Correlation of hull length and speed

That addage is true. It’s the greatest compliment and goal of every teacher - when the student becomes the master. Good job.

As noted above, there are too many comlicating factors for a simple answer. For very low energy inputs a half sphere is the fastest shape. At elite athlete energy inputs a long kayak is faster. Hull speed is a significant factor for short boats with poor streamlining, but more streamlined boats have such small wakes that paddling up the wake is not a major impediment to speed, and as previously noted trim adjustments can reduce the impact of that further. A narrower boat will always be faster up to the limit of draft exceeding 1/2 beam or so (assuming comparable surface and streamlined shape) but short narrow boats have poorer stability. For an individual paddler the questions of “what boat can I paddle most comfortably at speed X?” and “what boat will allow me to traverse distance Y at effort level Z in the least time?” can be answered only experimentally, but reasonable starting points can be suggested based on paddler characteristics and the specifics of X, Y, and Z.

A canoe is about mobility on water - smooth silent efficiency. A craft that let’s us move at a leisurely pace, alone or with a partner. It’ll do that and more.

A kayak is about speed and eefficiency. It can go out in conditions that send most other boats back to land. When waves knock it over, the paddler can roll right back. It’ll do that and more, but above all, it’s speed.

As long as kayaks are made, someone will mill over the formulas to figure out how to use a paddle to propel the sleek boat even faster. It normal and natural. People love speed, and when it comes to speed, we can add a motor, or pedals, or sails, but in the end, the paddle will remain the tool of choice. At least for me.

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No, not for everyone. You are talking about a large number of people but not for everyone who kayaks.

Kayaking for me and many others is about being in a boat that can go places where a canoe cannot go, at least a canoe in my hands. As long as I can get to where I want to go and back again before a storm comes in or dark I don’t give a rat’s ass about speed. Efficiency yes simply because expending unneeded energy paddling seems silly. Conservative planning and knowing the area you are paddling in well can usually significantly lower the risk of having to dash home ahead of a storm. Getting dark tends to be a fairly predictable event so easy to plan for.

Speed is not the most important factor for me these days, at all. It is the boat that can most reliably get me home if things bump up or I have a physical issue out there.

Celia, I agree. My point is that the formula about hull length and speed will be ongoing, forever. It doesn’t matter what boat a person prefers, or how they use it. It doesn’t matter whether a more people want to go down a river or out in the ocean or on a placid lake. Someone will always ask the question: what is the correlation of hull length and speed.

I don’t agree that kayaks are only about speed.

Kayaking is swimming with a boat. Being One with the water. Like a Porpoise. It is three dimensional { with a skirt } More in the water , than on the water. Speed for any kayak is directly related to stroke efficiency and paddle proficiency.

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Nor do I, but it is one area where they excel. While other paddlers are doing their thing, some are looking to push the envelope. I don’t need a racing kayak, and have no desire to race anybody. All I’m saying is that some ride waves, I use the boat for exercise. The topic hold my interest, and it doesn’t matter if others find it a can of worms. If another kayaker is interested in the topic, I’ll discuss it. Cars are made to drive kids to soccer games, but there are some drivers who love to tweek the car and improve the handling and speed. Same thing. Not everybody does it, but I’m not sure what to say when somebody say, everybody doesn’t like to go fast around corners.

One thing does puzzle me. The OP was interested in knowing if there was a way to calculate how much faster a longer narrower boat would be. Specifically naming the 12.5, 14.5, 17.5 Tsunamis. I’ve paddled them all and have detailed records over the same course, under different conditions of wind. Tide and current. I also own a Tsunami SP, a 140 Tsunami, two 140 Pungo, have access to two 120 Pungos and another 140 Tsunami. Either everybody already knows the answer and won’t reveal the secret because it’s elementary, or nobody else care. I get it. Just suprises me. I expected someone on a kayak forum to share an interest in the question. I do understand that people use kayaks to just hang out, fish, tour, and surf. I enjoy everyone’s stories. I’m starting to feel like a freak.

