Bray Yacht Design
& Research Ltd.

White Rock, B.C.
A Design Revolution

     If you have not bought a new powerboat in the last few years you may have missed a quiet revolution in design that has taken place. Subtle changes in hull design have produced performance benefits which were not possible previously. The newer boats plane a little more efficiently but the biggest change is in the cornering. Flat turns are very prominent in the new generation of vessels afloat. I first notice this change in a 21 foot production boat built by Hinterhoeller. It's performance was quite startling at the time it was first launched, and this vessel became quite popular in Eastern Canada. Today all the newest boats have picked up on this design revolution.

     These new boats have been developed through extensive model testing, research, and development. It has been found that by adding a wide chine flat there is an increased in the stability of the hull both at rest and underway. By adding a 10 degree down angle outboard to this flat they have also increased the effectiveness of the planing surface making the boat both smoother and faster for any given horse-power. At the same time spray is turned downward helping to lift the boat and making it dryer. In general, these boats are just plain wider than before and this has the added benefit of more interior space.

     The concept of wider power boats has been discussed for years but it was always felt that narrow hulls were faster. This is certainly true of displacement hulls, where the vessel's form is continually fighting an uphill battle to climb over a bow wave that is the product of it's own passage. No matter how much power is installed, the boat continues to create a bigger hill to climb, made possible by the large quantities of power being transmitted into the water. A tugboat is the classic example of this battle of the laws of nature. Broad, deep, powerful, and of course heavy, tugs can never get out of their own wake. Reduced weight allows a shallower, narrower hull requiring less power. The rowing shell is the antithesis of the tug. Light to the point of fragile, extremely narrow to the point of unstable, long and shallow, it is easily driven by low horsepower. Climbing it's own waves would be no problem if it created any waves to climb.

     The marine industry, being as conservative as it is, took forever to get over this concept and to this day still struggles with it. Wide and narrow have their place in combination with other factors, but in the new runabouts wide is definitely the winning combination. Planing hulls use an entirely different hull form in anticipation of not only climbing up the mountain of waves they create but surfing down the other side. In this way they resemble skimming dishes with large, shallow areas with which to lift themselves up and out of the water. Light weight is important but so is area. The broad bottom provides a wide area for the water to make initial contact. The water is forced to change direction downward, and this imparts an upward force on the hull bottom.

     Most boats have considerable deadrise to soften the ride in rough water by allowing some of that lifting force to slide out to the sides when the hull is pressed into a wave. By turning down the chine flats the water is deflected at the last minute as it exits the hull, imparting some of the escaping forces to the hull. Because this water and spray are up high it does not have the force that the solid green water does down at keel level and thereby provides a smooth but effective ride. The spray, being air and water combined, will compress to give a shock-absorbing ride. To some degree you could say these boats are riding on a cushion of spray created by their hull form. When cornering, the vessel starts to heel, placing additional pressure on the lower chine, and the spray is increased. On the higher side the pressure has dropped and of course the spray does as well. The chines automatically return the boat to more even keel. The boat tends to remain level and continues to turn at a very flat angle. Although a steep heel angle during a turn looks very dramatic, a flat angle is a lot more comfortable.

Well there it is in a nutshell. The next time you are out in your run-about, check out the turns. If you have an older boat check out everyone else's turns and think about a new boat! Progress is being made and you may be left behind. There is a quiet revolution afoot and you could be a part of it.

Patrick J. Bray
Naval Architect

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