# Dynamics of a raft in water



## KlaustheK (Mar 20, 2021)

My family recently built small boats to race in a farm ditch with some friends. We used 1" Styrofoam sheeting and popsicle sticks. I built mine in the shape of a raft, and my secret weapon was a small keel near the front that hung down into the "fastest" part of the current, and I covered the entire top surface of my boat with popsicle sticks, making it act like a heavily loaded raft. My wife built a very light cataraft. Hers won. I do think mine was taking advantage of the faster subsurface current, but I think that sitting deeper allowed mine to be affected more by eddies from the shore and other currents. Hers seemed to only be affected by the main current going down the center.


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## 81939 (Jun 16, 2020)

See Chapter 2 section 4 in Open Channel Hydraulics, Chow (1959)


http://web.ipb.ac.id/~erizal/hidrolika/Chow%20-%20OPEN%20CHANNEL%20HYDRAULICS.pdf



and Chapter VIII in the Textbook of Theoretical Naval Architecture, Attwood (1899)


http://www.survivorlibrary.com/library/text-book_of_theoretical_naval_architecture_1899.pdf



Seems like the idea of going faster conflicts with the concept of rafting, no?


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## Wallrat (Jan 19, 2021)

On our recent MFS trip, I was running 16’ Jaguarundis, Sam had 14’ Maravia tubes. I compared our relative drift rates several times. Both of us drifting pointed across the current , there was no discernible difference.
With both boats pointing downstream, I would overtake him time after time, even without either of us rowing. I decided it was due to the longer water line, and the higher theoretical hull speed. That’s a concept from sailing, where the farther apart the bow and stern waves are, the higher the theoretical hull speed is.


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## jamesthomas (Sep 12, 2010)

All I know is that my Hyside 16 ft cat seems fast compared to most other boats of a similar size. Seems like small rafts like 10-13 footers fly down the rio compared to me though. ???


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## upacreek (Mar 17, 2021)

Let me preface this by saying I didn't do great on the exam covering Naviar-Stokes nor am a some fluid mechanics guru, but maybe can at least sketch out the basics as (hopefully) understand them...sans the confusing math and diagrams. Perhaps the two main things consider for this quandary are 1) the resistance of water moving through a sloped open channel creating a velocity profile from the bottom boundary layer to the top that transitions from turbulent (slow) to a relatively laminar (fast) flow; 2) the sheer stress of the submerged cross-sectional area of the boat "slipping" with/against these varying currents nearest the surface of a non-compressible fluid (water). 

IIRC in a smooth and uniform channel, this translates to the fastest current being dead-center just below the surface where resistance is lowest; but rarely if ever does anything natural have that kind of predictable morphometry and smoothness since both roughness and non-uniformity generates all kinds of water column turbulence/eddies in the real world of rivers not to mention with bank and object interactions. When measuring the flow for example in a wadeable stream, you always measure velocity at 1/3 of the total depth to limit these boundary layer effects. Also as sort of an interesting aside, there are ways to estimate the bottom roughness that generates this turbulence and the resulting velocity profile using dimensionless coefficients like Manning's N, but it can also be field measured and tested via methods laid out by Wolmann, Cowan, etc. Anyhow, the reason a rivers roughness/eddies/turbulence likely matters is that the depth of your boats draft influences whether the turbulent vortices deeper down start acting against your forward movement, which is also a function of its mass vis-à-vis inertia as well as boat shape and surface smoothness to be imparted those forces by. This is where stuff like Reynolds #'s and hull dimensions start coming into play like with ship designs, then things only get trickier still when going beyond the frictional force to include those of wave and wind resistance along with their ever shifting vectors. I think Klaus made some excellent observations that sorta deduced all of this, but in the spirit of a relatively short answer would say it'd be pretty complicated to optimize your raft speed against the dynamic flows and forces of a river...especially when can just eye-ball the relative surface velocity in certain parts of the channel or thalweg using bubbles, flotsam, and so forth to find your speediest line. But then of course, what's the hurry?


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## Smoregon (Aug 14, 2017)

Endopotential said:


> Can't help but wonder how I can go faster downstream without expending extra energy


Use a motor.😁


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## Quiggle (Nov 18, 2012)

I’m no scientist but let me add my smart ass comment, I find that I move the fastest out of the group when I’m in the current,bow pointed where I want to go and my blades are in the water (this is the part most people forget). Oh yeah it also helps to keep your craft throughly hydrated, I find spilling 2-3 beers a day on her to be sufficient.