Maybe nobody has answered the question because the question as asked cannot be answered, or at least not answered simply. One can even debate what hull speed means. Is it maximum comfortable cruising speed? Maximum speed with an all-out effort by a very strong paddler?

The only simple and absolute correlation between hull length and speed that applies is that of maximal theoretical hull speed. Maximum theoretical hull speed relates simply to waterline length since maximum theoretical hull speed in knots is equal to 1.34 times the square root of waterline length in feet. But even that value is of limited relevance since, as has already been pointed out, many paddlers never get up to maximal hull speed and really strong paddlers can often exceed that speed.

Many other factors come into play besides paddler strength and endurance. These include the waterline length/beam ratio, the prismatic coefficient of the hull, draft per load, how sharp the water entry of the hull is, bow deadwood, and wetted surface area which itself depends on hull cross-sectional contour.

There really is no way to simply relate hull length with efficiency for any given paddler and load without taking into a bunch of other considerations.

I don’t have enough fingers and toes for that, but I did notice a strange correlation between the length of each boat and the difference in speed. It wasn’t a scientific calculation. I gues I essentially just stated the obvious, with the help of a WAG formula, but it was specific to those three models.

Oh boy…

There’s a metric ton of data that goes into this. And most of it only pertains to displacement hulls.

displacement hull for those that don’t know is a V - Bottomed hull. Most boats with a hard chine fall into the psuedo displacement category. Soft chines less so…

but suffice it to say to a point a longer hull will be a faster boat all other things being equal (Eg. Beam.)

here’s a simplistic hull speed calculator: Hull Speed Calculator — Optimal Vessel Speed

But none of these goes into hull efficiency.

so my Tsunami @ 17.5 (it’s actually 17.9) has a wetted waterline of about 17 ft. So my hull speed is 6.3 MPH.

And if I go all out I can hit about 6 mph. but this is all theoretical, since hull efficiency also plays into things. ALOT…

and requires some serious math…

but again as I said all things being equal (beam width) a longer boat will be more efficient, up until the co-efficient of drag gets too much for the under-powered propulsion unit. (Eg. Human being.)

On a good day you and I may be able to generate about 1/4 hp for short bursts.

So having said this…

My Tsunami @17.5ft has a hull speed of 6.3 mph, I’ve run races with that boat and over 10 miles I’ve averaged about 4.96 to 5 mph.

I also have a fishing boat that is 14 ft it’s hull speed comes in at 5.5 Mph, (its wetted lenght is 13 ft.) and has the exact beam width as my tsunami. On that over a 10 mile course it only averages 4.2 mph to 4.5 mph.

Now the 14 ft boat will absolutely get to it’s top speed much faster than my 17.5 foot one. shorter waterline and less drag, however once there it does take a bit more energy to maintain top speed, whereas the 17.5 foot boat takes longer to get to top speed, once there the amount of energy required for input to maintain it is much less.

all of these measurements were done with GPS on flat-water, with 50% upwind and 50% downwind.

So as you can see Hull speed is a nifty way to get an idea of the performance of a given boat, but it’s not really a measure of how fast you will actually be, no is it going to be a good indicator of how much easier to paddle it is.

I ran a 15 mile race this year (used my Tsunami 17.5) it took me 3.5 hours so my average was 4.2 mph however things to note the coast guard measured me on the downward leg 7 miles on the schuykill river at 6 knots. It feels like flat water but… you are going with the current. however the next 8 were significantly slower as I was now running up the Delaware River and there was 18" of chop to deal with (and my rudder broke.) So I had to course correct with the paddle as well as apply thrust. but still I came in 30 minutes ahead of everyone else. Yes kids I won the race.

so here’s the TL/DR - longer is faster at the same beam width and easier to paddle once you are up to speed, harder until you get there.

Your link does not show the waterline length/waterline width ratios for different kayaks. The link below provides the L:W ratios for a large number of kayaks under the Sound Rowers boat classes. The list is quite a few years old now so a lot of kayaks that have come since are not included.

