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Boats

See How They Run, Part I

Spectator - October 2001 - Smooth-Running Hulls

Spectator — October 2001

By Tom Fexas


See How They Run, Part I
The elusive “nice-running boat.”


 More of this Feature
• Part 1: Spectator
• Part 2: Spectator
 
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• Spectator Index

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I am in a boatyard surrounded by many vessels on the hard. Strangely, the bottoms of all the boats look exactly like mattresses complete with the swirled stitching and faded pastel flower patterns. The bottoms are soft to the touch. Then I awake.

While I am sure shrinks out there will have their own twisted analysis of this dream, I tend to be a practical sort, and to me the meaning is simple. I have always been obsessed by the way boats go through the water–especially through rough water–and these mattress-bottom boats were simply a manifestation of my desire to produce smooth-running hulls. Take that, Sigmund!

GREYHOUNDS AND DACHSHUNDS
What does it take to produce a "nice-running boat?"

The greyhound is one of the most graceful animals at speed, and the dachshund is one of the clumsiest. The greyhound makes long, fluid strides as it effortlessly moves forward. The dachshund takes many choppy steps and struggles for headway. The greyhound/dachshund analogy applies to boats of all sizes at all speeds. Offshore speedboats, trawlers, and everything in between can be greyhounds or dachshunds.

You can determine how a boat runs by simply analyzing a picture of her underway. The picture on this page is of one of our 43-foot Mikelsons running at about 20 knots. I have superimposed numbers on the photo to illustrate the two critical aspects of a boat’s performance. This information applies to monohulls at speeds up to about 40 knots.

CLEAVAGE
Cleavage is not what you see when your wife or girlfriend bends down. It is simply how a hull cuts the water. Clean-running boats (greyhounds) slide through the water, while sloppy-running boats (dachshunds) plow through it. One can determine this by the wave pattern forward (#1). Greyhounds throw the bow wave aft and out, while the dachshunds push water ahead in various amounts, depending upon their hull shape.

Entry angle (a measure of forebody sharpness) is key here. The finer the entry of the vessel, the cleaner she will cleave through the water and the smoother she will ride. This applies to both slow and fast boats. Entry angles are best made as sharp as possible while still allowing reasonable accommodations forward. Long, skinny hulls can have entry angles of only 8 or 9 degrees, while wider boats might have angles of 15 to 25 degrees.

Next page > Part 2: Trimmings > Page 1, 2

This article originally appeared in the June 2003 issue of Power & Motoryacht magazine.

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