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## Endopotential (May 14, 2019)

Thanks for the awesome replies! Attwood is a classic, even after 120+ years.

So I'm guessing that the qualities that favor dead drift speed get in the way if you're actively rowing? ie a deeper draft that helps catch the faster laminar flow ends up imparting resistance once you start rowing it past a certain speed?

Some people just enjoy the scenery floating by, which is a huge part of the fun. I often wonder what is under the surface which propels us along.


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## 81939 (Jun 16, 2020)

There are really two components of this scenario- 1) finding the location in the river with the highest velocity flow and 2) optimizing vessel drag for the water and air.

1) In "physics world" (in a vacuum, neglecting the air/water boundary, and perfectly uniform channel, bottom/water boundary condition)) you are correct that area in the river with the highest flow velocity is the immediate subsurface in the center of the channel, immediate being the key word. Immediate is synonymous to an infinitesimally small horizontal slice of the vertical velocity profile. The further below the surface, the lower the velocity. So theoretically speaking, if you were floating in this system, you would want to float as shallow as possible to only expose your raft to the highest velocity flow. In this scenario (no air resistance and at a dead drift) the boundary condition between the raft bottom and flow would eventually become static as there is no relative difference in velocity between your raft and the flow so the characteristics of the raft, besides draft wouldn't matter.

As upacreek pointed out, natural river channels are not uniform and there are many different variables that contribute to the flow velocity profile. Throw in the wind/water boundary condition, waves, vortex shed, the coanda effect, surface tension, and viscosity differences in silty rivers it gets more convoluted and less predictable. Simplistically speaking, the area of the river with the highest velocity flow, depending on the wind is likely just below the surface in the deepest part of the channel. The more wind opposite to the current the deeper below the surface that may be.


Then there is drag.

2) So you have lots of different scenarios with drag. If your goal is to dead float as fast as possible with no wind, you would want to increase your drag coefficient relative to the highest velocity flow with the water and decrease it with the air. Optimizing this ratio of air to water drag for maximum velocity becomes a pretty difficult multivariable calculus problem if you were to actually do it. Putting a paddle blade in the water probably helps depending on where you put it. In really windy conditions a drogue might help (water parachute).

If you have a tailwind, this does a 180. Increase your drag with the air and decrease your drag with the water and you just might be able to move faster than the current without expending energy.

If you choose to paddle, the efficiency of your hull becomes the limiting factor to maximum hull speed. I'd bet that with rafts, cats, etc. the fineness ratio (length to with), and skin friction drag are the primary opposing forces to the maximum power output of the human rower. I'm not sure how applicable theoretical hull speed is to non-ridged hulls such as inflatable rafts and cats as the the hull may deform to the wave it's actually creating. Several people have mentioned cats being faster than rafts. I believe this is due to the improved fineness ratio of cat tubes but I could be wrong.

Science!

_Edited for spelling and revision of theoretical hull speed applicability because I'm not so sure after thinking about it more._


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## jbolson (Apr 6, 2005)

Dakib_SW said:


> Science!


Actually these discussions are engineering, for what it matters.


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## 81939 (Jun 16, 2020)

jbolson said:


> Actually these discussions are engineering, for what it matters.


Debatable. I'd say it depends on what part of the discussion you are referring to. Describing the highest velocity flow is probably science. Optimizing float efficiency - more engineering. It's al based on scientific principals though - even the engineering.


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## jbolson (Apr 6, 2005)

To add to the actual discussion, with regard to boat design and forward acceleration. A long boat without much rocker will track straighter and thus be faster (less side to side movement). Then consider the resistance of the boat up front. Less frontal area equals less water resistance and will be faster. I think both these concepts support cats being faster to accelerate compared to regular rafts. Now the draft is a negative for acceleration, and an overloaded cat is a pig because of greatly increased draft. Of course you also sacrifice turning ability with these attributes. Always tradeoffs...


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## jamesthomas (Sep 12, 2010)

Wow, you guys are going all kinds of Dr Science here. Love it.


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## Heywood (Apr 12, 2019)

I think the importance of being on or near the surface is not as important as some may think. Here is a velocity profile diagram. 










The curve really flattens towards the surface and is probably a negligible difference when you think about 2 to 6 inches of draft for a loaded raft in river over 3 ft deep (or more). Any raft on the surface (ish) is going to be at the top end of the open channel velocity. 

