How close do you have to be to benefit from drafting
 

Marsh Jones wrote:

Drafting works on a bike because if you are behind, you are riding in a
[...]
This isn't the case in a canoe.
Drafting in a canoe or kayak is using the waves generated by ... the boats
around you.

You are correct, of course, drafting on a bike is "hiding" behind the lead rider
whereas drafting in a paddled boat is riding a wave. Totally different
concepts. You don't need to get too complicated to explain that.

Most fla****er canoe/kayak racing takes place at, or near
'hull speed'.

Over short courses (Olympic ICF class boats) the race is at speeds well in
excess of hull speed - over twice hull speed is routine. That only demonstrates
that hull speed is entirely arbitrary and is nothing resembling a speed limit.

In longer races, that level of power output can't be maintained by mere humans,
so the speeds drop to lower levels.

Mike

How close do you have to be to benefit from drafting
 

Thanks. I took enough math to know you are right.

Michael Daly wrote:
ace wrote:

Would this be true for different atomospheric pressures? Is this a
rough rule of thumb? I dont see how there can be such a tidy formula
for something as variable as air. what if air was replaced by carbon
dioxide.


The formula applies for any gas, any density. Check any book on fluid mechanics
or aerodynamics or Wikipedia: http://en.wikipedia.org/wiki/Drag_%28physics%29.

Mike

How close do you have to be to benefit from drafting
 

My kayak seemingly creates no waves; only a smooth v-shaped channel
that trails the boat. It seems to iron out the choppy waves. Isnt there
an advantage to follow in the wake when leading boat irons out choppy
waves. It is almost always a bit choppy in the ocean.


Michael Daly wrote:
Marsh Jones wrote:

Drafting works on a bike because if you are behind, you are riding in a
[...]
This isn't the case in a canoe.
Drafting in a canoe or kayak is using the waves generated by ... the boats
around you.

You are correct, of course, drafting on a bike is "hiding" behind the lead rider
whereas drafting in a paddled boat is riding a wave. Totally different
concepts. You don't need to get too complicated to explain that.

Most fla****er canoe/kayak racing takes place at, or near
'hull speed'.

Over short courses (Olympic ICF class boats) the race is at speeds well in
excess of hull speed - over twice hull speed is routine. That only demonstrates
that hull speed is entirely arbitrary and is nothing resembling a speed limit.

In longer races, that level of power output can't be maintained by mere humans,
so the speeds drop to lower levels.

Mike

How close do you have to be to benefit from drafting
 

I paddle on a stretch of river where ther is a racing club. They have
the ribver bouyed for racign in lanes. I don't know how they calcuate
the width but I suspect it's wide enough to keep boats from interfering
with each other. Also, the two hulls on a sailing catamaran have to
have open water between them at least 1/3 of the waterline length of
the hulls. If racing without lanes I'd try putting about 1/3 of the
boat length between the boats abeam before making a move to overtake.
Might work.


Over short courses (Olympic ICF class boats) the race is at speeds well in
excess of hull speed - over twice hull speed is routine. That only demonstrates
that hull speed is entirely arbitrary and is nothing resembling a speed limit.

In longer races, that level of power output can't be maintained by mere humans,
so the speeds drop to lower levels.

Mike

Mike is right on. Froude's formula was developed, I believe, for the
British navy (taxes at work) in the days of sail. They were fat heavy
boats with low power. Sailboats need to be fat so the wind doesn't roll
them over. Canoes and kayaks are long, narrow light boats with
proportionally more power. They slice though their own bow wave and
don't sit in their transverse wave. Kayaks are only half as wide as
canoes so they are faster although they are more prone to roll over.
Even more extreme are catamaran hulls and two are needed to keep from
rolling over. I don't know the actual limits to Froude's formula or if
there is an adjusment factor incorporating light displacement and
extreme length-to-beam ratio. When more power was available from
internal combustion engines the British navy did get Nathaniel
Herreshoff to design long narrow light displacemet boats with little
armour or munitions for racing into harbours and dropping torpedoes or
spies and racing out again. Nothing but aircraft could catch them. The
Brits called them Fairmile, the yanks PT (patrol torpedo).

