Hello.
Can someone please explain why longer is faster, seems like it would have more drag.
Also, is longer more stable in choppy water.
I am currently in a 10.4 wide body kayak.
thank you so much
Dave
basics
Whenever you’re in doubt about a theory, just make an example to the extreme, and you will have your answer. Imaging paddling a kayak 4 feet wide and 5 feet long. Then you get in one 2 feet wide and 8 feet long. You are plowing less water (you are creating less bow wake) and the narrower boat cuts through the water resistance with less energy.
Now if the person weighs 200 lbs. and you make the 2’ W x 8’ long boat a little longer, the boat sinks less deep in the water and will take less energy to propel it. The ratio of less resistance outweighs the increased surface friction. There will be a point when the surface friction resistance will gradually outweigh the gain that the increased flotation will deliver and then thats it, unless you start rounding the bottom more and on and on. Then you sacrifice stability for less wetted surface and gain speed again.
is longer more stable
Longer in general will not prove more stable. To go to the extreme again, if the goal was simply the ability to stay upright in rough water, something short like a whitewater boat would probably serve a person best. But they’re designed to be carried by current to their destination, and to be playful along the way. Sea kayaks are designed to travel efficiently through rough water and, by comparison to WW boats, are designed to track straight. So again, the compromises. A beginner just might stay upright longer in a short, stable recreational kayak than a sea kayak as conditions gradually became rougher. But if you get into it a little more, pick up skills, start to experience confident, efficient travel in gradually rougher conditions, you start to appreciate the compromises.
“stable” is in the eyses of the paddler!
If you get a longer kayak, when you first get it, it will not be nearly as “stable” to you as your short wide kayak, but after a learning curve, and a period of paddling it, you will be just as comfy in it as you are now in your short one.
Then you will realize that it is faster, more responsive, and more enjoyable than your shorter boat.
But … don’t just run out and buy a longer yak with out first trying a bunch of them out.
Jack L
If you hold width about constant and
increase length, longer will be more stable. And, a longer, narrower kayak may be more stable than a shorter, wider one. Not always, sometimes.
The wetted area of a boat does affect drag. Making a boat longer will tend to increase speed because of the width to length ratio. But the increased wetted area will partly offset the benefit of the length increase. People who paddle very hard often favor longer boats, which can have a higher speed potential. But others may prefer a compromise length, because for the “easy” paddler, the lower wetted area can mean less energy expended per mile traveled.
Displacement hull speed
http://www.sailingusa.info/cal__hull_speed.htm
Suggest you read this:
http://www.irbs.com/bowditch/
Hull drag
– Last Updated: Nov-03-10 2:42 PM EST –
The statement that 'long hulls are faster' is one of those vaguely worded statements that can be considered "true with an if", or "false with a but".
The speed of a given hull is created by the differential between the applied paddling force and the fluid drag on the hull. More force = you go faster for a given drag value. More drag = you go slower for a given paddling force.
Hull drag is primarily caused by fluid friction and wave drag (there's also form drag, which I am going to ignore). At low speeds, friction is the dominant source of drag. Shorter boats have less surface area in the water, so generate less friction drag. Therefore at low speeds, shorter boats are easier to paddle.
At speeds approaching 'hull speed', wave drag dominates. Hull speed is a theoretical maximum speed of a given displacement hull, and is proportional to the square root of the waterline length of the kayak. A longer kayak will have correspondingly higher 'hull speed', so a higher maximum speed it can attain. This speed is attained at considerable expenditure of energy, however.
The hull speed of a 14 foot kayak is about 5 knots. For a 17 foot kayak, it's 5.5 knots. If the extra 0.5 knots is important to you, then you want the longer boat.
The crossover between the dominance of friction vs. wave drag is in the range of 3-4 knots. So if you paddle at typical touring speeds of say, 2.5 - 3.5 knots, then a shorter boat will be easier to paddle. By this I mean, for a given paddling effort, a shorter boat will be faster. Equivalently, at say, 3 knots, a shorter boat will take less effort to paddle.
To be sure, a short boat needs to be designed well to track acceptably, which is an important point. And if you're going to be in an environment with long wavelength swell or waves, a longer boat will feel much more secure and be easier to drive.
There are clearly many tradeoffs involved, as others have said.
Trying to understand this one …
Sorry, but sounds like a contradiction:
“The hull speed of a 14 foot kayak is about 5 knots. For a 17 foot kayak, it’s 5.5 knots. If the extra 0.5 knots is important to you, then you want the longer boat. "
" So if you paddle at typical touring speeds of say, 2.5 - 3.5 knots, then a shorter boat will be easier to paddle. By this I mean, for a given paddling effort, a shorter boat will be faster.”
