- Consider a very long boat vs a very short one of the same beam and hull shape. The long boat will have greater wetted surface area, so that at low speed will have greater friction drag. However, at high speed the long boat will have less wave-making drag. If you plot total drag vs speed for both, the plots will cross. Below that crossing speed the longer (faster?) boat is slower. That is why a weaker paddler will sometimes find the slower boat to actually be faster.
- Hull speed does not apply to kayaks in any practical way. Many published plots of speed vs drag for different boats show gradual increase in the slope of the drag curve with increasing speed. Nothing remarkable happens to this curve at hull speed.
- The above apply to flat water only. Wind and waves/chop can affect a boats speed so that a faster flat water boat may be slower in a cross wind, or up, or down wind.
- A boat with too much rocker may be slower up or down wind than one with less rocker.
- A boat with too little rocker may be slower up or down wind that one with more rocker.
- A boat with a rudder will generally be faster in conditions than the same same boat without.
- A boat that surfs well and easily catches waves will be faster down wind only if the paddler knows how to catch the waves. It may or may not be faster up wind.
I agree with some of what you said, but how sure are you that hull speed does not apply in any practical way? Sure, it's not an absolute number since power output is not absolute, but it seems that only racers in specialized boats ever regularly reach or exceed that speed. I'd like to see one of those graphs that supposedly shows that hull speed is not a reasonable estimate of maximum speed. I suspect that you are not correctly relating the required propulsive force to the amount of force that a human can actually generate with a paddle (even though I have never measured that force myself, I *do* have a real-world example that illustrates how small that force actually is, but won't waste space with that right now).
If it were not true that the force needed to exceed hull speed is beyond what most of us can produce, how can it be that going 1 mph slower than hull speed is something that anybody can do with the kind of effort that can be maintained for hours on end, but maintaining a speed that is just 0.3 mph slower than hull speed is beyond the ability of most people? As my best personal example of that, I can't maintain hull speed in either of my rowboats, neither of which is a slouch when it comes to speed, and that's in spite of the great advantage in maximum power that the body can exert via oars as compared to a paddle. As an example illustrating this power, I can accelerate the shorter of these boats from a dead stop to more than 5.0 mph in the space of about six feet, yet exerting that much power continuously will not get the boat above hull speed (which is only 0.5 mph faster in this case). Bottom line: If what you say about hull speed is true, strong, fit paddlers ought to be able to go much faster than they ever actually do in the kinds of boats that most people are paddling.
One quick note: The significance of that "gradual increase in slope" that you speak of is what you are overlooking. The steady increase in slope indicates an exponential relationship between the paddler's exerted force and speed. As a paddler, you don't have all that much reserve in available propulsive force as it is, and when the need for that force increases exponentially with speed, you'll find yourself reaching your limit within a very small range of potential speeds. A really strong person will be able to push the speed a little higher, but to do so will require a hugely disproportionate increase in force. It's that disproportional nature of the increase that makes this so applicable in the real world.
OK I’ll play!
I recently picked up a Taran16 as a faster kayak. Before then I had 2 full seasons with an Alaw Bach and some time in my wife’s Impex Force3. Before then I spent many years in a Pintail. All 4 are very different boats. I consider myself a pretty strong aggressive paddler and a solid wave surfer.
- Can’t really argue with that. I have not paddled the Taran18 but folks who have say that it is a faster boat, but not by that much. The 16 is actually 16’8" while the 18 is the full 18" so not that much longer. The 18 is also .25" wider, other than that it’s the same exact design.
- I can’t comment on the plots but from experience when I’m hammering on flat water it does feel like the kayak planes a bit making it easier to maintain the higher speed. I don’t feel like I’m going that much faster when I redline though. I certainly don’t feel like there is a magical hull speed that I can live in.
- Absolutely. IMO wind and waves are were the better rough water boat has an advantage. Better secondary stability keeps you from having to brace. A boat with low windage keeps you from having to correct. A skeg, or better yet a rudder, really keep you from having to correct. A good surfing boat will help you use the waves to your advantage, etc.
4&5. I found that boats with more rocker tend to weather cock or lee cock easier but there are other factors such as hull shape that combine with rocker to cause this. When going directly upwind the amount of rocker doesn’t seem to come into play.
