How close do you have to be to benefit from drafting
 

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.

I was in a short adventure race this weekend . There was 4kms of sea
kayaking. I ended up in a soltice gts. The rudder was stuck down so I
had to use rudder. The cockpit was small and i could not move my legs.
My feet were on the pedals but just kind of resting on the pedals. When
I paddle with my small kayak with fixed rudders , I really shove my
feet hard down on pedals.

How close do you have to be to benefit from drafting
 

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.

I was in a short adventure race this weekend . There was 4kms of sea
kayaking. I ended up in a soltice gts. The rudder was stuck down so I
had to use rudder. The cockpit was small and i could not move my legs.
My feet were on the pedals but just kind of resting on the pedals. When
I paddle with my small kayak with fixed rudders , I really shove my
feet hard down on pedals.

At 10 feet, you were probably sitting on the uphill side of the wave.
In order to effectively draft, you either need to be a little further
back, or a lot further forward. Depending on the actual speed, and the
boats involved, you will usually find you are running into the guy's
rudder unless you slide just a little to the side (which works fine as a
rule). At 1/2 wavelength, you'll be paddling uphill, and actually
working harder than paddling by yourself. A lot of it is just plain
feel and experience, but if you practice riding with another boat,
you'll find it gets easier as time goes on.

The GTS is a tough boat to fit in if you've got big feet or big legs.
That keyhole makes it a little tough. But - you just need to adjust
your technique a bit so that your feet are both pushing against the
pedals and more flexing than actually moving. Tough, I know, but it can
be done. Also, you might try paddling without shoes - just socks. I
know - the beach is a little cold, but you may find you get better fit
under the deck that way. It only takes a few seconds to slip running
shoes or cycling shoes on at the end, but running shoe heels really
cramp your feet in a lot of boats.

Marsh

How close do you have to be to benefit from drafting
 

Thanks! You are a regular gold mine of information. Part of the problem
is that I do most of my training in a 10 ft boat with huge cockpit and
rigid pedals. I'm thinking of getting a mirror on my glasses so I can
see what is happening behind me. I'll have to revise my drafting
procedure.

Marsh Jones 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.

I was in a short adventure race this weekend . There was 4kms of sea
kayaking. I ended up in a soltice gts. The rudder was stuck down so I
had to use rudder. The cockpit was small and i could not move my legs.
My feet were on the pedals but just kind of resting on the pedals. When
I paddle with my small kayak with fixed rudders , I really shove my
feet hard down on pedals.

At 10 feet, you were probably sitting on the uphill side of the wave.
In order to effectively draft, you either need to be a little further
back, or a lot further forward. Depending on the actual speed, and the
boats involved, you will usually find you are running into the guy's
rudder unless you slide just a little to the side (which works fine as a
rule). At 1/2 wavelength, you'll be paddling uphill, and actually
working harder than paddling by yourself. A lot of it is just plain
feel and experience, but if you practice riding with another boat,
you'll find it gets easier as time goes on.

The GTS is a tough boat to fit in if you've got big feet or big legs.
That keyhole makes it a little tough. But - you just need to adjust
your technique a bit so that your feet are both pushing against the
pedals and more flexing than actually moving. Tough, I know, but it can
be done. Also, you might try paddling without shoes - just socks. I
know - the beach is a little cold, but you may find you get better fit
under the deck that way. It only takes a few seconds to slip running
shoes or cycling shoes on at the end, but running shoe heels really
cramp your feet in a lot of boats.

Marsh

How close do you have to be to benefit from drafting
 

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.

How close do you have to be to benefit from drafting
 

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

How close do you have to be to benefit from drafting
 

Interesting. You must have majored in accounting or mis. I might buy a
couple small toy boats and work this out in bath tub.

Marsh Jones wrote:
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

How close do you have to be to benefit from drafting
 

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

How close do you have to be to benefit from drafting
 

"On a bike, the force due to wind resistance increases with the square
of the
velocity. "

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.

How close do you have to be to benefit from drafting
 

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

How close do you have to be to benefit from drafting
 

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
 

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