Hostellers Sailing Club

Extracts from message lists

Introduction

Here are a few of the more informative messages I have come accross on the internet, eg from the 'openboat' list which a number of DCA members write to. The following is a small and rather random selection, there are plenty of other messages equally of interest but I probably missed them since I don’t find time to read more than a fraction of the hundreds of messages sent to lists such as egroups 'Openboat'.

I have added my own headings/comments in red, the rest is just the original message unaltered. Obviously opinions expressed in these messages are not necessarily shared by myself or by other members of the HSC

...snip..... It's not that difficult, but it's not as simple as a distance measurement, either. It's conservation of mass, conservation of momentum, conservation of energy, and what makes a fluid different from a solid. Conservation laws are all about balancing the books - accounting for what goes in and out of every little spot and making the flow picture consistent across the whole system. What you call cause and what you call effect is largely a matter of what you know already and what you're trying to figure out. It all has to be consistent.

- In a solid, shear stress is proportional to strain. That's why a solid resists being deformed and springs back when it's not being forced. In a fluid, shear stress is proportional to the time rate of change of strain. It has no resistance to being bent, squeezed, or twisted into any shape you want. But if you're moving through it, so the fluid has to get out of the way, then it resists. As soon as you stop, there's no resistance - it doesn't spring back and stays in whatever shape it's in. So all the net forces in a fluid are related to velocity in some way.

- Conservation of mass says that the time rate of change of the mass inside any volume you want to name is equal to all the mass flow into the volume minus all the mass flow out of the volume. Just like your bathtub with the drain open and the water running. If you're talking steady flow, then the time rate of change of the mass in any volume is zero and you have to balance the books on what goes in and what goes out. So if fluid is exiting one place rapidly, there has to be more area letting in the slower fluid to make up for it. In math terms:

rho*Vn*A = constant

(rho=density, Vn = velocity component normal to the area, A). It's conservation of mass that makes the flow speed up when you put your thumb over the end of the hose - the area goes down and the density doesn't change , so the velocity goes up.

- Conservation of momentum says that the change in momentum of the fluid is equal to the net force that's being applied. And the change in rotation of the fluid is equal to the moment that's being applied. So if you're producing lift, which by definition acts at right angles to the freestream velocity, then there's fluid that's being pushed aside. And if you're experiencing drag, there's fluid that's being slowed down - or dragged along with you, depending on your point of view.

The combination of no resistance to shear and not rotating if there's no applied moment lets fluid adopt a kind of weird motion called irrotational flow. Say the water's swirling down the drain of your bathtub and you drop a match on the surface of the water kind of in the middle where it's not next to the wall and it's not right in the center of the swirl. You will see it going in circles, but it's not rotating - if the head was pointed North when you dropped it, it stays pointing pretty much North even as it goes around in a circle. That wouldn't happen if you dropped it on a record player (remember those?). In that case, if it was pointed toward the center, it would continue to point at the center and would point at all points of the compass as it went around. Both matches are going around in a circle, but the match in the water isn't rotating while the match on the record is. That's because the water in the middle isn't rotating - it's irrotational.

Now the only way that can happen is if the fluid in the center is moving faster than the fluid farther out. That shears the living bejeezzus out of it, but it doesn't care because it's a fluid! The record is a solid, and it's not going to shear so it describes rigid body motion where the velocity drops off as you get to the center. Right at the center of the fluid, though, is a core that has solid body rotation - if it didn't, the velocity would be infinite there. So if you drop your match on the center of the swirl, it will spin like a top.

So in irrotational flow, the fluid can be going straight, it can be going in a circle, whatever. But it's not rotating. Remember conservation of momentum? If the fluid is going in a circle, there has to be a centripetal force acting on it. Which brings me to...

