Hull Speed Calculator and Waterline Length
Waterline length is the length of the boat from bow to stern where it sits in the water. In other words, as it floats in the water, if you were to mark the point on the bow where the water touched and marked the point on the stern where the water touched and then measured that distance this would be the waterline length.
By the way, if you were to connect the marks you made at the bow and stern and paint a line on the boat’s hull you would have created what is called a “boot stripe”. The boot stripe is the line painted on many boats which separates the bottom which is underwater from the sides that are above the water.
Yes, the length of the water line does affect the speed of some boats. In particular displacement type boats. These are boats that have a large underwater profile such as sailboats and trawlers. Since these types of boats are not able to get up on top of the water and plane they are pushing a tremendous amount of water.
How Does the Bow Wave Affect Hull Speed?
If you were to watch a displacement vessel move through the water you would notice that they create both a bow wave and a stern wave as they push through the water . The vessel’s bow wave wavelength increases as the boat’s speed increases. The faster the boat goes, the larger these two waves become until at some point they become a single wave. It is at this point that the boat has reached its “hull speed”. That means that this is as fast as it can go. It can’t go faster because it is caught in this wave. The longer the boat, the faster it can “theoretically” go because it takes longer for the bow and stern wave to become one wave. You can calculate the hull speeds or displacement speed of a displacement vessel with the following formula.
The Hull Speed Formula
Hull speed = 1.34 X (Square root of waterline length)
For example, if your displacement sailboat was 36 feet long the hull speed would be calculated as follows:
square root of 36 = 6
6 X 1.34 = 8.04 kts. hull speed
What Difference Does Hull Speed Make?
You can actually learn a lot about how a vessel travels when you understand hull speed. At hull speed, those bow and stern waves are synchronized and that’s important. Something called constructive interference happens and that means the boat is travelling as efficiently as it possibly can.
There’s nothing stopping you from exceeding hull speed. It’s not like the speed of your car which is determined by the engine and other performance factors. A sail boat or a boat under power could very reasonably exceed hull speed. However, it’s not going to perform well when it does so. Hull speed is basically your boat’s sweet spot, the speed at which it does the best it can. The speedometer won’t show you this.
If you exceed maximum hull speed, you’re going to see problems arise. You’ve likely seen it before if not in your boat than in someone else’s. The boat begins to plane and the bow will rise out of the water. That indicates the boat speed is simply too fast.
Once your boat starts planing, it’s going to have to struggle to keep that pace. Whether it’s powered by a motor or by sails, much of the energy going into thrust will now be wasted. So you’ll be pushing harder and achieving less. This can stress the engine and waste fuel. It also makes it more difficult to manage the sails on a boat under wind power.
Because of this, hull speed is general the optimal speed for any boat and you don’t want to exceed it if it can be avoided. You’ll save time, money and energy if you know your hull speed and are able to stick to it as much as possible.
On a related topic, you may wonder just how fast a boat can go. Are there speed limits? As mentioned above, displacement type boats are limited to how fast they can go.
Is There Another Hull Besides Displacement Hull Vessels?
Another type of boat is the planing vessel. Planing vessels are designed to actually rise up and ride on top of the water when power is applied. These vessels have flatter bottoms than displacement hulls.
They require considerably more horsepower to get the boat up on top of the water, but they can attain much higher speeds because of the reduced friction of moving on top of the water rather than through the water.
Some planing vessels are used for professional boat racing and can reach speeds over 100 miles per hour. Recreational planing vessels may reach speeds of 25-40 miles per hour.
On the water you will also find speed limits just as you do on land. You should be on the lookout for speed limits and abide by them. Also, just like in a car, you should never operate at a speed that is unsafe for the current conditions. You should constantly be on the lookout for other traffic, visibility, wave action around you, and other elements which may require a reduced speed.
No Wake Zones
Not every body of water is going to have a speed limit posted on a buoy for you to see. Some places are listed simply as no wake zones, for instance. In order to ensure no wake you’d need to be at a fairly slow pace, around 5 miles per hour, probably. This is a speed used around swimming areas, bridges, piers, docks and so on. These are places where excess speed would be a serious safety concern. This doesn’t give you a hard and fast number, but it does indicate you need to watch your speed. If your vessel is going fast enough to produce a disruptive wake, then you are violating the rules.
The Bottom Line
When you can calculate hull speed, it can be a lot of help. You’ll be better able to understand how your boat should be performing on the water and can optimize your speed as a result. Anything that can make your boat run more efficiently while taking stress of the engine and lowering fuel costs can’t be a bad idea, right?