August 14, 2014

Short bow means faster boat

HAVE YOU EVER wondered what shape of bow is most seaworthy for a sailboat? For example, does a raked bow, or a spoon bow, better fit the shape of an oncoming wave? Does this mean that the bow will have more buoyancy than a bow that is more upright — a plumb bow, or even a tumble-home bow, as some catboats have? And is buoyancy important in a bow? Does it stop a boat on the run in heavy weather from plunging deep into the swell ahead and causing a pitchpole?

You’ve probably noticed that modern production sailboats often feature bows (and sterns) that are shorter and more upright.  Ted Brewer, the well-known boat designer, says in his book Understanding Boat Design (International Marine):

“The long spoon bow, now rarely seen except on meter boats, was de rigueur on sailing yachts for many years because it reduced the handicap rating, yet picked up waterline length and speed as the boat heeled in a press of wind. However, this was only an advantage when the racing rule favored a short waterline; on two boats of the same overall length, a short bow automatically gives a longer waterline and a potentially faster boat.”

Brewer adds that the shorter waterline of the spoon bow does have the advantage of reducing wetted surface when the vessel is running upright or slightly heeled, as opposed to the constant, greater wetted surface of the long-waterline, short-stemmed yacht.

However, “modern sailing yachts reduce wetted surface by fin keels and spade rudders, not by spoon bows,” he points out.

As to the question of seaworthiness, the extra-long overhangs seen on some classes such as the 30 Square Meter are widely regarded as safe only in reasonably calm water. Modest spoon, clipper, and raked bows appear to be the safest for boats designed to cross oceans. All the same, there’s no bow that I know of that can prevent a pitchpole if conditions are right.  (Or, should I say, wrong.) Even heavy-displacement Colin Archers, with their bluff buoyant bows, have been known to capsize heel-over-head.

Today’s Thought
Regardless of type, every boat is a compromise of four basic factors: seaworthiness, comfort, performance, and cost.
— Ted Brewer, Understanding Boat Design

Blessed are the pure in mind, for they shall inhibit the earth.

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


Sixbears said...

How do scows fit in with all this? -or do they just give purists heart attacks?

John Vigor said...

Sixbears, scows are just go-fast planks. They make no concession to seaworthiness or accommodations. They are all about speed. I had a brief spell as bailer-boy in a 25-foot scow when I was in my teens and it was lots of fun. I later capsized a friend's 20-foot scow, which then grounded on a sandbank and pounded itself to bits in a storm. I wasn't very popular and I haven't sailed on a scow since but I do remember the thrills.
John V.