- Conservation of energy. Bookkeeping of the theromodynamics. You gotta keep track of the potential energy, the kinetic energy, the rotational energy, thermal energy, the chemical energy, the heat that's added and the work that's extracted. We're not talking hypersonics and dissociated flows here and we're not burning or cooling anything, so the chemical energy is frozen and there's no heat being added. For our discussions about lift and drag, we can leave out the potential energy. It plays a big role at the free surface, but that's not the topic of discussion right now. So we're down to the thermal energy, rotational energy, kinetic energy and work.

For the moment, put aside rotational energy (irrotational flow) and just consider the balance of thermal energy and kinetic. If there's no work going on, the sum of thermal energy and kinetic energy has to be the same as the fluid flows along. That's where the famous Bernoulli formula comes in. Total pressure, PT, is the sum of the thermal energy (which shows up as pressure for our constant-temperature flow) and kinetic energy.

PT = P + 1/2 * rho * V^2.

So if we consider the same blob of fluid and we don't do any work on it and we don't heat it and we don't rotate it, then the total pressure remains the same and there's this relationship between velocity and pressure at point (V0, P0) to the velocity and pressure at another point (V1, P1):

PT = P0 + 1/2 * rho * V0^2 = P1 + 1/2 * rho * V1^2

(P1 - P0)/(1/2 * rho * V0^2) = 1 - (V1/V0)^2

If point 0 just happens to be the conditions far upstream of the boat, and we're considering the case of a boat in a uniform flow, then all the blobs going by the boat start out with the same pressure and velocity (P0, V0). In that case the quantity on the left, above is called the pressure coefficient, Cp, which is a nondimensional measure of the change in pressure from the freestream to the point under consideration. It also uniquely defines the ratio of the local velocity to the freestream. When P1 is less than P0, Cp is negative and V1 is faster than V1. When P1 equals P0, Cp = 0 and V1 = V2. When V1 is zero, Cp = 1 and P1 = PT, the total pressure. PROVIDED the flow is irrotational and we haven't done anything to change its total energy. It's really just an exchange between the thermal energy and kinetic energy.

Now go back to the swirl in the bathtub and the match going around in the irrotational flow. Remember conservation of momentum? If the flow is changing direction, that's a change in momentum. So there must be a centripetal force being applied to it. The blob of fluid the match is sitting on has higher pressure on the outside and lower pressure on the inside. Which means there's a higher velocity on the inside than on the outside, according to the conservation of energy formulae above. Which is what we said before when describing irrotational flow. So it all checks. It's all consistent.

So now let's look at a sail, and for simplicity, I'll take a cat rig. Start with the uniform flow of the wind. The flow that's parallel to the sail's got no obstacle. The flow can't go through the sail, so the component of the wind that is normal to the surface of the sail has to change. If we make the sail board flat and line it up with the wind, the wind only has to go around the thickness of the rig and there's not much change. But if we start to incline it, the component of the wind that's normal to the sail is increasing. At 90 degrees angle of attack, all of the wind is normal to the sail. So angle of attack enters the picture. If we camber the sail, the direction that's normal to the surface is different at each point, so we have to apply the no-flow-normal-to-the-surface thing at each individual location. But that's just a detail.

So we've got the uniform flow and we've got to cancel out the part of it that's normal to the sail somehow. The flow pattern that's consistent with that is to put a row of vortices - little bathtub swirlies - along the camber line of the sail. We can adjust their strength so that where they cross the camber line they exactly cancel out the normal component of the free stream. Now the swirlie at the trailing edge has it easy. But the swirlie just forward of that has to be stronger because it's on the back side of the first swirlie who's flow has turned around and is now crossing the camber line from windward to leeward. So the second one has to be a little stronger than the first, and so on to the leading edge where things get really intense. But when you add up all their contributions, they just cancel out the flow that was trying to cross the sail. And when you step away from the sail and look at their contribution elsewhere, they all add a component to the velocity that is headed front-to-back on the lee side and back-to-front on the windward side. Add that to the freestream, and voila, the flow is faster on the lee side and slower on the windward side.

