A reader recently asked, "Are there any rules or formula to follow when starting a new boat design or are they created more by eye and experience? If the former, can you briefly explain what they cover and historically where they came from?"
This is an interesting question but one that could fill several volumes if answered in detail! However, Here is an abridged overview of the situation and where we came from. First, let's take a brief look at the historical base of modern naval architecture.
Ships and boats have been around for LONG time. Their design was then indeed one of eye and limited experience. But a few thinking people tried to learn the effects of various changes in hull shape through model testing—and a couple of famous names come to mind.
Around 1500, Leonardo di Vinci reportedly made 3 models and tested them, while one of the first known Americans was Benjamin Franklin in 1764. But it was a William Froude in England who was the first to discover a way to correctly upscale the model data for full size craft. He was born 200 years ago, on November 28th 1810.
Froude's initial involvement with ships was to study dynamic stability but then he got a commission to try and create more efficient hull shapes. The Admiralty funded the first test tank in his home town of Torquay, UK and he was soon testing models and devising a way to compare them with the full scale ship—now known as his Law of Comparison and involved the now famous 'Froude Number' or Fn.
In its dimensional form, Fn is also known as the Speed/Length Ratio and is equal to Velocity (in knots), divided by the square root of the Waterline Length (in feet). It's really worth remembering this ratio, as it enables floating boats of vastly different sizes to be compared, as far as many of their characteristics are concerned.
Between 1868 and 1874, Froude went on to test all sorts of hulls and the first 'bible' on ship design was written based on many of his discoveries. Although more recent tests throughout the USA, Europe and now even Asia, have further refined the data, Froude's principles have basically remained intact.
He created numerous Coefficients as ways to compare different shapes and tested displacement forms with varying proportions and ratios. He also did a series of tests on flat planing surfaces with steps in them, spurred by ideas from a Rev. Ramus. He also discovered that hull resistance was primarily made up of two components that varied independently from each other… namely frictional resistance and wave-making resistance and devised ways to calculate each from model tests. For the former, he did an extensive series of tests with surfaces of different types to establish frictional coefficients that are still considered valid today.
Around 1886, a man named D.W. Taylor, a graduate from the US naval academy, went to England to study at the Royal Naval College and learned of Froude's work.
Once back in the US, he had Washington build an even larger test tank (1900) and then conducted a more extensive series of tests with an updated ship form, now known by naval architects world wide as the Taylor Series.
Later, a systematic series for classic planing hulls were conducted in England and called the Series 62 and these covered a fairly wide range of lengths and breadths.
In 1900, there were only 5 known model test tanks in the world. But there are now over 100, so many other Test Series have followed, and each provides a wealth of information for naval architects worldwide, as to what effect various proportions have on resistance, dynamic stability and sea kindliness.
One of the first test series to interest multihull designers was one presented by E.P. Clement in 1961, covering the test results for planing catamaran hulls. Although there is no time or space to discuss any of these tests here, many of them are now available on the web.
As far as modern multihulls are concerned, perhaps no one has used model test data more extensively than the renowned UK designer John Shuttleworth, and his early trimaran Brittany Ferries GB once held the cross-Atlantic record.
Editors note: Michael plans to interview Mr. Shuttleworth in the coming weeks and we will be publishing this exchange as it becomes available.
Formulae and Coefficients
As noted above, the Froude Speed/Length ratio is very significant in boat design. Most descriptions and findings re hull resistance are directly related to it. For example it has been shown that a displacement hull creates a wave equal to its length at a S/L ratio of 1.34 and at that point, there's such a hump in the resistant curve that most ships cannot exceed it without a change in shape. Creating a flat planing surface, to give lift and effectively extend the boat's length through a flat wake aft, typically does this, but this can only be achieved with enough continuous power, something a sailboat cannot guarantee.
Other Coefficients of interest to the multihull designer are dimensional ones like the slenderness ration L/b, or the Prismatic Coefficient, (the volume of displacement divided by the product of maximum underwater cross-sectional area × L), which allows a designer to assess and compare the fullness of the boat ends. There are also basic ones like Length to Beam, Sail Area to Displacement and many other useful ways to compare one design with another, for performance, stability and sail balance. But these coefficients and ratios only serve to establish guidelines when designing by comparison and a lot of experience needs to be added-in to adjust these in the right way, as the purpose and size of any new design is considered.
Working from a series of controlled model tests could certainly help create better designs, but sadly, model testing has become very expensive and too few multihull designers avail themselves of the services.
Although a number of very interesting and revealing test series have been conducted in the last 20 years, few of them are out in the public domain. This means, that most multihull designers are tweaking older designs little by little, to hopefully arrive at something better.
It's been a safe way to go, and has produced some really high performing craft, but there is always the possibility that some aspects have been overlooked or that changes are canceling each other out and only very controlled tests can help to identify such issues.
Ships, by comparison, are almost always developed after reference to model tank tests—either through specific ones, or to the standard test series that now exist and are readily available. Even small boat designers could learn more from examining these tests, as through the power of the Speed/Length ratio, data can be readily downsized and anyway, most test models are just 10-20 feet long!!
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