Technical » Six Kinds of Cats and Two Kinds of Tris

Release Date: 1/8/2010

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Six Kinds of Cats

and

Two Kinds of Tris

by Calvin H. Markwood

The analytical method used in Analysis and Comparisons of Cruising Multihulls 2009[1] to compare various catamarans and trimarans can also be used to compare design approaches of various designers. Its a sort of reverse engineering thing...look at the results and deduce the approach. Performance and stability are the primary characteristics that can be inferred and indications of how the designer achieves them are often apparent. What cannot be inferred are proions for living while sailing, esthetics, windage and others. Drawings and pictures in the magazines, books and plans provide much of that.



The catamaran designers are Brett Crowther, Kurt Hughes, Derek Kelsall, the team of Morrelli and Melvin, John Shuttleworth, and Chris White. The trimaran designers are Ian Farrier and Dick Newick. The data used length, waterline length (where presented), beam, sail area and displacement were taken from the design books and plans the designers provide for sale plus magazines and web sites. Where waterline length was not available, it was scaled from the drawings. Also, the beam between hull centerlines was needed for reasons explained below. It too was scaled from the drawings.



The data was entered into an Excel spreadsheet and several design, performance and stability characteristics were calculated and plotted on charts for comparison. The data on the charts have unique symbols and colors for each designer. A computer-generated trend line is inserted for each designer to make comparisons easier, but should be used with the cautionthe trend lines represent a composite of all of the boats by each designer. Some designers feature a broad mix of cruisers, racing cruisers and racers; others feature primarily cruisers.



Note that some of the trend lines are linear or straight line and some are power or curved. (See the labels in the chart legends.) The equations used to compute the various parameters determine which to use. For example, if a boat is scaled up from 40 to 44 feet (10%) in overall length, the beam will be increased linearly, also by 10%, but the sail area will be increased by the square (power of 2) of the length increase, or 1.102 = 1.21, or 21%. So a graph of beam to length would be linear, while a graph of sail area to length will be power.



Featured Design Parameters



The following statements are snapshot descriptions of the boat parameters listed and computed in the spreadsheet and presented on the various charts. Their full meaning and derivation are discussed in Analysis and Comparisons of Cruising Multihulls.



Hull length-to-beam ratio (Lwl/Bh) This ratio is the measure of hull slimness at the waterline which is significant for performance. Good catamaran cruisers have values from 8-12, racers from 12 to significantly higher. Farriers trimaran main hulls are in the lower range of 6.8-7.5 because they carry the full displacement in the main hull. Newicks tris appear to have the outriggers slightly immersed, resulting in higher Lwl/Bh ratios quoted by the designer than the estimating tool used in the spreadsheet gives.



Boat overall beam-to-length ratio (Boa/Loa) This ratio gives the overall horizontal footprint or size of the boat. It is useful for determining whether a boat will fit into a given slip or not, but the next parameter is more important for the way the boat sails.



Hull centerline beam-to-waterline length ratio (Bcl/Lwl) This ratio is significant for performance and stability. Too small will result in drag due to wave interactions between the hulls as well as low resistance to capsize. For catamarans, this is the distance between centerlines of the two hulls. For trimarans, it is the distance between the main hull and one outrigger for performance considerations and between the centerlines of the two outriggers for stability considerations.



Displacement (D) Displacement is significant for performance and stability. Heavy boats require wide hulls that will reduce performance over light slim ones. However, a heavier boat will be harder to overturn (Bcl and SA being equal.)



Sail Area (SA) Sail area is significant for performance and stability. Sail area provides the power to drive the boat and the force to cause capsize.



Sail Area vs. Displacement (SA vs. D) This gives a first picture of power to weight and thus an indication of both acceleration and top speed, but the next parameter is better.



Sail Area-to-Displacement ratio (SA/D) High sail area for a given displacement (high SA/D ratio) indicates the ability of the boat to accelerate and determines overall speed potential of a boat. The raw numbers have no specific meaning, but comparison of the numbers of various boats is meaningful. Note however, that boat length (Lwl) is not included in this formula but is significant for determining and comparing boat performance, so this ratio is not conclusive unless the boats are very close to the same length. See, however, Performance Index and Base SpeedTM[2] below.