As others have pointed out the flow is much more complex than a 1d vertical profile. Here is a 2 d diagram of flows in different channels. 










The bottom right is where we mostly find ourselves and its obvious that its variable with fast parts and slow parts. Now imagine the reality of 3d flow. The path of highest velocity is twisting and turning and changing velocities as we accelerate over a rifle or slow down in a pool. There are eddies and cross currents and boils. All of these things are interacting with your raft/cat bottom and ultimately its very complex. How do these interactions change your speed? 

The stuff above is pretty settled and the rest below is my thoughts/opinion which is probably wrong. 

As Dakib mentioned drag is the primary way your hull and the river interact and I agree with his idea that max drag will help the boat match the current speed at a dead drift. Conversely that drag can be something to overcome when accelerating, slowing down or going changing direction from the current. So a raft/cat with lots of draft (ie. overloaded) will probably have the most drag and take advantage of the high velocity current compared to say a unloaded drift boat which will kind of skim along the surface without much interaction/drag from the current. Length and a longer water line can cause it to track straighter and keep better aligned in that narrow strip of fast current. I used to load up my 13.5' raft with way too much stuff and I could cruise in flatwater without needing to row because (my opinion) the current could really push my half sunken boat along. During windy flatwater paddle outs in the Royal Gorge we would partially deflate our floors and have the guests sit on the floor to increase drag on the bottom of the boat to catch the current and reduce air drag. Meanwhile our theoretical drift boat is getting blown upstream.

When it comes to how fast you drift down a river you have to think about keeping your craft in that twisting and turning thread of faster current, provided you have some drag to take advantage of it. Here is where momentum becomes important (opinion). Back in my overloaded raft I have used drag to take advantage of the current on the long straight stretches of Ruby Horse thief but now I am entering a curvy section. I have a heavy raft moving fast (mass x acceleration=momentum) and I shoot straight out of the current and hit the shore as the river bends becase my raft wants to keep its momentum in the direction it is moving (inertia). The current has changed directions but my momentum easily overcomes the drag (trying to keep me in the current) and I now have to work really hard to overcome my momentum and the drag if I dont want to hit the shore. Theoretically in a lighter craft with less momentum the drag forces could redirect the boat and keep it in the fast current. However, I am just guessing that no boat has the right combination of drag and low momentum to fully allow the current to keep you on the fast line all the time. So we have to row or paddle to keep ourselves in the sweet spot so the current does most of the work. 

my hands are tired now and I am probably wrong but that is how I see it.


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## dpwater (Aug 2, 2011)

Great thread and thoughts. I'm thinking gradient if you want to go fast and non stop. Though for those pool drops staying with the current is critical. Cats are fast till the wind factor. Yet it seems from observation that PVC rafts have less drag and go faster in general. What's the material difference?


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## johng (Apr 25, 2005)

I think Heywood nailed it. Very nice post! There's theory, which is really interesting, and then there's experience, which is often much more practical. Like so many things in life, there are a lot of details, and they matter.


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## Rightoarleft (Feb 5, 2021)

This is the kind of stuff I think about when I fall asleep. IMHO, nobody truly understands river hydrodynamics. We can become familiar through study and observation but here's the problem --humans think in two dimensions. Fluid dynamics is a three dimensional concept. We hardly understand how a wing works, and wings have been the focus of intense study for a century. Contrast the simplicity of a wing with the complexity of a river channel and you start to grasp the magnitude of our lack. 

This is what we love about river. Out of infinite complexity comes a sort of order, and with that we dance. As far as being fast, when I see somebody pushing miles it tells me they don't really want to be there. If you can't slow down and take a river as it comes, you can find somebody else to run with because it won't be me. 

And with that, I have spared you all the wordy hydrodynamic theories that reside in my head. Maybe a later day.


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## Endopotential (May 14, 2019)

Rightoarleft said:


> This is what we love about river. Out of infinite complexity comes a sort of order, and with that we dance.


Ah, you are a poet and a wise man. Well said. 

That pretty much satisfies my idle curiosity. Thanks everyone for your posts.


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## whip (Oct 23, 2003)

Should not this thread be titled: Dynamics of water in a raft?


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## Endopotential (May 14, 2019)

whip said:


> Should not this thread be titled: Dynamics of water in a raft?


Are you talking about the clear snowmelt stuff, or the carbonated hop barley variety?


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