Friday, December 28, 2012

Air Subduction in Kayak Paddles and cavitation in lee boards

I don't know if subduction is the right word to describe the phenomenon of air getting sucked down along the back side of a kayak paddle, but there I am using it.  Dictionaries indicate that common usage of subduction only applies to the geologic phenomenon of one piece of the earth's crust getting shoved under another but here I am with no one to stop me and so I am expanding the usage of subduction.
I mention subduction because it is a phenomenon that impacts paddle efficiency adversely but looks so normal when it happens that it's easy to miss entirely.  Subduction happens because pulling the blade of a paddle through the water creates a low pressure region on the back side of the paddle.  If the pressure is low enough, air will get sucked down along the blade of the paddle in response to the lower than atmospheric pressure there.  The result is reduced efficiency of the paddle.  Why? Because the amount of thrust you get from the paddle depends on the pressure difference between the front and the back of the blade.
Furthermore, this phenomenon seems to happen only with paddles that have long narrow blades.  At the start of the stroke with a long bladed paddle, only part of the blade is in the water and part of it is still out of the water, as you pull on the paddle, low pressure is created on the back side of the blade and the relatively flat surface of the blade that is sticking out of the water directs air down the back of the blade.  I suspect that water coming off both edges of the blade creates stable vorteces on both sides of the blade with a low pressure area in the middle that becomes the pathway for air subduction.
Commercial paddles with short, wide blades don't generate this phenomenon because the blade is fully immersed in the water before power is applied to it and the only thing sticking out of the water is the circular loom which does not produce any pathway for air to travel down to the low pressure area behind the blade.
But the phenomenon of air subduction with a long skinny blade is not unavoidable.  Depending on how the paddle is held and moved through the water the phenomenon can be avoided.  If the paddle blade is moved sideways as well as straight back, water moving across the face of the blade will not form the vortices that allow air to travel down the face of the paddle.
Since air subduction is dependent on low pressure on the back of the paddle blade, the lower the pressure, the higher the likelihood of subduction. Pressure is force divided by area so the same amount of force on a smaller area will create a greater pressure differential. And since on paddles with long narrow blades, only part of the full blade area is submersed at the start of the stroke, much more of a pressure differential is created across the face of the blade than in a paddle with a short wide blade where the whole blade is already fully submersed at the start of the stroke.

Lee board on a shallow draft Dutch craft.

The lee board deployed.  The angle at which the board is deployed controls the amount of lateral resistance that the board supplies.  The backward rake also allows the board to kick back and out of the way harmlessly should it hit bottom.
Not surprisingly the phenomenon of air subduction can appear anywhere that you move a  fin shaped object through the water like for instance a lee board on a sail boat.  The purpose of the lee board is to provide lateral resistance to a sail boat when it is sailing at an angle to the wind.  Some sail boats use keels to achieve the same results but lee boards are more handy in places where the water is shallow.  And they don't take up space in the cargo area of the boat like a centerboard does.  Lee boards are generally used on relatively slow moving boats which cargo boats generally are. And the triangular shape of the lee boards on Dutch craft puts most of the surface area of the board toward the bottom of the board, keeping the top of the board relatively narrow to minimize air subduction.
A slightly related phenomenon to air subduction is cavitation.  Unlike air subduction which happens when a foil pierces the surface of the water, cavitation generally happens when a foil is moving rapidly while fully submerged. If the speed of the foil is sufficient, it can create pressures low enough to cause water to vaporize. If you remember your high school physics, the temperature at which water turns from a liquid to a gas gets lower and lower as pressure drops.  The net effect of water turning to vapor is the same as air getting sucked into the water, it reduces the efficiency of the moving foil.  Generally, cavitation occurs in high speed propellers, but it can also happen in vertical fins on fast moving water craft like the one I posted about a few days ago.  Read more about it on the Vestas sail rocket site.