Displacement-Length (D/(.01Lwl)3) Displacement-length is a measure of hull fullness (as contrasted with waterline slimness for Lwl/Bh). Two hulls with the same Lwl/Bh can have significantly different Displacement length values.



Performance Index (0.5*(Lwl*SA/D)1/2) This index gives an estimate of the relative boat speed to wind speed on a reach, i.e., a figure of .8 says a boat will do 8 knots in a 10 knot wind. It takes into consideration boat length, sail area and displacement, all of the primary parameters that affect performance.



Bruce Number (SA1/2/D1/3) This figure is similar to the Sail Area to Displacement Ratio. In fact, it is its square root. Chris White uses it as an indicator of how robustly a boat is rigged[3]. As shown in the figure, it gives an indication of whether the sail area is chosen with safety or performance in mind.



Base SpeedTM (1.7*Lwl0.5*SA0.353/D0.253) This index gives an estimate of the average speed of a boat over 24 hours under average (or a variety of) conditions. It, too, takes into consideration boat length, sail area and displacement. It has been used to handicap fleets of sailboats of all typesmonohulls, catamarans and trimarans.



Stability Index (Compares the stabilizing moment of the weight of the boat on its hulls to the overturning moment of the wind on the sails) This index allows comparison of the relative stability of several boats. It takes into consideration sail area, beam between hull centerlines and displacement.



Stability Wind Speed[4] or Stability Speed (Similar comparison to Stability Index) This index gives an estimate of the wind speed in knots at which hull lifting will occur. Like Stability Index, it takes into consideration boat length, sail area and displacement. For trimarans, this speed is when the main hull should lift and the full displacement is imposed on the leeward outrigger. The point of sailing is not stated for this index, however, for fast multihulls, the relative wind will generally be on or near the beam when this index applies. It does not apply to conditions when pitch poling, or capsizing over the bows, is the threat.






Discussion of the Designs



Performance comes first, right? The four Crowther racing designs stand toward the top of the Performance Index chart and the Base Speed chart[5]. Shuttleworth's 8M Micro appears to be a day-sailer with sleeping space, so it is not surprising to see it high in performance as well. Shuttleworths Tektron 50 is listed as a "High Performance Racer Cruiser" and appears to be able to live up to the claim. Two of Hughes' boats and three more of Crowther's round out the set that fall above the 1.0 Performance Index figure for high performance boats. Farriers and Newicks trimarans are also right near the top, with Newicks higher. The remainder are in the cruising category rather than the racing category, with Crowther's and Shuttleworth's boats scoring generally above Kelsall's and Hughes'. Chris Whites are generally middle of the pack.



In Analysis and Comparisons of Cruising Multihulls, the trimarans show performance superiority over the broader data bases of catamarans when compared at 40 foot waterline length. This is probably because the catamaran data base contains a large number of boats this size that are designed for charter, with extra attention being paid to living amenities and stability (comfort and safety). The closeness of the Farrier and Newick trimarans to the Crowther and Shuttleworth catamarans in these performance measures shows that these should be competitive boats.


Insight into the relatively low placement of Hughes' boats in the Performance Index chart is most readily obtained from the Sail Area vs. Length chart. Their sail areas are about 30% below the trend line from over 350 cruising catamarans analyzed in Analysis and Comparisons of Cruising Multihulls, and the lowest of the designs compared here. While this under-powered condition hurts performance, it will show up favorably in the Stability Index and Stability Speed charts. Kelsall's boats are not under-powered by comparison with those of Shuttleworth, Crowther, Farrier and Newick, so there is a reason other than sail area for their relatively low performance figures. This is revealed in the following discussion.


The Displacement vs. Length and Lwl/Bh vs. Lwl charts show that all designers to have a range of light to heavy boats. (Note: the Lwl/Bh guidelines on the Displacement vs. Length chart do not apply to the trimarans.) The designers books show a variety of cruisers, cruising racers, racing cruisers and pure racers, so this is not surprising. Hughes designs seem to have the least scatter for the catamarans, with length to hull beam ratios generally in the range of 10 to 12. This is in the high performance range for boats with cruising capability. If they had "average" sail areas, his boats would have very high performance. Kelsall's boats appear to be heavier than the others, providing the answer to his relatively low Performance Indices.


