1.3 times the Square Root of the LWL

Arrgggg … now you know why pirates love that phrase.  None of them like math, and yet some math is actually important.  So what is 1.3 times the square root of the LWL?  It’s the theoretical hull speed of a boat with a displacement hull.  In simpler terms, it’s the fastest that particular boat can go.  As a boat starts to move thru the water, a bow wave develops.  The faster you go, the bigger the wave, and the further back along the hull it goes.  The boat has to climb that wave to move thru the water.  When the wave gets to a certain size and position along the hull, the boat can no longer climb the wave, and won’t go any faster.  That is the hull speed.

As an example, Final Fantasy is a Beneteau Oceanis 35.  The length of the waterline (LWL) is 31.8 feet.  The square root of 31.8 is 5.6.  1.3 X 5.6 = 7.3.  The hull speed of Final Fantasy is 7.3 knots, and that assumes a calm sea, no current, clean bottom, and a few other factors.  The boat won’t go any faster than that, no matter how hard you push the motor.

Recently, we received a call in the office reporting there was something wrong with Final Fantasy.  Seems that the engine was turning at 3500 RPM (throttle pushed as far as it would go) and the boat was only going a little over 7 knots.  Sorry, boat won’t go any faster, no matter how big a hurry the captain might be in.

Oh well, no harm, no foul … right?  Not so much.  Yanmar (the manufacturer of the motor) indicates the maximum RPM is 3500, and only for very short periods of time.  More than that will harm the motor.  Maximum cruising RPM is listed as 3000 RPM, and even that shouldn’t be used for extended periods of time.  It is better to assume 2500 to 2700 as a maximum cruising RPM.  Anything over that creates problems.  Fuel consumption increases dramatically.  Engine temperature starts to rise.  Oil thins down (because of the increased temperatures) resulting in reduced lubrication and greater oil consumption.  And, you probably hit hull speed by 2500, so the increased RPM is doing you no good at all.

In closing, please don’t think every boat can safely be pushed along at 2700 RPM.  It just isn’t true.  Every boat, motor, transmission, propeller combination has its own optimal cruising RPM.  My boat for example has a max of 2200 RPM, with an 1800 RPM cruising speed.  At 1800 RPM I consume .6 gallons of fuel per hour at close to hull speed with the motor temp sitting steady at 175 degrees.  At 1900 RPM the temp starts to drift up, hitting 180 very quickly and slowly continuing up from there the longer I keep that RPM.  At 2000 RPM my fuel consumption increases to 1 gal per hour, and the engine temp will quickly hit 200 degrees.  The extra half knot of speed I might get just isn’t worth it.

If you have any questions or doubts about the best cruising RPM for whatever club boat you happen to be on, please call the office.  We would be happy to help.

Note from Matt: One of the things we often note with less experienced sailors is that they simply forget to throttle down once they are done maneuvering. In other words, they back out of the slip, transition to forward using enough throttle to get the boat moving the other direction, and then forget about the throttle while they steer the boat. By the time they make it to the end of the fairway, they have picked up too much speed and are going faster that anyone is comfortable with. Don’t forget to throttle back down once you have steerage-way and keep that speed under control. Look at the docks and boats you are passing to be aware of your speed. The same is true when crossing the bay in open water. Look at the bubbles, debris, or other objects in the water. If you are fighting a strong current the GPS may say you are only going 4 knots, but you could be doing 2 or 3 more over the water. As Don said, pushing the throttle won’t speed you up, it’ll only stress the engine. Proper planning will avoid hurry and avoiding hurry will help us keep our boats in top shape for the generous boat owners who allow us to use them! Remember, we can only keep awesome boats in the fleet and attract new ones if boat owners are potential boat owners are happy when they visit and see them!

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