Every sailor is interested in how to make his or her boat goes as fast under sail as it can safely.  Most of us know that the type of boat, keel, propeller, and sails have an affect on performance of our boat.  But, what specific parameters are important in speed, comfort, and capsize resistance?  But how would you objectively compare the performance of two different boats that you might be considering buying?  Let's start with a simple comparison of two values for each boat.

Start by collecting some basic design information on the boats, including the the waterline length, sail area, and displacement (weight).   This information can usually be found in the owners manual or your boat broker may have this information.  Now we can calculate two key performance parameters, hull speed and sail area-to-displacement ratio. 

The first performance parameter of interest is the boat's hull speed.  The maximum speed (hull speed) of a cruising sailboat or any displacement (non-planing) hulled boat is dependent only on the length of the boat at the waterline.  This is due to the formation of wave trains along the sides of the boat and the transfer of energy into the waves.  The next time you are out motoring, watch the waves your boat makes.  When you start out slowly, there will be many small waves along the sides of the boat,  as well as your bow wave and a wave off the stern.  As you increase speed to about half throttle you will notice that the bow and stern waves are growing larger, and you may have one or two other major waves off the sides.  As you go to maximum throttle, the side waves will slide aft until they combine with the stern wave, so at hull speed (maximum) there is a large bow wave, a trough along the sides, and a large stern wave.  If you have any throttle left, all it will do is make the waves bigger, but you will not go appreciably faster.  Of course, if you have a small race boat or a maxi with a planing hull, then more throttle (or wind) will cause the boat to climb over the bow wave and begin planing over the top of the water and you will be able to go faster than hull speed.   Your boat's hull speed can be calculated (you knew this was going to involve math, didn't you?)  by the following formula...

My boat is 47 feet overall, but only 36 feet on the waterline.  So my hull speed = 1.34 x sqrt(36) = 8.04 kts.  This is why bigger boats can go faster than smaller displacement boats.  This is also why Navy destroyers are long and lean for faster hull speed.  What would the hull speed for a 400 ft. destroyer be?  Hull Speed = 1.34 x sqrt (400) = 26.8 kts.   So don't go racing a destroyer.

The net of this is that any boat that has a 36 foot waterline length (like a Catalina 38) will have the same hull speed as my boat.  The next key performance parameter deals with the power that we get from our sails and the weight that that power must move.   This parameter is called sail area/displacement ratio, sometimes written SA/D: 

Take your boat's displacement and divide it by 64.  Raise this number to the 0.666 power (you still remember how to use a calculator, right?).  Take your sail area and divide it by this number to get SA/D.  You should get a number between 14 to 21 or so; a bigger number indicates more effective "horsepower". 

With these two parameters, you can do a first cut comparison of two or more boats.  When I race offshore, I always do these calculations for my competitors to see who is the biggest threat.   A boat with a higher hull speed and a higher SA/D will be able to beat me if we go head to head.  But that is what handicaps are for!

Here are the hull speeds and SA/D for some common cruising boats.  Note that two very different boats, the Catalina 38 and the Vagabond 47 have almost exactly the same parameters and would be closely matched on an off-the-wind race. Going into the wind would be different matter, as the fin keel and tighter sheeting angles of the Catalina would let them go to windward much better.   In the table, also note that the Catalina 38 has a higher hull speed than the IP 40.  This is because the Catalina, while 2 feet shorter overall, has a longer waterline length than the IP 40.

Back to Home Page