TF Jones in his two books discusses long narrow hulls. He likes to
write about light boats that go fast with low power. All such boats are
notable for their small wakes. They disturb little water as they pass.

How close do you have to be to benefit from drafting
 

Wm Watt wrote:

I don't know the actual limits to Froude's formula or if
there is an adjusment factor incorporating light displacement and
extreme length-to-beam ratio.

The mistake people make is to assume that Froude's formula for hull speed
actually represents a meaningful number for analysis or design. It is simply an
observation that there is a speed-length ratio where the bow wavelength is the
same as the waterline length. It is only useful in comparing two nominally
identical hulls of different length. It is of no real value otherwise. Marine
architects and engineers do not use hull speed for design.

In real vessels, if you tow them and measure the bow wavelength and then
determine the speed at which it equals the waterline length, you will find that
is is not likely to be precisely 1.34. It may be more or less, depending on the
shape of the hull.

If you look at a graph of speed versus resistance measured from a towing tank
test, you cannot find a point on the graph that represents "hull speed". The
curve is smooth and shows no change in magnitude or slope that would show where
hull speed occurs. There is no manifestation that would suggest a rapid
increase in resistance. There is no indication that the vessels is "climbing
its bow wave".

Vessels do not climb their bow wave - you cannot climb a wave that you create.
That would be like holding a rope up with your left hand and claiming you can
climb it with your right. You cannot push through the bow wave for the same
reason. What happens is that the vessels changes apparent trim angle to match
the wave and you continue pushing the water out of the way. This starts with
_any_ motion of the vessel - it does not start at hull speed. The faster you
go, the more energy it takes.

It's too bad that the term used is "hull speed". It does not represent the
speed of the hull. I wish the term would go away as it has generated far more
bull**** than meaningful discussion on boat performance.

Mike

How close do you have to be to benefit from drafting
 

Michael Daly wrote:
Wm Watt wrote:

I don't know the actual limits to Froude's formula or if
there is an adjusment factor incorporating light displacement and
extreme length-to-beam ratio.

The mistake people make is to assume that Froude's formula for hull
speed actually represents a meaningful number for analysis or design.
It is simply an observation that there is a speed-length ratio where the
bow wavelength is the same as the waterline length. It is only useful
in comparing two nominally identical hulls of different length. It is
of no real value otherwise. Marine architects and engineers do not use
hull speed for design.

In real vessels, if you tow them and measure the bow wavelength and then
determine the speed at which it equals the waterline length, you will
find that is is not likely to be precisely 1.34. It may be more or
less, depending on the shape of the hull.

If you look at a graph of speed versus resistance measured from a towing
tank test, you cannot find a point on the graph that represents "hull
speed". The curve is smooth and shows no change in magnitude or slope
that would show where hull speed occurs. There is no manifestation that
would suggest a rapid increase in resistance. There is no indication
that the vessels is "climbing its bow wave".

Vessels do not climb their bow wave - you cannot climb a wave that you
create. That would be like holding a rope up with your left hand and
claiming you can climb it with your right. You cannot push through the
bow wave for the same reason. What happens is that the vessels changes
apparent trim angle to match the wave and you continue pushing the water
out of the way. This starts with _any_ motion of the vessel - it does
not start at hull speed. The faster you go, the more energy it takes.

It's too bad that the term used is "hull speed". It does not represent
the speed of the hull. I wish the term would go away as it has
generated far more bull**** than meaningful discussion on boat performance.

Mike
Mike,

I totally agree, and I stand chastised and corrected for using the term
"hullspunik" or whatever. Since it is by your definition impossible to
climb over the wave, what is happening when the point at which the hull
separates from the water moves from up on the knuckle of the bow to a
point several inches (or more) behind the knuckle? BTW, at this point
my energy output to sustain this position has decreased below max, and
the speed of the boat exceeds the speed at which this max energy output
occurs.

