Kayak Speed Question

Paddlers' Place
#9

Paddling resistance is primarily the sum of skin friction and wave-making resistance. There used to be technical articles published in Sea Kayaker that plotted both types of drag vs speed for commercial kayaks, but they were not measured values. Rather, they were the result of software - this can be good or bad, but the plots were not verified by experimental testing.

Re the two main components, skin friction is proportional to wetted surface area and velocity squared, i.e. it increases exponentially in a fairly predictable manner.

Wave-making resistance is less predictable since it is heavily based on details of hull shape, but it does increase with speed too. For a given displacement hull, wave-making resistance will increase rapidly over a range of speeds depending on its shape and length. The speed range is colloquially identified as the ‘hull speed’, although there is not a fixed point where the transition happens.

The transition happens at lower speeds for shorter hulls, i.e. longer hulls have higher ‘hull speed,’ which just means the transition happens at higher speed. At a given speed, wave-making resistance can be very high for a short hull, but significantly less for a hull that’s ‘long enough’ to still be operating below the transition point. In this case, the full resistance value will be less for the longer boat, which is the point of long skinny racing hulls.

At lower speeds, when a short and long hull are both operating well below the transition to high wave-making resistance, the shorter hull generally will have less total resistance because of its lesser wetted surface.

This is just my opinion, but I think for ‘normal’ paddlers at the 2.5 to 3 mph range, a 14 ft hull can be noticeably easier to paddle than a 17 ft hull. Not always, of course. This is the point of Brian Shultz’s article which is worth a read:
Choosing a skin-on-frame kayak - Cape Falcon Kayak (capefalconkayaks.com)

Having said all that, there are all sorts of reasons to opt for a longer hull, e.g. for choppy conditions, better tracking in the ocean, the ability to paddle fast when necessary, etc.

Sorry, this turned out to be pretty long-winded. I just now checked the Guillemot article, a much more complete explanation. I’m not sure I agree with the kinetic energy argument, but there are usually multiple ways to reason through fluid mechanic analyses - I’m definitely going to think about that.

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#10

My CD EXTREME / NOMAD is easy at any speed.

#11

Wilderness Systems Arctic Hawk moves out pretty well

#12

If you have the boat and ability to travel 30 miles in a day, then for a day trip on a body of water like the Chesapeake Bay where water access can be limited in some areas, you can explore places that many people can’t get to.

Wind and waves are he enemy of time and distance.

#13

fun discussion. It would nice if there was a theoretical hull speed included in the specs of touring/sea kayaks. I realized it would be mostly theoretical, but as long as same method was used on all boats it would at least give us an idea of trade offs. I’d love to see some kind of measurement of how much faster the Stratos 14.5 is than the 12.5 or how does Stratos 14.5 compare speed wise to WS Tsunami 145 or how does same hull in roto molded plastic compare to ABS or composite. I still haven’t had the opportunity to experience a true touring kayak yet. Saving up my nickels while I enjoy my old Blackwater 12.5 on flat and my RPM on WW.

#14

Funny you mention those 2 kayaks… I’m eyeing the tsunami 14.5 and Stratos 14.5 and wondering which is faster.
Reviews I’ve read hint at possibly the tsunami. Its wider but also means the stratos will sit lower and have additional plastic in the water, esp with its rocker.

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#15

If you get a chance to actually demo both boats, I’d love to hear a first hand account of someone that’s actually paddled both. None available for demos in my area. Especially right now. My guess was that Stratos would be faster due to being more narrow, but you make a good point about displacement of hull. I’d also love to know what other boats are considered comparable to these two in class and price.

#16

I’m primarily on rivers and small lakes. I’d love to have a higher performance boat to cover some miles, but I also don’t want it to be so long that it’s difficult to get back into narrow creeks. One long boat buddy of mine told me to go long and just learn to paddle backwards, lol.

#17

Ah nice. I’m near the coast but want to use it for everything. Mostly lakes and slow rivers, but able to shoot some cl II+ rapids on occasion as well as island hop off the coast. I may end up just getting a sea kayak and a rec lake+river generalist…but for every boat i get for myself, i also need to get one for the wife.

