short and thin kayak? (armchair kayak design question)

Builder's Forum
1

Hi,

It's been a while since I've been on the board. Hope everyones having a good time. I have an odd question based on an observation I had when I went out with a friend on some sit-on-tops. 

We were out in some confused seas, by the breakwater of our local marina, and I was just sitting there, loosey-goosey and allowing my kayak to do it's thing underneith me. I looked over, and my friend had just about flipped over his sit-on-top (I didn't know that was possible, I've stood up and walked around on these things) After coming to help him, I watched him more carefuly, and realized that he wasn't allowing the kayak to just roll under him, but was rather, sticking rigidly to it, so that when the boat rocked, he did to, instead of just pivoting at the pelvis and keeping his torso upright. 

I am, unfortunately, a bit of an autodidact, and what I know of kayaking is just what I've learned from messing about, a few videos, and talking to people on here. However, I'm also a martial artist and motorcycle racer, and it occured to me that it might just be the ability to losen up and relax, and allow the motorcycle/kayak/person throwing me across the mat to do their thing, and just go with the flow that helped me deal better with the confused seas than my friend who was a bit more rigid. 

So, that's what got me thinking... 

 

Kayaks tend to come in 2 general shapes: short and fat, and long and skinny. 

A shorter, smaller kayak is going to have less wetted area (unless it makes up for its shortness with more width) A longer kayak has a longer waterline, but that only matters as you approach hull speed. If you're a lazy/slow/weak paddler like myself and very seldom approach hull speed even on short boats, this is probably moot, but having less wetted area reduced resistance regardless of speed. So, to this ends, a shorter boat might be easier for a slower paddler to paddle at speeds under it's hull speed. (is this right so far?) 

Now, the narrow boat has 2 advantages. The first being that it's (lack of) bredth reduces wetted area, but the second "advantage" to my way of thinking, is it's inherent lack of stability. It will more easily roll and wiggle, and allow you to throw it around with a pivot of the waist than a more stable boat would, which is more inclined to do what the water is doing, and if you don't like it, too bad. (is this right ?) I think this might be a large part of why I prefered the longer narrower boats to the shorter more stable boats. I certainly don't paddle anywhere near hull speed on either of them, but if you're loose and relaxed, the narrower boat seems to feel more connected, and easier to move around, providing a somewhat more organic and connected feel than the more stable boat. It's more like wearing a boat than being in a boat. Is this realistic, Or is it more likely that the narrow boats I've been in might have had cocpits that better fit me better, and any boat with a tight cocpit will feel the same way?

If the above are both correct, I was wondering, since a number of boats seem to have a lot of excess boyancy so one could trim some length or width from them, would a short, narrow boat be a decent idea for a rec boat? Or would anything significantly short and narrow (say, 16' x 19") be so small that to get enough boyancy to hold an adult it would have to sit so low in the water as to negate any real advantage?  

 Thanks :)

 

-- James

 

 

2

This is a well discussed topic on other forums. Bryan Schulz has a nice essay on the length/width debate, too.

One thing that you'll find is that almost all commercial kayak designs are very wide in comparison with most traditional Greenland kayaks, which were very often under 20" in beam (though pretty long). Most of my kayaks have been narrower than commercial boats - notably my slightly modified Yost Sea Rover, which ended up at 16' x 17.5" (still not extreme, at all, by traditional standards). So I can say, yes, you wear these boats, and they are very responsive, easy to paddle, and fun. My Sea Rover's lack of stability was not an advantage, however, but more something to be dealt with and gotten used to.

To my mind, the usual definition of a "rec" kayak is one that takes little investment in skill or equipment to get out on the water to have fun in. A skinny and/or unstable boat is not particularly suited to this! A short and stable boat would be - and that's why they are so popular.

If you've got the skills/athleticism/desire to deal with a short, narrow craft, then they are definitely superior to the usual rec boat (in my opinion - and depending on what you use them for, of course).

And, yes, a well outfitted cockpit will allow you to control many a boat very easily. Just look at all the fat, oddly shaped whitewater kayaks out there, all being thrown around every which way with one's hips. Even some SOT's can be rolled with good thigh straps (I've done it).

