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Log in or Sign up. Boat Design Net. Hello all A few notes on wetted surface area WSA. The notes use only feet, pounds and long tons as I believe readers will be more familiar with them as opposed to metres, kilos and tonnes. I wrote these notes because I have seen on several sites various questions about WSA and the effect on speed, which has more WSA � a catamaran or a monohull, plus several other queries and they are all answered.

I have reviewed many Internet sites and in all reviews of speed, it is assumed that the reader is a designer or a naval architect NA who is familiar with calculations and has the background knowledge. I put myself in the position of someone who wants to build a boat perhaps or buy a kit and asks what is WSA all about and how much paint. Simple questions and I had trouble finding. If you know of anyone who would like some clarification on WSA please point them to build your own excel boat area thread.

WSA has three major applications for vessels Speed and power calculations of displacement hulls Painting areas, in bands etc Anodes, determine how many of a certain size based on area There is hardly any difference except that the hull area must be calculated up to the load waterline and possibly the light waterline for power, up to the various changes in the hull paint scheme colours for paint area and something similar for anodes.

These notes will cover only formulas. It is PITA in spades and not really necessary as the others will suffice. Before using a CAD system, you must understand the things that can go wrong in all the methods if you do not follow some basic rules about where curves end - including the effect of knuckles caused build your own excel boat area chines. If the CAD build your own excel boat area does not describe this, the answers will be wrong unless the method is specifically adapted to your hull form and the manual explicitly declares.

The last method, by integration, is probably not possible as the function will not be known but it is exact with a known function. Formulas These are not exact, there are many formulas, and they all disagree to some extent or. Some specialise in a particular ship type and others are general.

Although called the midship area ratio the area should be taken as the greatest area which usually occurs at the widest beam. For small craft, weights are usually all in pounds and ships are usually in long tons. Because most books give values in long tons rather than pounds the same is done.

Small boats use 1 cubic foot of SW weighs 64 lb, build your own excel boat area FW use The value is 3. Multiply this by 0. These are dangerous if you use them for the first time without anything else to compare them. For example, is it the main hull or build your own excel boat area it include bilge keels, rudders and any other appendages to the hull?

What is the form or the general shape or the proportions? All these have an effect on the answer. But if you work with only one boat type you will be able to calculate the coefficients using known designs.

To be fair, all the formulas need cross-checking. The general formulas take more account of the possible variations in the hull shape etc but they too still need to be checked. All the following formulas show the wetted surface of only the hull and you must add the appendage areas for the grand total.

Rudders, keels, bilge keels are straight-forward but the immersed part of a transom must also be added as the formulas all assume the area curve closes at the waterline ends. For example, the SNAME publication Principles of Naval Arcitecture gives good examples of the difficult secant method etc and Taylor is the only approximation given, see Section 7 of vol 1.

Taylor was instrumental in opening up the first US model-testing tank and he published his brilliant Speed and Power of Ships the full edition in based on investigations carried out during the previous 20 years or so. Before making the expensive models each was drawn to a large scale and the areas were carefully calculated. From these areas Taylor deduced his famous formula. Some designers rarely used it because of limited beam constraints for barge forms and the chore of continually referring to his chart that is shown in the illustration.

The Formula assumes build your own excel boat area the Displacement is in salt water so even if your vessel is in fresh water your must correct the displacement. Their formula takes care of dimensional distortions by making use of what is termed the Displacement Length Coefficient. I have often wondered why Taylor did not use something like this for areas since he made extensive use of it in his method of determining the ship resistance.

The chart has two curves drawn, one is the base values and the other in light red is the trend-line fitted by Excel. The lines closely overlap so you can see the trend-line is a good fit. The formula coefficients for the simple quadratic are shown as is the goddness of fit coefficient R squared. The next one was deduced by Froude, as a young man he was a junior engineer working for I. Many people had conducted experiments with models but it is Froude who laid the scientific groundwork for what is done nowadays.

He not only explained the frictional drag of a ship he published one of the very early formulas for WSA. This was the period when many of the major Yards in Britain had build your own excel boat area own tanks and were experimenting build your own excel boat area the early days of model testing in the rush to find more efficient hulls. It gives good estimates of the WSA of even the most extremely proportioned forms.

Probably one of the most recent formulas is the one developed by Holtrop and Mennen as part of their now classic paper on speed and power. They were researchers at the Dutch testing establishment at Wageningen now known as Marin.