Now we knew it had to work out that way anyway, because anybody can see that if you angle the sail, it will deflect air to the side. And that's a change in momentum. And that means there's a force at right angles to the flow - the lift force (conservation of momentum). And the air that gets pushed to the side has to be replaced by other air (conservation of mass). And so you got a whole lot of air curving to the side and that centripetal pressure gradient (lift) with low pressure inside the curve means the air inside the curve is flowing faster than the flow outside the curve (conservation of energy).

So if you start way to the lee side of the sail where the wind is essentially undisturbed and work your way to the back side of the sail, the velocity has to be speeding up because you're moving to the inside of the curve. When you reach the lee side of the sail, it's really whipping along. And if you start way out to the windward side of the sail where the wind is essentially undisturbed and work your way toward the sail, the velocity has to be dropping because your headed toward the outside of the curve. By the time you get to the sail, things are looking kind of sluggish. So conservation of momentum says the air will be faster than the wind on the lee side of the sail and slower than the wind on the windward side of the sail.

But, hey, that's the same thing produced by the fact that the flow can't go through the sail, so the sail acts just like a sheet of vortices - you get higher speeds on the lee side and lower speeds on the windward side. And the fast flow near the sail produces a pressure drop on the lee side while the slow flow near the sail produces an increase in pressure. And the difference in pressure between windward side and lee side, when added up over the whole surface of the sail, just equals the lift force that was required to turn the flow to follow parallel to the sail, depending on the sail's angle of attack and camber. So it all checks. It's all consistent.

Until you get right to the surface of the sail. Because an interesting thing happens there. The air that actually comes in contact with the sail sticks to it. It doesn't move along the surface. When you look up at it from the boat, it's not moving. Zip, nada, velocity zero. But just a short distance from the sail, the flow is really moving. Stopped on the inside, moving on the outside - that's like the phonograph record. Solid body rotation. The boundary layer is rotational flow. There's a change in rotational energy. The energy balance isn't the same as in the irrotational flow on the outside. In the boundary layer, pressure and velocity aren't related any more. In fact, the pressure is pretty much constant from the outside of the boundary layer to the surface. Which is handy, because you can calculate the velocities and pressures outside the boundary layer and then apply those pressures to the surface. It's especially handy because the simple Bernoulli relationship above doesn't hold in the boundary layer or in the wake from the boundary layer because it's rotational flow.

If you plot the velocity as a function of distance from the surface, it starts off as a linear variation and then blends into whatever velocity exists outside the boundary layer. The slope of that curve is the velocity gradient. And the shear stress is proportional to the gradient at the surface. The higher the velocity outside and the thinner the boundary layer, the higher the sheer stress. The shear stress slows down the flow in the boundary layer next to the surface. And that layer slows down the flow just of it. Which slows down the flow outside of that. So the boundary layer grows as the flow moves downstream, with the influence of the surface diffusing outward. It's like running into a traffic jam on the freeway - the traffic is stopped at the accident by the side of the road, and while the inside lanes keep going fast for a while, the outer lanes get slowed one by one until eventually even the inside lane runs into brake lights. Add up the shear stress over the entire surface and you have the skin friction.

The skin friction is a force. So there has to be a change in momentum in the flow. The change in momentum is the slowing of the flow as it enters the boundary layer. When the flow leaves the leech, it's still slowed. If you took a cross section of the flow velocity across the wake you'd see a dent in the profile where the slow flow was being shed by the boundary layer. In fact, one of the most accurate ways of measuring the drag of an airfoil in the wind tunnel is to measure just such a profile and add up the momentum deficit in that dent.

The cool thing about this is the rotational flow of the boundary layer is just what's needed to act as the cores for the swirlies that provided the circulating flow outside of the boundary layer. So it's strange but true that both the lift and the drag both have their origin in the boundary layer. And the fact that the flow can't penetrate the solid sail. And the way the direction of that no penetration is controlled by the thickness, camber and angle of attack. And they way the whole flow picture stays balanced with respect to the conservation laws.

