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taking it on the chin(e)

csabi

Design 101

Chines, chines, chines! Anarchy will you find a pro to explain why we have the chine instead of arcs on boats like Ran, Comanche, and all the open 60s??? Grey McGown Fort Worth.

chine lloydThe selection of a chined or round bilged hull form is often not a clear cut choice and a number of competing factors need to be considered. For grand-prix offshore focused designs like the Volvo 65’s, Volvo 70’s, IMOCA 60’s, Class 40’s etc. we are seeing designs that approach or even exceed the power to weight of the highest performance dinghies [18’ skiffs, i14’s …]. These designs spend a lot of time in the planing boat zone so considering powerboat and skiff-like features such as chines makes a lot of sense.

As you move into this high sail area, high stability, light displacement design space, the chined hull form starts become very appealing. For a given beam, the chine lets us maximize form stability when heeled, while the flatter sections that result provide dynamic lift that assist the boat into the transitional planing regime and provide a more optimal shape for high speed. On these wider boats, the chine also acts to give some directional stability to the boat when sailing at heel, which assists handling and allows for higher average speeds especially on the short-handed boats. There are an infinite number of takes on how to blend the chines into the sections forward, but the objective is to develop a shape that is fine enough to go through waves well but one that also provides dynamic and buoyant lift to keep the bow up when sailing at speed and heel. Remember that unlike a powerboat we have to resist a sizeable pitching moment from the sails and this needs both a hull that reacts well with speed and the help of crew weight and stack aft and the use of water ballast in some cases. The quest for dynamic bow up trim has been one drive for the use of spray strakes and other sectional features forward that we have seen implemented on a number of VO70’s and IMOCA 60’s.

For classes like the TP52’s or IRC 72’s that sail more windward-leeward races and have lower average speeds, the small amount of low speed added drag from the chine makes the choice a little more nuanced. We see explorations of shapes both with chines and with rounded off chines or lifted high hard bilges. The choice to go with chines is all about rule limitations, the type of racing and the targeted sailing conditions.

That covers the high performance choices you say, but why is it that so many of the latest production cruising boats have chines?

Clearly these boats aren’t designed to plane and sail around the ocean at high speed. No doubt, some of it is fashion bringing some of the high performance features down to the masses and giving the boats a straighter, more modern look. Beyond a fashion statement there are some advantages in allowing wider total beam aft, resulting in more deck space and the ability in smaller models to fit twin wheels. For cruising boats, chines can be complementary to the function of twin rudders. Performance differences aside, cruising boats often appreciate the control that twin rudders and chines provide when the boat is overpowered. For these heavier cruising boats there is a drag penalty due to the chine but it allows a big increase in form stability and a much larger interior volume for accommodations. Compare a modern 35’ cruising boat with one from even 5 years ago and the amount of usable interior volume and deck space that’s been added is staggering. The chine helps us achieve this interior volume without significant waterline beam increases. This keeps the low speed and low heel drag manageable while taking advantage of the control and righting moment of the chines at higher heel angles. The increased form stability also helps us achieve stability requirements without piling on keel weight that is a significant cost center.

The bottom line is there are no hard and fast rules about where and when to use a chined hull shape. As with all the other parts of a yacht design, nothing comes for free and each design needs to balance the performance trade-offs with the design objectives to come up with the right solution. – Britt Ward, Farr Yacht Design. Photo thanks to Mark Lloyd.

 

csabi

December 12th, 2014

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