Hull Speed

High gloss does not repel water
It simply does not ATTRACT and HOLD water. It APPEARS to repel water because of the beading. I want my boat to be slippery and slick, like a fish. I always scout the shore before I launch and look for dead fish to rub on my hull.

My next project, I call it: “Silence of the Carp”, is actually to make a ‘second skin’ for my hull out of dead fish.

About the non-joking part, …

– Last Updated: Sep-03-09 11:05 AM EST –

... if water beads on the surface, that surface DOES repel water, and I believe what angstrom says about the result of that is correct.

To address the joking part, fish slime probably would be a good speed-enhancer for your hull, again, for the reasons described by angstrom. But notice that fish slime does NOT cause water to bead-up, but instead, a drop of water applied to something coated in fish slime (such as a fish!) will instantly spread out into a sheet.

Oh yeah, remember to get your fish slime from LIVE fish, not dead ones. The slime will have washed away if the fish has been dead for very long. That does make the project a little more daunting. ;)

I think
the idea is that an ideal surface gives a fairly gradual velocity gradient in the boundry layer. Ideal surface roughness or hydrophylic/lubricating coatings – natural or synthetic – are different ways of achiving this. A glossy/waxed finish supposedly makes a more abrupt velocity gradient, possibly resulting in higher drag.

There is something fishy about slickness
I’m not following the logic. If the hull surface is slick and “repels” water, e.g. car wax, then doesn’t it mean that water does not stick to it? If yes, doesn’t that in turn mean that the friction b/w hull and water is less than the friction b/w “layers of water”, and hence the frictional resistance should be less with a slick hull compared to a rough hull that keeps a layer of water sticking to it???

Apparently it’s not that simple

– Last Updated: Sep-03-09 2:14 PM EST –

Angstrom has explained this pretty well, I think, so perhaps you should read his posts again.

As I understand the concept, a "darned-near-microscopic" layer of water clinging to a slightly rough surface, or to a hydrophilic surface, creates a "false surface" (composed of a water film) which is more forgiving to the movement of water against it than is the case for water sliding along a solid surface. In other words, that film of water "clinging" to the hull "feels more slippery" to passing water than the hull itself, even more so than an exceptionally smooth and water-repellent hull.

As angstrom says, the reason for this may be that the boundary layer between the solid hull and the water passing across it is "spread out" over a greater thickness, allowing a more-gradual transition zone between the water that is streaming by the hull and the hull itself, which in effect, makes that transition zone more "slippery".

Let's say a boat is moving 5 mph through the water. With an "ideal" smooth and water-repellent surface, water must slide along the surface of the hull at 5 mph. However, if there is a thin layer of water "following" the hull, the outer edges of that water film can slide along to some extent while "following" the water streaming past the hull, while water that is deeper within that thin film, right against the hull, stays where it is. Thus, there is NO single point where water must pass another object, be it the hull or water clinging to the hull, at the full 5 mph. Instead, at any given point within the film or at the outer edge of the film, the actual speed difference between various "bits" of water or between "bits" of water and the hull is substatially less than the boat's travel speed, and thus, the friction is reduced. In effect, friction is reduced in a manner that is somewhat similar to simply reducing the speed at which the hull goes through the water.

That's a very long way of describing the "transition zone" of velocity of water alongside the hull of which anstrom spoke.

That part I uderstood, but I do not see the logic in it or why it is so. That was my question.

If by definition something does not stick, then it slips -;). I saw the “empirical” data on the 400 grit roughness, so I do not doubt that is the case, but just it does not make sense logically, becasue probably I am missing some of the finer points not expressed in the post…

“No slip” condition
There is a condition called “no slip, no penetration” that always applies between a fluid and a solid. The molecules of water in contact with the molecules of the solid are always stuck together. Wax, gel coat, you name it, they’re always stuck. The next layer of water molecules will be able to move relative to the stuck water molecules, but not completely freely. This continues on up through what we call the boundary layer. The more gradual the boundary layer, the less each successive layer needs to move relative to the previous one, hence reducing drag.

There is current research in the field of fish slime. More accurately, the injection of a slick polymer (much like disolved gel-caps) into the liquid via a porous hull. It works, but much like riblets, no one knows quite why. There are at least a couple people at U of Michigan working on it.

That’s exactly the part I was addressing

– Last Updated: Sep-03-09 2:35 PM EST –

I did re-word my post slightly during the time you responded. Maybe that will make a difference. In any case, I WAS trying to explain the actual reason that this would work. I don't see why that wasn't apparent.

Here's a thought problem for you to illustrate the situation another way. You know how the tracks on a crawler tractor work? Imagine if you could make a boat hull where the entire wetted surface "crawled" toward the rear like the exposed side of a conveyor belt as you paddled. If you paddled at six mph, and "turned on" the crawling hull such that the outer surface of the boat moved to the rear at three mph, the boundary between your hull and the water would have a speed difference of only half of your travel speed - you'd be paddling at six mph but experiencing only as much friction as when paddling at three mph. This conveyor-belt analogy illustrates a very exaggerated transition zone between the hull and the water which passes by it. The analogy is even more accurate if the "conveyor belt" movement of the outder surface of the hull was driven entirely by friction with the water, and the "conveyor belt" was supported on nearly frictionless bearings.

