Boating NZ : FREE TO READ April 2014
subscribe online at www.mags4gifts.co.nz/boating-nz 55 Conversely, if the form runs more efficiently with reduced trim at lighter displacements, the designer may locate the fuel tank aft as the stern will lighten as the fuel is burned. Trim tabs, interceptors or engine tilt will help trim down the bow; the latter can also help trim up the bow, but all come with a drag penalty – see Rule Two. The deadweight of the boat – structure, accommodation, engines and hardware – is a fixed amount, and the hull must have sufficient volume at rest to support that weight – see Rule Four. This immersed volume increases with added beam or greater deadrise. If there is too much volume for weight or too much horsepower for the boat’s weight/deadrise combination, the chines will be close to the water’s surface and, as the speed increases, the chines may lift clear of the water. This may initiate chine walking as the boat, being unstable on its narrower planing body, falls over on to one chine, picks up lift from that chine and rolls back the other way. Chine walking can range from uncomfortable to alarming to dangerous. A useful number provided by the designer’s software is chine length and chine immersion. This tells us how much chine is immersed at a given speed, and when we are near a threshold of insufficient chine for stability. Maintaining a usable length of chine, sufficiently immersed, requires a moderate trim angle – otherwise the chine would run parallel to the water and generate no lift – so it is critical to balance the amount of lift and its location relative to the vessel’s weight and its LCG. Chine design is something of an art. Chine design is an art, and every designer approaches it differently. However there are some generally agreed principles that apply. If chines are too wide, the vessel will have a harsh ride; if too narrow, the vessel could lack dynamic roll stability and potentially suffer additional drag as spray climbs the topsides. A chine width somewhere between 2.5% and 5% of the local beam seems to work well. A little downturn of no more than 5 degrees, while having little effect on performance, does assist in turning spray down. Relating the chine line in profile to the keel line is hotly debated, but generally maintaining a near-prismatic form; ie, an extruded shape of constant section, in the aft third of the boat is beneficial so the keel line and chine line should run near-parallel to each other in plan and profile view. To ensure adequate chine immersion when underway, a good rule is to avoid the chine sweeping up above the waterline aft of midships when the vessel is at rest – which brings us back to the immersed volume and matching that to the vessel weight – see Rule Four. The amount of chine sweep in profile should be related to the HIGH AND LOW CHINES CHANGING CHINE LINES The chine sweep, amount of chine immersion and overall width and angler of the chine downturn differs between designers and between models within a range, as do the number, angle and width of spray rails/strakes, but most boat designers work within recognised parameters. Changing the chine line radically alters the shape of a boat’s hull and its handling characteristics. Rule One: Speed and control are mutually exclusive Rule Two: You don’t get ’owt for nowt – or, where there’s lift, there’s drag Rule Three: Beauty is in the eye of the credit card holder Rule Four: Isaac Newton always wins; Archimedes and Bernouli just comply Rule Five: Compromise management is the name of the game Stimson’s maxims on yacht design REGULAR_Future concepts_April14.indd 55 19/03/2014 1:07:30 p.m.
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