How close do you have to be to benefit from drafting
 

Marsh Jones wrote:

what is happening when the point at which the hull
separates from the water moves from up on the knuckle of the bow to a
point several inches (or more) behind the knuckle?

I don't know what you mean by knuckle. What kind of canoe/kayak are you talking
about?

Mike

How close do you have to be to benefit from drafting
 

Michael Daly wrote:
Marsh Jones wrote:

what is happening when the point at which the hull separates from the
water moves from up on the knuckle of the bow to a point several
inches (or more) behind the knuckle?

I don't know what you mean by knuckle. What kind of canoe/kayak are you
talking about?

Mike
Racing boats, in particular, since this whole thread started talking
about racing. Most racing canoes and kayaks have a very sharp break
between a fairly vertical bow and the 'keel line'. Look at the bow on
the Stratus for example
(http://www.wenonah.com/CDKayak/image...Stratus18.jpg). There is a
very well defined knuckle at the keel end of the bow. Many non-race
oriented boats will have a much softer turn to this point, which does
make it easier to turn _as a rule_ -assuming they have the rest of the
design sorted out.

That knuckle.

Marsh

How close do you have to be to benefit from drafting
 

I agree now "slicing through it's own bow wave" is an incorrect
impression I got from reading TF Jones. He has built plywood mulithulls
and may have got the impression from watching the point of separation
between the hull and the laminar flow move aft as speed increases.
Michael is right about the left hand and the right hand even when the
left hand does not know what the right hand is doing as may sometimes
be the case. :)



Michael Daly wrote:


---

Vessels do not climb their bow wave - you cannot climb a wave that you create.
That would be like holding a rope up with your left hand and claiming you can
climb it with your right. You cannot push through the bow wave for the same
reason. What happens is that the vessels changes apparent trim angle to match
the wave and you continue pushing the water out of the way. This starts with
_any_ motion of the vessel - it does not start at hull speed. The faster you
go, the more energy it takes.


Yes, now I see it's another mistaken impression from observing the
stern depressed by it's own wave. One shouldn't believe everything one
reads. How many fat-assed boats have been built to increase bouyancy
aft to compensate! :)

I think I see the correct interpretation now. It's elementary physics.
The force required to push each molecule of water out of the way,
starting at rest and accelerating to some terminal velocity, is

F = MA = MD/T**2 , where M = mass of water molecule, D = distance
pushed

As the boat changes speed neither the mass of the water molecule nor
the distance it gets pushed changes so the force is proportional to he
inverse square of the time in which the water molecule has to get out
of the way of the boat. As the boat slows the time increases and the
required force diminishes, as the boat speeds up the time decreases and
the required force increases.

The total required force is the sum over all the water molecules moved
out of the way which depends on where each is in relation to the hull.
The sum over all the water molecules is the volume of water pushed out
of the way but that also increases and decreases with the boat speed so
the sum includes another time factor. That seems to imply the force
required to push all the water out of the way is a function of the cube
of the boat's speed instead of the square. I just thought of that time
factor on the way over to the public library to type this into the
computer so have not thought it through.

Two long held impressions changed in one afternoon is an interesting
event for which I am grateful to Michael.

How close do you have to be to benefit from drafting
 

Marsh Jones wrote:

That knuckle.

Ahh, the forefoot :-)

I can't explain what you described. When I've seen racing kayaks in
competition, the forefoot is usually buried and the foredeck can be seen rising
and plunging due to the action of the paddler.

The only time I notice the separation point moving aft with speed is when I'm
riding a big roller - I know I'm on the wave for a short bit of surfing if I can
see the separation point move aft to a certain point and the sound is just
right. On a breaker, this doesn't happen as I have to try to avoid purling.

Mike