The Stability Index and Stability Wind Speed charts show many things: 1) Crowther's cruising boats are comparatively low in stability because they are generously powered and have comparatively narrow beams. His racers have generous beams, but are very highly powered, as you would expect racers to be. 2) Newicks trimarans tend to be low in stability speed as well, owing to light weight and generous sail areas. 3) Hughes' boats tend to have the highest stability as a consequence (benefit?) of low sail area and good beam, despite the fact that they are comparatively light. Several of them are large day charter boats that are particularly low on sail area, probably to assure safety. This may be observed on the SA/D vs. Length chart as well. 4) Kelsall's boats are also high in stability, but in this case it is a benefit of greater displacement. Their beams are comparatively narrow (see the Bcl/Lwl ratio chart), so there is no unusual stability contribution there. 5) Shuttleworth's boats are substantially beamier but have higher SA/D (on the SA/D vs. Length chart) than Crowther's, with the result that they have about the same stability. 6) Farrier, Morrelli and Melvin, and Whites boats are middle of the pack, indicating a good, conservative balance of performance and safety. Farriers trimarans are also in the middle as their greater beam (strikingly greater as shown on the Bcl/Lwl chart) compensates for their high SA/D ratio.


Other Characteristics Not Analyzed



Hull shapes and underwater appendages can also be observed in the design books to characterize the seven designers' boats. Crowther's hull shapes are hardest to discern, but if he has continued from his late father's designs, they are rounded, possibly elliptical in cross section, canted outward at the keels to gain stability without the weight penalty of making the whole boat wider. High aspect ratio dagger boards are apparent in his designs.

Hughes' designs are generally rounded with high aspect ratio boards as well. Kelsall uses a flattened, rounded hull shape and fixed, low aspect ratio keels. Shuttleworth's hulls also are generally rounded, but with high aspect ratio boards for the high performance boats and fixed, low aspect ratio keels for the cruisers. Whites boats have rounded hulls and high aspect ratio boards. Farriers trimarans have fairly wide main hulls, prompting the following comment from him: But don't assume narrow hulls as being fast - they just sink faster in the water and cannot carry a load. Newicks trimarans have narrower main hulls, generally approaching semi-circular, and sometimes strangely shaped outriggers. They appear to have planing surfaces on the outside with rounded insides.





So, how to sum it up?



The data indicates that Crowther's designs are the most varied, from spidery racing machines to what appear to be a couple of pretty stogy cruisers (SP35 and SP40). Hughes seems to be pressing for modest performance with stability, offering light, beamy, lightly-powered designs. Kelsall's boats are in the same class as the others, but would probably suffer from being heavier. Shuttleworth is pressing for performance with stability, displaying light, beamy, moderately powered designs. White is making safe, luxury cruisers with good performance. Farriers designs are extremely popular, judging from their prominence in boating magazines and venues. Newicks designs are consistently good performers with a long history of success and one-of-a-kind popularity.



So much for analysis! Wouldn't it be great to see some actual racing data to confirm these conclusions?





Calvin H. Markwood

e-mail: multihull.analysis@comcast.net



[1]It is strongly recommended that the article Multihulls 2009 be read and understood before reading this article. Although this article is intended to stand alone, a fuller discussion of the various design, performance and stability parameters is contained in the other article. That article and its companion catamaran and trimaran data base performance and stability graphs can be found on the Multihull Dynamics, Inc. website under the Articles and Books selection.

[2]Base Speed: A Simple Measure for Estimating Multihull Performance by Richard Boehmer, Multihull International, No. 225, April 1989 pp. 108-110.

[3]The Cruising Multihull by Chris White, p. 54, International Marine, 1997

[4]Called Stability in Wind in Multihull Designs by John Shuttleworth Yacht Designs Ltd., 1998, p. 37.

[5]Note that Power on the charts means a trendline that is to second or greater power, NOT a power (motor driven) boat.

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