Seems to me like you would be a good fit for a more traditional style boat - but maybe your pal in the SOT would be happier with what he has (?).

Dave Gentry

3

From what I've read in articles by boat designers over the years (I'm not a boat designer, but I love to contribute to these interminable debates amongst us amateurs!):

You are correct that a longer, narrower kayak, with its higher wetted area will tend to be easier to paddle at high speed, and harder to paddle at low speed, ceteris paribus. Is the crossover speed between these two realms generally just below the hull speed of the short, fatter kayak, as you surmise?  I'll have to leave that to the experts.

But I do think that most people have an exaggerated idea of the importance of hull speed to a kayak.

First, if you look at the slope of speed/drag curve for any kayak, I believe you will find that it is increasing both below hull speed, where the "hull speed effect" is increasing as bow and stern waves move closer to in-phase, and above hull speed, where the effect is decreasing, as bow and stern waves move farther out of phase. There is no noticeable inflection point near hull speed.

Furthermore, max speed for a racing kayak is much higher than hull speed: around 170% of hull speed.  There is a popular misconception that a kayak can't go faster than hull speed, or that it would need to plane to do so.  Both are absolutely false. 

4

One other comment: a long, narrow kayak tends to have higher wetted area than a more rounded hull, not lower as you had speculated.

5

Camper,

You're comparing size to shape in your last post and that doesn't really work all that well. The roundness of the hull is an attribute of all  kayaks, long & thin or short & fat.

Why don't you try again and tell us what you really mean?

In the meantime, since wetted area is a result of length, width, depth, shape, and in some cases, hydrodynamic flow (as well as the trim of the kayak in the water), it's quite possible for a physically smaller kayak to have a much larger wetted area than a larger one.

Laszlo

6

Yes, when I said all things being equal, I wasn't at all clear.

I meant "including displacement."  So I am comparing two boats which have the same displacement and have very similar hull shapes, but one is somewhat longer and somewhat narrower.  Boat designers sometimes start with a rough idea of displacement (from planned number of people, gear, etc.), among other things, and then start to make tradeoffs--I'd like her to be fast, so I'll make it a little longer and thinner, or, I want her to be more stable, so I'll do the opposite.  

It's my understanding that as you move from a hull with all three dimensions close to the same, to a shape with one dimension longer and longer compared to the others, holding the volume constant, that the wetted area goes up.

WoodenBoat had a good article on this stuff a few months back, which I can  probably find if I think of it.

Of course there are many factors that affect the speed/drag curves of boats.  I am only addressing one in isolation because i thought that was what the original post was asking about.

 

 

7

Among all the discussion I think one part of the origional question has been lost.  If your friend is sitting stiffly on the boat and it gets tipped 30 degrees, the weight of his upper body also gets shifted and eventually goes deyond the balance point.  If you sit with a "loose waist" and let the boat tip and flow under you with the waves and you keep your trunk upright the boat can tip quite far without becoming unbalanced.  It is not a matter for forcing the boat under you or forcing it not to tip but of balancing/ dancing with the waves.  There may be times (such as tipping the boat onto its side part way to "carve a turn" or when rolling the boat when a "hip snap" will be needed to force the boat but mostly, at least in my exerience (moderate) its more letting it move with the water and you  letting it move while keeping your body upright. 

 

then there is bracing or using the paddle allow you to lean beyond the stability point, or to get back to stability if you are already off balance but that is another discussion.

2cents worth. 

Ed

 

8

Yes, til your note, we've ignored a very interesting part of the original post.  And I totally agree with you (and with the original post) about how a skilled kayaker makes a boat stable, just as an experienced backseater on a motorbike makes life easier for the pilot.  Or how those dyed-in-the-wool bicycle guys Orville and Wilbur made an inherently unstable choice for the elevator placement (the bow instead of the stern!) a great idea in practice.

 It has been noted on this forum in the past that the best boat for you is one that feels a little tippy on the demo run, since in a very short time it will feel just right.  Your body learns to make instantaneous unconscious adjustments, what control system and audio engineers call "negative feedback".