The bottom line can be summarized by another analogy: No matter how smooth and polished you make the bottom of an object that you wish to drag along the ground, it will never drag as easily as it will if you place it on rollers. The little bit of circulating "slippage" between film of water clinging to the hull and the water passing by in the slipstream functions a bit like rollers between a solid object and the ground passing beneath (again, it could be a lot more complex, but that's the general idea).

Well there you go…
Just throw handfulls of gel caps in front of you as you paddle, or rig a dispenser on the front of your boat (are you listening NRS?).

Problem solved.

Coating nonsense
I have to weigh in on the difference between wetting or non-wetting coatings. This is all nonsense. The no-slip condition, which is empirically observed in all flows, trumps all considerations of the hydrophilic, -phobic nature of the coating. Once a surface is submerged, it is wet - period. Every surface carries along a layer of water attached to it - no exceptions except possibly for hypersonic flows in rarified atmospheres.

The only difference a hyro-philic/phobic coating might make is in the direction of the fluid meniscus at the air-water interface - this effect will be negligible excepts for water-walking insects. The claims on the high-speed coating site are bogus, and the test results shown there are meaningless. Believe me, if there was some sort of hair-spray coating that could give a 40% increase in range, then every oil tanker on the planet would be getting this coating without fail, but they don’t, because it is BS.

At the time of my graduate work in the mid-90s, I worked a bit on riblets - streamwise v-grooves to minimize drag. Their function was to influence the size of near-wall vortices to modify boundary layer development. They worked a little - the main problem with them is that they must be tailored to a specific speed, and must be aligned with the flow. Change in speed and direction reduces their effectiveness, so they tend to not be worth the effort. The current crop of banned swimsuits have riblets, granted, but it’s my understanding that their main advantage is an increase in the swimmer’s buoyancy, and physical compression of the flesh of the body and appendages.

Also at that time, there was waning interest in polymer injection into the boundary layer. Polymers do reduce drag by modifying the vortex structure in the middle portion of the boundary layer; the mechanism was not agreed upon at the time. The bottom line is that it might reduce drag, but you have to carry big tanks of chemicals to release into the water - not too pretty. Your submarine may go fast, but the snail-trail will give you away.

As far as shark/dolphin hydrodynamics, comparisons to boats are problematic. Yes they have slime and/or riblets, and they also have elastic/compliant skin (effect not understood). But the biggest problem is that animal swimming is a fully unsteady, mostly periodic flow regime, which is not comparable to steady flow over a rigid hull - they are fundamentally different.

I remember reading some research from the 50s and 60s where stuffed birds and penguins were mounted in wind and water tunnels, and the researchers were shocked, shocked to find that they could not explain flight from the data. I guess the fact that animals flap and wiggle as they fly slipped their minds - oh well.

Anyway, unsteady periodic fluid flow tends to be dominated by vortex dynamics which are notoriously difficult to sort out. Intuition on what is happening is often wildly incorrect - experiments are generally necessary. Bird flight (especially take off) was not really understood until the vortex dynamics were worked out.

For a kayak, I will reiterate what has been stated by others - the smoother the better. All the riblets in the world won’t help when you take your first paddle stroke and your boat yaws 5 degrees - oops, the riblets don’t line up with the flow anymore. The key is to keep the little bumps small and below the high-speed flow. Re: the Moody Diagram - note that it is for pipe flow, not external flow so is not quantitatively applicable to kayaks. In principle, the trends are the same however. There is a quantity called roughness length for a boundary layer. Bumps smaller than the roughness length do not affect the boundary layer much. It depends on speed - the faster you go the smaller the roughness should be to be negligible.

Lastly, I will reiterate what Dressmeister said, there is a lack of good texts describing boundary layers. I used Panton in my thesis, but it is still somewhat advanced. There is a good book by a biologist: “Life in Moving Fluids” by Steven Vogel. It is written for biology majors so is less mathematical than most, and he is truly a gifted science writer and describes fluid flow extremely well. As a plus, he deals with birds, fish, plants, waves and a whole bunch of things that are of interest to paddlers - I highly recommend it for the motivated non-student who wants to learn more about general fluid mechanics, turbulence, boundary layers, etc.

Cheers, Carl

what about a gel-cap dispenser on the bow?

This part that you wrote (below) helps, provided the relationship between friction and velocity is non-linear. If the friction is linearly increasing with speed, the it should not matter at the least if you do it in 1 boudary or 5.

"Thus, there is NO single point where water must pass another object, be it the hull or water clinging to the hull, at the full 5 mph. Instead, at any given point within the film or at the outer edge of the film, the actual speed difference between various “bits” of water or between “bits” of water and the hull is substatially less than the boat’s travel speed, and thus, the friction is reduced. "

I think the other theory about the vortices/micro hydraulics (?) makes sense too, and it seems to imply a different mechanism for decreasing friction - by breaking the uniformity of the layer and thus somehow reducing the amount of water that needs to be dragged along…

Guideboatguy, you are describing every boundary layer here - in every flow there is a thin film of fluid clinging to the solid surface.

