Saturday, 25 April 2009

Hull asymmetry

"I saw the new pics this morning. It must be exciting to see it that way at last. In one photo I can even see the "Kiribati dimple". Isn't it time for you to release this dirty little secret into the blogosphere?" Gary Dierking

O.K. Gary you asked for it!

I've been asked this question many times, over many years and after much practice, research and reading I think I'm ready to answer.

The so called Kiribati dimple is a feature of some of the variants of proa characteristics found around the Pacific past and present and is most pronounced in the proas of Kiribati.

In the late nineties I was fortunate enough to have been able to converse with several of the great aero-hydro dynamics experts of our time. In Auckland these were professor Peter Jackson and Tom Schnackenberg. These conversations lead to hours of reading in the engineering library at the University of Auckland. I was preparing research material for my thesis to contribute to an advanced degree in design.

Kuchemann was a German professor of aerodynamics invited to continue his research in the United States after the Second World War. During the 1950's he was experimenting with supersonic aircraft design for McConnell Douglas. One of his papers talked about delta wing flow dynamics. This was interesting enough to me however I then came across some experimental wind tunnel work on supersonic fuselage design. The "Coke bottle" profile of several jet fighter bombers solved the problem of shock wave development at the wing root where the wings joined the fuselage. This is an area of dramatic increase in fluid displacement. It was found that where this secondary drag inducing shock wave developed, the fuselage could be "waisted" or reduced in circumference to improve the overall aerodynamic efficiency of the airframe at sub and supersonic speed. It was explained to me that supersonic air has similar flow characteristics to water when compared to fluid flow over a hull.

I made the assumption that a hull with significant rocker and combined proximity to the widest section of the hull of an asymmetric proa (or any displacement sailboat hull for that matter) would benefit from a waist on the high pressure (or leeward) side in order to reduce drag.
I had already had my curiosity aroused on this subject by J.S. Taylor many years before when Mike and I were building Takapu. Taylor published a series of articles in the NZ Seaspray Magazine in which he wrote about his experiments on the hull of a proa with just such a dimple. I recall him relating his experiments which involved the filling and fairing of the dimple on his trial hull. The hull was slower and lifted less after the treatment than before!

Mike and I have since that time incorporated the dimple in our proa design work with good and satisfying results. Both Takapu and Toroa are faster than their predicted maximum hull speed, based on prismatic coefficient calculations, by a significant factor. Furthermore we have found that our hulls are not as susceptible to broaching when sailing down wind in a following sea.

Of course the artists right side of my brain also has a say. Intuitively I know that in nature pure symmetry is anathema in living organisms. Where symmetry appears to present itself there, on closer detailed inspection (on the other side of the coin), you will find variation.

Both the United States Navy and Jaques Cousteau carried out experiments on the form drag of fish and marine mammals. When towed (dead) at their known respective speeds, the from drag of their streamlined bodies was far greater than their musculature could possibly propel them.
I believe that we have been looking at the dynamics wrongly.
No fish dolphin, shark or seal is ever symmetrical when swimming at speed. They are always at any given moment either asymmetric one way, or the other. Whilst asymmetric at the extreme extent of flex the body is curved on one side and hollow on the other (in the case of a dolphin the same is true though of course the asymmetry is in the horizontal plane). Whilst in the fully curved stage of motion, at speed the lift generated is slightly forward of lateral in a similar way that a sail generates lift. The body is inclined to generate a zone of low pressure on each oscillation which reduces drag and effort.

I believe that the canoe builders of Oceania either observed this phenomenon or generations of builders stumbled on the benefits of asymmetry thousands of years ago. Either way the fact remains that it works and it works very well. I would not consider building a shunting proa without this feature when there is so much to gain and so little to loose.

Harmen R Hielkema.

Thursday, 23 April 2009

Toroa paint project

Remember to click on the images if you want a larger view.

This image clearly shows the beak head with the tack peg protruding from the lee side, there's one at each end to arrest the rig as it shunts from one bow to the other.

Ben coving internal corners (viewed from Toroa's leeward starboard hand end).

Sanded and ready for primer

Ben glassing the boom sprit end

Priming the kiato

Ama primed

Hull and decks primed

Here you really see for the first time the asymmetry of Toroa's centerboard dagger rudder slots & hull. Note the hollow in the lee (left hand) waterline.

Sunday, 12 April 2009

April update

image by Juha Merila

These images are of the Toroa, or Mollymawk, after which a headland in the area I grew up was named by the ancient Maori.
Regrettably these beautiful birds are no longer found on our mainland, they have dispersed themselves to remote offshore islands. I believe that there are less than 300 breeding pairs left.

Toroa is completely glassed over and after a heavy sanding is ready for a coat of primer.
Things are progressing more slowly presently as I have had to find full time employment.
I'm so drained after a days work I find it impossible to get out to the proa project at night. Ben Tombs (a local boat builder and musician friend, Ben plays wood wind instuments) has kindly been assisting me with the really toxic stuff and has got me over a major hurdle. I've decided to get him back to finish the paint aspect of the project. I'll post some images soon to illustrate the progress shortly.
Not too long now and we'll be ready to launch.
I've assembled the luff spars which are second hand carbon sailboard spars. The luff spar has been glued butt to butt with a sleeve, the boom has one spar telescoped inside the other.

The trailer has its new draw bar extension and is now waiting for me to take it to the local vehicle service center for a warrant of fitness.

Last thing on the list is the colour scheme which will mimic the colours of the mollymawk.

More later


Toroa by Harmen Hielkema & Mike Toy.

Header Photo: Toroa at Rawene by Julie Holton.

This blog is dedicated to the memory of my father Roelof Hielkema who instilled in me the willingness to learn.
These pages are intended to inform and add to the growing body of knowledge concerning the Canoe Culture of the Pacific, past, present & future, from the Tupuna, the Ancestors of the Pacific cultures to the people of the world.

These pages contain Images and text relating to our two proas, Toroa & Takapu, some history relating to our experiments & experiences.

The dissertation that I posted on this blog in April 2008 "Takapu The Proa" was written by me in 1997 in response to an assignment that I was set whilst studying for my design degree. The dissertation covers many issues that a proa enthusiast may benefit from reading about.

Waka define culture as culture defines waka

Waka reflect the individuality and uniqueness of a society which in turn is governed by the geography, geology, topography, climate, location, resources, isolation, origin, flora, fauna, flotsam, jetsam, etc.

Waka are our link to the past, they have shaped our present and define our future.

Waka are the vessels of knowledge, physical and mental development, freedom of bondage to the land, key to our inquisitiveness, expressions of our ingenuity and courage, our love of shape and form, the seat of our power.

Waka are the source of our material culture, from which all processes are derived.

Waka are who and what we are.