The L:W is probably as good as any simple number in determining the relative speed of different hulls. Unfortunately, the waterline lengths and widths are not provided by most manufacturers. Furthermore, these values will in practice vary with load but the L:W ratios are generally calculated at a standard waterline depth.

The L:W ratios are again a pretty good way to roughly judge how fast one kayak is compared to another but won’t tell you how much faster a kayak with an L:W of 9 would be than one with an L:W of 8 for any given paddler.

It does show why fast sea kayaks need to be pretty long, however. There is a limit to how narrow a kayak can be since it has to accommodate the paddler. It is hard to make a sea kayak with a waterline width of less than say 20" because a lot of paddlers can’t fit in them and they can feel very unstable to many. Many sea kayaks have very proud stems so that the waterline length can be up to a couple of feet less than the length overall.

In order for a kayak with a waterline width of 20 inches to meet the L:W ratio of 9.25 that is the lower limit for the fast sea kayak Sound Rowers class it would have to have a waterline length of at least 15’ 5". That usually means a length overall of 16’ or greater.

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Is this like my cousin Vinnie.

Vessel speed is inversely related to the width of the boat and correlated with the length of the water line. So a catamaran does better than a single hull and a trimaran does even better. Boats with rocker may need a larger paddler to get full water line, and boats with V bottoms may get wider with a larger paddler. This truth was taught to me by the founder of west side boat shop, who in his 70’s was still doing 40 miles a week in upstate New York. Doug Bushnell was also years ago in the Olympics, and is a naval engineer. Perhaps more important for those of you that can do 8+ MPH, turbulence will virtually stop a wide boat well below that pace. Finally the paddle can be more important than the boat.

West Side Boat Shop News: West Side Boat Shop Acquired by Dave The Kayaker LLC – Dave The Kayaker | The new home of West Side Boat Shop kayaks

I have tested the phenomenon myself over the years.

I have a Blue Finn glass 37lb, 21” wide, 19’ long and it is about ≈9% slower than the

EPIC V10 Sport carbon-kevlar 27 lb, 19”, 20’ which in turn is ≈9% slower than the

S1X carbon-kevlar 20 lb, 17”, 21’

I am 5’7” and about 150 lb and am getting close to Doug’s age when I knew him. I use an Epic small mid-wing paddle.

For a high speed flat water 10 km run, I can average low 3:10’s for 500 meter spits in the S1X; in the Finn it’s low 3:40’s and the EPIC just under 3:30.

Physics isn’t the only factor affecting speed. Stability and turbulence are key. If the waves or wind impact, I cannot safely use the S1X with Florida gators waiting for an error, and if I want to be able to take breaks and have some food I can’t take the EPIC in the always bouncy Ocean. Many champion racers therefore will use a slower boat if the weather is rough.

I have been thinking and it hurts, but here it is:

The correlation of speed with longer boats is because a longer boat creates a longer period of the bow wave. Think of this like swells versus chop in big water. You can ride the longer period of a swell, but usually struggle with chop and its shorter wave period.

There is a limit, the Westside X-par missile is 24 ft., an Olympic boat is 17 ft. In theory the Missile should be faster, but human muscle power can make a shorter hull faster.

Hull design can also make a shorter boat faster, if it enters and leaves the water more efficiently.

More important for an individual is that the fastest boat, for them, is the boat they are most comfortable in, given the circumstances.

Most folks can go faster with a better paddle, rather than a different boat, because of the learning curve for the new boat.

Oh, and if Bud is of equal paddling efficiency as I am, he would go faster in either his boats or mine.

It is because I am 5’11" and close to 180. His weight would cause less wetted surface than I do, so there would be less friction on the hull.

CraigF, you dun good. That spell of thinking was very good for your brain. Nice explanation.

Indeed. The integrated propulsion/navigation biosystem is the crux of all that is magic
Peace J

My 2 cents worth (may be worth just about that much too)

Length is only part of the very complex equation. I am reading up now on kayak building and design and the overview is easy, but the execution is far from simple. I want to start building them in the future so I am “in school” now.