Great Info!
Lots of interesting stuff there! I provided my way of explaining the boundary-layer theory mentioned by angstrom, but pointing out real-world flaws in any “ideal” design is pretty cool stuff. All that stuff about the ever-changing shape of a swimming animal as it propels itself through air or water, and how it’s the nature of those movements which propel it (rather than the sort of thing that can be duplicated by anchor points inside a wind tunnel or flow tank) is a complication that most people who study those things are aware of, but trying to actually duplicate what goes on with live animals by using models is pretty hopeless, at least right now.

340 mile race
Blog from a participant

Site for the race

Men’s solo was done by santo albright in 44 hours 54 minutes (over 7.5mph average) and the mens tandem was just under 39 hours for better than 8.7 mph average - an average over three hundred and forty miles - so I assume they did a good bit over it over 9mph.

That explains it!!!

– Last Updated: Sep-03-09 6:19 PM EST –

Have you seen how fast the Missouri River flows? There are plenty of places on that river where a fast paddler could easily double his speed relative the fixed objects on shore or the river bottom, compared to the speed that he is capbable of making his boat move through still water. There are many parts of the Missouri River that are too fast for good paddlers to make any progress at all going upstream, unless they hug inside turns and take advantage of slackwater and eddys.

Again, hull speed only deals with the speed that a displacement boat moves through the water. Added "ground speed" due to movement of the water itself across the earth is not a hydrodynamic effect (that's the same reason you can't legitimately break a track sprinting record by running on a big conveyor belt, which doesn't increase your *actual* ability to run fast). That's not to say none of these guys ever exceed hull speed, only that they are not exceeding it by anything close to the margin or the long time periods that you implied. The Missouri River moves really fast, making anyone who paddles downstream cover a lot of miles in a short time (the current speed is even faster, *on average* and relative to the quantity of flow (which is probably a lot less nowadays so the actual speed comparison may not always be correct), nowadays than in olden times, as a result of channel-straightening and containment to accomodate barge traffic).

When I take my guide-boat down the Wisconsin River, I can easily manage a steady GPS speed of 7 mph, often faster than 8 mph if I work hard, but as I've already mentioned, the absolute fastest that boat will move *through the water* is 6.0 mph. The Wisconsin River is much slower than the Missouri River, so hitting 9 mph on the Missouri River doesn't sound all that spectacular.


In my 16’ sea kayak I have clocked myself with a gps at these speeds in small surf running diagonally. 3 foot waves. Nothing real crazy. Super fun. Theoretical hull speed works fantastic in a tank. Kind of gets blown away outside the lab. It is easy to double theoretical hull speed with a wave behind you.

Catch a favorable 30mph wind with a hull that likes to surf and you could skip along at well above hull speed easily for long periods of time.

Just remember what hull speed “is”

– Last Updated: Sep-04-09 9:54 AM EST –

It is not a maximum speed limit. It's the speed at which your boat sits precisely between two waves of its own creation, so to go any faster, the amount of power output necessary starts increasing at much steeper rate. Almost any boat will start to plane, rather than be a pure-displacement boat, once pushed substantially beyond hull speed, and paddlers just don't have the power to do that in most cases.

When surfing, you are using gravity to power your boat, and gravity "is a lot stronger than you are" if the wave face is steep enough. It's no different than putting a motor on your boat, and everyone knows that small motor boats can go faster than hull speed. This is NOT the same as "blowing away the theory of hull speed" outside a test tank. It's simply a case where you've applied loads of extra power to make the boat climb out of that trapped-between-two-waves situation so it can start going faster.

Hull speed relativity must be considered
with the relativity linked to the amount of power required to maintain the “optimum” hull speed.

I believe that regardless of the shape, finish, composite, and all else, that a hull has an “optimum”

hull speed, as GuideGuy sort of stated. Any hull can be forced to a faster speed, but the amount of power required will likely be disproportionate to the power required to maintain optimum hull speed.

If the industry could standardize power required, let’s say about 10 foot pounds required to keep canoe xyz moving at its optimum hull speed of 4mph, then the buying paddlers would know what amount of paddling work is required at what optimum speed. It would still be up to the padder to have an idea of their capability with paddling power, but this would let a 10 foot pound person know what paddle craft would be best for them.

For example, a Coleman 17’ canoe might require 30 foot pounds of paddling power to maintain a 3 mph speed.

A Grumman aluminum 17’ canoe might require 20 foot pounds of paddling power to maintain the same 3 mph,

and a kevlar cruiser might require 10 pounds for the same 3 mph.

To throw in another variable, hull speed is likely to also be proportionate to gliding distance. To standardize this theory the amount of weight in a canoe should be kept the same according to its category, with day tripping about 400 pounds, and tripping about 600 pounds. The canoe should be paddled at hull speed in flat water, and the paddling stopped. The canoe with the longer glide will also be the canoe with the higher hull speed, and lower required foot pounds of paddling power to maintain hull speed.

That’s my .02. Happy thinking about paddling!

yup. I understand
I was just thinking out loud and responding to the post that said most people will never reach hull speed.