A Question for the technically minded

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8 years 11 months ago #17789 by Scode
I would like to know how overall boat length affects speed. Is it a case of longer ski equals faster ski?

The reason I ask is after seeing Tim Jacobs paddling his Nelo 560 to second place in fairly flat conditions in the recent World Champs. He is giving up close to 80cm on most elite level boats doing the rounds these days.

I would think for intermediate level paddlers it wouldn't make a huge difference but at the pointy end of elite paddling every little advantage or disadvantage means a big difference in overall performance.

I guess the benefits of the shorter ski is increased manoeuvrability when chasing runs. It seems that most manufacturers have hit a "sweet spot" of around 6.4 meters for their elite craft. Is Nelo challenging this theory? Or is it just a one off experiment ski?

Cheers

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8 years 11 months ago #17791 by sAsLEX
en.wikipedia.org/wiki/Froude_number

the science of paddling series covers a lot of the answers better than i can put it www.surfski.info/getting-started/tips-ot...addling-part-iv.html

look for the other articles at the bottom of that page
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8 years 11 months ago #17792 by Scode
After reading that article by Ralph Baker I would now love to get my hands on a ski about 5.9m long but with the beam of an elite level ski at around 43cm. I guess a Swordfish at 6.1m and 45cm would be as close as it gets right now.

Cheers

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8 years 11 months ago #17795 by Kestrel
Actually, the Carbonology Atom is 5.95m and 44cm, and is still probably THE fastest surf ski on the planet, despite the design being several years old now. Way ahead of its time, and still the gold standard for speed and agility.

I think most surf ski manufacturers are up at 6.4m or so for their elite skis, not because they're faster, but because they work well in open-ocean swell for the "big guys", the prototypical 6' / 90kg bruisers who make up the top echelon of pros. In other words, they're tailor-made to win the Molokai race. But for the average "normal" paddler who isn't a pro male and is paddling flatter water or shorter swells (which is normally the case, despite our fantasies), they're actually too long and have too much volume. For most of us, they're just plain too big.

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8 years 11 months ago #17796 by mtnsutton
To simplify what is going on, with a consistent cross sectional area or beam, the hull speed of a boat will get higher as the length increases. So theoretically you can go faster in the longest boat possible but... The wetted surface area of the hull is also increased which allows more frictional area and means that it takes more power to maintain the same speed. The balancing act begins. what is the perfect length for the speed that you can potentially achieve? This is why surfskis are longer than flat water K1 race boats. In flat water there is no wave to push you to a higher speed so there is no need for the additional length.

