Surfski for strip building...

Monday, May 20, 2013 (Spindrift, surfski), 21 comments

My wood strip surfski has been brewing for a very long time. It started with the Sea Racer. As the name implies, it was designed as a very fast sea racer with long distance touring capacity. I found the inspiration in Alaska and the plans were finalized during spring 2006.

I started building the prototype during the summer and had the hull and deck done when my calendar started to overflow with jobs and assignments and at the same time, requests for classes and talks (the Sea Racer parts was used for strip building demonstrations).

Other builders were more ambitious and Sea Racers got launched – almost 50 til now – and they seemed to deliver: tippy, super fast and quite able i rough seas – sometimes in unexpected contexts. Some examples: here, here, here and here.

I got requests for commercial designs, while the Sea Racer prototype collected dust in the workshop. First was Nordic kayaks. After a couple of successful multisport kayaks (Rocket and Rocket plus), they suggested a surfski for nordic waters. The Sea Racer prototype parts were redefined as test devices for surfski ideas. After some going back and forth between the drawing table and a newly built surfski prototype, the Fusion went into production.

Now requests for a strip version started coming in via mail and over the phone, and I confidently answered "Yes, working on it...". Which was true – a rough sketch was recently saved in the "bright ideas" folder in the computer.

Seabird Designs requested a line of surfskis, and the Wave line found its way to the kayaks stores.

Three new surfskis for Nordic Kayaks were next in turn, but by this time Fredrik Lindström were doing most of the actual design work. I helped in refining the software output into working drawings and worked on the visual design and decorating scheme.

...more and more questions about the coming strip surfski ;-)

Roar Berge got tired of waiting and built a surfski version of the Sea Racer.

Jan-Olof Karlsson offered to build a prototype of the surfski, from the not quite finished plans. So I adjusted the dimensions for Jan-Olof and he cleared the workshop and got started.

A new company contacted me with an idea that had been in my mind for a while: a shorter, less specialized surfski for "common" usage as a touring craft – with the best qualities from both sea kayaks and surfskis.

Then a week ago Alexandre Marcotte in Canada (he built a Black Pearl a couple of years ago) took the bold step of actually ordering and paying in advance for the surfski plans – and that, of course was the final kick. I had to clear the workspace from all urgent assignments and deadlines swishing by, and get those plans done!

So here it is, finally after six years, a strip surfski! It is of course a much much better surfski than the one I started drawing back then – I have learned a lot in those years, the surfski concept has evolved and the market has grown. It is designed as an intermediate ski – not an elite ski nor a beginner skis (even if it will be well within reach of the ambitious beginner as well). It can be built with an under-hull rudder for racing/excercise or an aft mounted rudder and hatches for touring. The dimensions are 608x48 cm and the length can be adjusted as usual during setup, with advice on the plans for hulls between 576 and 640 cm. The cockpit structure may seem complicated to build, but three suggested methods are indicated on the plans: strip, foam or carbon/fiberglass.

A shorter and wider version will be available later – in about six years... (just joking ;-)

I do not plan to include a top level elite ski. Very few amateur builders would be able to match a professionally built elite ski tipping the scale at approx 10-11 kg – and those who would, won't need my plans anyway. If you want to compete in the surfski top worldwide circus, I suggest you buy a Nordic Kayaks Nitro, or one of the other top skis.

Sea racer or surfski?

What is the real difference between the Sea Racer and the Spindrift? Or in general – what separates surfskis from sea kayaks or multisport kayaks?

Well, it turns out the only sure sign is the cockpit configuration. In everything else there are lots of crossover crafts floating around, defying categorization: sea kayaks with surfski maneuverability and surfski with the calm movements of sea kayaks and all kinds of hull configurations.

Surfskis were from the start developed to run off the wind on large swell reaching extreme speeds, and to surf the wave fronts in control. To keep the position and direction on those waves, extreme maneuverability was needed. To control the craft, precise steering and a large rudder was necessary. The stem was high to avoid diving in the gigantic waves of the native waters in Hawaii, South Africa, California, Australia – while the stern often became little more than a long tail with a rudder. The fore part was deep and narrow moving the lateral center forward to balance the large rudder, while the aft part was wide and shallow to prevent the stern from submerging in speeds close to or above hull speed. Surfskis became elite tools for tough oceans races. But eventually the market evolved to include also crafts more suited to "normal" paddlers with touring ambitions.

