Saturday 2 October 2010

Han's Comment on my blog post "Takapu the Proa a Dissertation"

Hi Han

I read about the Gondola in a "Wooden Boat Magazine" article many years ago. Since then I have wanted to study one closely, in action, to see what other benefits there may be from the asymmetry.

From my experience with the Proa I have learned that good design (especially design features with a long successful history) usually solves at least three problems at once.

I believe that this hull shape could simply offset the thrust of the oar but I believe that the answer may not be so simple, it may also have a more fundamental reason. One reason will almost certainly be the need for extra buoyancy over the side that the oarsman stands, on this long slender hull, to keep the boat level for the comfort of passengers.

Another will be the need to counteract a tendency that long slender hulls have to broaching. My experience with long symmetrical canoe hulls is that they have a strong tendency to track off course when a small force acts across the line of least resistance, i.e. wind or wave action. I believe the reason for this broaching action is the result of a pressure differential that gets started when the hull turns through the flow and water speeds up around the outside of the turning circle. This in turn generates lift which exacerbates the turning moment into a logarithmic spiral. The result is almost impossible to correct with any kind of lateral counter force like a long sweep or paddle (a disastrous situation in the congested busy waterways of Venice!).

With an asymmetric hull the pressure differential is constant and therefore more predictable for the oarsman to counteract. Lastly I believe that the oarsman exerts a slight diagonal force in the thrust sweep of his oar which results in lift from the rounded side of the hull. This thrust/lift combination in turn reduces the amount of effort required to move the hull through the water, the same phenomenon that a fish utilises when swimming.

Re rudderless steering.

The Patin Catala of Spain is a great example which has evolved into a very successful sailing class.  http://woodenboat.com/boat/?p=1358

Best

Harmen
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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.