Wm Watt wrote:
ace wrote:
I was following behind this guy about 10 ft for entire race. It took
quite a bit of effort to steer to stay behind him. I tried hard to get
closer but for some reason I couldnt. I guess I could have got behind
someone going slower.


Theoretically, if both paddlers and boats are perfectly matched, then
some small variable like wind or bow wave will make the difference.
However you have to ignore the advantage to the following paddler of
following a pace boat until the time comes for the final sprint. In
practice I think no paddlers and boats are perfectly matched.

In more extreme conditions position is important. Other posters have
written of cars and bicycles. When driving from Ottawa to Florida in a
small car, tucking in behind a semi on the I95 and driving in it's slip
stream conserves fuel. You just hope none of the watermellons on those
trailers in the Carolinas fall off the back. Drving from Ottawa to
Vancouver in a small car, trying to pass a semi in a head wind on the
praries my car could not break through the "bow wave". I tried a few
times comming up beside the cab but that was a far as I got. These were
low powered cars, Dodge Colt and Ford Festiva.

As far as paddlers staying together, it's probably because of the steep
gradient on the exponential hull speed curve. Small increments in speed
require large differences in paddler power which cannot be sustained
for long. I'm sure kayaks and canoes can be designed so the curve takes
off at given rates of power, cusomizing the hull to an individual
paddler's strength and endureance. I don't know if anyone actually does
this.

Sailors race under much the same conditions and have developed standard
strategies for competing in close quarters. Winning is as much about
strategy and position as boat speed. Boat speed is a necessary, but not
a sufficient, condition.


Your Festiva example is pretty good for aerodynamic drag and drafting -
and yeah - drafting semis in a Festiva is extreme. We used to do it in
an old VW microbus:-)

Low speed hydrodynamics is also an extreme condition. When you get two
racing boats, or boats racing, that are similar enough in speed and
strength that one doesn't just 'up&leave' drafting happens.

On a smaller scale, it breaks down somewhat. In bicycles, you aren't
moving enough fluid medium (air) to come near 'hull speed' - it's a
matter of how much power you can generate to make the bike go. My
"proof" is that while I can still briefly go 30MPH on the flat, I
can(used to) go 64MPH down a mountain. There is no 'hull speed'
limitation until a bike is moving faster than mere mortals dare tread.

Boats, in a very viscous medium (water) behave quite differently. Water
behaves quite differently from air. Being a business major and all, I
don't have enough strong science to explain all the theory about waves -
my lab is the river/lake. It's pretty easy to get a racing canoe or
reasonably fast kayak to hull speed. And as has been covered here, hull
speed is simply the speed at which the 1st wave period is the same
length as the boat. There is also a second wave, third, forth etc
continuing backwards from the boat. These waves "Vee" away from the
hull at some angle - which is at least partially defined by the shape of
the hull. The amplitude of waves is affected by several factors - shape
and resistance/displacement of the hull, depth of water, and probably a
few that I've left out. It isn't just as simple as calculating a conic
shape with a given displacement. It's how the boat sits in the water at
speed in various conditions, how tapers back in, and how it behaves with
other boats. A USCA Cruiser is the same length and width at the
gunwhale as a Pro canoe, but they have different underwater shapes and
draft very differently.

Drafting on a bike is pretty straightforward. Assuming the same speed
for both bikes, it takes more watts to draft at 20feet back (3 lengths)
than it does at 10, 5, etc. At 6" from the back wheel, you feel like
it's a pretty free ride. In racing boats, the draft is much more
difficult. At 17', you are stuck behind the 2nd wave, and are working
your butt off to get closer, but you can sit at 18' pretty easily and
maintain position unless it gets shallow all of the sudden. If you have
enough strength to punch thru, you can get on the downhill face of the
wave and close quite quickly to the stern ride. This takes some skill
to hold position, because the boat generally wants to wander away from
directly behind (as opposed to bike where the low pressure zone is felt,
and makes it easier to stay there)

However, as the depth goes from infinite (+20') to 4-5 feet, the waves
will shorten and steepen, and you can easily get stuck teetering on top
of a big stern roller and wind up going backwards quickly. This 2-5'
water is known as suckwater to most canoe racers. It sucks to paddle
in, and the stern is 'sucked' down - because you are now paddling up the
hill of your bow wave that's being reflected off the bottom quite
noticeably.

So the next (and usually best) place to be is on the same wave as the
lead boat - side wake. No similar position exists for bikes. Side
waking takes much more experience and work, because you are now
constantly getting sucked in to the other boat or pushed out. In a
canoe, it usually means riding with the bow paddler somewhere around the
center thwart, but different boats and conditions move this up or back.
However, this is about the only successful place from which to mount an
attack, especially in shallow water. Again, it's an empirical sense,
but I think riding the side wake also may increase the effective 'hull
speed' of the two boats together by moving the stern wave peak further
back. [Anybody want to tank test this?]

Popping a canoe or kayak occurs when you get the boat up and over the
bow wave. Done well, drafting can continue here, done poorly and the
poor sucker in back can be walking when they suddenly wind up being
pushed onto the beach or run out of water in the belly of the wave.
Racing canoes love 6" deep sand bottom rivers.

So there's a short and possibly refutable summary of why drafting on a
bike can't directly be compared to drafting/riding in a boat. By all
means, poke holes in it, clarify points, and elaborate on the science.

Marsh