Stability as a function of ???

I’m playing around with Kayak Foundry software and I think I got the gist of it. One thing that it does not calculate it seems is the stability of the boat.

So, my question is how is stability (mostly secondary secondary) affected by boat length? Also, is stability affected by rocker?

For example, if a 12 foot boat is 21" at the waterline will it be equally stable as a 17 footer at 21" waterline? Same paddler, same displacement, same general hull shape (round-ish bottom, full bow and stern for max waterline - looks much like Epic 16x or 18x or Kayak Pro Nemo/Marlin).

More length means more stable,
other things being equal. I don’t know the “rules” for the effect of rocker, but it would seem that if you add rocker and pull the ends of the boat out of the water, you would be reducing stability. Yet I have high rocker boats that are quite stable.

just a guess
I’ll be curious to hear what the boat design experts say.

If I can remember my engineering basics my guess would be that stability is proportional to the distance from the hull centerline to the centroid of the geometry of the hull shape…times the surface area of the hull. The centroid is just the imaginary “center of gravity” of the geometric shape (which must change with lean angle unless one has a flat bottom boat).

For example if you had a rectangular boat (with a sharp vee hull) 2 feet wide and 12 feet long then the hull surface area is 24 square feet and you’d lean on half the area (12 sq ft) and the effective “center of pressure” when you lean the boat would be 6 inches away from the hull centerline but as the hull shape gets more like a real boat then the center of pressure moves closer to the centerline of boat and you give up leverage and stability. Those sponsons that they sell for fishermen must make the “effective” lever arm much longer and that’s where the stability comes from and they’d also add some extra surface area too which would help. In principle the 17 footer would be more stable than 12 footer since there would be more surface area to “push back” when you lean and the center of pressure would be about the same (depending on exact hull shapes). I’d think that rocker would just decrease the effective surface area a bit but the effect should be relatively small since there’s only a small effect on surface area and the effect works against a short level arm since the nds of the boat have small surface area.

Make sense? I hope I pass the test; I’d be happy with a B.

Increasing rocker would have the effect

– Last Updated: Nov-15-08 6:55 PM EST –

. . . . of lowering the center of gravity of the boat. All else being equal; padler weight, length, beam; more rocker wil lower the seat closer to or even below the waterline. This will make you feel more stable.

Center of bouyancy
Stability is a function of how the center of buoyancy moves in relation to the CG with heal angle.

I’ll have to read about these…
My presumption that displacement is the same, width at the widest is the same. The difference would be I suppose that the short boat will go deeper in the water, while the longer would sink less. Not sure how this would affect stability when off-center though…

can you explain
the center of buoyancy concept?


– Last Updated: Nov-16-08 8:01 PM EST –

Here's what I Googled that I think explains a lot:

The Guillemot article is especially useful I think in explaining the concepts. That said, I still have not quite put my mind around the exact relationship, relative to my question because the center of gravity in a shorter boat (which sinks deeper than a longer boat of the same displacement and width) sits lower than the center of gravity in the longer boat for the same paddler. Thus, the fact that the center of buoyancy of the shorter boat is lower (bad) is offset by the fact that the center of gravity is lower too. However, I think a longer boat will end-up more stable, just not sure how much exactly...

Can someone roughly predict how wide a 12 footer needs to be to be as stable as a 19 footer that is 21" wide (assume similar hull shapes and same displacement and paddler)?

stability is in the cockpit
Guillemont has excellent software. Please consider how with practice you can learn to paddle a very tippy boat that seems humanly impossible. There are people who paddle a west side thunderbolt(round bottom and 15 in waterline width) in ocean races such as blackburn. It is about leaning on the paddle. The racer, boat and paddle are one. Because the paddle is over 6 ft long it is as though the boat is 6 ft wide. Never underestimate the ability of the brain to be stretched and do what seems scientifically impossible. Each second is divided into many parts. It is like a centerfielder who sees the ball off the bat and runs to a spot on the warning track. As the last instant he sticks out his glove and YES he caught it.

Imagine the exhiliration of a fast narrow boat having gone thru huge waves with all that turbulence. My 20 in wide eft has gone thru big waves mcadoods

The software might be able to compute
an objective value for stability with a given seat height, but the stability for you does depend on your weight and height. Even then what you are used to has a huge effect on the perceived stability.

A 23 inch wide boat might feel like a barge to a surf ski guy, but might be tippy to someone coming from a 30 inch wide SOT.

If you have paddled boats similar to what you want, their hull shapes should give you an idea of what design will make you smile. Or pick a kit design that might be close to get your feet wet (CLC or Pygmy).


OK - let me be clear
I want to compare stability vs. length for a hull that is optimized for speed in calm conditions at either length. Meaning round bottom, full bow and stern, little rocker.

Take me out of the equation, please. Assume the center of gravity is a dead weight, fixed at something like 2 feet above the seat -;). A little lower in the shorter boat.

Given that, how much wider, if any, would the short boat need to be to have the same secondary stability as the longer boat? Also assume similar above the water profile of the hull, so that when leaned it creates similar buoyancy increases in either boat, provided the widths are equivalent.

Again, due to the lower center of gravity and higher center of buyoyancy of the shorter boat, I can see it may need to be a little wider to compensate. But since I am not talking a 7’ boat but more like 12-14’ compared to 17-19’ feet longer one, it is not immediately clear to me without actually doing the math if I indeed need to increas the width or the lowered center of gravity would be enough to offset the higher center of buyoyancy… If someone has software or experience to tell me where the cutoff might be, please do.

Kayak Foundry
I’m surprised that Kayak Foundry doesn’t calculate stability as it is a fundamental issue in boat design.

Maybe your not looking for the right thing. Do your see “righting moment” anywhere?

Looking at the web site:

I see stability listed as the first bullet.