Wednesday, December 26, 2012

Skin on Frame Housing - the yurt gets a roof

Progress on the yurt continues.  Much of the progress is imperceptible to anyone but myself but the progress is there.  As it turns out, my initial suspicion that my first yurt would be a prototype for working the kinks out of the design turned out to be true. So let me share what I've discovered so far.
 I got the crown on the central ring of the yurt.  I used some kayak rib stock, 1/4 inch by 3/4 inch white oak, socketed into the central ring and lashed together with tarred seine twine. The idea behind the crown is that it will be open under normal conditions but covered when it is raining or heat needs to be kept in the yurt.
Another discovery I made after the initial assembly of the yurt was that for the sake of getting a good setup it helped to trace a circle on the ground first before erecting the walls so that the walls would form a perfect circle.  The perfect circle is essential so that all the rafters are the same distance from the central ring.
I also discovered that I had much more wood in the central ring than I needed and so it was too heavy and difficult to get into position when no help was available. What I did to remedy the weight problem was to cut some of the extra wood away from the center of the ring.  It was way stronger than it needed to be. 
 The next step was to cut out some fabric for the roof.  It took me some time to work out the trigonometry for the roof cover shape, but I did it.  But I have learned to distrust theory and test with an actual life size cover.  The other reason for getting the cover in place was that a series of rain storms were coming into the area and that would be a good test to see how the cover would stand up to the wind.
Here's an inside shot of the cover.  The wind lifted it some and pushed it off center.  Part of the problem is that the shape of the cover is like a pie with a wedge cut out of it and the edges adjacent to the missing wedge get pulled together and glued or sewn resulting in the desired pyramidal shape.
The cover is made out of some sort of vinyl covered fabric that is used for advertising banners.  A friend gave me a bunch of these banners and I cut them up to fit the roof. The fabric has two problems.  One is that it is very heavy and the other is that it is very stiff.  The combination of weight and stiffness makes it very difficult to pull this stuff up on the roof.  The weight also works against its portability.
On the plus side, the cover survived the storm and did not get blown off.  I also discovered that once again, there is nothing like building something to test a design because until you do, it's next to impossible to know what sort of problems the design has.  So far, none of the design elements have been complete failures but lots of them can use improvement.
The list so far.
The wall battens are slightly too flexible.  A bit more stiffness is advisable even at the expense of greater weight.
The skin is too stiff and heavy.  Lighter weight is advisable.
Other commitments have kept me away from the yurt, but the learning continues.  I will continue to post whenever there is progress or even when design ideas fail.

Sail Speed Records

New speed records for sail powered water craft are being set constantly. The latest by the Vestas sailrocket is 65.45 knots.  Of course these are records set under ideal conditions.  One should not expect cargo carrying sail boats to reach these kinds of speeds.  Still, regardless of limitations these records are impressive and raise the question, what makes these speeds possible and why weren't they achieved in the past? In other words, what has changed?
I suspect the answer to the question is multifaceted.  Perhaps the biggest impediment to new boat designs was tradition.  It took a while to apply airplane technology to sail design.  It also took a while to bring the weight of a sailing craft down enough so a sail could generate enough power to make the sailing craft rise on a hydrofoil.  Speed in a watercraft seems to be limited by friction between the craft and the water and minimizing that makes new records possible.
But in the past, new, faster sail boats evolved from working craft that had to operate under all conditions and not just special race course conditions.  One thing that is apparent from watching the videos of sail records being set is that the wind seems to be off shore with the craft racing alongshore, thereby avoiding waves. 
And there are any number of other things that a useful work boat has to be able to do that a racing craft cannot do.  Still, new ideas are always welcome.  I am looking forward to seeing recreational versions of these craft.  Who knows, perhaps a skin on frame model that does 20 knots would be a fun experiment.  Planing hull, no hydrofoils. 
The little experimenting that I have done with putting sails on skin on frame kayaks has taught me that the inherent flexibility of skin on frame hull limits how much sail one can carry.  The faster the boat is expected to go, the stiffer it needs to be, or in any case, stiffer than a boat that travels at 4 knots.

