swedeform+skeg= :(

I recently did some research. A swedeform hull creates laminar flow disruption at the rear of a kayak (as opposed to the front on a fishform hull). The skeg creates lateral resistance by going down into uninterrupted water to counteract the disrupted water at the stern.

Does this explain why the short skeg on my swedeform Valley Etain feels almost useless?

On my fishform Valley Club RM, the skeg which could extend down 12" and 90 deg could make the kayak go from weathercocking all the way to leecocking.

It just seems like heresy to add a rudder to a Brit style kayak!

Fish vs. Swede
Hull designs are surprisingly complex systems. Some good discussion here:


It may not answer your question directly, but you’ll see that many factors beside fish form or Swede form factor into the tightness or looseness of the stern. I haven’t paddled an Etain – it just may be that the design as a whole has a relatively locked-in stern, such that skeg or rudder won’t have as noticeable an effect compared to a hull with a looser stern

Phil, this seems questionable.
“A swedeform hull creates laminar flow disruption at the rear of a kayak (as opposed to the front on a fishform hull”

There is laminar flow disruption behind the moving hull with any common hull design, fishform or swedeform. For faster, easier cruising, swedeform extends the bow for more efficient parting of the waters, while shortening the stern a bit because the flow behind the middle of the boat is disrupted anyway, and trying to help it return smoothly is a misinvestment.

In my whitewater boats, swedeform has meant the bow is more “locked in” while the stern is freer to wander. Fishform means the bow is less locked in, and I can change course at the bow without having to lean back to loosen the bow’s bite on the water.

I haven’t dealt with skegs, except when I owned a 27’ single scull. It was strongly tubular and hence inclined not to turn, but had a big metal skeg under the stern portion. My particular single had a tiny skeg under the bow portion. I think that suggests that your kayak with the little skeg might have benefited from a bit more.

g2d is right, the laminar/turbulent transition will depend on a lot of different factors, but the Swede-/fish-form distinction is not going to be relevant, in my opinion. I don’t imagine a laminar boundary layers exists for more than a foot or so on any hull, although I don’t think anyone knows this for sure.

too simplified
The flow becomes turbulent within a couple feet for any of hulls.

Transition to turbulent flow may be determined using dimensionless Reynolds number, read about it here - http://en.wikipedia.org/wiki/Reynolds_number

Turbulent flows can be defined by Re>10^6 ( 1,000,000 )

Re=(speed)*(Length)/(kinematic viscosity)

For water 1/(kinematic viscosity)=10^6

(speed) may be taken at 1 m/s ~ 2kt, then the flow will become turbulent at around 1 meter, 3 ft.

Still too simplified

– Last Updated: Jun-07-13 12:41 PM EST –

The data you reference is correct --- but only for steady flows. From personal experience, it is difficult to reproduce these results unless you have a very finely tuned flow apparatus, with well-conditioned, low-turbulence inflow.

A kayak hull is an inherently unsteady flow regime. While being propelled, a hull experiences unsteady motion in all six degrees of freedom due to the asymmetric application of paddle forces, in both time and space:

surge - there is generally significant variation in forward velocity during the paddle stroke.

heave and sway - there is vertical and horizontal motion, although this motion is less significant (but still not zero).

yaw, roll and pitch - the hull rotates in all three planes, to varying degrees.

All of these motions will likely trigger turbulence earlier. Locally, some of the motions may result in re-laminarization of the boundary layer, but not for long.

In any case, the parameters for steady flow will act as an upper limit for the transition to turbulence. To all this, add the fact that the water you're paddling through may already be turbulent due to wave action, local eddies, a wind-driven boundary layer, etc. and things obviously become quite hard to predict.

For example, for a heavily yawing hull (ignoring all the other unsteadiness), I would guess that the transition to turbulence happens immediately at the cutwater, alternating from side to side, with regions of separated flow following, etc.

So, as a rule of thumb, you can make the calculation given by suiram, then multiply by a factor to account for unsteadiness and water conditions. That factor will probably be in the range of 1/3 to 1/10 (or zero, in some cases).

oh yes
You are absolutely correct!

My back of the envelope math simply shows that the flow will most likely be turbulent before the cockpit area. In both swede and fish form boats skeg is, typically, behind the paddler, around stern hatch area, it is sitting in turbulent flow.

Simply blaming ineffectiveness of skeg because of turbulence due to this or other hull is a gross simplification.

It is more likely than not that the boat is not trimmed properly. OP might benefit from repositioning the seat.

Ah yes,
…the original post, I forgot about that :wink:

Your point about trim is probably well taken. There is no need for a skeg to extend into non-turbulent flow for it to function.

Still confused
Thank you all for your very detailed and technical replies.

It appears that the hydrodynamics of kayak hull design is more complex than just length, width, rocker, skeg/rudder, fish or swede form.

Without resorting to a quadratic equation or doing testing in a water tank, I’m trying to negate the tendancy to weathercock. I’ve moved the seat as far back as possible and I even placed once a 30lbs bag in the day hatch without much effect.

I was out yesterday on Lake Ontario in moderate wind and waves. The Etain is really stable and maneuverable. It just needs a lot of edging and paddle inputs when being broadsided by wind and waves. I guess this is just a quark I need to get used to. The skeg still has yet to prove itself as wether up or down it’s impact was not noticeable .

The Aquanaut was amazing at tracking but so so for quick direction changes.

I guess you can’t have it all!