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See How They Run, Part I Page 2

Spectator - October 2001 - Smooth-Running Hulls

Spectator — October 2001

By Tom Fexas


See How They Run, Part I
Part 2: Trimmings


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Some of you may be surprised to learn that shape and fineness of the entry affect both head-sea performance and following-sea performance. If the entry is too fine, following seas will raise the stern and depress the bow, and a deep forefoot will act like a rudder, producing bow steer that can lead to broaching. (Broaching occurs when the bow catches a wave and the stern is pushed around it, turning the hull and putting it broadside to the seas). This can be overcome by adding volume above the waterline, thereby combining a fine entry and enough buoyancy to prevent the bow from depressing. Dachshunds that push walls of water ahead of them are usually the result of pure and simple greed. As I have said many times before, greedy interior designers demand maximum floor space forward, dictating wide, blunt entries. This may be great for fitting damned emperor-size forward berths, but it will severely compromise performance. I will say it again: A boat should be designed from the outside in, not the inside out.

TRIMMINGS
Proper trim angles for high-speed vessels can run anywhere from two to four degrees (#2), depending on many factors. A hull running bow-high, looking like she’s preparing for a moon shot, is not a good thing. First, visibility over the bow is impaired. Second, initial impact with the waves occurs further aft, where the bottom is flatter, thereby producing a rougher ride. As a general rule of thumb, if one can see daylight under the forefoot at speed, the boat is not properly trimmed.

There are really two trim angles to be concerned with: cruising trim and, pardon the expression, "hump speed." We have all seen boats that, when power is applied, tip their noses high in the air and suck their tails down, struggling to get over the hump and onto plane. Hump angles for properly designed hulls range from three to six degrees. Generally hump speed is the speed at which the transom of a semiplaning or planing hull breaks clear of the water. The cleanest-running hulls make the transition from displacement to "over the hump" speeds effortlessly and almost without notice. They simply assume a trim angle of two to four degrees when power is applied and maintain that angle through hump speed into cruising speed. Poorly designed or overloaded hulls may require some help getting through the hump (which is similar to breaking the sound barrier). This is often done with trim tabs, and indeed, the subject of trim angles can not be fully analyzed without discussing trim tabs. Are they needed in a well-designed hull? The answer is a very clear yes and no.

Tests have shown that on a given hull, tab application may help at one speed but hurt at another. Trim tabs are also handy for trimming the boat athwartships in a beam wind and also allow one to adjust trim angles for different sea states. For example, in following seas a hull sometimes runs better with the bow up because this angle produces less steering by the forefoot.

Sometimes poor performance can be cured with some simple changes. A good part of my e-mail correspondence is from frustrated boat owners seeking to transform their dachshunds into greyhounds. Unfortunately, changes to hulls cannot be recommended simply based on a description of what the boat is doing. Before changes are prescribed, one must first ride in the boat, measure her speed, trim angle, etc. and take still and video pictures of the boat underway. So please, friends, refrain from sending correspondence informing me that your beloved Asea Dsea will not get on top, will not turn to port, and is pounding your molars loose, unless you are prepared to arrange sea trials and pay the freight. To be continued…

Tom Fexas is a naval architect and designer of powerboats in Stuart, Florida. His Web site is www.tomfexas.com.

Previous page > Part 1: Greyhounds and Dachshunds > Page 1, 2

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

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