Outback Yachts

Efficiency & Performance

Safe with a shoal draft, extremely economical, and amazingly versatile…the Outback 50 offers it all!

Typical yacht hulls invariably behave and perform according to their speed-length ratio (S/L), or speed in knots divided by the square root of the waterline length in feet. At an S/L of up to 1.1 or 1.2, a hull will operate comfortable and efficiently, creating a small wave train as it travels through the water. When the S/L exceeds 1.3, the hull is starting to create a wave longer than itself and its behavior changes markedly; the power consumption increases radically and the bow rises, or trims up, as the hull tries to climb up and out of its own wake. This undesirable behavior tends to peak at an S/L of around of 2, and if power is sufficient and the hull form appropriate to reach an S/L of 3 or higher, the hull will settle into full planing mode where it can generally ‘skim’ across the water fairly comfortably and efficiently.

The region between S/L of 1.3 and S/L of 3 is typically referred to as the ‘transition zone’ and is almost always problematic. Fortunately because of the Outback's design, a very desirable speed range is achieved, from say 10 to 20 knots. This is precisely the speed range the Outback, with a waterline length of approximately 52’, is intended to run.


Performance Graphs

click images to expand

Conclusions

The Outback hull form is performing essentially as desired. The hull’s shape and proportions have been arranged in such a way that the transition from displacement to planing modes is quite smooth. It is essentially a straight line with a very small ‘hump’ at 12.5-15 knots, which translates to an S/L of 1.7-2.1. Typically, on relatively fatter and more heavily loaded hulls, this resistance hump is much more pronounced.

The same can be seen in the Trim Angle curve, which peaks at 2 degrees and remains flat. More typically proportioned (i.e. ‘fatter’) planing hulls often trim up to 5 degrees while making the transition to planing speeds, which is why the curve was plotted on that scale; to show how relatively flat the Outback hull stays as speed increases. Aside from the benefits in resistance, this is very desirable from the standpoints of both comfort, the deck angle never goes too far from level, and safety, as visibility over the bow is not compromised.

The net result is a hull that will operate comfortably throughout its desired speed range, and does not require excessive amounts of power and fuel to do so. The speed-power curve shows speed potential of the boat in loaded condition with very modest twin engine packages. Trips of a few hundred miles can easily be made at a good clip of 12-15 knots or more, basically twice the speed of a typical cruising trawler. Slowing down to a very respectable 8 knots can extend the range out to about 1,000 miles, plenty for long trips out to the islands or up the coast.