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#18

I’m still a WW newby but based on what I’ve learned in both my rec boat and my WW boat the hull design of Stratos is going to be more forgiving and fun in current. The defined keel of WS that makes it track so well can catch a current or rock and cause a surprise tip.

#19

I have an Epic V5, 14 foot, and an Epic V7 17 foot. The V7 gets to speed easier, glides better, and is faster than the V5 with approximatly the same effort. This is personal observation. Also one of my paddling buddies has a V5 and we have done side by side comparisons. After one member of our group got a V7 and started leaving us in the dust is when I started thinking about geting one. I’ve had ten boats, that V7 is by far my favorite.

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#20

How fast are they?

#21

No need for a published spec, hull speed is easy. It’s just a brute force calculation, although you need to know the water line length of the boat (not overall length).

See Wikipedia, for instance - as you can see, it is NOT an exact value, hence the variation in coefficient. The truth is, it’s a one-significant-figure parameter, so a pretty coarse measure:

Hull speed
Hull speed1084×279 41.9 KB

#22

I believe what you are interested in is the same thing touched on in other posts.
Namely, effort to maintain a cruising speed, rather than all out top speed.
Here is what I believe, for what it’s worth.
Narrower is almost always faster.
There is a point of diminishing return for length, and it’s probably shorter than most people think.
The slower the speed, the less volume you want in the ends. And my understanding is that it doesn’t just mean width, it can also mean depth.
The slower the speed, the less asymmetrical you want the boat. There may even be a point where fish form requires less effort.

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#23

interesting, I guess I thought hull design would make more of a difference

#24

Hull design absolutely makes a difference. However, the theoretical parameter called hull speed does not take hull design into account - that’s why it’s not particularly useful as a hull descriptor or as a quantitative measure to compare boats. In fact, it’s no more useful than just stating the water line length of a boat.

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#25

Guillemot article is generally correct, but misses one important point in his explanation: for a given displacement, a longer hull not only has a more gradual entry (allowing for slower displacement of the water ahead), it also has lower frontal surface area presented to the water (resulting in less water displaced per foot travelled). An 8 foot boat 3 feet wide displacing 4 cubic feet and a simple diamond profile with no rocker has a frontal area of 4x12x12x12/96x2 square inches, or 144 square inches. At one foot per second forward motion it must accelerate one cubic foot per second to a velocity of 4.5 inches per second. Stretching the same profile to a 16 foot x18" boat we have 4x12x12x12/168x2 or 72 square inches. At one foot per second forward 0.5 cubic foot per second must be accelerated laterally to a velocity of 1.125 inches per second. This gives a 32 fold difference in energy expenditure for displacement between short and fat versus long and narrow at any speed. For simplicity I have ignored varying the draft of the hull, making the short boat the same width and sit deeper in the water would mitigate this to some extent. Specifically, the 8’x18" boat would accelerate 1 cubic foot per second to 2.25 inches per second, for an 8 fold difference in energy. I don’t know whether his drag curves reflect this difference or not.

#26

Oops, the second equation should be 4x12x12x12/192x2.

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#27

I’m having no luck understanding these calculations. Boats do not push water directly forward or directly sideways, and certainly not at a constant bulk velocity. Is this just spit-balling, or is it based on a method you’ve seen used before?

The Guillemot article is well written and scientifically accurate, the conclusions are well-established. The resistance plots were generated using software that includes water displacement by the hull - it is included in the form drag.

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#28

It is a simplification to conceptualize the impact of frontal surface area on resistance to forward motion. I should have labelled it as such. Simplification because, as you state, the water displaced is not all moving in a single direction at a single speed. With regard to the article from Guillemot, just noting that frontal surface area is a significant factor which he does not explicitly mention but has a large impact on the difference in resistance between longer and shorter boats. I agree overall the best article on boat length and paddling resistance I have seen.
With regard to the plots, I have not seen the math in the program, and probably couldn’t verify that it was correct (or incorrect) if I had. Presumably it is sound and empirically verified across a range of shapes, but I have seen all kinds of things calculated by programmers using the wrong inputs as starting points.