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Tuesday, January 22, 2013

Balancing Act

After testing four different prototypes, here is the latest version of the rudder cassettes that we will supply with your Paradox A Class Catamaran.


Once again, the optimum choice strikes a balance between different considerations. 
In fact, the design of this component is a great illustration of how the best solutions use the most appropriate materials and techniques for a given application, regardless of trends.

The cassette assembly requires very high dimensional accuracy since it sets the steering geometry.
At the same time it needs to be light, stiff, easily manufactured to reliably tight tolerances, and economical.

This assembly determines the position of the pivot axis relative to the rudder blade (fore-and-aft) and the angle of the blade relative to the tiller (in the horizontal plane).

The position of the pivot axis is critical to the balance of the rudder: The further back along the blade (more foil area ahead of the pivot axis) the more ‘compensation’. Meaning more area helping to turn the rudder relative to the area behind the axis pushing to straighten the rudder. 

On a swing-up rudder compensation can be adjusted by ‘kicking’ the rudder forward past vertical so that the bottom tip is ahead of the pivot axis. 
This solution is fine when the rudder is fully submerged, but it has the drawback that compensation will increase as the upper part of the blade exits the water. When only the tip is left in the water, the rudder will most probably be over-balanced, resulting in a light and ‘skittish’ feel through the tiller extension. At high speeds, especially when foiling, this can be dangerous. 
A Class cats so far have not had to confront this problem because stable foiling has been impossible. But the effect can still be felt when in displacement mode as immersion of the windward rudder varies.


Our solution maintains the correct compensation regardless of ride height and heel angle. 
Actually the rudders are designed to provide a more positive feel as they come out of the water. 
This is achieved by tapering the leading edge aft toward the rudder tips… 

Another advantage over kick-up rudders is that compensation does not change as the angle of attack of the horizontal foil on the rudder is altered (rake). 'T' or ‘+’ foils on the rudders of other boats cannot be tuned for horizontal angle without altering rudder compensation. 

Coming into the beach, kick-up rudders with winglets also cannot provide any control because they can only be fully up clear of the water or fully down. Our rudders can be partially retracted to still provide steerage in shallow water.



To achieve the goal of positioning the rudders accurately, there are important material and process considerations that are not immediately obvious. 
The bearing surface needs to be precise, have a low coefficient of friction, be dimensionally stable and able to hold the rudder without damaging its surface and trailing edge. 
The connection with the pivot axis has very high point loads that need to be reacted out into the cassette. 
The tiller needs to be supported at the correct angle both inboard and upward and be tough enough to withstand the occasional abuse such as a rough tack or jibe.

Satisfying these needs with carbon mouldings requires the build-up of considerable thickness in high load areas. 
The processes are necessarily complex because the shapes involved cannot be moulded in one piece. Dimensional accuracy is not ideal because of the nature of the process and materials. 

The relationship between any weight savings and the additional cost is so disproportionate that it raises doubts about the appropriateness of the material.
A carefully optimised machined aluminium fabrication results in a very efficient structure that is competitive in terms of weight and can be made reliably at a fraction of the cost. 
As a bonus there is a certain ‘aeronautical’ aesthetic that is unique and very pleasing.

Rake adjustment is through shims between the gudgeons and the transom.
Tapping plates inside the boat mean you can replace the shims without having to access the interior.

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