Finally, just as people change lanes to avoid the congestion in the slow lanes, the slowing flow in the boundary layer moves the outer flow outward. Conservation of mass again. Plunk a tiny box down on the surface straddling the boundary layer. the flow going out the downstream side of the box is slower than the flow coming into the upstream end. There has to be more area letting the flow out than is letting it in (rho*V1*A1 = rho*V2*A2, V2A1). So part of the flow that enters the front face of the box flows out the back and the rest of the flow goes out of the top of the ox - an outward flow. This outward flow effectively modifies the shape of the sail (the fancy term for it is the boundary layer displacement thickness). Which modifies the flow required to match the effective shape. Which modifies the pressures. The net effect of the change in pressure is a decrease in pressure over the aft part of the airfoil compared to what it would be if there were no boundary layer. This is called the pressure drag. The displacement thickness is different on the windward and leeward side because the velocities are different so the shear stress is different so the boundary layer development is different. This difference in effective shape side-to-side amounts to an effective change in camber, so the lift is affected, too.

So that's where lift and parasite drag (skin friction + pressure drag) come from. And it's all consistent and it all agrees with what's actually observed and you can actually use it to make predictions that work.

That's my story and I'm sticking to it.

Cheers,

Tom Speer me@tspeer.com http://www.tspeer.com

An informative email all about rowing our kind of boats (ie. Non competitive rowing not using sliding seats or outriggers)

Subject: Rowing a Wayfarer & Oars

Date: Wed, 22 Nov 2000 00:07:07 -0000
From: "Werner Cook" <werner@fsmail.net>

Here are some extracts from several sources about rowing and oars.
They come from Wayfarer News and from the DCA Bulletin.

From Wayfarer News Winter 1998 Issue 80:
By Bob Harland

For salt water a person is said to be rowing when using a pair of
oars, one in each hand, but pulling if using both hands on one oar.
For fresh water a person is said to be rowing if using one oar and
sculling with a pair of oars. (Interesting but a bit academic!?)

Gearing:
Gearing refers to the ratio between the length of oar inboard of the
pivot point (ie the rowlock) and that outboard of it. The greater the
gearing the longer and steadier the stroke. Most rowers can manage a
gear ratio of 1:3, above this most people will tire more quickly due
to difficulty in balancing the weight of the oar.

Length:
Starting with a gear ratio of 1:3, the inner length is restricted by
the width of the boat. if one person is going to pull (on salt
water!) we get a minimum inner length of 24 inches and a maximum of
say 30 inches. This equates to a minimum oar length of 8 feet. If
your oars are any less than eight feet then rowing is going to be
harder work than it need be. As it turns out, 9 to 10 foot oars are
about right for rowing a Wayfarer.
However there are limits to the length of oar that can be stowed in a
Wayfarer, depending on what mark of Wayfarer you have. You may find
that 8 feet is the maximum length you can accommodate. If you are
fortunate to have a World then 9 feet is no problem.

Material:
Sitka Spruce (from Canada) They are lighter and stronger but cost a
lot of money.

Blades:
Spoon shape is ideal for calm water and are the most efficient
(wastes less energy) but they are prone to 'catching crabs' in rough
water. In rough water the immersed area of blade must be variable and
a long narrow blade with flat surfaces is best.

Formula for finding out best oar length assuming ratio of 1:3

('B' is the beam of the boat between rowlocks and a small gap between
oars of say 6in)

3X(B/2-3in) + B/2-3in = 2B-1ft


From the DCA Bulletin (Date unknown) by Peter Bick:

Dinghies for all waters by Eric Coleman
Boats, Oars and Rowing by Peter Culler
Working Boats of Britain by Eric McKee
Small Boats by Phil Bolger
Sail and Oar by John Leather
Oars for Pleasure Rowing by Andrew Steever

A table of oar lengths :

An old established rule is that the length of the oar inboard and
outboard of the rowlock should be in proportion of 7:18. (thus a 5'
beam would mean 9'oars).