Sail Drone, further thoughts

In my original post on the sail drone, I expressed some pessimism that the thing would work.  In the meantime, the saildrone people put up some more information about their project and their designers.  Seems like they have enough experience to make the sail drone work.  One of the interesting things about the ocean is that the bigger the thing floating on it is, the more vulnerable it is to the forces of storms.  Small things like plastic bottles or coconuts can cross oceans with impunity.  So as long as a drone is small enough, it is probably impervious to damage. 
Still some problems remain for any autonomous drone.  The main one would seem to be collisions with ships and land masses.  Autonomy is one thing, avoiding collisions is another and adds complexity to the design.  The other problem with anything floating on water and containing electronic gear is water proofing.  As anyone who has ever bough a waterproof anything knows, waterproof is an optimistic term.  Water seems to be able to outwit almost any sealing technology so far invented.  Perhaps the most reliable technology is still the cork.  But we will see. I am keeping my eye on the sail drone website.

Saturday, December 1, 2012

Sail Drone

While driving to work a few days back I noticed some people hoisting a small red airplane looking thing from a trailer.  As the lettering on the wing of the thing indicated, it was a saildrone. When I got home I went to their website There wasn't anything there except a home page, but they have since fleshed it out some.  Anyway, some pictures of the saildrone.
 Here they are unloading the saildrone from its trailer.  At a distance it looks like the tail is at right angles to the main wing but that's an optical illusion.  The horizontal spar that the tail is mounted on is actually at an angle to the horizontal plane and that creates the illusion.
 That's a lot of crane to unload this small a thing, but the crane was already in place to take the Swedish Americas cup boat in and out of the water.  OK so some overkill is justified.
 The head on view shows the arrangement of the verticals.
And here it is sitting in the water.  Even with a little wind blowing, it heeled way over.  More ballast, I would think.  I also can't picture something this small operating autonomously in the ocean with breaking waves.  Snap that thing in pieces in no time and bend the heck out of any metal.  But I might be wrong.

Sunday, November 18, 2012

Swedish Americas Cup Boat

I thought I'd post some more pictures of the Swedish Americas cup boat.  Far as I know, it's the same as all the other contenders' boats except for the paint scheme.  These pictures have no particular significance other than that my shop is right next to Alameda's seaplane lagoon where the Swedish team is keeping their boats and so I see these boats on a pretty regular basis and occasionally feel compelled to take pictures of them when they are looking particularly impressive.

A shot of one of the smaller boats.  The Swedes have one with white lettering and one with red lettering.

On an afternoon when the wind is blowing, both of the Swedish boats are out on the bay with their support boats following behind.

A few Saturdays ago, the Swedes put their big boat in the water.  Crane at the right. 

A tighter shot of the big boat.  Crew members in red shirts hang around on shore, probably drinking something with alcohol in it.

Meanwhile, the smaller boats are still around.  The rainy season has started.  Hence the clouds.

The big boat being towed into the seaplane lagoon with the reserve fleet in the background.

Saturday, November 17, 2012

Double Ridged Unangan (Aleut) Kayak Paddle

A while back, Suzawa Ikunori of Japan posted some pictures of several Unangan paddles in a Japanese Museum. Unlike most of the Unangan paddles I had seen before which have one face which is more or less flat and one face which has a ridge running down the center,  these paddles had ridges running down both the front and back faces. 

The paddle is quite long assuming the the loom is the usual length of about 22 to 24 inches.

This photo shows three of these double ridged paddles.  The middle one seems to have some damage to the tip.

And this is a drawing of the paddle blade and the relative size of the ridges and the location of the blade edge.
It appears that the layout of the blades is similar to that of the single ridge paddles except that the normally slightly convex side of the blade also has a ridge carved into it.  I have taken one of my single ridged paddles which I found to be too stiff and heavy and have carved away at the flat face of it to create a low ridge on that face.  Testing of this configuration is still pending. 
One thing that double ridged paddles might eliminate is arguments about which is the right way to hold them, ridge forward or ridge backward.  But since the paddle is not symmetrical, there is still room for argument.  I hope to test my own version of the double ridged paddle soon and report on results.