I know this anecdotal, but by 17’ 6" Nordkapp LV consistently makes the same 6 mile jaunt in almost 30 minutes less than my old Tempest 165. And the 165 was about 15 minutes slower than the Tempest 175 it replaced.
I know there can be a lot of variables, like tide, wind, etc. But this is a regular course I have done dozens of times in each boat, so, give or take, there is a noticeable difference. Especially the T’s, which are the same hulls scaled for size.
Believe me, I know zip on boat design, hulls, or fluid dynamics. Just speaking as a guy paddling the boats. Appreciate your comments …
i think…
What carldelo was saying was at slow speeds, the shorter boat is easier to paddle (takes less effort), due to less wetted surface.
In your case you were probably paddling hard enough for the differences in length to make a difference in overall speed.
Differences
Well the difference in theoretical hull speed between a 16-1/2 boat and a 17-1/2 foot boat is only 3%. So I’d wager the difference in the average speeds of your boats (which I believe based on your long-term averaging) is based on the details of the hull shapes, rather than overall length.
For instance, what is the actual waterline length of your boats? My 15’3" QCC has a longer waterline length than many 17’ Greenland style boats.
Length to beam ratio is important, as are block coefficient, etc. etc. None of this is simple to figure out. Boat length itself is a very coarse measure.
stabilikty
Imagine a kayak with a sealed cockpit and no one in it. Imagine waves. Will the kayak flip over? Probably not, depending on the kayak, but it is very unlikely for a standard narrow width, long length sea kayak. Will it flip over with you in it? Not if you let it do its thing without interference (loose hips). Will you feel like it is stable? Not at first, but as you gain experience and understanding it will become very stable. Just let it happen.
long versus short kayak
Doesn’t a paddlers weight and height also have something to do with overall speed/stability ratios? I didn’t see this mentioned in any of the responses, only boat width and length.
In choppy water
To answer the second question of the OP.
Setting aside most other factors for a moment, the kayak lenght relative to the size/period of the chop does matter. The shape of the bow and stern (their volume) also matters a lot.
A kayak that has most of its volume near the cockpit with pinched and low volume ends will not be affected nearly as much in terms of stability compared to a kayak of the same length but with full ends and less volume in the center. The first kayak will tend to bob up and down a lot and the bow and stern will jump up or down a lot, but the part of it that creates stability the most (the wide center section) will be in the water most of the time.
A full-volume-ended kayak will be much more likely to be proped-up by either or both of its ends thus lifting the center up between wave crests, thus its stability side to side will be decreased.
The above is not a clear-cut + or - though. Either of the two kayak may outperform the other depending on specific needs and conditions…
A longer kayak will be less affected by small chop - it simply rides over it.
Put the same kayak in medium chop and it struggles since it can’t ride over the waves any more and they are too big to cut through effectively. In that situation a slightly shorter kayak may do better.
Make the chop even bigger/longer period and again the longer kayak is “better” as it has more top speed and there is no longer a need to cut through - a shorter kayak will simply be slower there for the same effort (of course, assuming we are trying to move at more than a snail pace).
The Coaster!
I once owned a Mariner Coaster and, looking back after fifteen years and several kayaks later, I think it was the best all-around, most versatile kayak I’ve ever paddled. Therr was no problem keeping up with a group; the 13 foot Coaster was just such an easy boat to paddle at a steady 3.5 knots. But at anything more tha 4 knots, she would start to settle down in the water and applying more effort was only a frustation. OTOH, running down the steep, short seas so typical of shallow Barnrgat Bay where I do most of my paddling, the little Coaster would get on a wave and surf it out and, with a few strokes, you were on the next wave. It was there that I could run away from the longer boats. It’s too bad that the Coaster was just my second boat [the first being an Aquaterrs Chinook] and I let myself get talked into trading it for a ‘proper’ long boat. The Coaster is no longer made and I haven’t seen anything on the current market [with the exception of certain SOF boats from Cape Ralcon kayaks] that begins to approach its qualities.
Some good commentary here
As for the Coaster, it is a superbly capable sea boat with a waterline about the same as many 16-17 ft. “proper” boats.
Science can ruin years of accepted marketing dogma and untruths. The most focused upon variables often have the least to do with real world performance. The more one understands the science behind all this the more they realise how silly and blatantly “wrong” so much of what’s been accepted in sea kayaking really is…
I’d go so far as to say many people are probably paddling the wrong boat for them based on bad information, and adherence to beliefs that are wrong.