Creativity Cures Complacency

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8 years 11 months ago #17798 by kayakchampeen
with all due respect to those who took a shot at answering the question. I'd like to point out a few oversimplifications. Firstly, the froude number mentioned in the paper only suggests a soft threshhold as it pertains to maximal speed. Most modern naval architects do not even recognize "hull speed" as a a term that has any meaning at all, although paddlers like to throw this around alot. While it is a fair generalization to say that if displacement and max waterline beam are held constant that a longer hull has lower wave making resistance at higher speeds, It is also fair to say that skis are made 21ft long so that they could be fast without being inordinately tippy. If one made a 21ft boat as narrow as a k-1 it would be completely unpaddleable. A shorter boat can be faster if the sectional shapes are more semicircular and parabolic to reduce frictional drag, which comes into play even at high speeds as the boat is accelerated with every stroke. Traditional thin-ship hydrodynamics fails here b/c it is difficult to model the roll, heave, pitch, yaw, and squat effects of a cyclical and reciprocal stroke instead of steady-stade propulsion. Most studies of displacement hulls presume a linear propulsion akin to a model being towed in a basin and miss the point entirely when it comes to high speed paddling. At some point in the stroke there is a bit of hydrodynamic lift applied to the boat that actually lessens the displacement to a degree (not planing). On a downwind run the boat is often in "semi-planing" mode (also a term with no real meaning in hull design) at this point other factors begin to affect speed more than paddle power. Smaller folks will obviously benefit from shorter boats, but I find that any boat over 20ft simply won't accelerate as well as a shorter boat with higher prismatic coefficient and less form drag, windage, and frictional resistance, even though the wave making resistance may be predicted to be less on the longer hull. Also, when the speeds are slower due to current, headwind, etc. the resistance curve is different at slower speeds and frictional resistance begins to dominate. A poster mentioned that surfing downwind may be the only time that a 6.5meter boat makes any sense at all, because the velocities are high enough to shift the resistance curve upward off the chart.
It doesn't surprise me that shorter boats can be surprisingly fast as I can keep up with V12's on flatwater (for a while at least!) in an 4.5 meter ICF wildwater boat that is suprisingly narrow at the waterline, has a higher prismatic coefficient than most skis, effectively no rocker, no rudder for added resistance, and will accelerate much faster than a ski.
That said, K1's are as short as they are b/c the rules limit the to 5.2meters long. This is why they have to be so skinny to achieve the speeds they do. They would certainly be closer to about 19ft if the rules allowed it. To state the obvious, One can make a 19ft ski much faster than a typical elite ski if it is designed to be more "K1 like". The likely tradeoffs being downwind speed and far greater tippiness. Also the nelo 560 for instance is more K1 like in it's outfitting than many ocean ski's making for greater possible application of power. I'm guessing 50-75% of the ski owners in the world are on a ski that is too long for their weight or intended application. Big boys in the big swell are the only people who can benefit from a 21.5ft boat of any kind. Until recently, all of the shorter skis were beamier and aimed squarely at intermediate paddlers. It is great to see someone designing smaller, faster skis for serious speed in other conditions and for smaller folks.
BTW Those ridiculous 23ft boats you see for adventure racing are a joke, they aren't any faster for any purpose and will soon be on the scrap heap.
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8 years 11 months ago #17801 by RedBack
^^^
What he said...

:-)

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8 years 11 months ago - 8 years 11 months ago #17802 by Hiro
The length of the ski has absolutely no effect. A red ski will make you go faster !
Last edit: 8 years 11 months ago by Hiro.

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8 years 11 months ago #17808 by Physio

Hiro wrote: The length of the ski has absolutely no effect. A red ski will make you go faster !

Black is the new red

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8 years 11 months ago #17827 by red_pepper
As a general rule of thumb, all else being equal, a shorter boat will have less waterline drag, but a longer boat has less shape drag (without going into a lot of the other details/variables). Generally the cross-over with our boats seems to come somewhere around the 4.5 mph range (where shape drag becomes more important than waterline drag and a longer boat will start to exhibit less resistance). Of course, if the boat gets too long a paddler won't have the strength to overcome the waterline drag. 21' seems to be the magic length for those resistance curves with a reasonable width for stability (for a 200 lb paddler). I own Ted Van Dusen's prototype Mohican, and here's what he had to say about the design of the boat:

Your Mohican was the first one that I made. After trying about a dozen kayaks over three years with the intent of finding one that that had a comfortable sitting position like an ICF race boat with a narrow foredeck that was about the same speed with good glide and was just stable enough to learn good technique and paddle in the winter without constant worry about having a chilling swim, I reluctantly decided that I had to take the time to design one to fill the void. I spent nearly two years working on the design to find the optimum balance of speed, comfort, stability, and seaworthiness that I was looking for. I was hoping to build a boat around 18 or 19 feet long, but was not able to get the resistance low enough at that length for my weight, so modeled designs until I reached 21 feet. I was fortunate to have excellent towing tank tests for over a dozen kayaks and design codes that I had developed using test data from rowing shells to fast slender ships.

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8 years 11 months ago #17829 by YBA/Jim Murray
I know Ted Van Dusen developed his own designs.

Interestingly, an ICF K2 is 6.5 metres long-21.3 feet. I wonder how many surfskis were developed from those hulls and adapted to ocean paddling, evolving by trial and error to the boats seen today. Paddlers not so long ago built and raced there own boats and paddles. Sometimes it was a spectacular failure :blush: , but after some time and effort a workable product was developed.
Jim

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