The formula is applicable for a wide range of forms and the published limits for powering are; 0. Examples These are examples using the various formulas for a selection of vessels where the results are declared and are verifiable. A Kayak from the site www. Another classic - Series Model tests of a range of forms in the same family with the range of Cb from 0. The coefficients and proportions of the original model are used not the values of the derived model.

All the data has been corrected to a standard length of Despite repeated searches I was unable to locate any pleasure craft or yachts that gave enough data to check the formulas against known results.

A table of the final check results etc is shown below in the illustration. The results by formula closest to the known values are in a different colour and boldly underlined. No firm conclusions should be drawn because such disparate vessels are used. This concludes the first part, the next posting will be more interesting as it looks at WSA of monohulls versus catamarans.

If anyone has any comments or queries � fire away! Mike DSep 22, If you're going to cover the wetted surface area of multihulls, I hope you plan to include the effects of heel and vertical sail build your own excel boat area, as well as the difference between trimarans and catamarans. Build your own excel boat area surface area Part 2 - monos or duos?

Hello all This posting concerns the wetted surface of a catamaran versus a monohull. All the aspects of stability, suitability, seaworthiness build your own excel boat area are not addressed only the WSA. All values in the same units as the earlier post � feet and pounds or long tons 2, I exclude the South Seas praos and the French catimaron although the latter may have inspired the very slow cat houseboats or inspired by them?

Monohulls were the standard from the earliest times to the late 19th Century when some catamarans were built as railroad ferries for service in the English Channel between the UK and France.

They were largely unsuccessful as the strength of the connections between the hulls and the deck could not be accurately calculated � the old story of a mathematicians taking a years.

Because of serious cracking and failures, the ships were pulled out of service and catamarans re-gained a bad reputation. There had been earlier attempts that were experimental rather than commercial such as those by a medical doctor, Sir William Petty, who discovered that a long narrow hull was faster than a stubby one but less stable.

So he attached two hulls, saw the wave patterns, discovered a great deal of useful information but nothing came of it. He built a 20 ft cat and a series of bigger, successful versions. In the US inthe Hobie appeared and cats never looked back � except at monohulls trying to catch.

All these catamarans have the build your own excel boat area thing in common � speed, and they all follow the same trend in that they are long and skinny compared to a monohull. As boats get bigger and longer that fix becomes impractical as hundreds or thousands of tons would be needed.

The only solution is increased beam and the use of lightweight materials topsides. When any immersed hull increases speed it must overcome two main resistances Frictional � caused by the hull rubbing the water Wave-making � the hull generates a set of waves that vary in size and distribution depending on the hull speed and the general shape and fullness.

As the speed-length ratio rises the wave-making resistance increases at a faster rate and soon dominates. At a speed-length ratio of around 1. Usually only small boats, warships, superfast ferries and racing craft travel build your own excel boat area higher speeds. But build your own excel boat area same 34 knots on a typical frigate or destroyer is about a 1.

The speed at which the wave-making resistance exceeds the frictional resistance depends on the length and shape of the hull. The tendency is therefore to make the cat generally longer than the monohull so it automatically becomes much slimmer. One cat hull is only half the volume of a monohull so it is naturally slimmer but the beam is usually further reduced which decreases the resistance. The frictional resistance has increased but the wave-making resistance has reduced and the total is less on the two hulls.

At very low speeds the frictional resistance will increase by a severe margin but the boat operates at high speeds all the time so the increase is operationally insignificant. There is a balance that must be achieved in that too long a hull will increase the weight as well as requiring a stronger hull because of higher loadings. As with many other naval architectural problems the best answer is a compromise between the conflicting factors taking account of cost as well as the technicalities.

In the first post I said that the Denny-Mumford formula is my favourite build your own excel boat area this is why. All the terms are known in the very basic design, no calculation of other hull form coefficients or characteristics is necessary and no coefficients need be looked up.

All my searching seemed to be on the lively areas so I was forced to use the diagram. It is straight-forward in spreadsheets because they all have routines to find the best fit of a formula to suit the data � in Excel this is Solver.

N is the number of new geometrically build your own excel boat area hulls geosimsso N is 2 for a cat. The build your own excel boat area monohull has the dimensions L, B etc and the new hulls have L1, B1. This means we can forget it because it is build your own excel boat area and we need only look at proportions.

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