- Agreed. The Taran is my 1st ruddered boat and I’m amazed at how much less energy I use with almost all types of paddling. I’d say the only times when having the rudder up could be faster are when paddling in very tight situations like windy creeks and rock gardens or when going upwind in extreme wind conditions.
- Agreed. I’ve found the rudder makes a huge difference as well. Both my Alaw Bach and my Taran are excellent at catching and holding onto waves. With the Taran I can steer with the rudder to hold onto the wave way longer. With the Alaw Bach I catch waves super easy, and it’s even more stable than the Taran, but as the wave tries to make you broach it’s very easy to lose the wave as you try and low brace and steer.
“Consider a very long boat vs a very short one of the same beam and hull shape” how is that possible?
I disagree with about half of the above, probably agree with the other half but I am not clear on what this post is trying to elicit. I am not going to wander into a discussion because it is too broad. But others may be looking for direction…
I hve “random thoughts”…
about how fast I used to be when I was younger !
Naval architecture is less tiring if a specific goal is set in the first oaragraph…‘here we discuss hull design for top speed’
Not same as ‘here we discuss hull design for maintaining high hull speeds for long distances’
So the area if discussion is defined and pins down available energy as a constant
Further, writing eg ‘moving upwind’ does not establish ‘upwind’ as a constant only a factor. Trying this conversion qualifies the post as humor.
The longer I keep a kayak the slower it gets…
…and our thoughts aren’t based on conjecture.
Google ‘kayak drag vs speed’ and look under ‘images’ for a plot of total resistance vs speed for different kayaks. It is clear from the plots that no particular point can be labeled hull speed. In other words, from the actual plots one cannot determine whether a particular speed is above or below hull speed.
Note also that the speed increment due to a given increase in effort decreases as speed increases. I agree that the drag increase is exponential, probably close to an exponent of 3, but this has nothing to do with hull speed.
Taran and Q700
I have a QCC 700, at 18 feet similar to your Taran. Since I do not generally ‘hammer’ it as you do, my speed seldom exceeds 4 kts. QCC also has a 600
at 16’8" but with the same beam as the 700. I have been wondering if the shorter 600 would be more efficient for me at speeds less than 4 kts due to its lower wetted area. But I like the handling of the 700 in wind and chop.
I would be interested to know if your boat is actually planing, or starting to. I was under the impression that no one has the power to change a kayak from a displacement hull to a planing hull. Maybe you could get someone to go-pro you while hammering.
This post is trying to elicit a better understanding of kayak design and performance.
Here is how our reasoning differs:
Hull speed is not defined by the shape of the graph of propulsive force vs speed, and to look at such a graph to try to "pick" hull speed is a totally backward approach. Hull speed is defined by the maximum speed at which waves of a particular wavelength can travel. Since the boat is between those two waves, the wave's travel speed becomes a very effective limit to how fast a boat that is propelled by a limited amount of force can go. For a very sleek boats propelled by a very motivated paddler, hull speed can be exceeded by some degree, but for ordinary touring kayaks paddled by ordinary people, hull speed is indeed a very effective limit to how fast the boat will travel. I suggest that you do some experiments yourself with a GPS, and you will see that once you get within about 0.5 mph of hull speed, expending huge amounts of extra effort accomplishes almost nothing. This is something that is about as applicable to the real world as it can be. You'll find that hull speed is not subjective or academic (or whatever else you like to think right now). You may not be limited to *exactly* hull speed, but it'll be pretty darned close!
You are right that the exponential nature of the increase in required effort with increasing speed is not entirely related to hull speed, but this is another backward approach to understanding the problem. It still remains an unavoidable fact that trying to make your boat go faster than the waves that it produces (the condition at hull speed) requires almost super-human strength. Last night when searching for force-vs-speed graphs, I ran across a website describing this phenomenon in a way that might ring true for you more effectively than my method of describing it seems to be doing. I may look for that later and give you the link (and yes, it was an article about kayaking, not just basic naval architecture).
