Michael Daly wrote:
Marsh Jones wrote:

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.

On a bike, the force due to wind resistance increases with the square of
the velocity. Add to that the rolling and friction resistance and the
overall effect is pretty much the same as resistance felt by a boat in
water. Take a look at the drag versus velocity characteristics of a
boat and note the somewhat arbitrary point chosen for "hull speed". Try
Marchaj's "Sailing Theory and Practice" at your library. Then compare
it to a graph of total resistance versus velocity for a bike and see if
you can find a meaningful difference.

If you want to see a kayak or canoe move faster than hull speed without
a lot of paddle effort, try surfing a big wave. That's just like riding
a bike downhill.

Boats and bikes pretty much follow the same behavior. Both are moving
in fluids and whether air or water, the physics is the same.

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.

I promise not to write about business if you stop making up physics. :-)

Mike
Mike,

OK, my analogies between bike and boat suck. And I'm just explaining
the physics badly. I deliberately left out rolling resistance and
laminar flow and all that stuff *bacause it isn't important to drafting
in a boat*.
Drafting works on a bike because if you are behind, you are riding in a
lower pressure area and that the effectiveness of this draft increases
fairly smoothly as you get closer. There is no *noticable* period to
the wave coming off a bike - just an increase in resistance which makes
you put out more effort.
This isn't the case in a canoe. Yes, hull speed is fairly arbitrary,
and yes, it's pretty easy - assuming you have the skills, power and boat
design - to surf a wave and exceed hull speed without extra effort from
the human motor - but that throws a new item in the equation - gravity.
You are paddling downhill, and use the effects of gravity to overcome
the bow wave and surf/plane. That's not what I'm talking about.
Drafting in a canoe or kayak is using the waves generated by your boat
and the boats around you. It is very, very different than riding a
bicycle. Most fla****er canoe/kayak racing takes place at, or near
'hull speed'. Not surfing, but at a point where the power to make the
boat go faster increases so rapidly that normal human beings cannot
sustain that effort. The only exception to this is when you get shallow
enough that you can plane a canoe of flat water by overcoming the bow
wave, and even then the amount of effort required to sustain that is
quite high, and difficult for even the best to maintain for more than a
few minutes.
Arbitrary, yep. Different for different boats, yep. And a big
difference from just playing with the physics of a single boat. Since I
don't have a copy of Marchaj's book, I can't compare the graphs you
cite, and I'm quite certain of their validity. But I doubt there is much
written in there about the effect of sitting 1/2 boat length off the
stern and just to leeward of the lead boat in a one design race. It
just isn't a place to be in a sailboat. Very different application, and
the position of the trail boat relative to the wave generated by the
lead boat is meaningless compared to the fact that the lead boat is
stealing the wind.

Marsh