Friday, November 16, 2012

Google Maps Water Policy

As residents of the Western United States are probably aware, there have been a number of drought years with negative results on both the crops that need irrigation and on recreational users that like to go waterskiing on the state's reservoirs.

Just recently we returned from a vacation in Sequoia National Park and on our way out we passed Lake Kaweah, one of the many reservoirs at the edge of the Sierra Nevada mountains.  What we saw at the east end of the lake looking west was this. A mostly dry lake.  The remaining water was mostly near the dam.  The whole reservoir looked like it was at no more than 20 percent capacity.

Back home, I went on google maps to see what amount of water was actually left.  Surprisingly, on google maps, the lake looked 100 percent full.
In a way, it made sense for the map makers to fill the lake with virtual water since on maps, the convention is to show lakes up to design capacity.  So it also make sense to make the pictures match the maps. But on zooming in, I also found that while map makers wanted to show water where it was supposed to be and not where it actually was they did at the same time have a desire to show the latest road data.

And here it is, real roads under virtual water.  The deal is, if people are going to be able to launch their boats, you're going to have to build roads up to the edge of where the water actually is, that is several tens of feet below the surface of the virtual lake.
So map makers past fake water on top of the dry lake bed.

Curiously, in this particular case,  the surface of the virtual water had a cool vortex right in the middle of one of the bays.
I was intrigued and curious if Google followed the same policy of topping off reservoirs with virtual water all around the state of California.  I moved over to San Luis Reservoir just west of Los Banos.  This reservoir was also way down the last time I drove by it. 

The long view again showed a lake topped off to the brim.

But the closeups showed an un-doctored view, bathtub rings showing previous lake levels and trees growing on former lake bottom.
Research on this topic is continuing.

Why Build Your Own Kayak?

A friend of mine recently posted a picture of a customized car from circa 1970 Houston, Texas.  What struck me about the photo was that the owner of the car clearly wasn't satisfied with the amount of chrome that designers back in Detroit were putting on cars at the time, so he took matters into his own hands and added chrome to a level that he felt was appropriate.

And this same sort of urge, not the urge for more chrome, but the urge for more cool is what led me to start building my own kayaks. It was not about money, it was simply the desire for a superior esthetic over what commercial manufacturers were capable of delivering. Others may disagree.  Many prefer the look of shiny plastic.  I prefer the cool of skin on frame.

Wednesday, October 31, 2012

The Water We Eat

One of the things that traveling does is give one a broader perspective on the world than is available from staying in one place.  My recent trip to Kings Canyon National Park was no exception.  One insight I had on this particular trip was about water, where it comes from and where it ultimately goes.
We spent most of a week camping right next to Kings River which drains all the water that collects in Kings Canyon. The river at this time of the year, October, does not carry much water since it hasn't rained all summer and the melt-water from glaciers higher up in the Sierras is minimal. 

Still, he water of the river is cold and clear even late in Summer when temperatures during the day still rise into the nineties.
And as is usual for me when staying in a place for a while, I become curious about it, specifically about the Kings River.  Where does it go?  I don't remember where I got the answer, but the answer is, into a reservoir downstream in the foot-hills of the Sierra from which the water goes to the city of Fresno and to the surrounding farmland for irrigation. And so, the water of the river next to which we camped never reaches the ocean. And even before the reservoir was built and before the land in the valley was farmed, the river seldom reached the ocean. Most year it drained into the shallow Tulare Lake which most years had no outlet. Only in unusually wet years did the water of the lake overflow into the San Joaquin River which flows into San Francisco Bay.
But now, the Kings River irrigates farm land and the water that first fell as snow in the Sierra Nevada range ends up in almonds, peaches, plums, oranges and other produce, much of which is trucked out of the state via interstate highways and gets consumed far from its origins by people who have no clue that the water in the peach that they are eating may have also cooled my body on a hot day in October.

