Hull speed

[Bob Robeson]

Hull speed a result of the hydrodynamic forces at work. Like previous posts, this theoretical speed is determined by the (length of the water line)cubed, times 1.34.As a displacement hull goes through the water it creates a pressure wave. You can see this wave on any boat moving though the water. The faster the boat goes the longer the wave gets. Hull speed is reached when the wave (the high sides) becomes the same length as the boat. When you are going slower than hull speed the boat is going down the backside of the wave (down hill), requiring little power.To go faster than hull speed takes power, lots of power. As the wave gets longer, the boat must try and climb the front part of the wave. It’s like going up hill. And faster you go the steeper the hill, but if you add enough power you can go faster. Run down wind, put up a huge spinnaker, adding tons of power and you can go considerably faster than hull speed.Hope this helpsBobC-36

 

[Ron Dague]

Quick Correction

Almost correct, take the square root of the length of the boat waterline, times 1.34. (Not the cube of the length) So, my h34, has a waterline of approximately 27 feet. Square root of 27, is 5.2. Multiply times 1.34 and the theoretical hullspeed is about 7kts.Extra horsepower in the form of additional sails will not overcome that speed by much, because as the speed increases in an algebraic fashion, the downward pressure on the rear of the boat increases geometrically. A little surfing on the downside of the waves, current, etc. can help you cheat a little. Lighter boats might plane a little. Also, some hull designs would utilize a slighty higher constant than 1.34. This is most evident if you try to pile on too much sail in heavy wind conditions. At some point, you don't go any faster. The stern kind of squats down, and the pressures on the rig increase dramatically, even dangerously. (A displacement hull can be sunk by towing at speeds above "theoretical hull speed". Ask any towboat captain!)

 

[Bob Robeson]

opps

cubed, sq. root opps. Thanks for the correction.Sails can easily over power the hull speed and almost all performance cruisers and racers can sail faster down wind with spinnaker than the hull speed.It is easy for a 30-ft boat with a hull speed of 6 kts to do well over 10 kts with spinnaker.Example: the awards the Newport 30 got for anyone hitting 12 kts.Our C-36 with a hull speed of 7.5 regularly hits 8 and I have had it over 9 kts without the spinnaker and many people have done 12 to 14 kts under certain conditions.Hull speed only refers to the pressure wave being the same length as the boat. When the pressure wave and boat are the same length you are at hull speed, not top speed.You are certainly right about lots of other factors like the type of displacement (light, moderate, heavy, etc.) and how much extra power a boat can handle.Any you are right about the dangers of over powering your boat.Neat discussionBob