What is waterline length and does it affect boat speed?

Chris Riley by Chris Riley Updated on June 11, 2021. In

Boat Safe is a community supported site. We may earn comission from links on this page, but we have confidence in all recommended products.

That is a very good question. 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 effect 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 type of boats are not able to get up on top of the water and plane they are pushing a tremendous amount of water. planing2.gif (72584 bytes)

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 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 speed of a displacement vessel with the following formula.

Hull speed = 1.34 X (Square root of LWL)  *LWL is length waterline

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

Along the same topic of speed, Corey asks: How fast can a boat go, are there speed limits? This is another good question, Corey. As mentioned above, displacement type boats are limited to how fast they can go. However, 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.
planemov2.gif (56642 bytes)
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 vessel 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.

About Chris

Outdoors, I’m in my element, especially in the water. I know the importance of being geared up for anything. I do the deep digital dive, researching gear, boats and knowhow and love keeping my readership at the helm of their passions.


Leave a Reply

Your email address will not be published. Required fields are marked *

More in kids

Required Equipment for Boats

Parents get help at BoatSafeKids

The History of Ship Figureheads

The Niña, the Pinta and the Santa Maria

All content is © Copyright 2020. All rights reserved.