The Giant Sequoia and Some Random Thoughts on Trees and Wood

It is one thing to see wood at a lumber yard and it is quite another thing to see it in the form of a living tree.  Unlike the stuff in the lumber yard which gives little evidence of where it came from, the living tree obviously speaks for itself.  It is not a product but an individual being with a considerable amount of character that makes it distinct from others of its kind, especially when old.
The musings I am about to deliver on the nature of wood were prompted by a trip to Kings Canyon and Sequoia National Parks.  The two parks are adjacent to each other, with Sequoia to the south and Kings Canyon to the north.  The only way to approach these parks by car is from the Central Valley of California.  Kings Canyon is adjacent to the city of Fresno and Sequoia is adjacent to the city of Visalia. Giant Sequoias can be found in both parks, although Sequoia National Park has more of them. Coming from the Central Valley which is flat and almost devoid of trees except for fruit trees in the orchards, it is hard to imagine that one is about to enter a mountainous zone where the huge sequoias live. 
But on to some pictures. I would like to say off the start that it is impossible to convey the massiveness of these giant trees in photographs.  The best one can do is show part of the tree with a human or two next to it for scale.  But even then, the photograph cannot capture the sense of presence that these large trees convey. Nevertheless, here are some photographs.
This trunk of a Sequoia has been hollowed out by fire and decay and so it is possible to walk through and along the trunk. 
Here is another sequoia that has fallen.  Someone cut a hole through it that one can walk through. The hole by the way is about seven feet tall.

Here is a picture of the tree named the General Sherman.  This is the world's most massive living being.  It is suspected that some of the trees cut down when logging first began were larger than this, but General Sherman is currently the largest individual.  Its diameter at the base is in excess of 30 feet. 

 And here is the root ball of a recumbent sequoia, 30 feet from side to side.
 And using a deer for scale.
 And more humans for scale.
 And a tree hugging human for scale.
Even the branches of these trees are huge.  This one fell off a tree and the park service left it laying where it fell to give people an idea of their large size.  The largest branches on the General Sherman are about 7 feet in diameter.

The sequoias are not the tallest trees in the world.  That distinction goes to the coastal redwoods.  Neither are they the oldest trees. That distinction goes to the bristlecone pines of the White Mountains, one valley to the east of the Sierra Nevada.  But the sequoias are the most massive trees. Their trunks seem to be columns of almost constant diameter rising some 300 feet into the air and then seeming to end abruptly in the older specimens.  

Someone took a slice of a sequoia trunk and propped it up at an angle for the edification of the public. This slice represents roughly 2000 years of growth.  Age is determined by counting the annual growth rings.  The age of standing trees has to be estimated because there apparently is not any equipment that can remove sample cores of the requisite length, roughly 18 feet needed to count all the rings from the center of the tree to its edge.

 This is a side view of that specimen.


And here is a closeup of the growth rings in that slice.  The top of the picture is closer to the center of the tree where the older wood resides.  The crack running horizontally through the picture is a fire scar.  Moving downward toward the crack, the growth rings get ever closer together, an indication of ever diminishing growth.  After the fire, there is an explosion of new growth that again diminishes over time.  Apparently, fires burn the vegetable matter on the ground and release nutrients into the soil which spur new growth for a few decades until most of the nutrients are once again tied up in vegetable matter until the next fire comes along.  Also, note the little circular impressions just above the crack.  These are probably cores taken for carbon 14 dating and then plugged. 
When botanists try to estimate the age of trees based on diameter, they look at slices like this one and try to get an average count of growth rings per inch since as this picture shows, ring density changes with every fire and also with annual precipitation.

There was no sign indicating what sort of wood these park benches were made out of, but I suspect that they were made of sequoia wood. Yes, they haven;t stopped logging sequoias.  Not all of them are protected. Some of them are on private property and some of them are in national forests which are administered by the US Department of Agriculture which regards forests as cropland meant to be harvested. Not an editorial comment by the way. If we want to make stuff out of wood, we have to cut down trees.  More on that at the end.

