Hullspeed and the
Speed/Length Ratio

So what gives one boat better hullspeed than another? This question was pondered long and hard by William Froude (1810 to 1869), a British engineer who had a special fascination with the sea and ships.

Funded by the Admiralty, who were clearly very keen to get some answers to this question, he built a tank testing facility at Torquay, where he experimented with various model hull forms.

As an early expert in model analysis he was well acquainted with the 'law of mechanical similitude', which demonstrates among other things that there are few linear relationships in hull design.

So just what is the answer?

Let's take a look...

Hullspeed and the Matchbox Analogy

Hullspeed and the Matchbox Analogy

Consider your hull as a matchbox - not wonderfully efficient hydrodynamically, but stick with it for a moment.

Dissatisfied with the constraints of matchbox living, you decide to double its size. You add another matchbox ahead to double its length, two alongside to double its beam and four on top to double its draft.

Now wetted area has increased by four, volume and displacement by eight and stability - as the product of its mass and acceleration - has increased sixteenfold.

So by doubling a hull's dimensions, wetted area is squared, displacement is cubed and stability increases by the power of four.

With this knowledge and that gained by carefully measuring applied force and resultant movement, Froude was able to both calculate and demonstrate that a relationship existed between hull speed and waterline length - that relationship being known and described in the metric world as 'Froude Numbers'.

The Speed/Length Ratio

However, most of us more accustomed to units of feet and knots are probably more familiar with the Froude Number's close relation - the Speed/Length Ratio.

The Speed/Length Ratio

S/L Ratio = hullspeed (in knots) divided by the square root of the waterline length (in feet)

This discovery enabled Froude to compare the performance of boats of different length. For example a 25ft sailboat moving at 5 knots would have the same S/L Ratio at a 100ft patrol boat steaming along at 10knots, and consequently both would develop the same resistance per ton of displacement at those speeds.

For Froude's models, having no rig above the waterline to create windage, this resistance was caused by two principal factors; hull drag and wave making resistance.

Maximum Hull Speed

Maximum hull speed (in knots) = 1.34 x the square root of the waterline length (in feet)

Waterline Length

20 feet

25 feet

30 feet

35 feet

40 feet

45 feet

50 feet

Max Hull Speed

6.0 knots

6.7 knots

7.3 knots

7.9 knots

8.5 knots

9.0 knots

9.5 knots

These figures relate to a boat in displacement mode. If sufficient power can be applied to overcome hull drag and enable the boat to plane, then other criteria will affect ultimate hullspeed.

You are here: Sailboat Cruising > Sailboat Design > Hull Speed

New! Comments

Have your say about what you just read! Leave me a comment in the box below.

Recent Articles

  1. Sailboat Design Categories for Ocean, Offshore, Inshore and Sheltered Waters

    Oct 19, 16 09:45 AM

    These four internationally recognised sailboat design categories are defined by the wave and wind conditions likely to be experienced in the circumstances under which the boat might be used.

    Read More

  2. Get LED Boat Lights and Forget About Battery Drain!

    Oct 17, 16 08:59 AM

    LED Boat Lights are definitely the way to go for both cabin lighting and navigation lights - around 90% savings in power consumption and 50,000 hours before they need replacing!

    Read More

  3. How Many Sailboat Sails Do We Really Need For Cruising and Racing?

    Oct 16, 16 03:35 AM

    Our sailboat sails are becoming increasing hi-tech and correspondingly expensive, so how do we get value for money from our sailmakers without sacrificing quality and efficiency?

    Read More

Didn't find what you're looking for?
           Use this search feature to find it... 

Custom Search