leong

Sir Godfrey, The purpose of this thread was not to choose the best kayak (which, of course, is ill defined); instead, the purpose was to solicit from real users how hull shape, length, beam, overhang, etc., affect boat performance. With this goal in mind and with all due respect, I don’t see anything wrong with assessing the similarities and differences between two disparate kayaks (like the Epic 18 and the Explorer).

The Epic 18 used in the video has a water-line length of approximately 18 feet; the conventional kayak looks like it has a water-line length of about 15 feet (I'm guessing, perhaps someone can identify the conventional boat). Perhaps the longer water-line length (and not the lower rocker or less overhang) of the Epic accounts for most of the superior performance shown in the video. A better comparison might be between two kayaks of the same water-line length. For example, the water-line length of my Falcon 18 is about 16 feet and the overall length is 18 feet. The water-line length of the Epic 16 is about 16 feet and the overall length is also about 16 feet It would be interesting to see a video comparing two such kayaks (i.e. of comparable water-line lengths) plowing through waves.

Scott, Here are two relevant links: http://www.kayakshops.com/kayak_speed.htm http://www.seakayakermag.com/2007/07e-news...r/fastkayak.htm

See Title for additional information.

To nit pick: You say every one of these volunteers would have died very quickly without a PFD. Maybe or maybe not? We have no way of knowing this. But, either way, it isn't relevant to the gasp reflex, which the video really didn't discuss. Perhaps the ninth volunteer (if she were there) would have suffered a gasp reflex. The gasp reflex may not be a universal reaction. But neither is it universally true that all people in cold water will drown unless they are wearing a PFD. Obviously, it depends on how far away from safety and other factors. But, it's a good video. It would have been a lot better if it discussed the gasp reflex in a little more detail..

I think it is misleading because of what it left out. Although not intentional, this well done video can be worse than a big lie; it misleads, yet it cannot be pinned on anyone. A serious gag reflex before the completion of a roll (or coming up for air after falling off a skiff) could result in early death. Clearly, this is not a minor point. It should have been emphasized in the video.

The 50 lb. pull was with the boat sideways to the wind and current. Even Ern couldn't have msde headway against a 50 lb. static drag.

11 AM is good. Looks windy but I'm game unless you want to switch to the Concord river in Bedford. Directions to Gloucester HS: Route 128 North to Grant Circle (a little past the bridge over the Annisquam) Enter Roundabout and take first exit onto Washington St. In about 0.3 miles turn right onto Centennial Ave In about 0.4 miles turn right when you see the HS (Leslie O Johnson Rd) Drive counterclockwise around the HS to the boat ramp (if the way to the boat ramp is blocked drive back out to Centennial, make left and look for a left to get you to the water) Note: this is different than the NSPN directions (it avoids crossing the drawbridge over the Blyman canal)

You beat me to the punch Ern. How about leaving from Gloucester HS (Annisquam River) instead? That way we can paddle out to the river mouth on the north side and see what the conditions are. If good, we can paddle east towards Folly Cove or west towards Cranes beach. If the conditions are really bad we could hang around in the river or perhaps go into Gloucester harbor.

Actually, I have raced the old Epic (not the new 18X with integral rudder) twice on flat water at the weekly Charles River races. My times were comparable to my Falcon 18; but I think I might have done better if I were more used to the Epic. But, as I've been trying to say over and over again, my question is more theoretical: All I want to know is if there is any validity to statements such as: “Our experience and testing have shown that long overhanging bows offer no benefit.� If the statement is valid, then why are overhanging bows part of most boats? No time for Navier-Stokes today Rick ... the options market beckons.

Been there, done that. But it doesn't answer the fundamental question I posted from Epic: “Our experience and testing have shown that long overhanging bows offer no benefit.†Is it true? Is it hype? Perhaps there is no answer. I'm working on a possible explanation which is related to how the overhangs are often in the water when the conditions are rough. But, because of the non-linearity of the Navier-Stokes equations, it's difficuult to arrive at a stable numerical solution.

