Re: Tippo Meter

Skis are basically unstable. There is no righting moment on a ski shape once it starts to heel, so it will capsize without a live paddler

That's the idea of having a 'heavy' weight in the seat to be 'the paddler'!

An interesting study in physics , hopefully it wouldn't be construed as a solid basis for chosing a ski.

I wouldn't suggest that, as nothing can beat paddling various ski's, but it could provide more information when comparing similar skis and if a newer model is 'around the corner' that has 'better stability' , perhaps it would be worth waiting.

Can someone explain the concepts of 'primary' and 'secondary' stability? How does this translate into what the paddler experiences on the water? Is it better to have more primary or more secondary stability, or is that paddler dependant? Does anyone have examples of which boats display more/less characteristics of each?

Primary stability is the stability that holds the boat up while it is near perfectly upright. Once you lean over somewhat, secondary stability is the stability that resists you leaning over more. Many surfskis have a lot of secondary stability such that the boat holds you up as you lean over more and more. For example, the Mako6 provides a lot of stability as you lean over more (even after the gunwale is under water, it is still pushing you up), but is a bit twitchy while you are perfectly upright. I had a solo canoe (known as the "Spencer Eagle") that has a good deal of initial (or primary) stability, but once you lean over a bit it no longer seems to hold you up (and you will tip in if you aren't careful).

In scientific terms, there is a certain force that the boat exhibits which resists rotation. You could draw a plot of this curve for the boat at every angle. There are software tools to give stability analysis of hull designs, but I don't have any experience with these.

A boat that has a perfectly circular cross-section can have good primary stability, but poor secondary stability. This is because the centre of bouyancy doesn't move when the boat rolls. An example might be a sprint kayak (near circular cross-sections), or a rowing shell.

A cross-section that is more oval or tending towards rectangular can have the same primary stability as above, but much better secondary stability because the centre of bouyancy will shift as the boat rolls. Most surf-ski's have a flat spot on the hull that creates more secondary stability. So this is something visual to look out for when comparing boats. Having said that, its virtually impossible to assess a boats stability by just looking at it. There are one or two other variables that come into the stability physics that require accurate measurement. So you should always paddle the boat, before passing judgement.

On a slight different note, if a circular cross-section is unstable why use it? Well, it minimises the wetted surface area which means more speed. This is not always the case, but as a general rule of thumb holds true. Stability versus Speed is basically the bottom line. Of course, everyone wants the fastest boat with stability. This is not impossible, but not easy either. Also, the human element makes it a classic design problem where there is more than one possible outcome, depending upon the strengths and weakenesses of the paddler. Yes, it can get a bit technical, but fascinating also. The current generation in surf-skis are seriously pushing the envelope in this department.

Thanks for those replies. I paddle a Mako 6 and Thomas's comments resonate with my experience.