Of course, there’s the whole “subjective, emotional thing” which is the driver for most.
length vs width
My intuition (which is likely wrong) always told me that the reason longer boats had higher hull speeds was because the boat would be more knife-like in the water because it could taper more gradually from bow to the widest point. But all the math and words I hear suggest that no the length is the key thing and only when such factors are near equal do the other aspects like width have an impact.
I would have thought that if you had an HV and LV version of a boat such that the LV was exactly the same, but scaled in all dimensions then the HV and LV would have similar top hull speeds, but it seems suggested that the HV would have a higher hull speed (though more surface drag so more effort at lower speeds).
Thoughts?
Hull speed
– Last Updated: Nov-05-10 6:01 PM EST –
Well, theoretical hull speed is the speed of translation of a wave with a wavelength equal to the boat length. The idea being that once the bow wake of your boat has lengthened to the point where it equals the length of your boat, further propulsive energy goes into making the wave amplitude higher, rather than speeding up the boat.
But this is a theoretical consideration. What happens as you approach hull speed is that the hull starts to enter the planing regime. At low speeds, hulls are supported by a buoyant force based on their displacement, which is why low-speed boats are called displacement hulls. Once you start to enter the planing regime, part of the support comes from deflection of water downward by the fast-moving hull. Once you're fully planing, the maximum speed goes way up, but the amount of energy needed is high.
So theoretical hull speed can be thought of as the beginning of the planing regime. The ability of a hull to plane is strongly dependent on its shape, and the transition to the planing regime is gradual, so the 'theoretical hull speed' is a fairly loose quantity. Mathematically speaking, it is generally defined as the point when the Froude number of the hull reaches 0.40, but I don't have the time to go there right now.
Just to ask this…
As to hull speed and why a boat has it (or doesn’t), you can go look at Sea Kayak magazine archives and find tests that’ll put numbers on hull resistance for various boats they’ve reviewed at various speeds. Suffice to say that the answer isn’t as clean as longer is always faster, or skinnier is always less stable etc. It really comes down to particular boats, their hull shape and their displacement. So I am not going to get involved in that.
However, I am curious about why you are asking this question. Are you thinking about going for a new boat, or paddling with a group and finding that you are working harder than you think you should be, that kind of thing? It may help to know if this is a passing thought or something has prompted the question.
generally longer has the potential
– Last Updated: Nov-05-10 6:46 PM EST –
to be faster, but as others have mentioned, the increased surface friction does cancel out the better length to width aspect ratio in longer/skinnier boats. Besides the paddler's skill, there are many little factors in hull designs besides the length and width.
Perfect example of 2 identical hull designs at different lengths: I wanted to build a wood/fiberglass kayak kit and settled on the Pygmy Arctic Tern design. Since I didn't want to waste lots of time, building effort, and money on a hull design that sucks, I did lots of hydrostatistic research on the 14 and 17 foot version. Same hull design, rocker, hard chine, and materials, with the 17' version only being 1/2" skinner.
Speed/Resistance according to the KAPER program:
Arctic Tern 14 with 250lbs payload
2 knots = 0.89 lbs effort
3 knots = 1.86
4 knots = 3.02
4.5 knots = 6.32
5 knots = 9.07
Arctic Tern 17 with 250lbs payload
2 knots = 0.95 lbs effort
3 knots = 1.98
4 knots = 3.66
4.5 knots = 5.31
5 knots = 7.98
So at slow or cruising speed it's easier to paddle the shorter version, but it hits a "wall" if you want to paddle fast and you're exponentially putting in a lot more effort. The more efficient aspect ratio of the longer boat overcomes the increased friction and has a higher potential top speed. Keep in mind that these are identical hull designs and easier to compare each other. Compared to sea or touring boats, rec boats are much wider with flatter bottom hull designs focused more on calm water stability and comfort. That solid feeling flat bottom becomes a liability when the waves start rolling!
Even though it's a little bit more effort to paddle, I tend to paddle fast and wanted camping gear space, so I built the 17' version and couldn't be happier. You have to decide on your paddling needs and preferences as well.
Not all HV and LV’s are the same …
In some cases, the LV is indeed a scaled down version of the regular boat, and an HV scaled up. But in other cases, the LV might be the same hull with a lower profile deck.
For example, I believe a Tempest 165 is a scaled down 175, so, at least theoretically, they should perform the same. I have read that some consider the Nordkapp LV to be a different boat, with more rocker, than the regular Nordkapp, ie NOT a scaled down version.