What does “perform well in waves or rough water conditions†mean? , Go fast, and cover a lot of miles through waves and rough water? Maneuver, roll, scull , play, and be nimble in waves and rough water? Answer: All of the above. Regarding the difference between a low rocker, blunt- bowed boat and a traditional kayak with rocker and overhang : Wouldn’t it be better to go and paddle the boats, find out what happens? Answer: For sea kayaking, theory (kayak design) informs practice (paddle the boat) and practice informs theory. Besides, there is not enough time this century to try all the boats. It’s better to have some rules of thumb and valid theory to guide you in choosing what boats to try. I remember a college text book which described extensive physiological studies of ruby throated hummingbirds , the findings of which were that they were physiologically incapable of storing the energy required to migrate across the Gulf of Mexico, from the Yucatan Peninsula to the Gulf coast of Texas, without sustenance. “But the birds, never having read the report, continue to do so every spring.†Answer: A valid scientific theory would have predicted that the birds were capable of the flight. Remember: In science, theory informs practice and practice informs theory. I would hope that, based on the practice (what the birds actually did), the biologists would have revisited their invalid theory of energy storage and corrected it accordingly.

Agreed. But how about a kayak with low-rocker, short waterline length and no overhang. How would that boat compare in maneuverability to a more traditional kayak of the same waterline length but more rocker and overhang? That is, is it the the rocker and/or overhang that provides the maneuverability or is it strictly the waterline length (assuming similar prismatic coefficient). Also, out in deep and rough water does the rocker and overhang help or not? The Epic pitch implies that it doesn't... from Epic: “A common misconception is that sea kayaks require extensive "rocker" or extended-bows to perform well in waves or rough water conditions.â€

Many of the race-oriented sea kayak manufacturers (for example; QCC, Kayakpro and Epic) make the claim that their kayaks are faster due to a higher waterline length to overall length ratio. They also implicitly or explicitly claim that rocker and overhang are overrated. For example from Epic: “Our experience and testing have shown that long overhanging bows offer no benefit.†“A common misconception is that sea kayaks require extensive "rocker" or extended-bows to perform well in waves or rough water conditions.†“Epic kayaks have ample rocker but minimal extended-bow.†“With a plumb bow and full waterline length the bow gradually lifts as it approaches the wave and lands more gently. The longer waterline length gives a much more efficient hull and the kayak performs better in all conditions.†From QCC: (with respect to overhang), “… sticking out there catching the wind or adding weight (serves) no purpose.†Although this is all probably valid with respect to speed for racing, the higher rocker and overhang of more traditional boats surely offer some other advantages. It’s easy to make a kayak with 1. a high waterline length to overall length ratio and 2. less rocker and 3. no overhang. Yet, most boats are designed with higher rocker and more overhang than, say, QCC boats. It can’t just be that the manufactures are building boats for the aesthetic beauty of overhangs and rocker at the cost of functional capability. In almost every case, extreme expeditions use kayaks with fairly high rocker and overhang; think of boats like the NDK Explorer or the Valley Nordkapp. I’m trying to determine the advantages and disadvantages of kayaks with more rocker and overhang versus those with a longer waterline length. The published literature only emphasizes hull speed, wetted surface area and tracking with respect to rocker, overhang and waterline length. Obviously, for kayaks in general, the longer the waterline length the higher the hull speed at the cost of being harder to turn. For a given overall length, high rocker and overhang make it easier to turn a boat at the cost of a lower hull speed (due to a shorter waterline length). Consider two cases for comparison: 1. Two kayaks with the same waterline length but with different rocker and/or overhang, and 2. two kayaks with the same overall lengths but with different rocker and/or overhang. Below are some possible advantages/disadvantages I can think of for both cases. Case 1. Keep the waterline length a constant (say16 feet) and stretch the overall length of the boat from 16 feet to say18 feet via increasing the rocker and/or the overhang. The advantages/disadvantages of this hypothetical stretching are: More cargo carrying capacity. More playful?? Able to handle higher loads because waterline length increases as load increases. Better handling in waves and rough water?? Worse tracking than it would have been if you also stretched the waterline length. Less efficient due to more of the boat sticking out to catch wind and adding weight. Less prone to pearling. More aesthetic beauty. Case 2. Keep the overall length a constant (say18 feet) and increase the waterline length of the boat from 16 feet to 18 feet by decreasing the rocker and/or reducing the overhang. The advantages/disadvantages of this hypothetical stretching are: Faster top speed (higher hull speed due to increased waterline length) Harder to turn and handle (less playful), but you can effectively increase rocker by leaning More prone to pearling?? Better tracking Harder to handle in waves and rough water?? More efficient due to less of boat sticking out to catch wind Less able to handle increased cargo loads (as boat sinks waterline length doesn’t increase much) Wetted surface area increases at the cost of more drag. Less aesthetic beauty Dear Reader: Are the two lists correct? Can you think of anything else that needs to be added to the lists above? What else needs to be considered?

Oops, I was talking about slow speed air drag. I meant to say: Yes, the force of air drag on a body moving slowly through the air is roughly proportional to the speed.