Here the park service has done a prescribed burn or someone dropped a cigarette. In any case, not much damage was done.  For a long time, the park service used to suppress fires but found that periodic fires are better than monster fires fueled by debris on the ground that has accumulated for too long.  Also, they found that sequoias will not reseed very well without fire.  Fire turns vegetable matter into ash, releasing nutrients for the seedlings to grow.  Fire also kills off young trees that would otherwise shade out the new seedlings. And fire also opens up the cones to release seeds.  So now the park service does prescribed burns which do little damage but reduce the amount of fuel that could produce more dangerous fires.

This picture shows young sequoias coming up with the older fire scarred sequoias in the background.  The ash and the light also allow ferns to grow.

Another view of the burned ground.  Yes the ground burns since the top six inches are a compacted mass of needles and small branches.
Almost every old tree shows fire scars. As a matter of fact, I have never seen so many burned trees as I saw in Kings Canyon National Park.  In the national forests, fire scarred trees are more rare since those are logged over on a more regular basis and fires tend to be suppressed because the trees are regarded as a crop, not a part of an ecosystem in which fire plays a beneficial role.
This is a side view of about eight inches of fibrous sequoia bark which lacks resin and therefore does not burn readily.  Bark on sequoias can be up to two feet thick and this is what lets them grow old in an environment where fires occur naturally every seventy years or so.
Here the fire burned into the trunk of the tree but the segment at the left is still alive.  Every yearn the bark spreads sideways to cover over scars and in time will cover them completely.

Here is a segment of  a branch. The heart wood is red and the sap wood is a lighter color.  The heart wood of the tree is dead.  The sapwood is the part of the tree that is still alive and growing and is where the tree adds more wood around the outside of the tree under the bark. For purposes of lumber we want heart wood because the sap wood decays more readily.
So I was curious what kind of wood sequoias are made out of or perhaps more properly, what kind of wood they grow.  Right on the ground near this broken branch above I found a small piece that had broken off when that branch came down.  It was red in color like that branch but on carving some off, I found that the red color was mostly a product of weathering and that the wood was a lighter color.  It was also surprisingly light, similar to redwood but with a consistency that felt more like cedar.  And then it occurred to me, why not?  If you are going to grow large, why not build yourself out of light wood.  Since most of the weight of the tree is in the trunk, the lighter it is, the larger you can build it.  Sequoias and other conifers don't have huge spreading branches like oaks and so most of their strength is needed to resist the compression of the mass that piles up on top of the roots, mostly in the trunk.  Hardwoods like oak on the other hand that put out large lateral spreading branches need stronger wood to resist bending forces and stronger wood is heavier.  And so boat builders like conifers because they produce long straight trunks that can be cut into long straight boards.

As a boat builder I use wood. And as a paddle maker I also use wood, especially wood from older trees since they are the ones that have the wood that is clear of knots. But to use wood without a knowledge of what happens to the forests where that wood is taken from is akin to buying a pork chop in a supermarket without knowing that one needs to kill pigs in order to make that pork chop.
The impact that the visit to the sequoias had on me was to see trees as individuals.  Young trees a few decades old, especially in large quantities tend to be rather similar in appearance and lack distinction as individuals.  Every tree seems to look like every other tree and so one can more readily regard them as a crop to be cut for use as lumber.  Old trees however, tend to be distinctive, each one different from the next and since in the case of the sequoias they may have been around for perhaps two thousand years or more, one is forced to realize that cutting one of those trees for human use deprives the rest of the world from the presence of such old trees for another two thousand years.  And so it seems that to cut such old trees is rather frivolous because the gain in lumber does not make up for the loss of the presence of the individual.  
And yet, as a wood worker, I use wood.  And since I make objects for other people, I want to give them a good product free of flaws and blemishes.  I want to give them something that looks like wood is something that is extruded by a machine. 
Perhaps I need to rethink my approach.  If I do not want to participate in the cutting of old trees, then perhaps I need to figure out a way to make paddles from younger trees whose wood is not quite as perfect and blemish free as that of older trees.  Stay tuned, I have some younger trees in mind that I will experiment with