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