Nick, of course you are quite correct. But several of my motives for posting were the following: 1. The referenced article was wrong (whether a typo or a misconception). But I've seen this same type of error so many times in the non-technical literature that I believe it was a misconception. 2. Yes, there is nothing wrong with publishing drag as a function of speed. However, many paddlers might equate paddling effort with drag; it is much more meaningful to equate effort to power. Example: A man can push the ground with a force of 150 pounds (his weight) for hours at a time by just standing still or walking on a flat surface. But he can't climb a steep hill at 4 MPH for very long. 3. Once you look at paddling power as a function of speed (for a given boat) you realize how much more effort = power is required to increase your speed. 4. Based on several of the posts in this thread it appears that there are misconceptions within the paddling community regarding the definitions of work, power and force. 5. Although too technical to discuss here, an understanding of how power increases with speed has some practical applications for me related to racing and/or pursuing paddlers ahead of me.

John, you were helpful and I my response was not intended as a put down in any way. Sorry if you took it as an insult. I look at the subject this way: Yes, the force of air drag on a body moving through the air is roughly proportional to the speed. However, I'd say the total drag on a kayak moving through the water at moderate speeds (say less then 4.5 knots) is roughly proportional to the square of the speed; hence, the power would be roughly proportional to the cube of the speed. For example, the proportionality constant for the Gulfstream is 0.1 (for speed in knots and force in pounds); i.e. Total drag force = 0.1 * speed squared. Using this formula for the Gulfstream I get speed vs. force of (the seakaykers published values of force are in parentheses): 2 ---> 0.4 (0.42) 3 ---> 0.9 (0.87) 4 ---> 1.6 (1.63) 4.5 -> 2.03 (2.25) So for kayaks at moderate speeds why not these rules of thumb: 1. Force to propel is proportional to the square of speed. 2. Power to propel is proportional to the cube of speed. For most purpose you are not interested in absolute estimates of force or power (so you don't need the specific proportionality constant for your boat) . You just want to know the percent increase of force or power needed to get some percent increase in speed. Feel free to chew me out directly at leon dot g at verizon dot net.

I don't know what standard predictions you are talking about? For some boats, increasing the speed from 1.5 Knots to 3 Knots requires about a 6-fold power increase. For the Gulfstream, increasing the speed from 2 to 4 knots requires a 7.8-fold increase in power.

No No No. A Kayak moving at a constant speed of 3 Knots in a straight line (i.e. moving at a constant velocity of 3 Knots in some direction) needs approximately a 14-fold increase in power to move at 6 Knots.

Everything you said John is correct and well said. But correctness doesn't imply relevance. All I said is that the author was mistaken when he said that it requires a four-fold increase in power to increase from 4 MPH to 8 MPH. Such a kayak would be worth $millions. Then I gave the example that it requires more than a 14-fold increase in power to take a Gulfstream from 3 to 6 Knots.

Lisa, Scott and I'll be there.

Yup, and even seakayakermagazine doesn't seem to get it since they publish force vs. speed.

Ern, we can discuss your question another time. But you missed the point of my question. No, it doesn't take 4 times the power to double the speed from 3 to 6 knots. Why is the effort (power) required about 14 times greater for a doubling of speed in the kayak example? No, there's nothing unusual about the Gulfstream. You'll also get about 14 for both of my boats (Falcon 18 and Seda Impulse) and almost all other kayaks (even yours) going from 3 to 6 knots!

I just read the following statement in the fitness paddling website http://race.fit2paddle.com/C1159474119/E20...5945/index.html A bit of science here, for the same boat, to double the speed, requires 4 times the horse power, or to get your kayak from 4 mph to 8 mph requires you to put out a 400% increase in effort. Of course, the obvious mistake here is that a 300% increase in power corresponds to 4 times the power (not 400% as stated). But this is not the interesting mistake. According to the data at http://www.unold.dk/paddling/articles/kayakvelocity.html the force required to propel a CD Gulfstream (for example) at 3 knots is 0.87 pounds and at 6 knots it is 6.24 pounds. Thus, the force alone goes up by a factor of 7.17. However, the power goes up by a factor of 14.34 (much more than the stated factor of 4). This will be quite trivial to some of you, but I believe many of the kayakers reading this will not see how I arrived at the 14.34 factor. As kayakers this is an important concept. It will help you understand why it's hard for a fast paddler to catch up when a slower paddler is given a headstart. Or it will help you understand why you sweat a lot more from a small increase in speed.

Take 128 to exit 14 and go west (I think it's a left turn) on route 133, then turn right on Harlow and soon turn left on Conomo Point Rd.