Boat Dimensions


What the Specs Tell Us about a Boat By Mike Huston


The Seattle Boat Show starts in just


a few days and if you are in the market for a boat (or just interested) you are likely to end up with a handful of spec sheets for different boats. These sheets usually contain gorgeous pictures of the boat and a list of numbers. These numbers are the subject of this month’s article. It is my hope to explain what these numbers tell us about a boat. Let’s start by defining the basic set


of numbers – the boat’s dimensions. I have also listed the units that these numbers should be in (in the US) for use in calculated ratios that follow.


Base Dimensions:


LOA – Length Overall (feet) LWL – Length at the waterline (feet) Beam – Width of boat at widest point (feet) Displacement – Weight of boat dry (lbs.) Ballast – Weight of keel (lbs.)


Draft – Depth of deepest part of boat (feet) Sail Area – Total area of mainsail and forepeak (area between mast/forestay). Note: this does not take into account any overlap area of a genoa. (sq ft) Mast Height – Height of mast above the waterline (feet)


Engine – usually the manufacture, fuel type and horsepower are listed


Water – Capacity of fresh water tanks (gal.) Fuel – Capacity of fuel tankage (gal.)


Calculated Ratios:


SA/D – Sail area to displacement ratio D/L – Displacement to Length ratio B/D – Ballast to displacement ratio L/B – Length to beam ratio


Okay, these are most of the


commonly reported numbers, let’s look at what they tell us. Some of these numbers, while informative, have little direct impact on performance while others have a major impact in predicting performance. I’d like to start by discussing the performance ones first, and the most important of these is length at the waterline (LWL). Most keelboats have displacement hulls, meaning they do not plane (i.e. lift up and slide over the water) instead, they push through the water. And the speed at which you can, with relative ease, push a displacement boat is directly related to its LWL. This speed is called the theoretical hull speed, which can be calculated with this formula: √LW Lx1.34 = speed in knots For example a boat with a 36 ft.


waterline will have a theoretical hull speed of 6x1.34 or 8.04 knots. Be aware that the LWL changes


when a boat heels over. How much it changes depends on the shape of the hull so this impact is hard to know


without a sea trial. But this explains why some boats can sail faster than they will go under power. A boat sailing faster than it travels under power can also be caused by an undersized engine and/or propeller. This leads nicely into engine


horsepower. Having an engine whose size matches the boat is important. On most modern models this is not much of an issue as the manufacturers usually do a good job of this. But on older boats it can be a concern, especially if you plan to cruise our local waters where wind can be inconsistent. Other factors, such as the propeller, come into play but suffice it to say that having an engine capable of pushing the boat at hull speed is preferable. For more information on this subject see the Sailing Tips article in the September 2011 issue – it is available online at www.48north.com Sail area is the next big performance


factor. In heavy winds almost any sail plan will push a boat at hull speed but in light winds having more sail area can be a big advantage. But other factors, such as displacement, come into play. This is where the first of our ratios helps shed some light. The SA/D ratio is intended to give a relative number so that two similarly sized boats can be compared. The higher the value, the more power per unit of boat size. Boats with high numbers will accelerate faster and generally will sail faster in light air assuming other factors, like sail set, are similar. To give you a feel for this number here are some guide lines: heavy cruisers run from 10 to 15, cruiser/racers from 15 to 20 and numbers over 20 are usually found on boats designed for racing. If you are


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SA/D


√LW Lx1.34 = speed


D/L = D (0.01xLWL)3


LWL


LOA


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