to that angle. The initial slope of the graph is an indicator of primary stability, the steeper it is the more primary stability the boat has. After the graph peaks, secondary stability kicks in. The flatter the curve after the peak, the more secondary stability you have.

Now lets compare the Nordkapp.

Curve number 2 is pretty close to me without a load. As an empty day boat, the Aquanaut is clearly more stable. The initial slope of the curve is not very steep so no great effort is required to start edging it, the maximum righting moment is about 16 foot pounds at 25º of heel, but the righting moment remains positive out to about 45º, so you're still getting some support cranked over.

The Nordkapp on the other hand has a very gentle slope initially, and a maximum righting moment of about 3 foot pounds and going negative at about 25º. This means that the boat is almost effortless to edge and that if you want to edge it hard then you may need an element of bracing in your stroke.

Now lets look at curve number three, which is roughly me with a couple of weeks supplies. WOW, it looks like two different boats. The slope of the curve for the Aquanaut is now far steeper, if you want to edge it 40º you're going to need to apply over 70 foot pounds of force to do it. You'll also still have about 25 foot pounds of righting moment working to lift you out of the water when the boat is lying on its side. In other words it's now harder work to edge for control.

The Nordkapp on the other hand now has very comfortable primary stability, very predictable secondary stability, and yet it requires much less force to edge it. It also has the longer waterline length of the two, less wetted area, and a lower drag coefficient, that all adds up to a very efficient expedition boat and a day boat that is going to really polish skills. (Especially rolling skills I hear you say ? :) )

Is the Nordkapp really unstable though ? Lets take a look at another boat that is a striking looking boat too. If Darth Vader was going to paddle a kayak, this would be it. The original Stealth Kayak, the Tahe Marine "Greenland".

Where did curve number two go ? Hmmmm, the legend says "all values negative"........... Suddenly the Nordkapp's looking pretty stable ! It really is a case of swings and roundabouts. Stability is nice sometimes, and effortless edging and rolling can be nice too, but the rougher the sea gets, the nicer that effortless edging and rolling becomes.

The Inuit in Greenland designed their boats to handle well in some of the roughest seas on earth and they chose not to give them high levels of stability. It wasn't accidental, kayaks built in other areas were very different. They chose to build them to suit the conditions and they had many generations to get it right. There were other factors that they obviously felt were much more important than stability. Personally, I think the Nordkapp is a pretty good compromise for what I want to do. A high efficiency, high performance load carrier, that is responsive and lively enough to be a fun day boat as well.

Thank you Sea Kayaker Magazine, for the graphics, an invaluable resource for us paddlers.

[John Anderson]

Hi Owen, thanks for that, glad you found it interesting.

We really are spoilled for choice these days aren't we with something out there to suit everybody. You may well be right about the lack of Mirage reviews too :)

The Mirage is really a good example too, that a long skinny fast boat with a very efficient hull, does not necessarily have to require the balance of an olympic gymnast.

[Owen Walton]

John, With my tongue firmly planted in my cheek, I believe that the reason that Sea Kayaker has not got the figures for the Mirage 580 (or any of that marque's magnificent stable) is that they move too quickly for them to catch and measure them.
The other interesting factor from the SK analysis relates to speed. I won't go through all the technical detail, (as it is all their site,) but one of their general conclusions for us ordinary paddlers who want to maintain a reasonable, sustainable cruising speed is: "Paddlers who spend most of their time at cruising speeds of 3.0 to 3.5 knots (5.5 to 6.5 Km/hr) should focus their attention on boats with a low wetted surface and low Prismatic Coefficient". Whilst we can all go a bit faster than this, even whilst "cruising" on open water, I reckon 6.5 Km/hr is a good "smelling the roses" speed that even an old bloke like me can maintain for mile after mile.
I guess in summary, if you are a mere mortal and want a good cruising vessel, that has ALL the desirable attributes of good handling plus sustainable cruising speed, don't go for a long, skinny, tippy "racer". We are not in the class of Freyer or Clint.
I recommend all should read the article, understand the stability and speed factors, before choosing your next boat. Then all you have is: weight Vs cost, poly Vs composite, skeg Vs rudder, storage space Vs compactness, Aus built Vs import, etc etc. Then we have paddles: Euro Vs Wing Vs GP etc. Then there's the type of sail to choose etc, it just goes on and on. At least the subject will keep us debating and arguing into the foreseeable future.
Yet people still think they can just go down to their local camping store and "buy a kayak" - "surely a simple craft that was developed thousands of years ago can't be that complex."