January 15, 2013

Is there a hump in your sheer?

IF YOU WERE a yacht designer, what do you suppose the most important line in your drawings would be? The bow, perhaps? The stern? The waterline?

Well no, according to some of North America’s best-known designers, it’s the sheer line, the curve of the deck line from bow to stern as you look at it from the side.

Francis S. Kinney, in Skene’s Elements of Yacht Design. calls it “perhaps the one single line that crowns or damns the whole creation.”  Steve Killing, in Yacht Design Explained, calls it “the most prominent and significant line on a hull, It not only defines the character of the boat, but if done well will be (in truth must be) beautiful at any angle.”

And that’s the problem, apparently. When you’re drawing the sheer line by hand, it’s next to impossible to guarantee how it will appear from all angles.  “Sheers that look fine in each individual view can end up having some harsh curves when viewed in three dimensions,” says Killing.

Strangely enough, he claims that sometimes, if you look at a boat from a point off the starboard bow, the sheer can have a hump that appears seemingly out of nowhere about one quarter of the way back from the bow. But, conversely, if you look aft from the same vantage point. the sheer near the transom can appear much too straight.

Luckily, modern designers have found a tool that helps them find the perfect sheer line for any boat — the computer. “Many scoff at the use of computers in yacht design,” Killing adds, “dismissing them as inhuman, permitting none of the art required by the truly talented designer.”  But, in his experience, there is more opportunity for the designer to perfect the look of a boat on the computer screen than there ever was on paper.

A good design software program produces graphics with such speed and accuracy that a designer can now economically run through 10 subtly differing versions of a custom design and do a “walk around” on the screen with particular attention to problem spots that otherwise would have caused surprises and embarrassment on the workshop floor.  It also enables the designer to be sure that his all-important sheer line will always look right, adding beauty and elegance when viewed from any angle.

Today’s Thought
There is no excellent beauty that hath not some strangeness in the proportion.
— Francis Bacon

A dentist was just finishing up the annual examination of a millionaire Texan oilman.
“Perfect,” said the dentist. “Nothing wrong with your teeth.”
“Well drill anyway,” said the oilman. “I feel lucky today.”

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)

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