Sea kayaks on the other hand evolved towards strong tracking, rather than maneuverability. The designer sacrificed some maneuverablity for comfort on long passages – in automotive terms they have the soft somewhat imprecise steering with a healthy margin for errors of a family car, rather than the precise and attention-demanding direct steering of a race car. Of course some of the control is lost in the process. Have you tried to surf a steep wave diagonally in a sea kayak, you will know what I am talking about. But lately we have seen a lot of sea kayaks with almost surfski-like maneuverability and control.

A look at the lines of the Sea Racer and Spindrift reveals these differences. The Sea Racer hull is more symmetric, with LCB, LCF and CLA quite close to the center, indicating a hull that moves smoothly and predicably in the water (LCB = Longitudinal Center of Boyancy = the centroid of the underwater volume of the boat expressed as a longitudinal location. LCF = Longitudinal Center of Flotation = center of the waterplane, the "seesaw" pivot of the craft. CLA = the geometric center of the lateral plane). The seat is located slightly aft of the center, providing controllability without being overly dependant on the rudder (mildly understeering in automotive terms). The rocker is not very accentuated, which improves tracking. The kayak deck with cockpit rim/spraydeck keeps water out, allowing for a lower and less wind sensitive profile. For the same reason the freeboard midships can be lower, to allow a lower and less tiring paddle stroke on touring.

In the surfski the CLA is moved forward to balance the large rudder. The seat position slightly forward of the LCB is the key to the efficient and precise steering on wave fronts – in automotive terms the ski is slightly oversteered in low speeds, but becomes neutral in higher speeds since the longitudinal position of the hulls center of turning is related to speed.

The stations drawings show that the surfski is wider and higher than the Sea Racer. But the difference in volume and load capacity is not as large as it might seem – the higher Cp of the Sea Racer means that the volume is more evenly distributed along the hull, while the surfski has more volume concentrated midships. This suggests that the Sea Racer should be slightly faster on flat water, but the surfski should win in waves.

The shape of the sections look quite different. In the Sea Racer I wanted to optimize the intitial stability in what is a very narrow craft (43 cm), with the chines also enhancing steering control when the hull is leaned (a way to keep rudder turbulence to a minimum). For the wider surfski I chosed to minimize friction with elliptical sections below the waterline.

Software and the design process

The sectional shape is also to a degree influenced by different calculation algorithms. The Sea Racer was developed using Bearboat by Robert Livingstone for the hydrodynamic calculations. The main advantage of this is that the underbody defining vectors hang from the waterline and the keel, which makes it possible for the software to minimize the wetted surface for every set of design parameters chosen. It saves a lot of time finetuning the hull to low friction, but also means that the bottom and freeboard are two independent sets of curves, meeting at the waterline. On certain kinds of hulls this complicates getting a smooth natural curve from sheer to keel.

For the surfski I used Ross Leidy's KayakFoundry, forked from Bearboat, but with one important difference: the vectors hangs from the sheer and the keel. That means better overall control of the lines (including deck shape and stem and stern) compared to Bearboat, but the disadvantage that you have to work the wetted surface manually based on experience and through repeated iterations followed by checking the calculations. One further disadvantage is that KayakFoundry is just for Windows. I have to run it virtualized in Parallels on my iMac – it works, but on a not quite new Mac it runs noticeably slower.

Both softwares are easy to use and do an excellent job at their level, but the downside of the simplicity is the many limitations in what can be achieved (not a problem for the amateur builder, who would solve those issues in the construction phase rather than on the computer anyway). Therefore I use them for a quick zooming in on the hull shape I want, along with all the hydrodynamical data I need – getting within 95% of the final design in surpricingly short time and with adequate accuracy. From there I transfer the lines to Illustrator for the final manual finetuning, including both functional tweaking and the visual layout of lines and surfaces – which normally takes more time than the first 95% in the design software!

That may seem a tedious way to design a kayak, but the alternative – professional ship CAD design systems are very expensive and bloated with features of which I would use a small percentage (I do not need to know what happens to the ships stability when 600 tons of crude oil sloshes around in an half empty tank, nor at which parameters the risk of propeller cavitation becomes an issue ;-) and would take hundreds of hours training for me to master.

Spindrift

The surfski is named Spindrift – an apt name for a craft with an appetite for wind and waves.