In propeller driven aircraft it was easy to design and build larger more powerful engines in the late 30s and 40s. But making something more powerful didn’t always solve any problem because the 2 things a bigger engine need to move before anything else are #1 itself, and #2 the fuel needed to power such an engine. Bigger fuel tanks and bigger engines need bigger aircraft to hold them, which makes for more mass to overcome air resistance. So designing an engine that gave 2X the power and setting it into a plane that was 2X as large ended up giving designers a plane that used more gas and went as fast as what they had, but no faster, yet was a larger target and could not maneuver as well, making it an easier target to hit.
But when you have a better design of aircraft that can be fitted with a more powerful engine and not make the craft itself larger, THEN you get something noteworthy. (the P-51 Mustang of WW2 fame is a prime example. Switching from the Allison engine to the Rolls/Merlin engine made it into a legend)

So power is not the answer by itself. Efficiency of design is key. Same in kayaks.

In kayaks however YOU are the engine.

So a big powerful man may be far stronger then a smaller lighter paddler, and yet the smaller paddler in a smaller craft that he/she can push to the top end of that paddlers efficiency can be faster then the longer “faster” hull just because the bigger hull needs a certain amount of power to get it to it’s designed speed, but if the smaller paddler can’t give that amount of power (and keep it up over time too) the larger hull may be substantially slower for that paddler.
But put a paddler into the kayak who is not a lot heavier but substantially stronger and/or having a much better stroke technique and suddenly we see the longer narrower hulls jump ahead in speed.

So when I read all the posts before (many of which make excellent points) I am convinced that the question is too narrow in it’s scope.
I believe it’s far more about the condition of the paddler and their expertise than it is about the gear they use. Give an expert excellent gear and we really see impressive performance. But to me, the ones that impressed me the most are the ones showing what they can do with middle of the road gear and a “Walmart paddle” and they still outrun me. Why? Because they are an expert and I am not!

Also we need to define what we mean by ‘speed’. Like sprinters vs marathon runners. Who’s the faster of the 2? Depends on the way you measure it. Over a flat, short distance (say 440 yards) the sprinter is, and there is no competition. But make the run 22 miles long over mountains and the sprinter may come in “last” ----- because they may not come in at all.

So in discussing kayaks and hulls: we talk of speed, but over what amount of time and distance? And do we include camping gear too in our race?

A surf sky is faster then a touring kayak, but on rough water, very cold water, and for trips of 7-14 days the surf sky would “loose the race”…by default…because they would not show up for the competition at all. You can’t carry a week’s worth of food, water and camping gear on a surf sky. So which is “faster” over 160 miles of coastal wilderness?

Many people have a tendency to rate and score the “competitions” on an agenda that is not relevant to other people. Now there is nothing wrong with that per se, but the debates of “which is better” is often put forth without answering “better for what”? Any “race” in which only one person gets to set the rules is always going to be won by the rule maker.

Aficionados of any sport or activity love to delve into the gear and play with the toys, and derive great joy in finding the systems and equipment which they love the most. That’s why it’s fun. But there is not any deep wisdom in trying to make “converts” as if it were a devoted religion. The “best” kayak for someone wanting to make fast day trips is not going to suit the paddler that wants to go on 20 day long trips and neither of those 2 is going to give the devoted fisherman what he or she wants. And the flat back-water explorer is not gong to have a perfect kayak for their flat back-water needs with any of the previous 3.

So who’s right? Easy answer: All of the above!

My idea is to define the job before acquiring the “correct tool”.

No one kayak can do it all anymore than any one motor vehicle can do it all. There is a lot of room between a dune buggy and a Kenworth tractor trailer. Which is faster? (From what starting line to what finish line? Carrying what inside?) Heck the Kenworth can beat a dragster, IF the Kwnworth driver gets to make the rules!
(Race: from LA to N.Y City -----but the load needs to weight 30 tons and all must be moved together. Ready, set, go!)

Maybe we are asking the wrong question.

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