Most rowers can manage a gearing ratio of 1:3 which would accord with
a length of twice the beam, but tire more quickly if this is exceeded
even when the oar is counterbalanced inboard. He thought that oars
should not be completely balanced as they then lack feel. An extra 2
to 3 lbs should be required at the outer end. The blade can then be
immersed without effort.

Traditionally working oars were made of ash with the advantage that
they were ok without leathers. They were heavy though. Best quality
oars are made of spruce but are very expensive. In the old days oars
were leathered where they went through the rowlock horns, and the
leathering was usually at least 8" long. Nowadays you get moulded
short plastic collars which are horrid. If you decide to leather your
oars which is one way to improve them then try for at least 10"
lengths, you might like to row at times with the grips overlapping
which gives you greater leverage against the wind. Longer leathers
make this possible.
Whatever you do, don't have anything to do with plastic rowlocks and
sockets, they are an abomination. Use bronze if you can afford them
but there is nothing wrong with galvanised iron.

On inland waters you are taught to row with long strokes catching
your water early; this is fine in smooth water but not rough. If
there is a sea running and particularly if you are against the wind
then keep your strokes short, using a quick in and out action.

A touch of vaseline or tallow on leather and rowlock stem makes
rowing much more agreeable.

Again from the DCA Bulletin (date unknown) by John Rawson

Oar handles don't normally overlap, about 1" gap is allowed so that
you don't smash your thumbs, should you put them over the end of the
oar.

A gearing ratio of 1:2 is very low and 1:3 is very high. Usually you
are around 1:1.5 to 1:1.75 or so. (I THINK THIS IS A TYPO AND SHOULD
READ 1:2.5 TO 1:2.75) Fast boats have high gearing but boats that
need fierce acceleration need low gearing.

The area of the oar blade is about one square foot, related to the
strength of the oarsman. The shaft usually finishes to just over 2"
diameter, again related to strength. The wood should be a softwood,
ie faultless Sitka, as this is less flexible than a hardwood shaft of
the same strength. Flexibility feels uncomfortable but the energy
used in bending the shaft is not lost.

At rest the oar blade should be covered by the water. This needs the
centre of gravity to be so placed that the oar will drop if released.
A downward force needs to be applied to the handle to raise the blade
from the water. This force on the handle should be about 4 lbs.

Now for my own thoughts:

I have a standard set of commercial oars (with plastic collars) about
7ft 6" long which just about fits on the floor boards either side of
the centre board case. (of the Wayfarer - ed) I have about 1 to 2 inches spare space. I use
galvanised rowlocks. I must say that to get the oars to dip into the
water properly the bottom of the oar shaft rubs on the gunnel. Rowing
is not comfortable and the plastic collars are getting a little
chewed up. Ideally I would need to have longer oars which then mean a
storage problem. I have seen some Wayfarer owners who stow their oars
partially on the side bench and poking forward under the foredeck.
This always looks very untidy to me and I think the oars so arranged
might get in the way of things. However this arrangment allows for
much longer oars.

I will always carry a set of oars though. If all else fails, (the
sails and the O/B motor) I will still have some form of 'motive'
power, even as inefficient as it is. A single oar can be an emergency
rudder and a depth sounder. I have seen a Wayfarer propelled with a
single oar on the transom (sculling).

If I were to sort things out a little better and money was no limit I
would have a set of oars that could be 'broken' down into two parts
for ease of storage. The method of securely and simply jointing them
would need to be high quality. They would be of Sitka Spruce, perhaps
laminated for stability, leathered and have epoxy reinforcing to the
blade tips. The rowlocks would be bronze (self lubricating and much
smoother than standard galvanised cast iron). Some form of foot rest
on the floor board (detachable) would make rowing more comfortable, I
really think you need somthing to push your feet against when rowing.
I might even fit a rowlock somewhere on the transom so that the
option of sculling could be explored and of course it would make a
very quick jury rig emergency rudder.