For what it's worth, it seems that for kayaks and similar boats, no one produces graphs of force vs speed that go much above hull speed (I say that because I didn't see any while looking at quite a few sources last night). I bet that if they did, you'd see a "backing-off" of the rate at which required force increases with speed, and then indeed, the effect of wave-limited speed would have an influence on the shape of the graph that you could see and recognize. This is speculation on my part, but it's based on many years of operating small motorboats and observing motorboats of all sizes, as well as what I recall reading in an article about motorboat fuel economy some years back. As a motorboat's speed increases (and it doesn't matter what the hull shape is), there's a period of "wallowing in the hole" which occurs during the phase of barely exceeding hull speed, and this speed has been shown to provide, by far, the most inefficient use of fuel. Since fuel economy increases substantially once the boat's speed gets a little faster and it starts to plane, that's a pretty good indication that there's a break in that steadily-changing slope of the graphs you have been speaking about, IF those graphs include speeds greater than hull speed. I believe that looking at graphs which are limited only to hull speed and below, instead of including speeds that are both faster and slower, is the only reason you are not seeing anything distinctive in the graph's shape. You can't say that a particular feature does not exist on the graph if the portion of the graph that you need is missing.
We can go slow together with…
our old kayaks !
The hull speed formula 1.34 x root(LWL) yields 5.69 kts for an 18 ft kayak, 5.52 for a 17 ft kayak, and 5.36 kts for a 16 ft kayak. Very small differences. Sea Kayaker published curves of speed vs drag; originally based on tank tests, later based on calculations of both Broze and John Winters. The curves are up to speeds of 6 or 7 kts and nothing particular can be seen at the slightly lower hull speeds. So the portion of the graph that is needed is right there, not missing.
Very few paddlers can cruise at hull speed for any distance in a 16 to 18 ft boat. So I don’t see how hull speed is of practical value in choosing a boat. Maybe your motivated paddler could exceed hull speed in a sprint. I think that for any distance one is limited to speeds less than hull speed.
If one has measured speed drag curves, or the later Broze/Winters calculated curves (which are believed to be reasonably representative), then one would be better advised to look at the actual curves, which take into account the wave dynamics you accurately describe.
For the most part, we are just looking at this in different ways, more than we are disagreeing, and here is that difference in a nutshell.
Your points are all based on boat-length examples that would suggest that boats don’t differ very much in length, but in the real world, they do. In fact, there are thousands of times as many 10- to 13-foot kayaks being paddled these days than the 16- to 18-foot boats of your examples.
Sure, no one paddles at hull speed, but a person’s chosen cruising speed is very much influenced by hull speed, simply because people tend to paddle no faster than whatever speed makes their effort effective. So, when you throw shorter boats into the mix, the way hull speed affects the distance that you can travel in a day is far too big of a factor to ignore. I get the impression that you do not paddle shorter boats or paddle with people that do (and that’s perfectly okay), but if you did, you would have perceived what I was getting at immediately.
As to what you say about the portion of the graph above hull speed being present in those cases you cited, that’s not really true in the way I was trying to address. The top end of the speed range you describe is still well within that “wallowing in the hole” range of speeds where the boat is basically trapped between two waves (the boat doesn’t magically escape that trap by going a mere 1 or 2 knots faster). Granted, there’s not much use in plotting drag curves at higher speeds because only by surfing will a paddler experience them, and when doing that, gravity does all the work. But based on motorboat fuel economy and performance variations with speed, I’m certain that there will be a “blip” in the curve once speed increases to the point that the trap of the boat’s own wake has been escaped. The necessary propulsive force as speed increases will always become greater, but the rate at which it increases will be less, once the boat is no longer stuck in a hole of its own creation.
I’m not sure if my boats are slower, but
… they are definitely heavier!
Not only do the boats get slower and heavier, the cars we tote them on get taller!
So why don’t our funds do the same…?
But somehow, we all still seem to manage to be able to
-Frank in Miami
HMM… THAT FORMULA…
...seems to miss a SIGNIFICANT variable: beam!
As I read somewhere here on P-NET in an old thread on fundamentals of the same topic, compare a 19" wide ski, a 24" wide tandem SINK, and a 54" wide jon boat...
Formula suggests same resultant for all three...
But YOU try to sit in each and
-Frsnk in Miami
I hadn't paddled my rec boat for quite awhile, because I don't paddle that much on the river I used to use it on and I don't paddle that often on the small lake where I use it. But I just wanted to go do a quicky on the nearby little lake, so I took the rec boat.
I used to enjoy the rec boat from time to time, but after just sea kayaks for quite awhile, paddling the rec boat was a chore. Trying to push the rec boat up to a speed that feels reasonable makes it a real workout, so there is that.
So a warning to new paddlers: Don't be trying out long skinny sea kayaks if you are only prepared to go with something shorter and wider.